Dynamical dark energy from spacetime-symmetry breaking -- late-time behaviour and phantom crossing

TL;DR

This paper explores how explicit spacetime-symmetry breaking influences late-time cosmological dynamics, leading to dynamical dark energy with potential phantom crossing and violations of the null energy condition, while comparing to observational constraints.

Contribution

It introduces a model where explicit spacetime-symmetry breaking results in a logarithmic dynamical dark energy component, and analyzes its implications for cosmic evolution and observational constraints.

Findings

Dark energy exhibits divergencies and phantom behavior at certain symmetry-breaking values.

Constraints from gravitational-wave propagation can be relaxed by considering the sign of the symmetry-breaking coefficient.

The model can reduce existing constraints on symmetry-breaking coefficients by half while maintaining NEC conditions.

Abstract

We investigate the late-time cosmological dynamics in a simple case of explicit spacetime-symmetry breaking. By expanding in a small symmetry-breaking coefficient we are able to write the Friedmann equations as $\Lambda$CDM + dynamical dark energy, which we show contains logarithmic dependence of the scale factor. We find that the dark energy equation of state displays divergencies and phantom behaviour for certain values of the symmetry-breaking coefficient, where the NEC is also broken. We discuss the adiabatic sound speed of dark energy and compare the model to current constraints using the Chevallier-Polarski-Linder parametrisation. Remarkably, although the constraints on the same symmetry-breaking coefficient from e.g. gravitational-wave propagation are orders of magnitude stronger than what we obtain in this paper, we are able to cut those constraints, which are more or less symmetric around zero, in half by showing that same coefficient must be negative (or zero) if one wishes to keep the NEC intact.