Precision cosmology at high redshift with the Lyman-alpha forest

Tracking down the expansion of the Universe through cosmic time

The Milky Way, with all its stars and planets, is just one of the millions of galaxies in a universe that has been expanding since the Big Bang. Our Universe started as a hot dense plasma, that cooled down as the expansion continued. The distribution of matter was initially very homogeneous, but gravity turned small density fluctuations into the large structures that we see in the local Universe: galaxies, groups of galaxies and super clusters.

Fifteen years ago, two different teams of astrophysicists discovered that the Universe is not only expanding, but its expansion is also accelerating. That was a big surprise! When you throw a ball up in the air, you expect gravity to pull it down, reduce its ascending speed, and eventually bring it back to you. Somehow, this does not seem to apply on cosmological scales, and the expansion of the Universe is not slowing down, but speeding up.

The apparent acceleration of the Universe is one of the most important and challenging questions in physics. One possible explanation might involve revising the current theory of gravity, general relativity, derived by Albert Einstein exactly one hundred years ago. Alternatively, the acceleration could be caused by the presence of a new exotic component of the Universe, referred to as dark energy, that would act as a repelling force.

In order to find out the actual cause of the acceleration, we need to study how the expansion of the Universe has changed over time, going as far back as possible to the past. Due to the finite speed of light, when we look at a distant galaxy we are seeing it as it was millions of years ago, when the light was emitted. The further the galaxy we observe, the earlier in time we are looking at. So if we want to study the Universe when it was younger, we have to observe the most remote regions of the Universe.

The main goal of this project is to measure, as precisely as possible, the expansion of the Universe when it was only one fourth of its current age (estimated to be of 13.8 billion years). To do so, I will study the light coming from a rare type of ultra-luminous galaxies, called quasars, that are so bright that we can observe them even when they are much further away than other galaxies.

Not all the light that quasars emit arrives to Earth. Even though the space between galaxies is almost empty, a small amount of gas (mostly hydrogen and helium) is enough to mask some of the light. This gas creates "shadows" in the images of very distant quasars, and we can use these to quantify the density of gas between the quasars and us. With this technique, known as the Lyman alpha forest, we are able to make very large maps of the distribution of matter in the Universe, even in the space between galaxies. These maps will allow me to study the expansion of the Universe with even more accuracy.

This compelling research will help us deepen our understanding of the Universe we live in, how it was formed, and the laws that govern it.