## Can gravitational waves be waylaid by gravity?

Yesterday, I learnt the answer is ‘yes’. Gravitational waves can be gravitationally lensed. It seems obvious once you think about it, but not something that strikes you (assuming you’re not a physicist) right away.

When physicists solve problems relating to the spacetime continuum, they imagine it as a four-dimensional manifold: three of space and one of time. Objects exist in the bulk of this manifold and visualisations like the one below are what two-dimensional slices of the continuum look like. This unified picture of space and time was a significant advancement in the history of physics.

While Hendrik Lorentz and Hermann Minkowski first noticed this feature in the early 20th century, they did so only to rationalise empirical data. Albert Einstein was the first physicist to fully figure out the why of it, through his theories of relativity.

Specifically, according to the general theory, massive objects bend the spacetime continuum around themselves. Because light passes through the continuum, its path bends along the continuum when passing near massive bodies. Seen head-on, a massive object – like a black hole – appears to encircle a light-source in its background in a ring of light. This is because the black hole’s mass has caused spacetime to curve around the black hole, creating a cosmic mirage of the light emitted by the object in its background (see video below) as seen by the observer. By focusing light flowing in different directions around it towards one point, the black hole has effectively behaved like a lens.

So much is true of light, which is a form of electromagnetic radiation. And just the way electrically charged particles emit such radiation when they accelerate, massive particles emit gravitational waves when they accelerate. These gravitational waves are said to carry gravitational energy.

Gravitational energy is effectively the potential energy of a body due to its mass. Put another way, a more massive object would pull a smaller body in its vicinity towards itself faster than a less massive object would. The difference between these abilities is quantified as a difference between the objects’ gravitational energies.

Such energy is released through the spacetime continuum when the mass of a massive object changes. For example, when two binary black holes combine to form a larger one, the larger one usually has less mass than the masses of the two lighter ones together. The difference arises because some of the mass has been converted into gravitational energy. In another example, when a massive object accelerates, it distorts its gravitational field; these distortions propagate outwards through the continuum as gravitational energy.

Scientists and engineers have constructed instruments on Earth to detect gravitational energy in the form of gravitational waves. When an object releases gravitational energy into the spacetime continuum, the energy ripples through the continuum the way a stone dropped in water instigates ripples on the surface. And just the way the ripples alternatively stretch and compress the water, gravitational waves alternatively stretch and compress the continuum as they move through it (at the speed of light).

Instruments like the twin Laser Interferometer Gravitational-wave Observatories (LIGO) are designed to pick up on these passing distortions while blocking out all others. That is, when LIGO records a distortion passing through the parts of the continuum where its detectors are located, scientists will know it has just detected a gravitational wave. Because the frequency of a wave is directly proportional to its energy, scientists can use the properties of the gravitational wave as measured by LIGO to deduce the properties of its original source.

(As you might have guessed, even a cat running across the room emits gravitational waves. However, the frequency of these waves is so very low that it is almost impossible to build instruments to measure them, nor are we likely to find such an exercise useful.)

I learnt today that it is also possible for instruments like LIGO to be able to detect the gravitational lensing of gravitational waves. When an object like a black hole warps the spacetime continuum around it, it lenses light – and it is easy to see how it would lens gravitational waves as well. The lensing effect is the result not of the black hole’s ‘direct’ interaction with light as much as its distortion of the continuum. Ergo, anything that traverses the continuum, including gravitational waves, is bound to be lensed by the black hole.

The human body evolved eyes to receive information encoded in visible light, so we can directly see lensed visible-light. However, we don’t possess any organs that would allow us to do the same thing with gravitational waves. Instead, we will need to use existing instruments, like LIGO, to detect these particular distortions. How do we do that?

When two black holes are rapidly revolving around each other, getting closer and closer, they shed more and more of their potential energy as gravitational waves. In effect, the frequency of these waves is quickly increasing together with their amplitude, and LIGO registers this as a chirp (see video below). Once the two black holes have merged, both frequency and amplitude drop to zero (because a solitary spinning black hole does not emit gravitational waves).

In the event of a lensing, however, LIGO will effectively detect two sets of gravitational waves. One set will arrive at LIGO straight from the source. The second set – originally sent off in a different direction – will become lensed towards LIGO. And because the lensed wave will effectively have travelled a longer distance, it will arrive a short while after the direct wave.

However, LIGO will not register two chirps; in fact, it will register no chirps at all. Instead, the direct wave and the lensed wave will interfere with each other inside the instrument to produce a characteristically mixed signal. By the laws of wave mechanics, this signal will have increasing frequency, as in the chirp, but uneven amplitude. If it were sonified, the signal’s sound would climb in pitch but have irregular volume.

A statistical analysis published in early 2018 (in a preprint paper) claimed that LIGO should be able to detect gravitationally lensed gravitational waves at the rate of about once per year (and the proposed Einstein Telescope, at about 80 per year!). A peer-reviewed paper published in January 2019 suggested that LIGO’s design specs allow it to detect lensing effects due to a black hole weighing 10-100,000-times as much as the Sun.

Just like ‘direct’ gravitational waves give away some information about their sources, lensed gravitational waves should also give something away about the objects that deflected them. So if we become able to use LIGO, and/or other gravitational wave detectors of the future, to detect gravitationally lensed gravitational waves, we will have the potential to learn even more about the universe’s inhabitants than gravitational-wave astronomy currently allows us to.

Thanks to inputs from Madhusudhan Raman, @ntavish, @alsogoesbyV and @vaa3.

## A shorter article about short gamma ray bursts lights up little

• Identify a simple and well-defined question
• Describe the question and answer it
• Get the fuck out

Writing with these three rules in mind makes for a good science article. You stick to the point, you know what details to include and what to leave out and, most importantly, you set straightforward expectations and meet them. The overall effect is for the reader to walk away feeling not disappointed. That’s always a happy ending.

Sadly, not everyone writes like this – rather, more broadly, not all news publishers think of science articles this way. For example, The Hindu regularly publishes science articles so packed with information – about the study as much as its authors – that you’re left confused about what you just read. Was it a profile or was it an explainer? It doesn’t matter because it failed either way.

The latest example of this kind of writing is an article about short gamma ray bursts. The binary neutron star merger known by the gravitational-wave event designation GW170817 was expected by astrophysicists to have unleashed a short gamma ray burst at the moment of collision – but data obtained of the event shows no signs of the expected radio signature. A group of scientists led by Kunal Mooley from Oxford University suggested this could be because GW170817 released a new kind of gamma ray burst.

BusinessLine (a business newspaper with the same publisher and top management as The Hindu) carried an article attempting to discuss all this. Sample the opening para, a mulch of facts and inaccuracies:

“The one located on the outskirts of Pune”? Sounds like everyone must know about it even if they don’t. “First-ever detection of gravitation waves”? Not really: gravitation waves, a.k.a. gravity waves and unlike gravitational waves, can be observed in Earth’s atmosphere. Also, the first-ever detection of gravitational waves came last year; what came in August was the first-ever detection of a neutron star merger. The three US scientists won the award for building LIGO, not detecting GW170817.

The rest of the article tries to simultaneously explain Mooley and co.’s interpretation of the data and also provide a glimpse of his educational trajectory. Why would I want to know he studied in Pune and Mumbai? Unless this is because the author wanted to drive home the India connection – which is all the more troubling because it plays up an aspect of the researcher’s identity that is irrelevant to their professional accomplishment. I’ve noticed many publications succumbing to this kind of thinking: if researcher is Indian, cover the paper/study/whatever irrespective of the legitimacy, strength and/or novelty of what they’re saying.

The science ought to take precedent, not the researcher’s identity. But when it doesn’t, you typically end up writing something that’s definitely not news and likely trash. You end up wrapping your national pride around a core of stupidity. I recommend the pages of ScoopwhoopThe QuintThe Better IndiaDailyOThe Times of India and The New Indian Express, among others, for examples. It’s also possible that the author was conscious about providing an India connection so readers in India took the article more seriously. I’ve made noise about such behaviour many times before, such as here: science shouldn’t be assessed, or enjoyed, solely according to what it can do for humankind.

Finally, it’s possible that the newspaper itself wanted to establish all details on record for posterity – but AFAIK, the BusinessLine is not a newspaper of record. This of course is a minor point.

By ditching the extraneous details, the author and the editor could’ve had the space to focus on the science more, using better language and without the painful economy of words it’s currently striving to. They could even have devoted some words to discussing whether other astronomers have disputed Mooley’s interpretation (they have), an exercise that would’ve made the article more reliable than it is. And to those who’re saying the article was probably kept short because there might not have been space in the newspaper, I’ve a bigger complaint: why wasn’t a short version published in print and a longer version online?

In all, I don’t think BusinessLine is taking its science journalism seriously. The time is past when they could’ve gotten ahead simply by being one of the few publications in the country to write articles about short gamma ray bursts. But given the complacency with which the article seems to have been composed and edited, maybe that time shouldn’t have existed in the first place. It surely doesn’t now.

Featured image: An artist’s illustration of a bright gamma-ray burst. Caption and credit: NASA/Wikimedia Commons.

## Onto drafting the gravitational history of the universe

It’s finally happening. As the world turns, as our little lives wear on, gravitational wave detectors quietly eavesdrop on secrets whispered by colliding blackholes and neutron stars in distant reaches of the cosmos, no big deal. It’s going to be just another day.

On November 15, the LIGO scientific collaboration confirmed the detection of the fifth set of gravitational waves, made originally on June 8, 2017, but announced only now. These waves were released by two blackholes of 12 and seven solar masses that collided about a billion lightyears away – a.k.a. about a billion years ago. The combined blackhole weighed 18 solar masses, so one solar mass’s worth of energy had been released in the form of gravitational waves.

The announcement was delayed because the LIGO teams had to work on processing two other, more spectacular detections. One of them involved the VIRGO detector in Italy for the first time; the second was the detection of gravitational waves from colliding neutron stars.

Even though the June 8 is run o’ the mill by now, it is unique because it stands for the blackholes of lowest mass eavesdropped on thus far by the twin LIGO detectors.

LIGO’s significance as a scientific experiment lies in the fact that it can detect collisions of blackholes with other blackholes. Because these objects don’t let any kind of radiation escape their prodigious gravitational pulls, their collisions don’t release any electromagnetic energy. As a result, conventional telescopes that work by detecting such radiation are blind to them. LIGO, however, detects gravitational waves emitted by the blackholes as they collide. Whereas electromagnetic radiation moves over the surface of the spacetime continuum and are thus susceptible to being trapped in blackholes, gravitational waves are ripples of the continuum itself and can escape from blackholes.

