My PhD thesis

I am working in the “Astroparticles and Cosmology” (APC) laboratory of Paris Diderot University and my supervisor is Jacques Delabrouille (here is a link to see his personal page).

My PhD subject is related to the discipline named cosmology, which is the science dealing with the study of the Universe as a whole since the Big Bang until today (on the contrary to astrophysics, which deals with the physics of the objects inside it).

One of the major problems of cosmology today is the energy content of the Universe. If one trusts General Relativity, all visible matter in the cosmos (stars, galaxies, clusters of galaxies,…) should count for only 4.6% of the total energy density of the Universe. The rest is hidden but physicists have evidences that the remaining 95.4% exist and have an influence on the Universe physics.

Then if we know about 4.6% of the Universe, what is the rest ? Cosmologists know that it is divided into two “dark” components : dark matter and dark energy. One can measure the gravitational influence of dark matter, on galaxy rotation curves for example, but this type of matter is nowhere to be seen. It appears in recent galaxy formation models that every galaxy should be surrounded by a dark matter halo, much larger than the radius of the host galaxy. The evidence of the presence of dark energy was first highlighted by Hubble’s observations which showed the recession of galaxies from us. Now we know that the Universe is expanding, and that this expansion is accelerated due to some negative pressure fluid (otherwise galaxies would merge together) that we interpret as dark energy. This interpretation might not be correct and that is a key question in cosmology.

Evolution of the energy content of the Universe from the Big Bang until today. Notice how dark energy is important today !

The so-called “cosmological parameters” express the energy densities of each component (dark energy, dark matter but also ordinary matter, radiations…) normalized to the critical energy density of the Universe. These parameters are then linked to the three possible curvatures for the Universe (open, closed, flat).

My PhD thesis aims at improving the constraints on the measure of cosmological parameters using galaxy clusters with data of the Planck satellite (European collaboration of ESA, launched in 2009 and that will stop during January 2012). The following document presents the Planck activity in the APC laboratory : poster_planck_v2.

Galaxy clusters are the largest gravitationally bound structures and therefore are of great interest to study the history of the Universe. For example, their direct observation at high distances (also far away in the past) permits the reconstruction of photons path and then gives information on the energy content, but also on tests of General Relativity. In my thesis I am particularly interested in their indirect observation through the Sunyaev-Zel’dovich effect.

Deep sky : galaxies and galaxy clusters

This effect corresponds to the interaction between Cosmic Microwave Background (CMB) photons and the hot gas inside galaxies in clusters. The CMB photons decoupled from matter only 380 000 years after the Big Bang and are then considered as the first light in the Universe. On their way, they might hit an electron of the cluster gas and, gaining energy, are observed differently from other CMB photons. This effect allows detection of clusters and their study (number counts, density, power spectrum,…) permits to put constraints on the cosmological parameters today.

In a nutshell, the goal of my PhD thesis is to analyze Planck data and improve cluster detection with SZ effect. Then, the new catalogs given by the satellite will serve to improve constraints on cosmological parameters.

The simulated Cosmic Microwave Background. The color differences correspond to temperature fluctuations, of the order of 1 for 100 000 !

Example of a cluster detected by Planck via SZ effect (left) and with X-rays (right).

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