Phenomenology of dark energy: exploring the space of theories with future redshift surveys

We use the effective field theory of dark energy to explore the space of modified gravity models which are capable of driving the present cosmic acceleration. We identify five universal functions of cosmic time that are enough to describe a wide range of theories containing a single scalar degree of freedom in addition to the metric. The first function (the effective equation of state) uniquely controls the expansion history of the universe. The remaining four functions appear in the linear cosmological perturbation equations, but only three of them regulate the growth history of large scale structures. We propose a specific parameterization of such functions in terms of characteristic coefficients that serve as coordinates in the space of modified gravity theories and can be effectively constrained by the next generation of cosmological experiments. We address in full generality the problem of the soundness of the theory against ghost-like and gradient instabilities and show how the space of non-pathological models shrinks when a more negative equation of state parameter is considered. This analysis allows us to locate a large class of stable theories that violate the null energy condition (i.e. super-acceleration models) and to recover, as particular subsets, various models considered so far. Finally, under the assumption that the true underlying cosmological model is the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) scenario, and relying on the figure of merit of EUCLID-like observations, we demonstrate that the theoretical requirement of stability significantly narrows the empirical likelihood, increasing the discriminatory power of data. We also find that the vast majority of these non-pathological theories generating the same expansion history as the $\Lambda$CDM model predict a different, lower, growth rate of cosmic structures.Comment: v1: 28 pages, 20 pdf figures. v2: 29 pages, minor improvements in the text, figures improve

Probing non-standard gravity with the growth index: a background independent analysis

Measurements of the growth index $\gamma(z)$ provide a clue as to whether Einstein's field equations encompass gravity also on large cosmic scales, those where the expansion of the universe accelerates. We show that the information encoded in this function can be satisfactorily parameterized using a small set of coefficients $\gamma_i$ in such a way that the true scaling of the growth index is recovered to better than $1\%$ in most dark energy and dark gravity models. We find that the likelihood of current data is maximal for $\gamma_0=0.74\pm 0.44$ and $\gamma_1=0.01\pm0.46$, a measurement compatible with the $\Lambda$CDM predictions. Moreover data favor models predicting slightly less growth of structures than the Planck LambdaCDM scenario. The main aim of the paper is to provide a prescription for routinely calculating, in an analytic way, the amplitude of the growth indices $\gamma_i$ in relevant cosmological scenarios, and to show that these parameters naturally define a space where predictions of alternative theories of gravity can be compared against growth data in a manner which is independent from the expansion history of the cosmological background. As the standard $\Omega$-plane provides a tool to identify different expansion histories $H(t)$ and their relation to various cosmological models, the $\gamma$-plane can thus be used to locate different growth rate histories $f(t)$ and their relation to alternatives model of gravity. As a result, we find that the Dvali-Gabadadze-Porrati gravity model is rejected with a $95\%$ confidence level. By simulating future data sets, such as those that a Euclid-like mission will provide, we also show how to tell apart LambdaCDM predictions from those of more extreme possibilities, such as smooth dark energy models, clustering quintessence or parameterized post-Friedmann cosmological models.Comment: 29 pages, 21 figure

Dark Matter Thermonuclear Supernova Ignition

We investigate local environmental effects from dark matter (DM) on thermonuclear supernovae (SNe Ia) using publicly available archival data of 224 low-redshift events, in an attempt to shed light on the SN Ia progenitor systems. SNe Ia are explosions of carbon-oxygen (CO) white dwarfs (WDs) that have recently been shown to explode at sub-Chandrasekhar masses; the ignition mechanism remains, however, unknown. Recently, it has been shown that both weakly interacting massive particles (WIMPs) and macroscopic DM candidates such as primordial black holes (PBHs) are capable of triggering the ignition. Here, we present a method to estimate the DM density and velocity dispersion in the vicinity of SN Ia events and nearby WDs; we argue that (i) WIMP ignition is highly unlikely, and that (ii) DM in the form of PBHs distributed according to a (quasi-) log-normal mass distribution with peak $\log_{10}(m_0/1$g$)=24.9\pm 0.9$ and width $\sigma= 3.3\pm 1.0$ is consistent with SN Ia data, the nearby population of WDs and roughly consistent with other constraints from the literature