Processes involving blackholes of a lower mass have been detected by conventional telescopes because these processes typically involve a light blackhole (5-20 solar masses) and a second object that is not a blackhole but instead usually a star. Mass emitted by the star is siphoned into the blackhole, and this movement releases X-rays that can be spotted by space telescopes like NASA Chandra.

So LIGO’s June 8 detection is unique because it signals a collision involving two light blackholes, until now the demesne of conventional astronomy alone. This also means that multi-messenger astronomy can join in on the fun should LIGO detect a collision of a star and a blackhole in the future. Multi-messenger astronomy is astronomy that uses up to four ‘messengers’, or channels of information, to study a single event. These channels are electromagnetic, gravitational, neutrino and cosmic rays.

The detection also signals that LIGO is sensitive to such low-mass events. The three other sets of gravitational waves LIGO has observed involved black holes of masses ranging from 20-25 solar masses to 60-65 solar masses. The previous record-holder for lowest mass collision was a detection made in December 2015, of two colliding blackholes weighing 14.2 and 7.5 solar masses.

One of the bigger reasons astronomy is fascinating is its ability to reveal so much about a source of radiation trillions of kilometres away using very little information. The same is true of the June 8 detection. According to the LIGO scientific collaboration’s assessment,

When massive stars reach the end of their lives, they lose large amounts of their mass due to stellar winds – flows of gas driven by the pressure of the star’s own radiation. The more ‘heavy’ elements like carbon and nitrogen that a star contains, the more mass it will lose before collapsing to form a black hole. So, the stars which produced GW170608’s [the official designation of the detection] black holes could have contained relatively large amounts of these elements, compared to the stellar progenitors of more massive black holes such as those observed in the GW150914 merger. … The overall amplitude of the signal allows the distance to the black holes to be estimated as 340 megaparsec, or 1.1 billion light years.

The circumstances of the discovery are also interesting. Quoting at length from a LIGO press release:

A month before this detection, LIGO paused its second observation run to open the vacuum systems at both sites and perform maintenance. While researchers at LIGO Livingston, in Louisiana, completed their maintenance and were ready to observe again after about two weeks, LIGO Hanford, in Washington, encountered additional problems that delayed its return to observing.

On the afternoon of June 7 (PDT), LIGO Hanford was finally able to stay online reliably and staff were making final preparations to once again “listen” for incoming gravitational waves. As part of these preparations, the team at Hanford was making routine adjustments to reduce the level of noise in the gravitational-wave data caused by angular motion of the main mirrors. To disentangle how much this angular motion affected the data, scientists shook the mirrors very slightly at specific frequencies. A few minutes into this procedure, GW170608 passed through Hanford’s interferometer, reaching Louisiana about 7 milliseconds later.

LIGO Livingston quickly reported the possible detection, but since Hanford’s detector was being worked on, its automated detection system was not engaged. While the procedure being performed affected LIGO Hanford’s ability to automatically analyse incoming data, it did not prevent LIGO Hanford from detecting gravitational waves. The procedure only affected a narrow frequency range, so LIGO researchers, having learned of the detection in Louisiana, were still able to look for and find the waves in the data after excluding those frequencies.

But what I’m most excited about is the quiet announcement. All of the gravitational wave detection announcements before this were accompanied by an embargo, lots of hype building up, press releases from various groups associated with the data analysis, and of course reporters scrambling under the radar to get their stories ready. There was none of that this time. This time, the LIGO scientific collaboration published their press release with links to the raw data and the preprint paper (submitted to the Astrophysical Journal Letters) on November 15. I found out about it when I stumbled upon a tweet from Sean Carroll.

And this is how it’s going to be, too. In the near future, the detectors – LIGO, VIRGO, etc. – are going to be gathering data in the background of our lives, like just another telescope doing its job. The detections are going to stop being a big deal: we know LIGO works the way it should. Fortunately for it, some of its more spectacular detections (colliding intermediary-mass blackholes and colliding neutron stars) were also made early in its life. What we can all look forward to now is reports of first-order derivatives from LIGO data.

In other words, we can stop focusing on Einstein’s theories of relativity (long overdue) and move on to what multiple gravitational wave detections can tell us about things we still don’t know. We can mine patterns out of the data, chart their variation across space, time and their sources, and begin the arduous task of drafting the gravitational history of the universe.

Featured image credit: Lovesevenforty/pixabay.

## Neutron stars

When the hype for the announcement of the previous GW detection was ramping up, I had a feeling LIGO was about to announce the detection of a neutron-star collision. It wasn’t to be – but in my excitement, I’d written a small part of the article. I’m sharing it below. I’d also recommend reading this post: The Secrets of How Planets Form.

Stars die. Sometimes, when that happens, their outer layers explode into space in a supernova. Their inner layers collapse inwards under the gravity of their own weight in a violent rush. If the starstuff can be packed dense enough, the collapse produces a blackhole – a volume of space where the laws of quantum mechanics and relativity break down and the particles of matter are plunged into a monumental identity crisis. However, if the dying star wasn’t heavy enough when it blew up, then the inward rush will create a very, very, very dense object – but not a blackhole: a neutron star.

Neutron stars are the densest objects in the universe that astronomers can observe. The only things we know are denser than them are blackholes.

You’d think observed means ‘saw’, but what is ‘seeing’ but the light – a form of electromagnetic energy – from an event reaching our eyes? We can’t directly ‘see’ blackholes collide because the collision doesn’t release any electromagnetic energy. So astronomers have built a special kind of eyes – called gravitational wave detectors – that can observe ripples of gravitational energy that the collision lets loose.

The Laser Interferometer Gravitational-wave Detector (LIGO) we already know about. Its twin eyes, located in Washington and Louisiana, US, have detected three blackhole-blackhole collisions thus far. Two of the scientists who helped build it are hot favourites to win the Nobel Prize for physics next week. The other set of eyes involved in the last find is Virgo, a detector in Italy.

You’ve been told that blackholes are freaks of nature. Heavy objects bend spacetime around themselves. Blackholes are freaks because they step it up: they fold it. They’re so heavy that when spacetime bends around them, it goes all the way around and becomes a three-dimensional loop. Thus, a blackhole traps one patch of the cosmos around a vanishingly small heart of darkness. Even light, if it comes close enough, becomes trapped in this loop and can never escape. This is why astronomers can’t observe blackholes directly, and use gravitational-wave detectors instead.

But neutron stars they can observe. They’re exactly what their names suggest: a ball of neutrons. And neutrons experience a force of nature called the strong nuclear force, and it can be 100,000 billion billion billion times stronger than gravity. This makes neutron stars extremely dense and altogether incredibly heavy as well. On their surface, a classic can of Coke will weigh 355,000 billion tonnes, a thousand-times heavier than all the humans on Earth combined.

Sometimes, a neutron star is ravaged by a powerful magnetic field. This field focuses charged particles on the neutron star’s surface into a tight beam of radiation shooting off into space. If the orb is also spinning, then this beam of radiation sweeps through space like the light from a lighthouse sweeps over the sea near it. Such neutron stars are called pulsars.

## The nomenclature of uncertainty

Many science articles in the past year dealt with observations falling short of the evidence threshold but which have been worth writing about simply because of the desperation behind them. Has this prompted science writers to think about the language they use?

The headline of a Nature article published on December 9 reads ‘LIGO black hole echoes hint at general relativity breakdown’. The article is about the prediction of three scientists that, should LIGO find ‘echoes’ of gravitational waves coming from blackhole-mergers, then it could be a sign of quantum-gravity forces at play.

It’s an exciting development because it presents a simple and currently accessible way of probing the universe for signs of phenomena that show a way to unite quantum physics and general relativity – phenomena that have been traditionally understood to be outside the reach of human experiments until LIGO.

The details of the pre-print paper the three scientists uploaded on arXiv were covered by a number of outlets, including The Wire. And The Wire‘s and Forbes‘s headlines were both questions: ‘Has LIGO already discovered evidence for quantum gravity?’ and ‘Has LIGO actually proved Einstein wrong – and found signs of quantum gravity?’, respectively. Other headlines include:

• Gravitational wave echoes might have just caused Einstein’s general theory of relativity to break down – IB Times
• A new discovery is challenging Einstein’s theory of relativity – Futurism
• Echoes in gravitational waves hint at a breakdown of Einstein’s general relativity – Science Alert
• Einstein’s theory of relativity is 100 years old, but may not last – Inverse

The headlines are relevant because: Though the body of a piece has the space to craft what nuance it needs to present the peg, the headline must cut to it as quickly and crisply as possible – while also catching the eye of a potential reader on the social media, an arena where all readers are being inundated with headlines vying for attention.

For example, with the quantum gravity pre-print paper, the headline has two specific responsibilities:

1. To be cognisant of the fact that scientists have found gravitational-wave echoes in LIGO data at the 2.9-sigma level of statistical significance. Note that 2.9 sigma is evidently short of the threshold at which some data counts as scientific evidence (and well short of that at which it counts as scientific fact – at least in high-energy physics). Nonetheless, it still presents a 1-in-270 chance of, as I’ve become fond of saying, an exciting thesis.
2. To make reading the article (which follows from the headline) seem like it might be time well spent. This isn’t exactly the same as catching a reader’s attention; instead, it comprises catching one’s attention and subsequently holding and justifying it continuously. In other words, the headline shouldn’t mislead, misguide or misinform, as well as remain constantly faithful to the excitement it harbours.

Now, the thing about covering scientific developments from around the world and then comparing one’s coverage to those from Europe or the USA is that, for publications in those countries, what an Indian writer might see as an international development is in fact a domestic development. So Nature, Scientific American, Forbes, Futurism, etc. are effectively touting local accomplishments that are immediately relevant to their readers. The Wire, on the other hand, has to bank on the ‘universal’ aspect and by extension on themes of global awareness, history and the potential internationality of Big Science.

This is why a reference to Einstein in the headline helps: everyone knows him. More importantly, everyone was recently made aware of how right his theories have been since they were formulated a century ago. So the idea of proving Einstein wrong – as The Wire‘s headline read – is eye-catching. Second, phrasing the headline as a question is a matter of convenience: because the quasi-discovery has a statistical significance of only 2.9 sigma, a question signals doubt.

But if you argued that a question is also a cop-out, I’d agree. A question in a headline can be interpreted in two ways: either as a question that has not been answered yet but ought to be or as a question that is answered in the body. More often than not and especially in the click-bait era, question-headlines are understood to be of the latter kind. This is why I changed The Wire copy’s headline from ‘What if LIGO actually proved Einstein wrong…’ to ‘Has LIGO actually proved Einstein wrong…’.

More importantly, the question is an escapism at least to me because it doesn’t accurately reflect the development itself. If one accounts for the fact that the pre-print paper explicitly states that gravitational-wave echoes have been found in LIGO data only at 2.9 sigma, there is no question: LIGO has not proved Einstein wrong, and this is established at the outset.

