Dark Energy

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.

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.

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.

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.

to appear in MNRAS

The understanding of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. Whether or not the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, many models have been proposed to solve this problem, both in the context of General Relativity and alternative theories of gravity. Actually, a further possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state (EOS), 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 EOS. Our high-redshift analysis is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts (GRBs) 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 EOS parameters we use the Markov Chain Monte Carlo Method (MCMC). It turns out that the dark energy equation of state is evolving for all the parametrizations that we considered. We finally compare our results with the ones obtained by previous cosmographic analysis performed on the same astronomical datasets, showing that the latter ones are sufficient to test and compare the new parametrizations.

2011

We study the possibility to extract model independent information about the dynamics of the universe by using Cosmography. We intend to explore it systematically, to learn about its limitations and its real possibilities. Here we are sticking to the series expansion approach on which Cosmography is based. We apply it to different data sets: Supernovae Type Ia (SNeIa), Hubble parameter extracted from differential galaxy ages, Gamma Ray Bursts (GRBs) and the Baryon Acoustic Oscillations (BAO) data. We go beyond past results in the literature extending the series expansion up to the fourth order in the scale factor, which implies the analysis of the deceleration, q_{0}, the jerk, j_{0} and the snap, s_{0}. We use the Markov Chain Monte Carlo Method (MCMC) to analyze the data statistically. We also try to relate direct results from Cosmography to dark energy (DE) dynamical models parameterized by the Chevalier-Polarski-Linder (CPL) model, extracting clues about the matter content and the dark energy parameters. 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 can help giving narrower constraints in parameter estimation, in particular for higher order cosmographic parameters (the jerk and the snap), with respect to the literature; d) both the estimation of the jerk and the DE parameters, reflect the possibility of a deviation from the LCDM cosmological model.

Physics Letters B, 2010

We study the possibility of detecting oscillating patterns in the equation of state (EoS) of the dark energy using different cosmological datasets. We follow a phenomenological approach and study three different oscillating models for the EoS, one of them periodic and the other two damped (proposed here for the first time). All the models are characterised by the amplitude value, the centre and the frequency of oscillations. In contrast to previous works in the literature, we do not fix the value of the frequency to a fiducial value related to the time extension of chosen datasets, but consider a discrete set of values, so to avoid arbitrariness and try and detect any possible time period in the EoS. We test the models using a recent collection of SNeIa, direct Hubble data and Gamma Ray Bursts data. Main results are: I. even if constraints on the amplitude are not too strong, we detect a trend of it versus the frequency, i.e. decreasing (and even negatives) amplitudes for higher frequencies; II. the centre of oscillation (which corresponds to the present value of the EoS parameter) is very well constrained, phantom behaviour is excluded at 1σ level and trend which is in agreement with the one for the amplitude appears; III. the frequency is hard to constrain, showing similar statistical validity for all the values of the discrete set chosen, but the best fit of all the scenarios considered is associated with a period which is in the redshift range depicted by our cosmological data. The "best" oscillating models are compared with ΛCDM using dimensionally consistent a Bayesian approach based information criterion and the conclusion reached is the non existence of significant evidence against dark energy oscillations.

Physical Review D, 2011

We study the possibility to extract model independent information about the dynamics of the universe by using Cosmography. We intend to explore it systematically, to learn about its limitations and its real possibilities. Here we are sticking to the series expansion approach on which Cosmography is based. We apply it to different data sets: Supernovae Type Ia (SNeIa), Hubble parameter extracted from differential galaxy ages, Gamma Ray Bursts (GRBs) and the Baryon Acoustic Oscillations (BAO) data. We go beyond past results in the literature extending the series expansion up to the fourth order in the scale factor, which implies the analysis of the deceleration, q0, the jerk, j0 and the snap, s0. We use the Markov Chain Monte Carlo Method (MCMC) to analyze the data statistically. We also try to relate direct results from Cosmography to dark energy (DE) dynamical models parameterized by the Chevalier-Polarski-Linder (CPL) model, extracting clues about the matter content and the dark energy parameters. 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 can help giving narrower constraints in parameter estimation, in particular for higher order cosmographic parameters (the jerk and the snap), with respect to the literature; d. both the estimation of the jerk and the DE parameters, reflect the possibility of a deviation from the ΛCDM cosmological model.

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 of the Dark Universe

Perhaps the most explored hypothesis for the accelerated cosmic expansion rate arises in the context of extra fields or modifications to General Relativity. A prevalent approach is to parameterize the expansion history through the equation of state, ω(z). We present a parametric form for ω(z) that can reproduce the generic behavior of the most widely used physical models for accelerated expansion with infrared corrections. The present proposal has at most 3 free parameters which can be mapped back to specific archetypal models for dark energy. We analyze in detail how different combinations of data can constrain the specific cases embedded in our form for ω(z). We implement our parametric equation for ω(z) to observations from CMB, the luminous distance of SNeIa, cosmic chronometers, and baryon acoustic oscillations identified in galaxies and in the Lymann-α forest. We find that the parameters can be well constrained by using different observational data sets. Our findings point to an oscillatory behavior consistent with an f (R)-like model or an unknown combination of scalar fields. When we let the three parameters vary freely, we find an EoS which oscillates around the phantom-dividing line, and, with over 99% of confidence, the cosmological constant solution is disfavored.