Deflection of remnant light from the big bang through cosmic filaments

Scientists gain a greater understanding of central problems of cosmology like dark matter and dark energy.

The deflection of starlight through the sun during the solar eclipse in 1919 showed the first success of Einstein's General Theory of Relativity proposed in 1915 and secured his place in history. Since then the idea of gravitational lensing - the bending of light due to the influence of mass - has been developed to provide some of the most precise measurements of our Universe.  Gravitational lensing also provides convincing evidence for the dark matter envelopes around galaxies, known as dark matter halos. These dark matters halos introduce an extremely small deflection in light and hence distort the image of background galaxies resulting in a circle to appear slightly elliptical to us. Such effect is extremely small and impossible to measure for a single galaxy.  We are therefore required to average them for millions of galaxies to detect any measurable signal.  This is called ‘Weak gravitational lensing’. This measurement leads to an understanding of how much galaxies grow with time by eating more and more of the matter around them. They also give us the ability to study how a light beam reacts to mass around it over cosmic time.

Another celebrated observation in the study of the universe is called Cosmic Microwave Background (CMB), which is the remaining light after the big bang. The accidental discovery of CMB by Penzias and Wilson in 1964 led to the award of the noble prize in 1978. The CMB has been at the forefront of cosmological information since its first discovery and is one of the most prominent pieces of evidence confirming general relativity.  What we see in this remnant is that it's almost the same in all directions with a tiny fluctuation in its brightness at the level of one part per million. Now the fluctuations themselves have a certain shape. When this remnant light travels from the early universe to us it passes through millions of galaxies and dark matter halos on the way and this introduces tiny deflections due to weak gravitational lensing and has been measured with very high confidence.

In principle, the deflection of light from CMB can be caused by all the matters in the universe and not necessarily just the matter around galaxies. But since galaxies live in the densest region of the Universe, the signal of such deflection will be strongest from those parts. But, in the standard model of the universe, the dark matter is not only in the halos around galaxies but also spread in lines connecting galaxies known as filaments. Therefore such filaments should also cause deflection of CMB light and hence will produce a measurable weak gravitational lensing signal. Recently a team of scientist from the University of Edinburgh, Carnegie Mellon University, Lawrence Berkeley National Lab, University of Washington and the University of Berkeley have detected such a signal for the first time. This is the first step in going beyond the standard weak lensing of CMB around galaxies and should lead to an improved understanding of central problems of cosmology like dark matter and dark energy.  This could have great potential to also test whether Einstein is correct in an environment around filaments where many models will show a different behaviour, even if they behave the same around galaxies.

Dr Shadab Alam reported:  "It is fascinating that we can actually measure these tiny deflections in the light through cosmic filament, as we expect from our current understanding of the universe."