Rather, the peg in this case is – for example – that physicists have proposed a way to look for evidence of quantum gravity using an experiment that is already running. This then could make for an article about the different kinds of physics that rule at different energy levels in the universe, and what levels of access humanity has to each.

So this story, and many others like it in the past year that all dealt with observations falling short of the evidence threshold but which have been worth writing about simply because of the desperation behind them, have – or could have – prompted science writers to think about the language they use. For example, the operative words/clause in the respective headlines listed above are:

• Nature – hint
• IB Times – might have just caused
• Futurism – challenging
• Inverse – may not

Granted that an informed skepticism is healthy for science and that all science writers must remain as familiar with this notion as with the language of doubt, uncertainty, probability (and wave physics, it seems). But it still is likely the case that writers grappling with high-energy physics have to be more familiar than others, dealing as the latest research does with – yes – hope and desperation.

Ultimately, I may not be the perfect judge of what words work best when it comes to the fidelity of syntax to sentiment; that’s why I used a question for a headline in the first place! But I’m very interested in knowing how writers choose and have been choosing their words, if there’s any friction at all (in the larger scheme) between the choice of words and the prevailing sentiments, and the best ways to deal with such situations.

PS: If you’re interested, here’s a piece in which I struggled for a bit to get the words right (and finally had to resort to using single-quotes).

Featured image credit: bongonian/Flickr, CC BY 2.0

## A universe out of sight

We’ve been able to find that the universe is expanding faster than we thought. The LHC has produced the most data on one day. Good news, right?

Two things before we begin:

1. The first subsection of this post assumes that humankind has colonised some distant extrasolar planet(s) within the observable universe, and that humanity won’t be wiped out in 5 billion years.
2. Both subsections assume a pessimistic outlook, and neither projections they dwell on might ever come to be while humanity still exists. Nonetheless, it’s still fun to consider them and their science, and, most importantly, their potential to fuel fiction.

Cosmology

Note: An edited version of this post has been published on The Wire.

A new study whose results were reported this morning made for a disconcerting read: it seems the universe is expanding 5-9% faster than we figured it was.

That the universe is expanding at all is disappointing, that it is growing in volume like a balloon and continuously birthing more emptiness within itself. Because of the suddenly larger distances between things, each passing day leaves us lonelier than we were yesterday. The universe’s expansion is accelerating, too, and that doesn’t simply mean objects getting farther away. It means some photons from those objects never reaching our telescopes despite travelling at lightspeed, doomed to yearn forever like Tantalus in Tartarus. At some point in the future, a part of the universe will become completely invisible to our telescopes, remaining that way no matter how hard we try.

And the darkness will only grow, until a day out of an Asimov story confronts us: a powerful telescope bearing witness to the last light of a star before it is stolen from us for all time. Even if such a day is far, far into the future – the effect of the universe’s expansion is perceptible only on intergalactic scales, as the Hubble constant indicates, and simply negligible within the Solar System – the day exists.

This is why we are uniquely positioned: to be able to see as much as we are able to see. At the same time, it is pointless to wonder how much more we are able to see than our successors because it calls into question what we have ever been able to see. Say the whole universe occupies a volume of X, that the part of it that remains accessible to us contains a volume Y, and what we are able to see today is Z. Then: Z < Y < X. We can dream of some future technological innovation that will engender a rapid expansion of what we are able to see, but with Y being what it is, we will likely forever play catch-up (unless we find tachyons, navigable wormholes, or the universe beginning to decelerate someday).

How fast is the universe expanding? There is a fixed number to this called the deceleration parameter:

q = – (1 + /H2),

where H is the Hubble constant and  is its first derivative. The Hubble constant is the speed at which an object one megaparsec from us is moving away at. So, if q is positive, the universe’s expansion is slowing down. If q is zero, then H is the time since the Big Bang. And if q is negative – as scientists have found to be the case – then the universe’s expansion is accelerating.

We measure the expansion of the universe from our position: on its surface (because, no, we’re not inside the universe). We look at light coming from distant objects, like supernovae; we work out how much that light is ‘red-shifted’; and we compare that to previous measurements. Here’s a rough guide.

What kind of objects do we use to measure these distances? Cosmologists prefer type Ia supernovae. In a type Ia supernova, a white-dwarf (the core of a dead stare made entirely of electrons) is slowly sucking in matter from an object orbiting it until it becomes hot enough to trigger fusion reaction. In the next few seconds, the reaction expels 1044 joules of energy, visible as a bright fleck in the gaze of a suitable telescope. Such explosions have a unique attribute: the mass of the white-dwarf that goes boom is uniform, which means type Ia supernova across the universe are almost equally bright. This is why cosmologists refer to them as ‘cosmic candles’. Based on how faint these candles are, you can tell how far away they are burning.

After a type Ia supernova occurs, photons set off from its surface toward a telescope on Earth. However, because the universe is continuously expanding, the distance between us and the supernova is continuously increasing. The effective interpretation is that the explosion appears to be moving away from us, becoming fainter. How much it has moved away is derived from the redshift. The wave nature of radiation allows us to think of light as having a frequency and a wavelength. When an object that is moving away from us emits light toward us, the waves of light appear to become stretched, i.e. the wavelength seems to become distended. If the light is in the visible part of the spectrum when starting out, then by the time it reached Earth, the increase in its wavelength will make it seem redder. And so the name.

The redshift, z – technically known as the cosmological redshift – can be calculated as:

z = (λobserved – λemitted)/λemitted

In English: the redshift is the factor by which the observed wavelength is changed from the emitted wavelength. If z = 1, then the observed wavelength is twice as much as the emitted wavelength. If z = 5, then the observed wavelength is six-times as much as the emitted wavelength. The farthest galaxy we know (MACS0647-JD) is estimated to be at a distance wherefrom = 10.7 (corresponding to 13.3 billion lightyears).

Anyway, z is used to calculate the cosmological scale-factor, a(t). This is the formula:

a(t) = 1/(1 + z)

a(t) is then used to calculate the distance between two objects:

d(t) = a(t) d0,

where d(t) is the distance between the two objects at time t and d0 is the distance between them at some reference time t0. Since the scale factor would be constant throughout the universe, d(t) and d0 can be stand-ins for the ‘size’ of the universe itself.

So, let’s say a type Ia supernova lit up at a redshift of 0.6. This gives a(t) = 0.625 = 5/8. So: d(t) = 5/8 * d0. In English, this means that the universe was 5/8th its current size when the supernova went off. Using z = 10.7, we infer that the universe was one-twelfth its current size when light started its journey from MACS0647-JD to reach us.

As it happens, residual radiation from the primordial universe is still around today – as the cosmic microwave background radiation. It originated 378,000 years after the Big Bang, following a period called the recombination epoch, 13.8 billion years ago. Its redshift is 1,089. Phew.

A curious redshift is z = 1.4, corresponding to a distance of about 4,200 megaparsec (~0.13 trillion trillion km). Objects that are already this far from us will be moving away faster than at the speed of light. However, this isn’t faster-than-light travel because it doesn’t involve travelling. It’s just a case of the distance between us and the object increasing at such a rate that, if that distance was once covered by light in time t0, light will now need t > t0 to cover it*. The corresponding a(t) = 0.42. I wonder at times if this is what Douglas Adams was referring to (… and at other times I don’t because the exact z at which this happens is 1.69, which means a(t) = 0.37. But it’s something to think about).

Ultimately, we will never be able to detect any electromagnetic radiation from before the recombination epoch 13.8 billion years ago; then again, the universe has since expanded, leaving the supposed edge of the observable universe 46.5 billion lightyears away in any direction. In the same vein, we can imagine there will be a distance (closing in) at which objects are moving away from us so fast that the photons from their surface never reach us. These objects will define the outermost edges of the potentially observable universe, nature’s paltry alms to our insatiable hunger.

Now, a gentle reminder that the universe is expanding a wee bit faster than we thought it was. This means that our theoretical predictions, founded on Einstein’s theories of relativity, have been wrong for some reason; perhaps we haven’t properly accounted for the effects of dark matter? This also means that, in an Asimovian tale, there could be a twist in the plot.

*When making such a measurement, Earthlings assume that Earth as seen from the object is at rest and that it’s the object that is moving. In other words: we measure the relative velocity. A third observer will notice both Earth and the object to be moving away, and her measurement of the velocity between us will be different.

Particle physics

If the news that our universe is expanding 5-9% faster than we thought sooner portends a stellar barrenness in the future, then another foretells a fecundity of opportunities: in the opening days of its 2016 run, the Large Hadron Collider produced more data in a single day than it did in the entirety of its first run (which led to the discovery of the Higgs boson).

Now, so much about the cosmos was easy to visualise, abiding as it all did with Einstein’s conceptualisation of physics: as inherently classical, and never violating the principles of locality and causality. However, Einstein’s physics explains only one of the two infinities that modern physics has been able to comprehend – the other being the world of subatomic particles. And the kind of physics that reigns over the particles isn’t classical in any sense, and sometimes takes liberties with locality and causality as well. At the same time, it isn’t arbitrary either. How then do we reconcile these two sides of quantum physics?

Through the rules of statistics. Take the example of the Higgs boson: it is not created every time two protons smash together, no matter how energetic the protons are. It is created at a fixed rate – once every ~X collisions. Even better: we say that whenever a Higgs boson forms, it decays to a group of specific particles one-Yth of the time. The value of Y is related to a number called the coupling constant. The lower Y is, the higher the coupling constant is, and more often will the Higgs boson decay into that group of particles. When estimating a coupling constant, theoretical physicists assess the various ways in which the decays can happen (e.g., Higgs boson → two photons).

A similar interpretation is that the coupling constant determines how strongly a particle and a force acting on that particle will interact. Between the electron and the electromagnetic force is the fine-structure constant,

α = e2/2ε0hc;

and between quarks and the strong nuclear force is the constant defining the strength of the asymptotic freedom:

αs(k2) = [β0ln(k22)]-1

So, if the LHC’s experiments require P (number of) Higgs bosons to make their measurements, and its detectors are tuned to detect that group of particles, then at least P-times-that-coupling-constant collisions ought to have happened. The LHC might be a bad example because it’s a machine on the Energy Frontier: it is tasked with attaining higher and higher energies so that, at the moment the protons collide, heavier and much shorter-lived particles can show themselves. A better example would be a machine on the Intensity Frontier: its aim would be to produce orders of magnitude more collisions to spot extremely rare processes, such as particles that are formed very rarely. Then again, it’s not as straightforward as just being prolific.

