Whether it started with a bang or a bounce, the Universe has evolved in what we currently observe from very different early conditions. Early on, it was hot and far denser than it is today. Scientists believe the structure of the Universe we live in — galaxies and clusters of galaxies — originates from tiny energy fluctuations in the first tiny fraction of a second of the long history of the Universe. Scientists hypothesize that those quantum fluctuations were vastly increased in size by a period of sudden and exponential expansion called inflation. While there are many models of inflation, they all predict that these fluctuations in density then grew by the action of gravity into the large-scale structures we see in the Universe today, like galaxies and clusters of galaxies and even larger-scale webs of galaxies.  The fluctuations also were imprinted in the heat of the early Universe; just as some patches of the Universe were a bit denser than others, some patches were a bit hotter.  And we can map these regions of higher and lower temperatures by observing what has been called “the first light,” the cosmic microwave background (CMB), the heat leftover from the hot early phase of the Universe. 

What is more, the same mechanism that originated density fluctuations may be also responsible for sourcing primordial gravitational waves (PGWs). These PGWs are perturbations of the space-time propagating as waves, similarly to those observed by LIGO-VIRGO. However, PGWs have a different origin, and are much more difficult to detect. They leave a characteristic imprint in the CMB, the so-called B-mode polarization. The detection of B-modes generated by inflationary mechanisms is one of the key goals of Simons Observatory.

Indeed, the nature and detailed structure of primordial fluctuations leave observable features in the subsequent properties of the Universe, including the fluctuations in the CMB.  Simons Observatory is designed to extract this information, and thus crack the mysteries of the early Universe!  With Simons Observatory, we will observe the CMB so accurately that we will be able to distinguish the special features of many models of inflation, so we can pick the right one.  The same accurate observations will test the currently accepted scenario for the evolution and properties of the Universe — the standard model of cosmology — with unprecedented precision.