Effective Dark Energy Equation Of State Research Articles

Generalized Brans-Dicke theories in light of evolving dark energy

The expansion history of the Universe reconstructed from a combination of recent data indicates a preference for a changing Dark Energy (DE) density. Moreover, the DE density appears to be increasing with cosmic time, with its equation of state being below -1 on average, and possibly crossing the so-called phantom divide. Scalar-tensor theories, in which the scalar field mediates a force between matter particles, offer a natural framework in which the effective DE equation of state can be less than -1 and cross the phantom barrier. We consider the generalized Brans-Dicke (GBD) class of scalar-tensor theories and reconstruct their Lagrangian given the effective DE density extracted from recent data. Then, given the reconstructed Lagrangian, we solve for the linear perturbations and investigate the characteristic signatures of these reconstructed GBD in the cosmological observables, such as the cosmic microwave background (CMB) anisotropy, the galaxy number counts, and their cross-correlations. In particular, we demonstrate that the Integrated Sachs-Wolfe (ISW) effect probed by the cross-correlation of CMB with the matter distribution can rule out scalar-tensor theories as the explanation of the observed DE dynamics independently from the laboratory and solar system fifth force constraints.

Priors on the effective dark energy equation of state in scalar-tensor theories

Constraining the dark energy (DE) equation of state, ${w}_{\mathrm{DE}}$, is one of the primary science goals of ongoing and future cosmological surveys. In practice, with imperfect data and incomplete redshift coverage, this requires making assumptions about the evolution of ${w}_{\mathrm{DE}}$ with redshift $z$. These assumptions can be manifested in a choice of a specific parametric form, which can potentially bias the outcome, or else one can reconstruct ${w}_{\mathrm{DE}}(z)$ nonparametrically, by specifying a prior covariance matrix that correlates values of ${w}_{\mathrm{DE}}$ at different redshifts. In this work, we derive the theoretical prior covariance for the effective DE equation of state predicted by general scalar-tensor theories with second order equations of motion (Horndeski theories). This is achieved by generating a large ensemble of possible scalar-tensor theories using a Monte Carlo methodology, including the application of physical viability conditions. We also separately consider the special subcase of the minimally coupled scalar field, or quintessence. The prior shows a preference for tracking behaviors in the most general case. Given the covariance matrix, theoretical priors on parameters of any specific parametrization of ${w}_{\mathrm{DE}}(z)$ can also be readily derived by projection.

Bi-scalar modified gravity and cosmology with conformal invariance

We investigate the cosmological applications of a bi-scalar modified gravity that exhibits partial conformal invariance, which could become full conformal invariance in the absence of the usual Einstein-Hilbert term and introducing additionally either the Weyl derivative or properly rescaled fields. Such a theory is constructed by considering the action of a non-minimally conformally-coupled scalar field, and adding a second scalar allowing for a nonminimal derivative coupling with the Einstein tensor and the energy-momentum tensor of the first field. At a cosmological framework we obtain an effective dark-energy sector constituted from both scalars. In the absence of an explicit matter sector we extract analytical solutions, which for some parameter regions correspond to an effective matter era and/or to an effective radiation era, thus the two scalars give rise to “mimetic dark matter” or to “dark radiation” respectively. In the case where an explicit matter sector is included we obtain a cosmological evolution in agreement with observations, that is a transition from matter to dark energy era, with the onset of cosmic acceleration. Furthermore, for particular parameter regions, the effective dark-energy equation of state can transit to the phantom regime at late times. These behaviors reveal the capabilities of the theory, since they arise purely from the novel, bi-scalar construction and the involved couplings between the two fields.

Reconstruction of dark energy and expansion dynamics using Gaussian processes

An important issue in cosmology is reconstructing the effective darkenergy equation of state directly from observations. With fewphysically motivated models, future dark energy studies cannot only bebased on constraining a dark energy parameter space, as the errorsfound depend strongly on the parametrisation considered. We present anew non-parametric approach to reconstructing the history of theexpansion rate and dark energy using Gaussian Processes, which is afully Bayesian approach for smoothing data. We present a pedagogicalintroduction to Gaussian Processes, and discuss how it can be used torobustly differentiate data in a suitable way. Using this method weshow that the Dark Energy Survey - Supernova Survey (DES) canaccurately recover a slowly evolving equation of state to σw = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimumerror of ±0.025 at the sweet-spot at z ∼ 0.16, provided theother parameters of the modelare known. Errors on the expansion history are an order of magnitudesmaller, yet make no assumptions about dark energy whatsoever. A codefor calculating functions and their first three derivatives usingGaussian processes has been developed and is available fordownload.

Interacting dark energy: Constraints and degeneracies

In standard cosmologies, dark energy (DE) interacts only gravitationally with dark matter (DM). There could be a nongravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use cosmic microwave background, baryon acoustic oscillation and supernova data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from cosmic microwave background anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the integrated Sachs-Wolfe signature is suppressed since the nonstandard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken.