Resumen de Beyond the standard initial conditions: cmb and cosmological degeneracies
In this thesis work I investigated two main topics: 1. Non Gaussianity from CMB secondary anisotropies In particular I studied the CMB bispectrum signal produced by the cross correlation of the lensing and the Integrated Sachs Wolfe/Rees Sciam effect.
2. Isocurvature modes and Baryon Acoustic Oscillations Namely I focused on the impact of an isocurvature contribution on the estimation of the sound horizon at recombination rs and on the expansion history inferred from CMB observations and the implications for BAO measurements.
1.Forthcoming Cosmic Microwave Background (CMB) experiments will open the possibility to detect higher-order correlations in the CMB temperature fluctuations beyond the power spectrum. Since gravitationally induced non-linearities are small at last scattering, the CMB is expected to be the best probe of the primordial non-Gaussianity. This means that it would be possible to study in detail eventual deviations from Gaussian initial conditions and gain an unique insight both into the physics of the early universe and into the understanding of the evolution and growth of structures throughout the imprint of the non-linear growth of structures on secondary anisotropies. Regarding this last issue in particular I investigated the primary-lensing-Rees Sciama bispectrum (LRS) which is the leading contribution to the CMB bispectrum with a blackbody frequency dependence. Both, lensing and the ISW and RS effect are related to the gravitational potential and thus are correlated, leading to a non-vanishing bispectrum signal. As throughout this thesis work, the label RS refers to the combined contribution of the linear effect (Integrated Sachs Wolfe ISW) and the non-linear Rees-Sciama one.
One of the most remarkable thing is that in the case of a cosmic variance limited and all sky experiment (which can be representative of an experiment with the nominal performance of Planck) the signal-to-noise (S/N) of the L-RS bispectrum is of order of 10 so that forthcoming experiments will have the statistical power to detect this signal with high statistical significance.
In principle it will then be possible to use this signal to extract cosmological information on the late time evolution of the universe. The overall signal depends on the balance of two competing contributions along the line of sight: the decaying gravitational potential fluctuations and the amplification due to non-linear gravity. For this reason the effect can be used to place strong constraints on cosmological parameters that determine the growth of structures: \Omega_m, dark energy parameters and sigma8.
2.The measurement of Baryonic Acoustic Oscillations from galaxy surveys is well known to be a robust and powerful tool to constrain dark energy. This method relies on the knowledge of the size of the acoustic horizon at radiation drag derived from Cosmic Microwave Background Anisotropy measurements. This issue turns out to be very delicate and important especially in view of the next generation of galaxy surveys which aims at probing with high accuracy the late time expansion and thus the nature of dark energy by means of BAO at low redshift (z < 2).
In this work I quantified the effect of non-standard initial conditions in the form of an isocurvature component on the determination of dark energy parameters from future BAO surveys. Although the simplest and basic adiabatic picture is widely accepted and provides an excellent fit to current data , there is not a priori reason to discard different and more general initial conditions, provided that current observations still allow for mixed adiabatic and isocurvature contributions. Analyzing the CMB data with the prior assumption of purely adiabatic initial conditions when the real universe contains even a small isocurvature contribution could lead to an incorrect determination of the cosmological parameters since the presence of an isocurvature component introduces new degenerations in the parameters space.
In particular, I found that the presence of an isocurvature contribution, of a magnitude still allowed by present data, can affect both the size of the sound horizon at radiation drag and the CMB inferred expansion history in a non-negligible way. Relying on the prior assumption of purely adiabatic initial conditions when the real universe contains an isocurvature contribution could lead in fact to an incorrect determination of the cosmological parameters. For the forecasts I used an experiment with the characteristics of the Planck mission for CMB and experiments with characteristics of BOSS and EUCLID for galaxy surveys.
The key point is that the systematic error introduced in the sound horizon at radiation drag or, more generally, in the estimation of the cosmological parameters, propagates into a systematic error on quantities like H(z) and DA(z) that can be comparable or larger than the statistical error over a wide range of redshift bins for future BAO experiments. In particular, if there is an isocurvature component (at a level still allowed by present data) but it is ignored in the CMB analysis, the sound horizon and cosmological parameters determination is biased, and, as a consequence, future surveys may incorrectly suggest deviations from a cosmological constant. In order to recover an unbiased determination of the sound horizon and dark energy parameters, a component of isocurvature perturbations must be included in the model when analyzing CMB data. Fortunately, doing so does not increase parameter errors significantly. Note that the fiducial value for the isocurvature parameter, fiso, chosen in this work is slightly smaller than the expected statistical error on fiso from Planck data.
Thus from the CMB data there will be no evidence for isocurvature. The great statistical power of forthcoming BAO data however means that the systematic error induced by such a small amount of isocurvature is much much larger than these surveys statistical errors.
I have also extended the analysis done in Chapter 4 to a joint analysis of a CMB experiment like Planck and a large scale structure Euclid-type galaxy survey for a cosmological model that includes an isocurvature fraction in the initial conditions, varying Dark Energy and curvature.
I found that adding the cosmological information coming from a galaxy survey like Euclid strongly reduces the degeneracies introduced by the presence of an isocurvature contribution with respect to a CMB-only analysis. Adding the LSS information solves all the degeneracies in the case of a small amount of isocurvature fraction fiso = -0.01 even for a general cosmological model with Dark Energy and curvature as parameters.
