Dates: 16, 23, 30 october and 6 november 2014 (14.15pm to 18pm)
Intervenant: Dr Jens CHLUBA, Associate Research Scientist, Johns Hopkins University.
Description: It was already realized in the early days of CMB cosmology that tiny deviations of the cosmic microwave background (CMB) spectrum from a blackbody -- commonly referred to as spectral distortions -- provide a powerful tool for studying the thermal history of our Universe. Since COBE/FIRAS, we know that the average CMB spectrum is extremely close to a perfect blackbody, with deviations limited to one part in ten thousand. Some quarter-century has past since then and technological advances in principle allow improving current constraints on the CMB spectrum by more than three orders of magnitude. In addition, there are several processes in the the early Universe that should create spectral distortions within reach of this new technology. It is thus time to return our attention to the physics of CMB spectral distortions and ask how and what can we learn by studying them. In these lectures, the physics going into the formation and evolution of CMB spectral distortions in the early Universe will be covered. We will start from the Boltzmann equations describing the basic interaction between photons and matter (Compton, double Compton and Bremsstrahlung) and discuss simple analytic solutions of the problem in the pre-recombination era. Particular attention will be given to the different types of distortions (mu, y and residual distortion) and methods for computing them efficiently. We will cover various energy release scenarios (e.g., decaying and annihilating particles), highlighting in particular the physics of the damping of small-scale acoustic modes and possible constraints on the primordial power spectrum. We will then move through the recombination era, highlighting the spectral features associated with hydrogen and helium as one of the important new probes of the epoch. In the post-recombination era, structures form and heat the medium, creating y-distortions at different scales. We will highlight the physics of Sunyaev-Zeldovich effect how we can learn about cluster ’gastrophysics’ and cosmology from this.