It’s like rolling an unbiased die. The chance that you’ll roll a four is 1/6 (i.e. the coupling constant) – but it could happen that if you roll the die six times, you never get a four. This is because the chance can also be represented as 10/60. Then again, you could roll the die 60 times and still never get a four (though the odds of that happened are even lower). So you decide to take it to the next level: you build a die-rolling machine that rolls the die a thousand times. You would surely have gotten some fours – but say you didn’t get fours one-sixth of the time. So you take it up a notch: you make the machine roll the die a million times. The odds of a four should by now start converging toward 1/6. This is how a particle accelerator-collider aims to work, and succeeds.

And this is why the LHC producing as much data as it already has this year is exciting news. That much data means a lot more opportunities for ‘new physics’ – phenomena beyond what our theories can currently explain – to manifest itself. Analysing all this data completely will take many years (physicists continue to publish papers based on results gleaned from data generated in the first run), and all of it will be useful in some way even if very little of it ends up contributing to new ideas.

Occasionally, an oddball will show up – like a pentaquark, a state of five quarks bound together. As particles in their own right, they might not be as exciting as the Higgs boson, but in the larger schemes of things, they have a role to call their own. For example, the existence of a pentaquark teaches physicists about what sorts of configurations of the strong nuclear force, which holds the quarks together, are really possible, and what sorts are not. However, let’s say the LHC data throws up nothing. What then?

Tumult is what. In the first run, the LHC used to smash two beams of billions of protons, each beam accelerated to 4 TeV and separated into 2,000+ bunches, head on at the rate of two opposing bunches every 50 nanoseconds. In the second run, after upgrades through early 2015, the LHC smashes bunches accelerated to 6.5 TeV once every 25 nanoseconds. In the process, the number of collisions per sq. cm per second increased tenfold, to 1 × 1034. These heightened numbers are so new physics has fewer places to hide; we are at the verge of desperation to tease them out, to plumb the weakest coupling constants, because existing theories have not been able to answer all of our questions about fundamental physics (why things are the way they are, etc.). And even the barest hint of something new, something we haven’t seen before, will:

• Tell us that we haven’t seen all that there is to see**, that there is yet more, and
• Validate this or that speculative theory over a host of others, and point us down a new path to tread

Axiomatically, these are the desiderata at stake should the LHC find nothing, even more so that it’s yielded a massive dataset. Of course, not all will be lost: larger, more powerful, more innovative colliders will be built – even as a disappointment will linger. Let’s imagine for a moment that all of them continue to find nothing, and that persistent day comes to be when the cosmos falls out of our reach, too. Wouldn’t that be maddening?

**I’m not sure of what an expanding universe’s effects on gravitational waves will be, but I presume it will be the same as its effect on electromagnetic radiation. Both are energy transmissions travelling on the universe’s surface at the speed of light, right? Do correct me if I’m wrong.

## What's the universe telling us post-LIGO?

Since the LIGO Scientific Collaboration announced the first direct detection of gravitational waves on February 11, 2016, there have been at least 51 scientific papers written up on the topic discussing a variety of possibilities. The earliest papers parallel the announcement’s two ostensible achievements:

1. Albert Einstein was right when he postulated the existence of gravitational waves in 1915, in his theory of general relativity.
2. LIGO’s working principle is valid – in other words, the observatory works.

The third achievement was more of a signal: that the era of gravitational astronomy has begun, an era in which humankind will be able to study objects in the universe based on the gravitational effects they have on their surroundings, on the spacetime continuum. And in keeping with this new possibility, many of the 51 papers explore what else we can figure about the two blackholes that merged and caused the waves that LIGO detected.

Here’s a categorised list of their (informed) hypotheses along with brief descriptions.

Okay, was it a legit detection? Does it fit the theory? And is LIGO awesome yet?

1. http://arxiv.org/abs/1602.08492 – “We summarise the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network Circulars, giving an overview of the participating facilities, the gravitational wave sky localisation coverage, the timeline and depth of the observations”
2. http://arxiv.org/abs/1602.06833 – “total-variation denoising techniques may thus offer an additional viable approach for waveform reconstruction”
3. http://arxiv.org/abs/1602.04782 – “The chirp signal from the gravitational-wave event GW150914 is used to place numerous first constraints on gravitational Lorentz violation”
4. http://arxiv.org/abs/1602.04779 – “We point out that GW150914 experienced a Shapiro delay due to the gravitational potential of the mass distribution along the line of sight of about 1800 days”
5. http://arxiv.org/abs/1602.04666 – “… we can design activities that directly involve the detection of GW150914, the designation of the Gravitation Wave signal detected on September 14, 2015, thereby engage the students in this exciting discovery directly. The activities naturally do not include the construction of a detector or the detection of gravitational waves. Instead, we design it to include analysis of the data from GW150914, which includes some interesting analysis activities for students of the introductory course.”
6. http://arxiv.org/abs/1602.04531 – “We find that the existence of GW150914 does not require enhanced double black hole formation in dense stellar clusters or via exotic evolutionary channels. … We predict that BH-BH mergers with total mass of 20-80 Msun are to be detected next.”
7. http://arxiv.org/abs/1602.04199 – “Based on our observations, we conclude that it is unlikely that GW150914 was caused by the core collapse of a supergiant in the LMC, consistent with the LIGO Collaboration analyses of the gravitational wave form as best described by a binary black hole merger”
8. http://arxiv.org/abs/1602.04198 – “We report initial results of a deep search for an optical counterpart to the gravitational wave event GW150914, the first trigger from the Advanced LIGO gravitational wave detectors”
9. http://arxiv.org/abs/1602.03847 – “The stochastic gravitational-wave background from binary black holes, created from the incoherent superposition of all the merging binaries in the Universe, could be higher than previously expected. Using the properties of GW150914, we estimate the energy density of such a background from binary black holes. … We conclude that this background is potentially measurable by the Advanced LIGO/Virgo detectors operating at their projected final sensitivity.”
10. http://arxiv.org/abs/1602.03845 – “In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors”
11. http://arxiv.org/abs/1602.03844 – “This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event”
12. http://arxiv.org/abs/1602.03843 – “We find that the reconstructed waveform is consistent with the signal from a binary black-hole merger with a chirp mass of ∼30M⊙ and a total mass before merger of ∼70M⊙ in the detector frame”
13. http://arxiv.org/abs/1602.03841 – “Within our statistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-field regime of gravity”
14. http://arxiv.org/abs/1602.03840 – Discusses the properties of the merger
15. http://arxiv.org/abs/1602.03839 – “GW150914 was observed with a matched filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 {sigma}”
16. http://arxiv.org/abs/1602.03838 – “At full sensitivity, the Advanced LIGO detectors are designed to deliver another factor of three improvement in the signal-to-noise ratio for binary black hole systems similar in masses to GW150914”

What were the particulate or energetic effects of the blackhole merger?

1. http://arxiv.org/abs/1602.08764 – “The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. We obtained radio data with the Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes.”
2. http://arxiv.org/abs/1602.08436 – “We discuss [high-energy neutrinos] emission in connection with the … event GW150914 which could be associated with a short gamma-ray burst detected by the Fermi Gamma-ray Burst Monitor (GBM) 0.4 s after the GW event and within localisation uncertainty of the GW event”
3. http://arxiv.org/abs/1602.07352 – “We argue that the physical constraints required by the association of the Fermi GBM signal contemporaneous with GW150914 are astrophysical highly implausible”
4. http://arxiv.org/abs/1602.06961 – “The recent detection of the gravitational wave source GW150914 by the LIGO collaboration motivates a speculative source for the origin of ultrahigh energy cosmic rays as a possible byproduct of the immense energies achieved in black hole mergers, provided that the black holes have spin … and there are relic magnetic fields and disk debris remaining from the formation of the black holes or from their accretion history”
5. http://arxiv.org/abs/1602.05529 – “We model the afterglow of the Fermi GBM event associated with LIGO detection GW150914, under the assumption that the gamma-ray are produced by a short GRB-like relativistic outflow”
6. http://arxiv.org/abs/1602.05411 – “We search for coincident neutrino candidates within the data recorded by the IceCube and ANTARES neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves.”
7. http://arxiv.org/abs/1602.05140 – “The presence of at least one neutron star has long been thought to be an essential element of the model: its tidal disruption provides the needed baryonic material whose rapid accretion onto the post-merger black hole powers the burst. The recent tentative detection by the Fermi satellite of a short GRB in association with the gravitational wave signal GW150914 produced by the merger of two black holes has shaken this standard paradigm.”
8. http://arxiv.org/abs/1602.05050 – “We find that the 1.4 GHz radio flux peaks at ∼1E5 sec after the burst trigger. The radio afterglow is detectable if the ambient matter is dense enough with density larger than ∼10E−2 cm^−3.”
9. http://arxiv.org/abs/1602.04764 – “The observation of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory event GW150914 may be used to constrain the possibility of Lorentz violation in graviton propagation”
10. http://arxiv.org/abs/1602.04735 – “Mergers of stellar-mass black holes are not expected to have electromagnetic counterparts. However, the Fermi GBM detector identified a gamma-ray transient 0.4 s after the gravitational wave (GW) signal GW150914 with consistent sky localisation”
11. http://arxiv.org/abs/1602.04337 – “We briefly show how the very recent LIGO gravitational wave observation GW150914, emitted by a binary black hole merger distant ∼1.3 [billion] ly from the Earth, tightens the phenomenological bound on a massive graviton or on the screening of gravity”
12. http://arxiv.org/abs/1602.04180 – “Our results constrain the ratio of the energy promptly released in gamma-rays in the direction of the observer to the gravitational wave energy”
13. http://arxiv.org/abs/1602.03846 – ‘Astrophysical Implications of the Binary Black-Hole Merger GW150914’

How fast did the gravitational waves move through spacetime?

1. http://arxiv.org/abs/1602.05882 – “Connaughton et al. report the discovery of a possible electromagnetic counterpart to the gravitational wave event GW150914 discovered by LIGO. Assuming that the EM and GW are emitted at the same instant, a constraint is placed on the ratio of the speeds of light and gravitational waves at the level of 1E-17.”
2. http://arxiv.org/abs/1602.04188 – “We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, c_gw ≲ 1.7 in the units of speed of light”
3. http://arxiv.org/abs/1602.04460 – “The difference between the gravitational wave velocity and the speed of the light is found to be smaller than a factor of 1E-17, nicely in agreement with the prediction of general relativity theory”

LIGO can tell us how other observatories could spot gravitational waves (and perform follow-ups checks of the merger LIGO picked up on)

1. http://arxiv.org/abs/1602.06951 – “We show that the black hole binary (BHB) coalescence rates inferred from the advanced LIGO detection of GW150914 imply an unexpectedly loud GW sky at milli-Hz frequencies accessible to the evolving Laser Interferometer Space Antenna (eLISA), with several outstanding consequences”
2. http://arxiv.org/abs/1602.04715 – “We discuss the prospects of eLISA for detecting gravitational waves from Galactic binary black holes similar to GW150914”
3. http://arxiv.org/abs/1602.04488 – “… the LAT observed the entire LIGO localisation region within ~70 minutes of the trigger, and thus enabled a comprehensive search for a gamma-ray counterpart to GW150904. The study of the LAT data presented here did not find any potential counterparts to GW150904”
4. http://arxiv.org/abs/1602.04156 – “We have searched for an optical counterpart to the first gravitational wave source discovered by the LIGO experiment, GW150914, using a combination of the Pan-STARRS1 wide-field telescope and the PESSTO spectroscopic follow-up programme”
5. http://arxiv.org/abs/1602.03920 – Probing “the connection between compact binary mergers and short Gamma-ray bursts”
6. http://arxiv.org/abs/1602.03868 – “We report on observations taken with the Swift satellite two days after the GW trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the gravitational wave error region and covered 4.7 square degrees.”

