The dark energy–dominated Universe

Monthly Notices of the Royal Astronomical Society, 2012

The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, one can try to constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires only a minimal set of assumptions, namely to specify the metric, and it does not rely on the dynamical equations for gravity. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series. The main results are: a) even if relying on a mathematical approximate assumption such as the scale factor series expansion in terms of time, cosmography can be extremely useful in assessing dynamical properties of the Universe; b) the deceleration parameter clearly confirms the present acceleration phase; c) the MCMC method provides stronger constraints for parameter estimation, in particular for higher order cosmographic parameters (the jerk and the snap), with respect to those presented in the literature; d) both the estimation of the jerk and the DE parameters, reflect the possibility of a deviation from the ΛCDM cosmological model; e) there are indications that the dark energy equation of state is evolving for all the parametrizations that we considered; f ) the q(z) reconstruction provided by our cosmographic analysis allows a transient acceleration.

Journal of Cosmology and Astroparticle Physics, 2014

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 Λ Cold Dark Matter (ΛCDM) 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 ΛCDM model predict a different, lower, growth rate of cosmic structures.

Physics Essays, 2017

This article is a continuation of a former article, taking account of recent observational results a.o. from the Planck mission. The adopted approach starts considering a hot big bang scenario for an universe with Friedmann-Lemaître-Robertson-Walker (FLRW) metric, the spatial curvature parameter k = 0 and the cosmological constant Λ= 0. It does not contradict the standard model (now called « Λ-CDM model ») except on a point : the acceleration of the expansion of the universe is not related to the cosmological constant but to a « dark energy » component α which is mostly constant in time and is a constitutive part of the Hubble parameter. Using a reversed method, it is shown that the scale factor evolves in a similar way as in the Λ-CDM model for the past epochs but that the acceleration in the future is less pronounced. The evolution of several cosmological parameters from the end of an inflationary stage till now is given as was done in the former article. The major difference is that, instead of imposing the pressure to be nil at present time in order to comply with the equation of state for the matter-dominated era, the pressure – density ratio is left free to be negative. The pressure then naturally becomes negative around 7.12 Gyrs after the big bang. It is shown that the contribution of dark energy is negligible during the radiation-dominated era but becomes of the order of the contribution of matter during the matter-dominated era which solves the coincidence problem.

Dark Matter in Astrophysics and Particle Physics - Proceedings of the 7th International Heidelberg Conference on Dark 2009, 2010

http://msor.victoria.ac.nz/ The Hubble relation between distance and redshift is a purely cosmographic relation that depends only on the symmetries of a FLRW spacetime, but does not intrinsically make any dynamical assumptions. This suggests that it should be possible to estimate the parameters defining the Hubble relation without making any dynamical assumptions. To test this idea, we perform a number of interrelated cosmographic fits to the legacy05 and gold06 supernova datasets, paying careful attention to the systematic uncertainties. Based on this supernova data, the "preponderance of evidence" certainly suggests an accelerating universe. However we would argue that (unless one uses additional dynamical and observational information, and makes additional theoretical assumptions) this conclusion is not currently supported "beyond reasonable doubt". As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation-representations in which acceleration versus deceleration reduces to the question of whether the relevant graph slopes up or down.

2012

The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, one can try to constrain the parameters using a cosmographic approach. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series.

International Journal of Modern Physics D, 2006

In this paper we review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating Universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.

The Astrophysical Journal, 2002

We consider ever-expanding Big Bang models with a cosmological constant, Λ, and investigate in detail the evolution of the observable part of the universe. We also discuss quintessence models from the same point of view.

arXiv: General Physics, 2013

The inflationary phase of the Universe is explored by proposing a toy model related to the scalar field, termed as {\it inflaton}. The potential part of the energy density in the said era is assumed to have a constant vacuum energy density part and a variable part containing the inflaton. The prime idea of the proposed model constructed in the framework of the closed Universe is based on a fact that the inflaton is the root cause of the orientation of the space. According to this model the expansion of the Universe in the inflationary epoch is not approximately rather exactly exponential in nature and thus it can solve some of the fundamental puzzles, viz. flatness as well as horizon problems. It is also predicted that the constant energy density part in the potential may be associated to the dark energy, which is eventually different from the vacuum energy, at least in the inflationary phase of the Universe. However, the model keeps room for the end of inflationary era.

Physics Letters B, 2008

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in Cosmology but also in high energy physics theories. Following this approach, we explore in this paper some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z over the entire cosmological evolution, i.e., z ∈ [−1, ∞). This parameterization allows us to divide the parametric plane (w0, w1) in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. A statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution H(z) is performed to check the observational viability of the EoS parameterization here proposed.

Monthly Notices of the Royal Astronomical Society, 2007

The variation of the expansion rate of the Universe with time produces an evolution in the cosmological redshift of distant sources (for example quasar Lyman-α absorption lines), that might be directly observed by future ultra stable, high-resolution spectrographs (such as CODEX) coupled to extremely large telescopes (such as European Southern Observatory's Extremely Large Telescope, ELT). This would open a new window to explore the physical mechanism responsible for the current acceleration of the Universe. We investigate the evolution of cosmological redshift from a variety of dark energy models, and compare it with simulated data. We perform a Fisher matrix analysis and discuss the prospects for constraining the parameters of these models and for discriminating among competing candidates. We find that, because of parameter degeneracies, and of the inherent technical difficulties involved in this kind of observations, the uncertainties on parameter reconstruction can be rather large unless strong external priors are assumed. However, the method could be a valuable complementary cosmological tool, and give important insights on the dynamics of dark energy, not obtainable using other probes.