Cosmological constraints on Lorentz violating dark energy

TL;DR

This paper investigates a Lorentz-violating dark energy model, ThetaCDM, analyzing its cosmological effects and constraining its parameters using observational data, finding tight bounds on deviations from standard cosmology.

Contribution

It provides a detailed analysis of the ThetaCDM model's cosmological implications and constrains its parameters using observational data, highlighting differences from LambdaCDM at the perturbation level.

Findings

Constraints on deviations from LambdaCDM are tight, with gravitational constant discrepancies limited to 1.8%.

The model affects CMB and matter fluctuation spectra through modifications in gravitational interactions.

The background evolution of ThetaCDM is indistinguishable from LambdaCDM.

Abstract

The role of Lorentz invariance as a fundamental symmetry of nature has been lately reconsidered in different approaches to quantum gravity. It is thus natural to study whether other puzzles of physics may be solved within these proposals. This may be the case for the cosmological constant problem. Indeed, it has been shown that breaking Lorentz invariance provides Lagrangians that can drive the current acceleration of the universe without experiencing large corrections from ultraviolet physics. In this work, we focus on the simplest model of this type, called ThetaCDM, and study its cosmological implications in detail. At the background level, this model cannot be distinguished from LambdaCDM. The differences appear at the level of perturbations. We show that in ThetaCDM, the spectrum of CMB anisotropies and matter fluctuations may be affected by a rescaling of the gravitational constant in the Poisson equation, by the presence of extra contributions to the anisotropic stress, and finally by the existence of extra clustering degrees of freedom. To explore these modifications accurately, we modify the Boltzmann code CLASS. We then use the parameter inference code Monte Python to confront ThetaCDM with data from WMAP-7, SPT and WiggleZ. We obtain strong bounds on the parameters accounting for deviations from LambdaCDM. In particular, we find that the discrepancy between the gravitational constants appearing in the Poisson and Friedmann equations is constrained at the level 1.8%.