Phenomenology of models beyond LCDM
Phénoménologie de modèles au-delà du modèle de concordance
Résumé
The prevailing paradigm in Cosmology is that of General Relativity with a cosmological constant Lambda - accounting for the late-time accelerated expansion of the Universe - and some form of non-relativistic (Cold) Dark Matter, responsible for seeding the potential wells at early times. Hence the Lambdarm CDM - or emph{Concordance Model of Cosmology}. This (phenomenological) model is in remarkable agreement with a wide variety of observational probes - over many different scales and epochs in the cosmic history. Because of its simplicity, a positive cosmological constant Lambda is quite appealing, but nevertheless poses the problem of its small value from a fundamental standpoint. In recent years, increasingly precise cosmological observations have reported a few statistically significant curiosities within the lcdm paradigm. The most interesting examples being the (sim 5sigma) discrepancy in the value of H_0 and the (sim2sigma) discrepancy in the amplitude of matter fluctuations sigma_8, as inferred by early and late-time probes. Due to this, many extensions to the simple Lambda picture have been proposed over the years, these go by the name of Dark Energy models.In this thesis, having high-energy physics considerations in mind, we explore various extensions to the standard lcdm paradigm and asses the viability of such models in light of recent and future observations. Our approach is rather phenomenological, aimed at capturing various types of behaviors while focusing on tools that can efficiently discriminate between the wide variety of Dark Energy and Modified Gravity models. One emph{potential smoking gun} is the emph{growth index of density perturbations} gamma. We study in detail the global behaviour of gamma(Omega_m), focusing on models that could lead to emph{a change in its slope} - in sharp contrast with the monotonically decreasing lcdm case. These include, an f(R)-inspired bump in G_{rm eff}(z), a varying w_{rm DE}(z), or more intricate (higher-dimensional) models, such as the Dvali-Gabadadze-Porrati (DGP) model. We also study its behaviour in the presence of an emph{Axion-like} (unclustered) component during matter domination and derive interesting (mathematical) properties.Finally, we explore the possibility of having a negative cosmological constant-dubbed lambda- in the dark sector. For these models to be viable, and accelerate the Universe at late-times, the dark sector should also contain an additional (effective) degree of freedom -dubbed X - such that Omega_{rm DE}=Omega_X+Omega_lambda. We consider various types of behaviours in the X-component, parametrized by a varying EoS w_X(a). We further test the viability of these models through a nested-sampling of the parameter space, and use Bayesian techniques to compare them to lcdm for model selection. We also comment on the implications of introducing a high-H_0 prior in separate runs.noindent Although we find no decisive evidence for Omega_{lambda,0}neq0, its presence remains viable as it hides behind an emph{effective} (positive) Lambda with w_Xsim -1. Models with higher evidence are found to be those with new physics (w_Xleq-1) appearing at large-z.A value of H_0 substantially higher than H_0sim 70; rm km.s^{-1}.Mpc^{-1} would be a decisive test of their viability.
Les observations des dernières décennies ont fait émerger un modèle standard en cosmologie, appelé modèle de concordance. Le paradigme qui s'est imposé grâce aux observations aux échelles cosmologiques est celui d'un univers contenant deux phases d'expansion accélérée. La première permet d'expliquer l'origine des galaxies et amas de galaxies que l'on observe de nos jours, tandis que la deuxième rend compte de l'expansion accélérée actuelle, confirmée par de nombreuses observations. Le mécanisme physique à l'origine de cette expansion accélérée reste une des plus grandes questions ouvertes en physique. Néanmoins, des observations provenant du rayonnement fossile cosmologique (Cosmic microwave background - CMB), des oscillations acoustiques baryoniques (Baryon Acoustic Oscillations -BAO), des supernovae du type Ia (SNeIa) et de nombreuses autres observations, semblent en accord avec l'introduction d'une constante cosmologique Lambda dans les équations du champ de la gravitation (les équations d'Einstein). Par ailleurs, une composante de matière ``sombre'' froide (non-relativiste) -appelée Cold Dark Matter ou CDM- est un ingrédient nécessaire pour expliquer la croissance et la formation des grandes structures dans l'Univers. Ce modèle repose aussi sur des hypothèses concernant les fluctuations cosmologiques primordiales (supposées d'origine inflationnaire). En fait, le modèle LCDM de base est entièrement caractérisé par 6 paramètres libres.
Origine : Version validée par le jury (STAR)