Probing patchy reionization via CMB, LSS and their cross-correlations

One of the most important predictions of the Big Bang theory is that the Universe will be filled with electromagnetic radiation as the remnant heat left-over, known as Cosmic Microwave Background (CMB) radiation. Through measurements of the CMB, a simple yet powerful model of the Universe has emerged, providing a precise estimate of its age, contents, topological properties and initial conditions. The latter are thought to be generated during a phase of transient vacuum energy in the very early Universe, the Inflation, driven by the physics of fundamental quantum fields. Although impressive, a question we hope to answer with future cosmological measurements is when cosmic Inflation happened by observing the imprints of those perturbations in the CMB. In particular, the spacetime metric itself is able to generate perturbations in terms of primordial gravitation waves, which perturb the CMB polarization in its curl (B) mode, at the last scattering onto electrons happened when CMB photons decoupled from the rest of the system forming neutral atoms (recombination), and later, hitting again electrons made free by the formation of first structures (reionization era). Cosmological gravitational waves generate B-modes at degree scales and different models of Inflation predict different amplitudes of the signal, which is parametrized in terms of the tensor-to-scalar ratio, r. Other cosmological sources of B-modes exist, such as the Gravitational lensing onto CMB photons travelling to us, deflected by forming cosmological structures, generating CMB anisotropies at the arcminute scale. Also, astrophysical contributions to B-modes, the so-called foregrounds, are capable of contaminating the signal from primordial gravitational waves. Despite these challenges, upcoming CMB experiments are set to detect a level 10^{-3}, as this level carries ultimate information of the Inflationary process itself. On the other side, Reionization is believed to have occurred when the first generations of stars and quasars produced sufficient amounts of UV and X-ray radiation to ionize the vast majority of neutral hydrogen in the Universe. We have measurements telling us when the process started and ended, but poor knowledge of its details because of the complex physics involved. The epoch of Reionization (EoR) is the key event to understand the intergalactic medium (IGM) evolution and subsequent structure formation. For this reason, the study of the reionization epoch has now become a central topic in Astrophysics and Cosmology. Precise measurements of the temperature and polarization anisotropies in CMB is one of the most promising probes of the EoR and hence, exploring fundamental questions in the field of Astrophysics and Cosmology. CMB observations currently constrain the Reionization measuring the optical depth, tau and its epoch parametrized by its mean redshift z. This thesis represents a step further in this analysis, investigating the spatial dependence of the process concerning the details of the involved astrophysical processes. Also, we consider how these new investigations modify the CMB signal induced by Reionization itself. The astrophysics determining how the gas of known particles in the Universe passed from being almost neutral to the ionized state is related to the properties of high redshift galaxies, which are the primary sources of ionizing photons. The spatial structure "Patchiness" of Reionization creates fluctuations in the electron density in different directions along the line of sight and generates secondary B-modes in the CMB, which are targets of the measurements of the ultimate CMB experiments in the next decade. Our methodology includes performing cross-correlation analyses of the fluctuations in optical depth with the brightness temperature in 21cm observations tracing neutral gas at high redshifts, along with its detectability from future CMB and 21cm probes. The measurements yield a determination of the sizes of the characteristic reionizing structures ("bubble") and ionization fraction as a function of redshifts. B-modes in the CMB is generated as a consequence of patchy Reionization due to the screening and scattering mechanisms. We investigate the amplitude of the signal and studied its contamination to primordial B modes. In particular, we exploit recent advances in the understanding of the reionization history through observations of the Lyman-alpha forest. We use high-dynamic-range radiative transfer simulations of cosmological Reionization that are calibrated to these data. These simulations allow us to calibrate the excursion set approach, providing the necessary validity check of this methodology. Our findings suggest that the contribution of Reionization to the search for primordial gravitational waves is unlikely to be a concern for sensitivities of planned and proposed experiments for most realistic models of the reionization history. The thesis is organized as follows. We review the basic physics of the CMB in Chapter 1 and provide definitions relevant for the following Chapters. In Chapter 2 we describe how to model Reionization providing the basics of star formation history in high redshift galaxies; also, we explain the statistical techniques for the reconstruction of the optical depth and forecast the detectability by future CMB experiments. In chapter 3, we studied an alternative probe of patchy Reionization by performing the cross-correlation of the optical depth field with the 21cm brightness temperature field. In chapter 4 we estimate the amplitude of the B-mode signal due to patchy Reionization by using high-dynamic-range radiative transfer simulation of Reionization; in particular, we study the contamination to the primordial B-mode signal from cosmological gravitational waves.