Any other blackhole mergers out there?

1. http://arxiv.org/abs/1603.00884 – “… we systematically vary model assumptions within existing uncertainties and study their effects on the evolution of blackholes in globular clusters and the final structural properties of [the clusters]”
2. http://arxiv.org/abs/1602.08767 – “We consider a system composed of ten black holes with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass blackholes are classified into four types.”
3. http://arxiv.org/abs/1602.05554 – “Here we derived the binary black hole merger rate for isolated binary systems based on the nearby ultra-luminous X-ray source (ULX) luminosity function (LF)”
4. http://arxiv.org/abs/1602.04226 – “We explore the evolution of stellar mass black hole binaries which are formed in self-gravitating active galactic nuclei disks”
5. http://arxiv.org/abs/1602.03842 – “Here we report on the constraints these observations place on the rate of binary blackhole coalescences. Considering only GW150914, assuming that all BBHs (BBH) in the universe have the same masses and spins as this event, imposing a false alarm threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of 2−53/Gpc^3/year (comoving frame)”
6. http://arxiv.org/abs/1602.03790 – “The masses inferred for the black holes in the binary progenitor of GW150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.”

We still know nothing about dark matter and dark energy… right?

1. http://arxiv.org/abs/1603.00699 – Asks what the LIGO find can tell us about the nature and strength of dark energy
2. http://arxiv.org/abs/1603.00464 – “We consider the possibility that the black-hole binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M⊙ ≲ M_bh ≲ 100M⊙ where primordial black holes may constitute the dark matter.”
3. http://arxiv.org/abs/1602.07670 – “We describe the minimal modification required for self-acceleration and show that its maximum likelihood yields a 2.4-sigma poorer fit to cosmological observations compared to a cosmological constant, which, although marginally still possible, questions the concept of cosmic acceleration”
4. http://arxiv.org/abs/1509.08458 – “… gravitational-wave cosmology breaks the dark degeneracy in observations of the large-scale structure between two fundamentally different explanations of cosmic acceleration – a cosmological constant and a scalar-tensor modification of gravity”

Could the blackhole merger have done anything strange?

1. http://arxiv.org/abs/1602.08759 – “After comparing the real and imaginary parts of the ringdown signal of GW150914 with the corresponding quantities for a variety of gravastars, and notwithstanding the very limited knowledge of the perturbative response of rotating gravastars, we conclude it is unlikely that GW150914 produced a rotating gravastar unless its surface is infinitesimally close to the event horizon”
2. http://arxiv.org/abs/1602.08086 – “The magnetospheric activity just before the merger made the FRB, and subsequently an undetected short GRB. The gravitational wave (GW) event GW150914 would be a sister of FRB 150418 in this second scenario. In both cases, one expects an exciting prospect of a GW/FRB/GRB associations.”
3. http://arxiv.org/abs/1602.06526 – “We apply the delay in timing of FERMI GMB transient occurred in coincidence with gravitational waves event GW150914 observed by LIGO to constrain the size of the spherical brane-universe expanding in multi-dimensional space-time”

Obviously some papers belong in more than one category; I’ve binned them according to which categories the unanswered questions in them would best belong in. And why did I draw up this list? Boredom had a bit of a role to begin with but as I picked up more papers, it became harder to keep track of the different avenues of research. And as even more papers crop up, I’ll probably return to – and update – this list, but until then I think there’s fodder here enough for dozens of blog posts.

## What’s the universe telling us post-LIGO?

Since the LIGO Scientific Collaboration announced the first direct detection of gravitational waves on February 11, 2016, there have been at least 51 scientific papers written up on the topic discussing a variety of possibilities. The earliest papers parallel the announcement’s two ostensible achievements:

1. Albert Einstein was right when he postulated the existence of gravitational waves in 1915, in his theory of general relativity.
2. LIGO’s working principle is valid – in other words, the observatory works.

The third achievement was more of a signal: that the era of gravitational astronomy has begun, an era in which humankind will be able to study objects in the universe based on the gravitational effects they have on their surroundings, on the spacetime continuum. And in keeping with this new possibility, many of the 51 papers explore what else we can figure about the two blackholes that merged and caused the waves that LIGO detected.

Here’s a categorised list of their (informed) hypotheses along with brief descriptions.

Okay, was it a legit detection? Does it fit the theory? And is LIGO awesome yet?

1. http://arxiv.org/abs/1602.08492 – “We summarise the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network Circulars, giving an overview of the participating facilities, the gravitational wave sky localisation coverage, the timeline and depth of the observations”
2. http://arxiv.org/abs/1602.06833 – “total-variation denoising techniques may thus offer an additional viable approach for waveform reconstruction”
3. http://arxiv.org/abs/1602.04782 – “The chirp signal from the gravitational-wave event GW150914 is used to place numerous first constraints on gravitational Lorentz violation”
4. http://arxiv.org/abs/1602.04779 – “We point out that GW150914 experienced a Shapiro delay due to the gravitational potential of the mass distribution along the line of sight of about 1800 days”
5. http://arxiv.org/abs/1602.04666 – “… we can design activities that directly involve the detection of GW150914, the designation of the Gravitation Wave signal detected on September 14, 2015, thereby engage the students in this exciting discovery directly. The activities naturally do not include the construction of a detector or the detection of gravitational waves. Instead, we design it to include analysis of the data from GW150914, which includes some interesting analysis activities for students of the introductory course.”
6. http://arxiv.org/abs/1602.04531 – “We find that the existence of GW150914 does not require enhanced double black hole formation in dense stellar clusters or via exotic evolutionary channels. … We predict that BH-BH mergers with total mass of 20-80 Msun are to be detected next.”
7. http://arxiv.org/abs/1602.04199 – “Based on our observations, we conclude that it is unlikely that GW150914 was caused by the core collapse of a supergiant in the LMC, consistent with the LIGO Collaboration analyses of the gravitational wave form as best described by a binary black hole merger”
8. http://arxiv.org/abs/1602.04198 – “We report initial results of a deep search for an optical counterpart to the gravitational wave event GW150914, the first trigger from the Advanced LIGO gravitational wave detectors”
9. http://arxiv.org/abs/1602.03847 – “The stochastic gravitational-wave background from binary black holes, created from the incoherent superposition of all the merging binaries in the Universe, could be higher than previously expected. Using the properties of GW150914, we estimate the energy density of such a background from binary black holes. … We conclude that this background is potentially measurable by the Advanced LIGO/Virgo detectors operating at their projected final sensitivity.”
10. http://arxiv.org/abs/1602.03845 – “In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors”
11. http://arxiv.org/abs/1602.03844 – “This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event”
12. http://arxiv.org/abs/1602.03843 – “We find that the reconstructed waveform is consistent with the signal from a binary black-hole merger with a chirp mass of ∼30M⊙ and a total mass before merger of ∼70M⊙ in the detector frame”
13. http://arxiv.org/abs/1602.03841 – “Within our statistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-field regime of gravity”
14. http://arxiv.org/abs/1602.03840 – Discusses the properties of the merger
15. http://arxiv.org/abs/1602.03839 – “GW150914 was observed with a matched filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 {sigma}”
16. http://arxiv.org/abs/1602.03838 – “At full sensitivity, the Advanced LIGO detectors are designed to deliver another factor of three improvement in the signal-to-noise ratio for binary black hole systems similar in masses to GW150914”

What were the particulate or energetic effects of the blackhole merger?

1. http://arxiv.org/abs/1602.08764 – “The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. We obtained radio data with the Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes.”
2. http://arxiv.org/abs/1602.08436 – “We discuss [high-energy neutrinos] emission in connection with the … event GW150914 which could be associated with a short gamma-ray burst detected by the Fermi Gamma-ray Burst Monitor (GBM) 0.4 s after the GW event and within localisation uncertainty of the GW event”
3. http://arxiv.org/abs/1602.07352 – “We argue that the physical constraints required by the association of the Fermi GBM signal contemporaneous with GW150914 are astrophysical highly implausible”
4. http://arxiv.org/abs/1602.06961 – “The recent detection of the gravitational wave source GW150914 by the LIGO collaboration motivates a speculative source for the origin of ultrahigh energy cosmic rays as a possible byproduct of the immense energies achieved in black hole mergers, provided that the black holes have spin … and there are relic magnetic fields and disk debris remaining from the formation of the black holes or from their accretion history”
5. http://arxiv.org/abs/1602.05529 – “We model the afterglow of the Fermi GBM event associated with LIGO detection GW150914, under the assumption that the gamma-ray are produced by a short GRB-like relativistic outflow”
6. http://arxiv.org/abs/1602.05411 – “We search for coincident neutrino candidates within the data recorded by the IceCube and ANTARES neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves.”
7. http://arxiv.org/abs/1602.05140 – “The presence of at least one neutron star has long been thought to be an essential element of the model: its tidal disruption provides the needed baryonic material whose rapid accretion onto the post-merger black hole powers the burst. The recent tentative detection by the Fermi satellite of a short GRB in association with the gravitational wave signal GW150914 produced by the merger of two black holes has shaken this standard paradigm.”
8. http://arxiv.org/abs/1602.05050 – “We find that the 1.4 GHz radio flux peaks at ∼1E5 sec after the burst trigger. The radio afterglow is detectable if the ambient matter is dense enough with density larger than ∼10E−2 cm^−3.”
9. http://arxiv.org/abs/1602.04764 – “The observation of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory event GW150914 may be used to constrain the possibility of Lorentz violation in graviton propagation”
10. http://arxiv.org/abs/1602.04735 – “Mergers of stellar-mass black holes are not expected to have electromagnetic counterparts. However, the Fermi GBM detector identified a gamma-ray transient 0.4 s after the gravitational wave (GW) signal GW150914 with consistent sky localisation”
11. http://arxiv.org/abs/1602.04337 – “We briefly show how the very recent LIGO gravitational wave observation GW150914, emitted by a binary black hole merger distant ∼1.3 [billion] ly from the Earth, tightens the phenomenological bound on a massive graviton or on the screening of gravity”
12. http://arxiv.org/abs/1602.04180 – “Our results constrain the ratio of the energy promptly released in gamma-rays in the direction of the observer to the gravitational wave energy”
13. http://arxiv.org/abs/1602.03846 – ‘Astrophysical Implications of the Binary Black-Hole Merger GW150914’

How fast did the gravitational waves move through spacetime?

