S8 tension: cosmologists can't agree on how clumpy the universe is
Cosmology is in for exciting times, going by the latest research that suggests the key to revealing the fundamental nature of the universe lies in finding out how clumpy it is.
Accepted theory has it that after the universe was explosively born in a 'Big Bang' in the void some 13.8 billion years ago, it expanded, engendering galaxies, star clusters, solar systems, and planets.
When scientists looked at the cosmic microwave background (CMB) — the radiation left over from the Big Bang itself — they saw an absolutely smooth glow across the sky. The early universe must have been remarkably uniform, they concluded, with only small variations in density (of about one part in 100,000 when it was 380,000 years old).
Primordial fluctuations
How did matter in the universe get to be so lumpy today after starting out so evenly? The 'lumps' we see in the universe arose from different chunks of matter like galaxies and dark matter — a hypothetical, invisible form of matter that doesn't interact with light or electromagnetic radiation and which makes up a significant portion of the universe — being pulled together by gravitational forces.
Over the years, cosmologists have tried to map the overall spread of matter through the early universe. In the standard cosmological model, called the Lambda Cold Dark Matter (ΛCDM) model, dark matter and dark energy — the mysterious force that drives the expansion of the universe — comprise about 95% of the universe. The interplay between these components influences how the primordial fluctuations evolved into the large-scale structures that we observe today.
Cosmologists use the term Sigma 8, or S8, to quantify the matter around us. This matter is made up of baryonic particles, such as protons and neutrons, that bunch up in different regions of space. The value of S8 is calculated by studying various regions of the universe. Each region is defined by an astronomical length scale of approximately 26 million light-years. Within these regions, cosmologists count the number of galaxies and other cosmic structures, such as galactic clusters and filaments, to assess the distribution of matter.
A higher value for S8 indicates more clustering with a greater amount of matter clumped together, while a lower value indicates a more uniform distribution of matter.
A problem arose when cosmologists used different ways to measure the value of S8 and came up with different estimates. This lack of agreement has come to be called the 'S8 tension' in astrophysics.
Cosmic-shear surveys
Astronomers have conducted galaxy surveys to determine the value of S8 . One method involves measuring the distortion in the shape of galaxies as seen from the earth: an effect known as cosmic shear. These distortions occur when starlight passes through a galactic cluster and is bent and amplified by gravitational forces, much like a magnifying glass does. Astronomers use this gravitational lensing to study indistinct epochs in the evolution of the universe. Cosmic-shear surveys help to map the diffusion of matter, including dark matter, in the universe so cosmologists can deduce the amplitude of matter fluctuations as quantified by S8.
The results of the latest such survey were recently published in the journal Physics by an international team of researchers from the University of Tokyo. They used the Hyper Suprime-Cam (HSC) — a camera installed on the Subaru Telescope in Hawaii — to collect data and came up with a value of 0.747 for S8, which tallies with the values found by previous surveys.
'The Subaru HSC survey is one of the deepest wide area surveys of the sky,' Surhud S. More, a co-author of the study and professor of astrophysics at the Inter-University Centre for Astronomy and Astrophysics in Pune, wrote in an email. He added that the researchers probed matter's distribution using the gravitational lensing effect down to small scales.
'We were able to show that any movement of ordinary matter, such as gas within the large-scale structure of the universe, will not be sufficient to explain the smaller value of the clumpiness which had been found in our previous study.'
In other words, the discrepancy in S8 has to do with the dark matter and dark energy that pervades the cosmos. While this reaffirms that all is well with the ΛCDM model, it does not dispel the S8 tension itself: studies like this were based on gravitational lensing to determine the value of S8 to be 0.747, which does not agree with the higher value predicted by data from the CMB.
Relic radiation
Cosmologists consider the CMB to be a better tool to look back in space and time. They have known for a long time that the surge of primordial matter in the CMB holds clues to the universe's origins in the form of 'ripples' generated by the expanding universe. These ripples resulted in lumps and bumps — future star clusters and galaxies — in the otherwise uniform fabric of space. These telltale galactic signatures were detected in 1992 by NASA's Cosmic Background Explorer satellite.
But with the S8 tension persisting, the ΛCDM model looks to be in need of modification — unless some as yet undiscovered systematics could affect such a conclusion.
As Prof. More said, 'One of the main difficulties in using deep surveys such as Subaru HSC is our lack of understanding of how fast the galaxies in these surveys are actually receding from us, quantified by the redshift [increase in wavelength] of certain lines in their spectrum. As the millions of galaxies used in these analyses are faint, one cannot analyse the spectrum of light of these galaxies to determine this redshift. This constitutes one of the major uncertainties that still remains unresolved before we start entirely doubting the standard theory of cosmology.'
A new view
Last year, data from the Dark Energy Spectroscopic Instrument in Arizona in the US suggested that the push of dark energy — represented by the cosmological constant lambda in the ΛCDM model — is weakening and that the universe may actually be decelerating over time.
The possibility of dark energy getting weaker means that the pace of expansion of the universe will eventually slow down and may, at some point, even turn negative. In that case, it is not inconceivable that the universe will collapse in on itself in a 'big crunch'.
In any case, the task of updating the ΛCDM model will become easier when the Rubin Legacy Survey of Space and Time (LSST) begins operating later this year. The LSST will launch from the Vera C. Rubin Observatory being built in northern Chile, using its camera — the largest ever built — to peer back in space and time like never before.
Who knows what answers this unparalleled wide-field astronomical survey of the universe, wider and deeper than all previous surveys combined, will provide to questions we can't even imagine now about the mysteries of the universe…
Prakash Chandra is a science writer.
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