The early universe was hot and dense. Matter was in the form of free electrons and atomic nuclei with light bouncing between them. After about 380 000 years of cosmic expansion, the temperature had dropped enough for the first stable atoms to exist. At that moment, photons started to propagate freely through the Universe. Today, billions of years later, we observe this afterglow of the Big Bang as a faint microwave radiation. This relic radiation isn’t completely uniform but varies in its intensity across the sky, reflecting tiny variations in the density of the primordial matter. Over time, and under the influence of gravity, these density fluctuations grew and the large-scale structure of the Universe was created.

‍

But, what created the fluctuations in the cosmic microwave background? What was the fundamental origin of the large-scale structure of the Universe? A remarkable fact about our Universe is that the structures we see around us are not distributed randomly, but rather display interesting spatial correlations. A central challenge of modern cosmology is to construct a consistent history of the Universe that explains these correlations. Observations suggest that the primordial correlations were created before the hot Big Bang, at energies far exceeding those probed by particle colliders. Members of LeCosPA are playing a leading role in the quest to understand the Universe under these extreme conditions.

Prof. Baumann and his group have made important contributions to the theory of cosmic inflation, a conjectured phase of accelerated expansion before the hot Big Bang. During the inflationary period, small quantum fluctuations were stretched to cosmological distances, rippling the fabric of spacetime in an apparently random, but correlated fashion. These correlations retain a memory of their genesis, providing us a rare glimpse of the Universe in its infancy. While inflation is a successful phenomenological model, it is not yet a complete theory. In particular, the physical origin of the inflationary expansion is still a mystery. LeCosPA's theory group continues to play an important role in the theory of the initial conditions of the hot Big Bang.

Inflation is likely to have been the highest energy observable phenomenon in Nature. This provides the opportunity to use the Universe as a gigantic cosmological collider operating at energies far exceeding those of ordinary colliders. New particles, with extremely large masses, can be spontaneously created by the rapid expansion of the spacetime and, through their decays, produce distinct correlations in the density after inflation. These correlations are tracers of the inflationary expansion history and therefore contain vital information about the physics of inflation. Prof. Baumann and collaborators have developed a bootstrap method to predict the signals of the cosmological collider.