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.

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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).

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 (?CDM) 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.