My research interests revolves around inflationary cosmology.
Owing to several observations that started with Edwin Hubble, we know that our universe is in expansion. On very large scales, the distance separating two objects grows, as the fabric of spacetime expands more and more. This means that, if we look back in the past, the universe was much smaller, hotter and denser than it is today.
The model that describes most accurately the history and evolution of the Universe today is called the Λ-CDM (or concordance) model. According to the theory, about thirteen billion years ago, all the matter and energy of the universe was forming an insanely dense, hot and homogeneous soup. Well, actually, the soup was not completely homogeneous; some inhomogeneities, however extremely minute, were present. And the existence of these inhomogeneities in our primordial soup had dramatic consequences. Indeed, as they were denser, they could attract more matter, which would make these regions even denser. Therefore, as billions of years passed, the overdense regions saw their density increase, while the underdense regions became less and less filled with matter. This led to the growth of large scale structures that we observe today, such as clusters of galaxies.
A relic of those very homogeneous times is the faint radio signal, called Cosmic Microwave Background (CMB), that we receive from all the directions in the sky. Here is a picture taken by the Planck satellite:
On this picture, we see a snapshot of the universe when it was approximately 380 thousand years old. The red and blue spots show the tiny differences in temperature (or in density) of the universe. At this time, the fluctuations in temperature are one part in a hundred thousand!
Therefore, from very small inhomogeneities present in the early universe, were born today’s galaxies and stars and nebula and all the rest. But where were those inhomogeneities coming from? This question can be answered by the paradigm of inflation, which describes a phase of exponentially accelerated expansion of our spacetime at the beginning of the Universe. While we don’t have strong observational evidence for inflation yet, it solves many of the problems of the Λ-CDM model of cosmology, and therefore many physicists are working on inflation.
Inflation didn’t last long, but was quite considerable; in about 10-32 seconds, the universe expanded by a factor of more than 1026! During that time, the small quantum fluctuations in density of the pre-inflationnary universe were brought to large, classical scales. And that’s how the primordial inhomogeneities were born!
So, we have a mechanism explaining the existence of the small inhomogeneities of the early universe. But there exists a large variety of ways to implement that mechanism. How do we set apart all the models that cosmologists came up with? By studying the statistics of the inhomogeneities.
In particular, we can look at correlation functions. These functions describe the correlation between two – or more – points in the sky that are separated by a specific angle. And what we see is that the statistics of the fluctuations is very well described by a Gaussian distribution. But small deviations from this Gaussian statistics, that we call non-Gaussianities, could tell us a lot about the history of the universe, and it would help tremendously in discriminating the different inflationary models. Therefore, cosmologists are really excited to observe non-Gaussianities in the near future!