1. http://arxiv.org/abs/1602.05882 – “Connaughton et al. report the discovery of a possible electromagnetic counterpart to the gravitational wave event GW150914 discovered by LIGO. Assuming that the EM and GW are emitted at the same instant, a constraint is placed on the ratio of the speeds of light and gravitational waves at the level of 1E-17.”
2. http://arxiv.org/abs/1602.04188 – “We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, c_gw ≲ 1.7 in the units of speed of light”
3. http://arxiv.org/abs/1602.04460 – “The difference between the gravitational wave velocity and the speed of the light is found to be smaller than a factor of 1E-17, nicely in agreement with the prediction of general relativity theory”

LIGO can tell us how other observatories could spot gravitational waves (and perform follow-ups checks of the merger LIGO picked up on)

1. http://arxiv.org/abs/1602.06951 – “We show that the black hole binary (BHB) coalescence rates inferred from the advanced LIGO detection of GW150914 imply an unexpectedly loud GW sky at milli-Hz frequencies accessible to the evolving Laser Interferometer Space Antenna (eLISA), with several outstanding consequences”
2. http://arxiv.org/abs/1602.04715 – “We discuss the prospects of eLISA for detecting gravitational waves from Galactic binary black holes similar to GW150914”
3. http://arxiv.org/abs/1602.04488 – “… the LAT observed the entire LIGO localisation region within ~70 minutes of the trigger, and thus enabled a comprehensive search for a gamma-ray counterpart to GW150904. The study of the LAT data presented here did not find any potential counterparts to GW150904”
4. http://arxiv.org/abs/1602.04156 – “We have searched for an optical counterpart to the first gravitational wave source discovered by the LIGO experiment, GW150914, using a combination of the Pan-STARRS1 wide-field telescope and the PESSTO spectroscopic follow-up programme”
5. http://arxiv.org/abs/1602.03920 – Probing “the connection between compact binary mergers and short Gamma-ray bursts”
6. http://arxiv.org/abs/1602.03868 – “We report on observations taken with the Swift satellite two days after the GW trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the gravitational wave error region and covered 4.7 square degrees.”

Any other blackhole mergers out there?

1. http://arxiv.org/abs/1603.00884 – “… we systematically vary model assumptions within existing uncertainties and study their effects on the evolution of blackholes in globular clusters and the final structural properties of [the clusters]”
2. http://arxiv.org/abs/1602.08767 – “We consider a system composed of ten black holes with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass blackholes are classified into four types.”
3. http://arxiv.org/abs/1602.05554 – “Here we derived the binary black hole merger rate for isolated binary systems based on the nearby ultra-luminous X-ray source (ULX) luminosity function (LF)”
4. http://arxiv.org/abs/1602.04226 – “We explore the evolution of stellar mass black hole binaries which are formed in self-gravitating active galactic nuclei disks”
5. http://arxiv.org/abs/1602.03842 – “Here we report on the constraints these observations place on the rate of binary blackhole coalescences. Considering only GW150914, assuming that all BBHs (BBH) in the universe have the same masses and spins as this event, imposing a false alarm threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of 2−53/Gpc^3/year (comoving frame)”
6. http://arxiv.org/abs/1602.03790 – “The masses inferred for the black holes in the binary progenitor of GW150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.”

We still know nothing about dark matter and dark energy… right?

1. http://arxiv.org/abs/1603.00699 – Asks what the LIGO find can tell us about the nature and strength of dark energy
2. http://arxiv.org/abs/1603.00464 – “We consider the possibility that the black-hole binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M⊙ ≲ M_bh ≲ 100M⊙ where primordial black holes may constitute the dark matter.”
3. http://arxiv.org/abs/1602.07670 – “We describe the minimal modification required for self-acceleration and show that its maximum likelihood yields a 2.4-sigma poorer fit to cosmological observations compared to a cosmological constant, which, although marginally still possible, questions the concept of cosmic acceleration”
4. http://arxiv.org/abs/1509.08458 – “… gravitational-wave cosmology breaks the dark degeneracy in observations of the large-scale structure between two fundamentally different explanations of cosmic acceleration – a cosmological constant and a scalar-tensor modification of gravity”

Could the blackhole merger have done anything strange?

1. http://arxiv.org/abs/1602.08759 – “After comparing the real and imaginary parts of the ringdown signal of GW150914 with the corresponding quantities for a variety of gravastars, and notwithstanding the very limited knowledge of the perturbative response of rotating gravastars, we conclude it is unlikely that GW150914 produced a rotating gravastar unless its surface is infinitesimally close to the event horizon”
2. http://arxiv.org/abs/1602.08086 – “The magnetospheric activity just before the merger made the FRB, and subsequently an undetected short GRB. The gravitational wave (GW) event GW150914 would be a sister of FRB 150418 in this second scenario. In both cases, one expects an exciting prospect of a GW/FRB/GRB associations.”
3. http://arxiv.org/abs/1602.06526 – “We apply the delay in timing of FERMI GMB transient occurred in coincidence with gravitational waves event GW150914 observed by LIGO to constrain the size of the spherical brane-universe expanding in multi-dimensional space-time”

Obviously some papers belong in more than one category; I’ve binned them according to which categories the unanswered questions in them would best belong in. And why did I draw up this list? Boredom had a bit of a role to begin with but as I picked up more papers, it became harder to keep track of the different avenues of research. And as even more papers crop up, I’ll probably return to – and update – this list, but until then I think there’s fodder here enough for dozens of blog posts.

## Roundup of missed stories – February 14, 2016

Developments I’d have liked to cover but haven’t been able to for lack of time.

Previous editions here.

1. Zika virus and the 2016 Olympics, Umrah and Hajj – “These mass gatherings provide an additional opportunity to undertake research on the transmission and prevention of Zika virus. Preparedness has been the key to success of recent Hajj mass gatherings held amid known risks, such as pandemic influenza A H1N1, MERS, and Ebola outbreaks. Lessons from Saudi Arabia’s success with hosting Hajj during declared pandemics can be helpful to Brazil and the Olympics organisers. The next 4 months will be a crucial period for both Brazilian and Saudi authorities to review emerging research findings on the natural history of Zika virus through expert consultations. International stakeholders can facilitate the needed advocacy and support.”

2. The incredible story of LIGO – “Dicke, a master at cutting through thorny mechanical dilemmas, also instilled in Weiss the value of solid experimental design. Returning to MIT as a professor, Weiss embraced the teachings of his mentors and became one of the world’s leading experts in high-precision measurements of gravity. The capstone of Weiss’s career is LIGO. Weiss developed the notion of using a special technique called laser interferometry to track minute movements of matter due to gravitational waves. Interferometry involves focused beams of light with well-defined frequencies (i.e., laser beams), traveling along separate paths, then coming together again. The pattern created when the beams reunite provides precise information about the difference in path length.”

3. LIGO’s detection of gravitational waves was predicated on already knowing what to look for – even though we’re finding it for the first time

3a. Cornell theorists affirm gravitational wave detection – ““You need big computers because the equations are so complicated,” explained Larry Kidder, senior research associate and a co-leader of the SXS collaboration. One calculation – with varying masses and spin rates – takes a supercomputer a full week to solve, running 24 hours a day. With different parameters, some calculations take months. SXS created a theoretical catalog of what the different possible gravitational waves would look like. Teukolsky said that the new LIGO paper shows the measured waves with an SXS wave superposed on top and in excellent agreement with the measurements. “That’s a very strong confirmation that these are gravitational waves that come from black holes – and that Einstein’s general theory of relativity is correct,” he said.”

3b. Gravitational waves found, black-hole models led the way – “”Even though the modeling and observations of these gravitational wave sources is difficult, requiring detailed, multi-physics models, the potential to study new realms of physics and understand new astrophysical transients is tremendous. Los Alamos is well-poised to solve these problems,” Fryer said. “Our program studying merger progenitors argued that the most-likely system would be a binary black hole system,” stated Fryer, “and it is gratifying to see that this first detection is exactly such a system. As aLIGO detects more of these mergers we will be able to probe aspects of stellar evolution.””

“That men and women can, in a few short years, take tiny smidges of data from often ill-behaved instruments around the world and judiciously combine them with a wide range of physical theory – including the demanding mathematical subtleties of general relativity – to form an account of something not only unimagined but unimaginable to anyone without the new mental equipment this joint endeavour provided: that seems to me a source of wonder greater than the vastest of astronomical numbers.”

“”These names such as Charlie Chaplin or Elvis Presley are not being arbitrarily thrown up but have come to be associated here with a lot of research. Prima facie, these artists are from the Roma community which have traces in India,” he said. “They migrated from here to Europe 2,500 years ago but till today they have preserved many of social and cultural traditions. We are attempting to bring out that commonality, and express our affinity towards the community.”

“One more observation, about definition: This case teaches us that defining plagiarism in terms of lifting someone else’s sequence of words is far too restrictive. If you will forgive me some technical notation, a sentence with words w1 … wk may also be reasonably suspected of being plagiarized if there is an unacknowledged source sentence x1 … xk such that, for most i between 1 and k, either (a) wi = xi or (b) wi is in the set of words presented in some thesaurus or synonym-dictionary as alternatives for xi or (c) wi and xi are both in the set of words presented in the entry for some third word (recall service and liturgy).”

“The major existing X-ray satellites are NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton, which both launched in 1999. These can analyse the constituent wavelengths of X-rays — the spectra — emitted by point-like objects such as stars. But ASTRO-H will be the first to provide high-resolution spectra for much more spread-out X-ray sources such as galaxy clusters, says Norbert Schartel, project scientist for XMM-Newton at ESA’s European Space Astronomy Centre outside Madrid, who is also a member of ASTRO-H’s ESA team.”

7. Hail, Clooney! – “… everyone was resigned to the fact that this was more or less a Clooney show. The actor was asked if he’d make a sequel to Syriana, the Oscar-winning film on petroleum politics that he produced. He said, “There is a lot wrong with the world, as we all know. But we are in a political period in our country today, and we’re not talking enough about the world. As filmmakers, we react to events. We don’t lead the way. The film happens years after the news story breaks. And you need a good story, good characters.” He spoke of his humanitarian work in Darfur (“it’s very close to me”) and how he’d like to make a film around the conflict. “But we haven’t found the proper script yet.” He said he was meeting Angela Merkel the next day.”

