Breaking down CMB Bharat

A consortium of Indian scientists has submitted a proposal to the national space agency for a new space science mission called CMB Bharat. Let’s break it down.

What is CMB Bharat?

According to Tarun Souradeep, a senior professor at the Inter-University Centre for Astronomy and Astrophysics, Pune, the proposal is for a “comprehensive next generation cosmic microwave background mission in international collaboration, with a major Indian contribution.”

What is the cosmic microwave background?

Very simply put, the cosmic microwave background (CMB) is radiation leftover from the time the first atoms formed in the universe, about 378,000 years after the Big Bang. It is the smoke of the ‘smoking gun’, as it were. It manifests as a temperature of 2.7 K in the emptiest regions of space. Without the CMB, these regions should have exhibited a temperature of 0 K. The ‘microwave’ in its name alludes to the radiation’s frequency: 160.23 GHz which falls in the microwave range.

As radiation that has been around since the dawn of space and time, it carries the signatures of various cosmic events that shaped the universe over the last 13.8 billion years. So scientists hoping to understand more about the universe’s evolution often turn to instruments that study the CMB.

What will CMB Bharat do?

Souradeep: “It proposes near-ultimate survey polarisation that would exhaust the primordial information in this ‘gold-mine’ for cosmology.”

The CMB contains different kinds of information, and each kind can be elicited depending on which instruments scientists use to study it. For example, the European Space Agency’s Planck space probe mapped the CMB’s small temperature variations throughout the universe. Based on this, scientists were able to obtain a clearer picture of how mass is distributed throughout space.

The other major feature of the CMB apart from its temperature is its polarisation. As electromagnetic radiation, the CMB is made up of electric and magnetic fields. When these fields bump into certain forces or objects in their path, the direction they’re pointing in changes. This flip is called a polarisation.

By studying how different parts of the CMB are polarised in different ways, scientists can understand what kind of events might have occurred to have caused those flips. It is essentially detective work to unravel the grandest mysteries ever to have existed.

The CMB Bharat proposal envisages an instrument that will study CMB polarisation to a greater extent than the Planck or NASA WMAP probes did – or, as Souradeep put it, to a “near-ultimate” extent. WMAP stands for Wilkinson Microwave Anisotropy Probe. Planck probed about 10% of the CMB’s polarisation while WMAP probed even less.

What kind of instrument will CMB Bharat be?

Souradeep said that it is an imager with “6,000 to 14,000 power detectors in the focal plane”. The focal plane is simply the plane along which the detectors will make their detections.

They will be maintained at a very low temperature, at much less than 1 K. This is because these instruments will emit heat during operation, which will have to be siphoned away lest it interfere with their observations.

As a result, they will be sensitive in the attowatt range – i.e. to energy changes of the order of 0.000000000000000001 joule per second.

What kind of discoveries will CMB Bharat stand to make?

Its goals are classified broadly as ultra-high energy and high energy.

The ultra-high energy regime refers to a very young universe in which its energy was packed so tightly together that gravitational and quantum mechanical effects didn’t express themselves separately, as they do today. Instead, they were thought to have manifested in the form of a unified ‘quantum gravity’.

Of course, we don’t know this for sure; and even if the universe went through this phase, we don’t really know what reality would have looked like. According to Souradeep, CMB Bharat is expected to be able to “reveal the first clear signature of quantum gravity and ultra-high-energy physics in the very early universe”.

It could also help understand the quantum mechanical counterpart of gravitational waves. These are ripples of energy flowing through the spacetime continuum, and are released when very massive bodies accelerate through the continuum.

The Laser Interferometer Gravitational-wave Observatories – known famously as LIGO – detected the classical, or gravitational, form of these waves and won its makers the Nobel Prize for physics in 2017. Their quantum mechanical side, should it exist, remains a mystery.

CMB Bharat’s high-energy regime refers to constituents of the particulate realm. Per Souradeep, the mission will explore problems in neutrino physics, including help determine how many kinds of neutrinos there actually are and the order of their masses; map the distribution of dark matter; and track baryons (composite particles like protons and neutrons) in the observable universe.

Additionally, the instrument will also be able to study the Milky Way galaxy’s astrophysical properties in greater detail.

What’s the status of CMB Bharat?

“The Indian Space Research Organisation has a programmatic approach to science projects,” Souradeep said. ISRO’s Space Science Programme made an ‘announcement of opportunity’ for future astronomy programmes in February 2017. Following this, he said, a “consortium of cosmology researchers” drafted a proposal for CMB Bharat in April that year.

“The project is under review and consideration.”

Souradeep told The Hindu, “Typically, ambitious space missions of this magnitude take over a decade [to] launch. We would like to be observing for 4-6 years and the time to final release of all data and release could extend to [about] five years.”

The Wire
January 27, 2019