Inflationary Cosmology: the Story of the Early Universe

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:

CMB by Planck
Credits: ESA and the Planck Collaboration

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!

Anne-Sylvie Deutsch

Author: Anne-Sylvie Deutsch

Anne-Sylvie Deutsch is a graduate student and works with Prof. Shandera on inflation and early universe cosmology. She studied for a Bachelor Degree in Physics at Université Catholique de Louvain in Belgium, and continued at the same university to complete a Master Degree in Particle Physics and Cosmology. During that time, she spent four months at the Laboratoire AstroParticules et Cosmologie in Paris, and wrote her master thesis Polarization of the Cosmic Microwave Background: E and B Modes Separation under the advising of Dr. Bucher and Prof. Ringeval. She then left her home country for another Master Degree in Quantum Field and Fundamental Forces at Imperial College London, where she wrote her second master thesis Effective Field Theories for Inflation under the supervision of Prof. Contaldi. In 2013, she came to Penn State to start her PhD program. She currently works on how the fact that we only have access to a finite range of distance (or modes) can affect the observed statistics of our universe in the context of inflation, and the bias it can induce on the determination of the parameters of the underlying particle physics.