“Who needs a more accurate numerical value of G (the current recommended value6 is 6.67408 ± 0.00031 × 10−11 kg−1 m3 s−2)? The short answer is, nobody, for the moment, but being apparently unable to converge on a value for G undermines our confidence in the metrology of small forces. Although it is true that the orbits of the planets depend on the product of G and the mass of the Sun — the structures of all astrophysical objects are determined by the balance of gravity and other forces produced by, for example, gas, photon or degeneracy pressure — ab initio models of the Sun are still an order of magnitude away from predicting a value of G at a level comparable with laboratory determinations. We do not need a value of G to test for departures from the inverse square law or the equivalence principle. There is as yet no prospect of a theory of quantum gravity that would predict a value for G that could be tested by experiment. Could these unresolved discrepancies in G hide some new physics?”

“We knew this is a sensitive area, and the editorial brought into the open what had been simmering under the surface. What we now have is an opportunity to have an open and frank discussion about data sharing. This is all about the patients. This is all about disease. We can’t let it be about anything else.”

“The problem with effective field theories that the authors address is that higher-dimension terms give rise to contributions that cannot be determined by factorization. In some effective field theories, however, these higher-dimension terms are connected to lower-dimension ones by symmetries. This is the case, for example, for nonlinear sigma models, which describe pion interactions at low energies. In this case, and in others, the symmetries are reflected in the behavior of the scattering amplitudes: they approach zero more rapidly as we take some of the external momenta to zero. Cheung et al. take advantage of this behavior to extend the applicability of Cauchy’s theorem to cases where the infinite-momentum condition fails to hold. Their work allows us to extend the idea of defining quantum field theories via physical principles, instead of via a Lagrangian, to an important class of effective field theories.”

“The history of innovation offers plenty of testaments — most of the people we celebrate as geniuses, whose breakthroughs forever changed our understanding of the world and our experience of life, labored under David Foster Wallace’s definition of true heroism — “minutes, hours, weeks, year upon year of the quiet, precise, judicious exercise of probity and care — with no one there to see or cheer.” Marie Curie toiled in her lab until excessive exposure to radiation begot the finitude of her flesh, wholly unprotected by her two Nobel Prizes. Trailblazing astronomer Maria Mitchell made herself “ill with fatigue” as she peered into the cosmos with her two-inch telescope well into the night, night after night. Thomas Edison tried material after material while looking for a stable filament for the first incandescent bulb, proclaiming: “I have not failed. I’ve just found ten thousand ways that won’t work.” And then there was light.”

“India should stop trying to build the Taj Mahal with borrowed bricks. We need a concerted effort at home to discover and manipulate relevant genes in indigenous organisms and crops (such as chickpea and rice). Indian microbial institutes should take up projects in this direction, because most of the currently used genes for transgenic generation are of microbial origin. That requires a change in direction from an Indian GM-food strategy that has traditionally aimed at quick product development instead of careful assessment of the underlying science. Such home-grown GM crops would also reduce reliance on transgenic technology produced by multinational companies, which is expensive and rarely optimized for the conditions of specific regions. Some GM crops designed abroad need more water than is usually available in some parts of India, for example, putting great stress on farmers. Indian scientists need better training in IP issues, especially when our researchers join foreign collaborations to examine and exploit the molecular biology of our natural resources. Otherwise, Indian researchers may get the scientific credit for discoveries but fail to claim the right to commercialize the products developed.”

“The ash-preserved ruins of Pompeii, more than any other source, have provided historians with a window into just what life in that time and place was like. A Day in Pompeii, an exhibition held at the Melbourne Museum in 2009, gave its more than 330,000 visitors a chance to experience Pompeii’s life even more vividly. The exhibition included a 3D theater installation that featured the animation above. Watch it, and you can see Pompeii brought back to life with computer-generated imagery — and then, in snapshots over the course of 48 hours, entombed by Vesuvius again.”

– “The span of a woman’s twenties—not just in urban India, but elsewhere too—is a period in which she can go from being an ingénue playing at power with older men to becoming, herself, a station of strength. “It is astonishing how strong you become, when you’ve spent a lot of time being other people’s weaknesses,” I write in one story, ‘Corvus’. A weakness—a flaw, a temptation, a mistake. Strength takes shape, invariably, through failure, including the failures of others. It happens, ultimately, through unrequited love, love at the wrong time, love afraid of the sound of its own name. And so, left to yourself in the absence of other scaffolding, you teach yourself how to build an Ark that you fill one by one by one by one with memory that petrifies into treasure, risk that alchemises into beauty, rupture that raptures into meaning. And then, by yourself, you pull its door closed.”

“… energy demand is growing rapidly, leading to construction of just about every form of electricity generation known. The two most populous of these economies — China and India — have great ambitions for nuclear power, and everything else. During 2014, China brought online 5.3 GW of nuclear power, 20.3 GW of wind turbine power, 21.8 GW of hydropower and 53.3 GW of power from thermal plants (mostly coal). Between September 2014 and September 2015, India commissioned a 1 GW nuclear reactor, coal plants generating 16 GW and wind and solar plants generating nearly 5 GW. In recent years, these two, and several other countries, have generated more energy from non-hydro renewables than nuclear energy25. In short, China, India and other developing countries are following an all-of-the-above strategy. As a result, although the overall capacity of nuclear energy might grow, globally the share of nuclear power in electricity generation will continue to drop (Fig. 4). Although costs may currently take a back seat to meeting demand, in the long run the same economic forces shaping the nuclear future in the developed world will limit nuclear growth in the developing world too.”

## Tracing the origins of Pu-244

Plutonium-244 could’ve come from neutron-star mergers, so knowing its abundance could reveal the rate at which such mergers happen.

Excerpt:

The heaviest naturally occurring elements are thought to form not when a star is alive but when it begins to die. Specifically, in the explosion that results when a star weighing 8x to 20x our Sun dies, in a core-collapse supernova (cc-SNe). In this process, the star first implodes to some extent before being rebounded outward in a violent throwing-off of its outer layers. The atoms of lighter elements in these layers could capture free neutrons and transmutate into an atom of a heavier one, called the r-process.

The rebound occurs because if the star’s core weighs less than about 5x our Sun (our entire Sun!), it doesn’t collapse into a blackhole but an intermediary state called a neutron star – a small and extremely dense ball composed almost entirely of neutrons.

Anyway, the expelled elements are dispersed through the interstellar medium, the region of space between stars. Therefrom, for example, they could become part of the ingredients of a new nebula or star, get picked up by passing comets or meteors, or eventually settle down on the surface of a distant planet. For example, the isotope of one such element – plutonium (Pu) – is found scattered among the sediments on the floor of Earth’s deepest seas: plutonium-244.

Based on multiple measurements of the amount of Pu-244 on the seafloor and in the interstellar medium, scientists know how the two amounts correlate over time. And based on astronomical observations, they also know how much Pu-244 each cc-SNe may have produced. But what has caught off recent scientists is that the amount of Pu-244 on Earth over time doesn’t match up with the rate at which cc-SNe occur in the Milky Way galaxy. That is, the amount of Pu-244 on Earth is 100 times lower than there would’ve been if all of it had to have come from cc-SNe.

So where is the remaining Pu-244?

Or, a team of astrophysicists from the Hebrew University, Jerusalem, realised, was so much Pu-244 not being produced in the first place?

## As the ripples in space-time blow through dust…

The last time a big announcement in science was followed by an at least partly successful furor to invalidate it was when physicists at the Gran Sasso National Laboratory, Italy, claimed to have recorded a few neutrinos travelling at faster than the speed of light. In this case, most if not all scientists know something had to be wrong. That nothing can clock such speeds except electromagnetic radiation is set in stone for all practical purposes.

Although astronomers from Harvard University’s Center for Astrophysics (CfA) made a more plausible claim on March 17 on having found evidence of primordial gravitational waves, they do have something in common with the superluminal-neutrinos announcement: prematurity. Since the announcement, it has emerged that the CfA team didn’t account for some observations that would’ve seriously disputed their claims even though, presumably, they were aware that such observations existed. Something like willful negligence…

Imagine receiving a tight slap to the right side of face. If there was good enough contact, the slapper’s fingers should be visible for some time on your right cheek before fading away. Your left cheek should bear mostly no signs of you having just been slapped. The CfA astronomers were trying to look for a similar fingerprint in a sea of energy found throughout the universe. If they found the fingerprint, they’d know the energy was polarized, or ‘slapped’, by primordial gravitational waves more than 13 billion years ago. To be specific, the gravitational waves – which are ripples in space-time – would only have polarized one of two components the energy contains: the B-mode (‘right cheek’), the other being the E-mode (‘left cheek’).

On March 17, CfA astronomers made the announcement that they’d found evidence of B-mode polarization using a telescope situated at the South Pole called BICEP2, hallelujah! Everyone was excited. Many journalists wrote articles without exercising sufficient caution, including me. Then, just the next day I found an astronomy blog that basically said, “Hold on right there…” The author’s contention was that CfA had looked only at certain parts of the sea of energy to come to their conclusion. The rest of the ‘cheek’ was still unexplored, and the blogger believed that if they checked out those areas, the fingerprints actually might not be there (for the life of me I can’t find the blog right now).

“Right from the time of BICEP2 announcement, some important lacunae have been nagging the serious-minded,” N.D. Hari Dass, an adjunct professor at Chennai Mathematical Institute told me. From the instrumental side, he said, there was the possibility of cross-talk between measurements of polarization and of temperature, and between measurements on the E-mode and on the B-mode. On the observational front, CfA simply hadn’t studied all parts of the sky – just one patch above the South Pole where B-mode polarization seemed significant. And they had studied that one patch by filtering for one specific temperature, not a range of temperatures.

“The effect should be frequency-independent if it were truly galactic,” Prof. Dass said.

But the biggest challenge came from quarters that questioned how CfA could confirm the ‘slappers’ were indeed primordial gravitational waves and not something else. Subir Sarkar, a physicist at Oxford University, and his colleagues were able to show that what BICEP2 saw to be B-mode polarization could actually have been from diffuse radio emissions from the Milky Way and magnetized dust. The pot was stirred further when the Planck space telescope team released a newly composed map of magnetic fields across the sky but blanked out the region where BICEP2 had made its observations.

There was reasonable, and it persists… More Planck data is expected by the end of the year and that might lay some contentions to rest.

On June 3, physicist Paul Steinhardt made a provocative claim in Nature: “The inflationary paradigm” – which accounts for B-mode polarization due to gravitational waves – “is fundamentally untestable, and hence scientifically meaningless”. Steinhardt was saying that the theory supposed to back the CfA quest was more like a tautology and that it would be true no matter the outcome. I asked Prof. Dass about this and he agreed.

