Observational Evidence of the Accelerated Expansion of the Universe

The discovery of cosmic acceleration is one of the most important developments in modern cosmology. The observation, thirteen years ago, that type Ia supernovae appear dimmer that they would have been in a decelerating universe followed by a series of independent observations involving galaxies and cluster of galaxies as well as the cosmic microwave background, all point in the same direction: we seem to be living in a flat universe whose expansion is currently undergoing an acceleration phase. In this paper, we review the various observational evidences, most of them gathered in the last decade, and the improvements expected from projects currently collecting data or in preparation.Comment: Accepted review article to appear in a special volume of the "Comptes Rendus de l'Acad\'emie des Sciences" about Dark Energy and Dark Matte

Constraining the Properties of Dark Energy

The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3 of the critical density, is very smoothly distributed, and has large negative pressure. Its nature is very much unknown. Most of its discernible consequences follow from its effect on evolution of the expansion rate of the Universe, which in turn affects the growth of density perturbations and the age of the Universe, and can be probed by the classical kinematic cosmological tests. Absent a compelling theoretical model (or even a class of models), we describe dark energy by an effective equation of state w=p_X/rho_X which is allowed to vary with time. We describe and compare different approaches for determining w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies and clusters, and CMB anisotropy, focusing particular attention on the use of a sample of several thousand type Ia supernova with redshifts z < 1.7, as might be gathered by the proposed SNAP satellite. Among other things, we derive optimal strategies for constraining cosmological parameters using type Ia supernovae. While in the near term CMB anisotropy will provide the first measurements of w, supernovae and number counts appear to have the most potential to probe dark energy.Comment: 6 pages, 3 figures; proceedings of 20th Texas Symposium on Relavistic Astrophysic

Probing decisive answers to dark energy questions from cosmic complementarity and lensing tomography

We study future constraints on dark energy parameters determined from several combinations of CMB experiments, supernova data, and weak lensing surveys with and without tomography. In this analysis, we look in particular for combinations that will bring the uncertainties to a level of precision tight enough (a few percent) to answer decisively some of the dark energy questions. We probe the dark energy using two variants of its equation of state, and its energy density.We consider a set of 13 cosmological and systematic parameters, and assume reasonable priors on the lensing and supernova systematics. We consider various lensing surveys: a wide survey with f_{sky}=0.7, and with 2 (WLT2) and 5 (WLT5) tomographic bins; a deep survey with 10 bins (WLT10). The constraints found from Planck, 2000 supernovae with z_max=0.8, and WLT2 are: {sigma(w_0)=0.086, sigma(w_1)=0.069}, {sigma(w_0)=0.088, sigma(w_a)=0.11}, and {sigma(E_1)=0.029, sigma(E_2)=0.065}. With 5 bins, we find {sigma(w_0)=0.04, sigma(w_1)=0.034}, {sigma(w_0)=0.041, sigma(w_a)=0.056}, and {sigma(E_1)=0.012, sigma(E_2)=0.049}. Finally, we find from Planck, 2000 supernovae with z_max=1.5, and WLT10 with f_{sky}=0.1: {sigma(w_0)=0.032, sigma(w_1)=0.027}, {sigma(w_0)=0.033, sigma(w_a)=0.040}, and {sigma(E_1)=0.01, sigma(E_2)=0.04}. Although some worries remain about other systematics, our study shows that after the combination of the 3 probes, lensing tomography with many redshift bins and large coverages of the sky has the potential to add key improvements to the dark energy parameter constraints. However, the requirement for very ambitious and sophisticated surveys in order to achieve some of the constraints or to improve them suggests the need for new tests to probe the nature of dark energy in addition to constraining its equation of state. (Abriged)Comment: 14 pages, 5 figures; matches MNRAS accepted versio

Decaying dark energy in light of the latest cosmological dataset

Decaying Dark Energy models modify the background evolution of the most common observables, such as the Hubble function, the luminosity distance and the Cosmic Microwave Background temperature-redshift scaling relation. We use the most recent observationally-determined datasets, including Supernovae Type Ia and Gamma Ray Bursts data, along with $H(z)$ and Cosmic Microwave Background temperature versus $z$ data and the reduced Cosmic Microwave Background parameters, to improve the previous constraints on these models. We perform a Monte Carlo Markov Chain analysis to constrain the parameter space, on the basis of two distinct methods. In view of the first method, the Hubble constant and the matter density are left to vary freely. In this case, our results are compatible with previous analyses associated with decaying Dark Energy models, as well as with the most recent description of the cosmological background. In view of the second method, we set the Hubble constant and the matter density to their best fit values obtained by the {\it Planck} satellite, reducing the parameter space to two dimensions, and improving the existent constraints on the model's parameters. Our results suggest that the accelerated expansion of the Universe is well described by the cosmological constant, and we argue that forthcoming observations will play a determinant role to constrain/rule out decaying Dark Energy.Comment: 15 pages, 3 figure, 2 table. Accepted in the Special Issue "Cosmological Inflation, Dark Matter and Dark Energy" on Symmetry Journa

The Accelerating Universe

In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy - component that causes the acceleration - is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter complements other articles in this book that describe theoretical understanding (or lack thereof) of the cause for the accelerating universe.Comment: Invited review chapter for book "Adventures in Cosmology" (ed. D. Goodstein) aimed at general scientists; 28 pages, 10 figure