Exploring the physics of cosmic acceleration

L'expansion accélérée de l'univers est devenu un fait établi que personne ne pouvait prévoir il y a encore une vingtaine d'années. Pour expliquer l'accélération cosmique, l'univers doit être composé de 75% d'énergie noire, une matière hypothétique à pression négative. Une alternative aussi vertigineuse consiste à modifier la relativité générale d'Einstein à l'échelle cosmique.Mes travaux de thèse portent sur la contrainte des modèles d'énergie noire et de gravité modifiée avec les données observationnelles provenant de la croissance linéaire des structures cosmologiques. Une méthode basée sur une nouvelle paramétrisation de l'index de croissance des perturbations linéaires cosmologiques permet d'analyser un grand nombre de modèles "accélératoires" en même temps. Nous avons évalué et validé cette méthode par une analyse systématique de sa précision et de sa performance. Mes résultats montrent que le modèle standard de la cosmologie (le modèle ΛCDM) reste en accord avec les données actuelles. Dans une étude approfondie, nous simulons les contraintes possibles avec les futures sondes cosmologiques de "précision" comme Euclid. Pour analyser encore plus de modèles en même temps, nous introduisons la théorie effective des champs de l'énergie noire (EFT) dans le formalisme développé auparavant. La EFT est un formalisme prometteur qui permet d'explorer d'une manière complète tous les modèles gravitationnels non-standards résultant de l'addition d'un degré de liberté supplémentaire dans l'équation d'Einstein. Nous proposons une paramétrisation de cette théorie que nous confrontons avec les données actuelles et futures.The accelerated expansion of the universe has become an established fact that nobody could foresee until twenty years ago. To explain the cosmic acceleration, the universe must be composed by 75% of dark energy, a hypothetical form of matter with negative pressure. Alternatively, Einstein's field equation must be modified on cosmic scales. During my thesis I have worked on the constraint of dark energy and modified gravity models with data coming from the observed growth rate of cosmic structures. We have introduced a method based on a new parametrization of the growth index of linear cosmological perturbations. An advantage is the possibility of a concurrent analysis of multiple accelerating models. We have evaluated and validated the method in a systematic precision and performance check. My results show that the standard model of cosmology (the ΛCDM model) remains consistent with current data. In an ongoing study, we have simulated future constraints for upcoming cosmological 'precision' probes like Euclid.In a second step, we introduce the effective field theory of dark energy (EFT) into our formalism. The EFT is a promising framework that allows to explore in a complete way all non-standard gravitational models that result from adding one degree of freedom in Einstein's field equation. Another advantage is its neat split of background and perturbation observables. We propose a parametrization of the EFT that we confront with current and simulated future constraints

On the dynamical emergence of de Sitter spacetime

17 pages, 2 figuresWe present and discuss an asynchronous coordinate system covering de Sitter spacetime, notably in a complete way in 1+1 dimensions. The new coordinates have several interesting cosmological properties: the worldlines of comoving ($x^i=const$) observers are geodesics, cosmic time is finite in the past, and the coordinates asymptotically tend to that of a flat Robertson & Walker model at large times. This analysis also provides an argument in favor of the natural emergence of an equation of state of the type $p=-\rho$ in the context of the standard cosmological model

Dark Matter Thermonuclear Supernova Ignition

We investigate local environmental effects from dark matter (DM) on thermonuclear supernovae (SNe Ia) using publicly available archival data of 224 low-redshift events, in an attempt to shed light on the SN Ia progenitor systems. SNe Ia are explosions of carbon-oxygen (CO) white dwarfs (WDs) that have recently been shown to explode at sub-Chandrasekhar masses; the ignition mechanism remains, however, unknown. Recently, it has been shown that both weakly interacting massive particles (WIMPs) and macroscopic DM candidates such as primordial black holes (PBHs) are capable of triggering the ignition. Here, we present a method to estimate the DM density and velocity dispersion in the vicinity of SN Ia events and nearby WDs; we argue that (i) WIMP ignition is highly unlikely, and that (ii) DM in the form of PBHs distributed according to a (quasi-) log-normal mass distribution with peak $\log_{10}(m_0/1$g$)=24.9\pm 0.9$ and width $\sigma= 3.3\pm 1.0$ is consistent with SN Ia data, the nearby population of WDs and roughly consistent with other constraints from the literature