“Inflation is a very big saga with various dimensions and consequences. One of Steinhardt’s
points is that the multiverse aspect” – which it allows for – “can never be falsified as every conceivable value predict will manifest,” he explained. “In other words, there are no predictions.” Turns out the Nature claim wasn’t provocative at all, implying CfA did not set itself well-defined goals to overcome these ‘axiomatic’ pitfalls or that it did but fell prey to sampling bias. At this point, Prof. Dass said, “current debates have reached some startling professional lows with each side blowing their own trumpets.

It wasn’t as if BICEP2 was the only telescope making these observations. Even in the week leading up to March 17, in fact, another South Pole telescope named Polarbear announced that it had found some evidence for B-mode polarization in the sky (see tweet below). The right thing to do now, then, would be to do what we’re starting to find very hard: be patient and be critical.

## Feeling the pulse of the space-time continuum

Humans have known about the force of gravity since ancient times. Yet, we are still exploring its true nature, how it works, and why it works the way it does.

The Copernican
April 17, 2014

Haaaaaave you met PSR B1913+16? The first three letters of its name indicate it’s a pulsating radio source, an object in the universe that gives off energy as radio waves at very specific periods. More commonly, such sources are known as pulsars, a portmanteau of pulsating stars.

When heavy stars run out of hydrogen to fuse into helium, they undergo a series of processes that sees them stripped off their once-splendid upper layers, leaving behind a core of matter called a neutron star. It is extremely dense, extremely hot, and spinning very fast. When it emits electromagnetic radiation in flashes, it is called a pulsar. PSR B1913+16 is one such pulsar, discovered in 1974, located in the constellation Aquila some 21,000 light-years from Earth.

Finding PSR B1913+16 earned its discoverers the Nobel Prize for physics in 1993 because this was no ordinary pulsar, and it was the first to be discovered of its kind: of binary stars. As the ‘B’ in its name indicates, it is locked in an epic pirouette with a nearby neutron star, the two spinning around each other with the orbit’s total diameter spanning one to five times that of our Sun.

Losing energy but how?

The discoverers were Americans Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst, and their prize-winning discovery didn’t culminate with just spotting the binary pulsar that has come to be named after them. Further, they found that the pulsar’s orbit was shrinking, meaning the system as a whole was losing energy. They found that they could also predict the rate at which the orbit was shrinking using the general theory of relativity.

In other words, PSR B1913+16 was losing energy as gravitational energy while proving a direct (natural) experiment to verify Albert Einstein’s monumental theory from a century ago. (That a human was able to intuit how two neutron stars orbiting each other trillions of miles away could lose energy is homage to the uniformity of the laws of physics. Through the vast darkness of space, we can strip away with our minds any strangeness of its farthest reaches because what is available on a speck of blue is what is available there, too.)

While gravitational energy, and gravitational waves with it, might seem like an esoteric concept, it is easily intuited as the gravitational analogue of electromagnetic energy (and electromagnetic waves). Electromagnetism and gravitation are the two most accessible of the four fundamental forces of nature. When a system of charged particles moves, it lets off electromagnetic energy and so becomes less energetic over time. Similarly, when a system of massive objects moves, it lets off gravitational energy… right?

“Yeah. Think of mass as charge,” says Tarun Souradeep, a professor at the Inter-University Centre for Astronomy and Astrophysics, Pune, India. “Electromagnetic waves come with two charges that can make up a dipole. But the conservation of momentum prevents gravitational radiation from having dipoles.”

According to Albert Einstein and his general theory of relativity, gravitation is a force born due to the curvature, or roundedness, of the space-time continuum: space-time bends around massive objects (an effect very noticeable during gravitational lensing). When massive objects accelerate through the continuum, they set off waves in it that travel at the speed of light. These are called gravitational waves.

“The efficiency of energy conversion – from the bodies into gravitational waves – is very high,” Prof. Souradeep clarifies. “But they’re difficult to detect because they don’t interact with matter.”

Albie’s still got it

In 2004, Joseph Taylor, Jr., and Joel Weisberg published a paper analysing 30 years of observations of PSR B1913+16, and found that general relativity was able to explain the rate of orbit contraction within an error of 0.2 per cent. Should you argue that the binary system could be losing its energy in many different ways, that the theory of general relativity is able to so accurately explain it means that the theory is involved, and in the form of gravitational waves.

Prof. Souradeep says, “According to Newtonian gravity, the gravitational pull of the Sun on Earth was instantaneous action at a distance. But now we know light takes eight minutes to come from the Sun to Earth, which means the star’s gravitational pull must also take eight minutes to affect Earth. This is why we have causality, with gravitational waves in a radiative mode.”

And this is proof that the waves exist, at least definitely in theory. They provide a simple, coherent explanation for a well-defined problem – like a hole in a giant jigsaw puzzle that we know only a certain kind of piece can fill. The fundamental particles called neutrinos were discovered through a similar process.

These particles, like gravitational waves, hardly interact with matter and are tenaciously elusive. Their discovery was predicted by the physicist Wolfgang Pauli in 1930. He needed such a particle to explain how the heavier neutron could decay into the lighter proton, the remaining mass (or energy) being carried away by an electron and a neutrino antiparticle. And the team that first observed neutrinos in an experiment, in 1942, did find it under these circumstances.

Waiting for a direct detection

On March 17, radio-astronomers from the Harvard-Smithsonian Centre for Astrophysics (CfA) announced a more recent finding that points to the existence of gravitational waves, albeit in a more powerful and ancient avatar. Using a telescope called BICEP2 located at the South Pole, they found the waves’ unique signature imprinted on the cosmic microwave background, a dim field of energy leftover from the Big Bang and visible to this day.

At the time, Chao-Lin Kuo, a co-leader of the BICEP2 collaboration, had said, “We have made the first direct image of gravitational waves, or ripples in space-time across the primordial sky, and verified a theory about the creation of the whole universe.”

Spotting the waves themselves, directly, in our human form is impossible. This is why the CfA discovery and the orbital characteristics of PSR B1913+16 are as direct detections as they get. In fact, finding one concise theory to explain actions and events in varied settings is a good way to surmise that such a theory could exist.

For instance, there is another experiment whose sole purpose has been to find gravitational waves, using laser. Its name is LIGO (Laser Interferometer Gravitational-wave Observatory). Its first phase operated from 2002 to 2010, and found no conclusive evidence of gravitational waves to report. Its second phase is due to start this year, in 2014, in an advanced form. On April 16, the LIGO collaboration put out a 20-minute documentary titled Passion for Understanding, about the “raw enthusiasm and excitement of those scientists and researchers who have dedicated their professional careers to this immense undertaking”.

The laser pendula

LIGO works like a pendulum to try and detect gravitational waves. With a pendulum, there is a suspended bob that goes back and forth between two points with a constant rhythm. Now, imagine there are two pendulums swinging parallel to each other but slightly out of phase, between two parallel lines 1 and 2. So when pendulum A reaches line 1, pendulum B hasn’t got there just yet, but it will soon enough.

When gravitational waves, comprising peaks and valleys of gravitational energy, surf through the space-time continuum, they induce corresponding crests and troughs that distort the metrics of space and passage of time in that area. When the two super-dense neutron stars that comprise PSR B1913+16 move around each other, they must be letting off gravitational waves in a similar manner, too.

When such a wave passes through the area where we are performing our pendulums experiment, they are likely to distort their arrival times to lines 1 and 2. Such a delay can be observed and recorded by sensitive instruments.

Analogously, LIGO uses beams of light generated by a laser at one point to bounce back and forth between mirrors for some time, and reconvene at a point. And instead of relying on the relatively clumsy mechanisms of swinging pendulums, scientists leverage the wave properties of light to make the measurement of a delay more precise.

At the beach, you’ll remember having seen waves forming in the distance, building up in height as they reach shallower depths, and then crashing in a spray of water on the shore. You might also have seen waves becoming bigger by combining. That is, when the crests of waves combine, they form a much bigger crest; when a crest and a trough combine, the effect is to cancel each other. (Of course this is an exaggeration. Matters are far less exact and pronounced on the beach.)

Similarly, the waves of laser light in LIGO are tuned such that, in the absence of a gravitational wave, what reaches the detector – an interferometer – is one crest and one trough, cancelling each other out and leaving no signal. In the presence of a gravitational wave, there is likely to be one crest and another crest, too, leaving behind a signal.

A blind spot

In an eight-year hunt for this signal, LIGO hasn’t found it. However, this isn’t the end because, like all waves, gravitational waves should also have a frequency, and it can be anywhere in a ginormous band if theoretical physicists are to be believed (and they are to be): between 10-7 and 1011 hertz. LIGO will help humankind figure out which frequency ranges can be ruled out.

In 2014, the observatory will also reawaken after four-years of being dormant and receiving upgrades to improve its sensitivity and accuracy. According to Prof. Souradeep, the latter now stands at 10-20 m. One more way in which LIGO is being equipped to find gravitational waves is by created a network of LIGO detectors around Earth. There are already two in the US, one in Europe, and one in Japan (although the Japanese LIGO uses a different technique).

But though the network improves our ability to detect gravitational waves, it presents another problem. “These detectors are on a single plane, making them blind to a few hundred degrees of the sky,” Prof. Souradeep says. This means the detectors will experience the effects of a gravitational wave but if it originated from a blind spot, they won’t be able to get a fix on its source: “It will be like trying to find MH370!” Fortunately, since 2010, there have been many ways proposed to solve this problem, and work on some of them is under way.

One of them is called eLISA, for Evolved Laser Interferometer Space Antenna. It will attempt to detect and measure gravitational waves by monitoring the locations of three spacecraft arranged in an equilateral triangle moving in a Sun-centric orbit. eLISA is expected to be launched only two decades from now, although a proof-of-concept mission has been planned by the European Space Agency for 2015.

Another solution is to install a LIGO detector on ground and outside the plane of the other three – such as in India. According to Prof. Souradeep, LIGO-India will reduce the size of the blind spot to a few tens of degrees – an order of magnitude improvement. The country’s Planning Commission has given its go-ahead for the project as a ‘mega-science project’ in the 12th Five Year Plan, and the Department of Atomic Energy, which is spearheading the project, has submitted a note to the Union Cabinet for approval. With the general elections going on in the country, physicists will have to wait until at least June or July to expect to get this final clearance.

Once cleared, of course, it will prove a big step forward not just for the Indian scientific community but also for the global one, marking the next big step – and possibly a more definitive one – in a journey that started with a strange pulsar 21,000 light-years away. As we get better at studying these waves, we have access to a universe visible not just in visible light, radio-waves, X-rays or neutrinos but also through its gravitational susurration – like feeling the pulse of the space-time continuum itself.