The interplay of energy and matter is simply wonderful because, given the presence of some intrinsic properties, the results of their encounters can be largely predicted. The presence of smoke indicates fire, the presence of shadows both darkness and light, the presence of winds a pressure gradient, the presence of mass a gravitational potential. And a special radiological extension of the last correlation gives rise to a phenomenon called gravitational redshift.

The wave-particle duality insists that electromagnetic radiation, if conceived as a stream of photons, can also be thought of as propagating as waves. All waves have two fundamental properties: wavelength and frequency. If a wave consists of a crest and a trough, the length of a crest-trough pair is its wavelength, and the number of wavelengths traversed by the wave in a second its frequency. Also, the energy contained in a wave is directly proportional to its frequency.

A wave undergoes a gravitational redshift when it moves from a region of lower gravitational potential to a region of higher gravitational potential. Such a potential gradient may be experienced when one moves away from a massive body, from regions of stronger to weaker gravitational pull (note the inverse variation). And when you think of radiation, such as light, moving from the surface of a star and toward a far-away observer, the light gets redshifted. The phenomenon was proposed, implicitly, in 1916 by Albert Einstein through his, and so called, Einstein Field Equations (EFE) that described the general theory of relativity (GR).

When radiation gets redshifted, its frequency gets reduced toward the red portion of the electromagnetic spectrum, hence the name. Agreed, the phenomenon is counter-intuitive. Usually, when the leash on an escaping object is loosened, the object speeds up. In the case of a redshift, however, the frequency is lowered (or the particle slowed).

The real wonder lies in the predictive power of such physics. It doesn’t matter whence the mass and what the wave: their interaction is always preceded and succeeded by a blueshift and a redshift. More, speaking from an application-oriented perspective, the radiation reaching Earth from outer space will always be redshifted. Consider it: the waves will have left the gravitational pull of some body behind on their way toward Earth. In thinking so, given some radiation, its source, and thus the radiation’s initial frequency, it becomes easy to calculate how much mass lies between the source and Earth.

A universal map of the cosmic microwave background (CMB) radiation as recorded by the Wilkinson Microwave Anisotropy Probe (WMAP) after its launch in 2011 (This map serves as a reliable benchmark against which to compare the locally observed frequencies of CMB)

As a naturally available resource, consider the cosmic microwave background (CMB) radiation. The CMB was born when the universe was around 379,000 years old, when the ionic plasma born moments after the Big Bang had cooled to a temperature at which electrons and protons could combine to form hydrogen atoms, leaving the photons decoupled from matter and loosened upon the universe as residual radiation (currently at a temperature of  2.72548 ± 0.00057 K).

And in the CMB-context, the Sachs-Wolfe effect is of two kinds: integrated and non-integrated. The non-integrated Sachs-Wolfe effect occurs at the surface-of-last-scattering, and the integrated version between the surface-of-last-scattering and Earth. The surface mentioned here can be thought of as an imaginary surface in space where the last matter-radiation decouplings occurred. What we’re interested in is the integrated Sachs-Wolfe effect.

Assuming that the photons have just left a star behind, and been gravitationally redshifted in the process, there is a lot of matter they could still encounter on their way to Earth even if our home planet maintains a clear line-of-sight to the star. This includes dust, stray rocks, gases blown around by stellar winds, and – if it does exist – dark energy.

The iconic Pillars of Creation, snapped by the Hubble Space Telescope on April 1, 1995, show columns of intergalactic dust in the midst of star-creation while also being eroded by starlight and stellar winds from other stars in the neighborhood.

Therefore, a great way to detect the presence of dark energy between two points in space would be easy, wouldn’t it? All we’d have to do is measure the redshift in radiation detected by a satellite in orbit around Earth coming from a selected region, and compare it with a map of that region. An analysis of the redshift “leftover” from subtracting the redshift due to matter should yield the amount of dark energy! (See also: WMAP)

This procedure was suggested in 1996 by Neil Turok and Robert Crittenden of the Perimeter Institute, Canada. However, after the first evidence of the integrated Sachs-Wolfe effect was detected in 2003, the correlation between the observed data and already-available maps was very low. This lead some skeptics to suggest that the effect could have instead been caused by space dust. The possibility of their being right was indeed high, until September 11, 2012, when their skepticism was almost conclusively refuted by a team of scientists from the University of Portsmouth and the LMU University Munich.

The study, lead by Tommaso Giannantonio and Crittenden, lasted two years and established at a confidence level of 5.4 sigma (or 99.996%) that the ’03 observation indeed corresponded to dark energy and not any other source of gravitational potential.

The phenomenological legacy of redshifts is derived from its special place in Einstein’s GR. The descriptive EFE first opened even the theoretical possibilities of such redshifts and their applications in astrophysics research.