Non-trivial gravitational waves and structure formation phenomenology from dark energy

TL;DR

This paper explores alternative dark energy models involving vector fields that can produce unique cosmological phenomenologies, such as non-trivial scalar effects and preferred directions, while evading gravitational wave constraints.

Contribution

It introduces vector field-based dark energy models with novel features like anisotropic perturbations and oscillating tensor modes, expanding beyond scalar field limitations.

Findings

Vector models can affect scalar modes without altering gravitational wave speed.

Existence of a second tensor mode capable of oscillation into gravitational waves.

Models with isotropic energy-momentum tensors despite anisotropic field configurations.

Abstract

The detection of the GW170817/GRB170817A event improved the constraints on the propagation speed of gravitational waves, thus placing possible variations caused by dark energy under restraint. For models based on scalar fields belonging to the family of Horndeski Lagrangians, non-minimal derivative couplings are now severely constrained, entailing a substantially limited phenomenology. In this work we want to stress that there is still a plethora of dark energy models that get around this obstacle while still providing interesting phenomenologies able to distinguish them from the standard cosmology. We focus on a class involving vector fields as a proxy, but our discussion is extensible to a broader class of models. In particular, we show the possibility of having a non-minimal derivative coupling giving a non-trivial effect on scalar modes without affecting gravitational waves and the possibility of having a second tensor mode that can oscillate into gravitational waves. We also present a novel class of configurations breaking rotational invariance but with an energy-momentum tensor that is isotropic on-shell. This peculiar feature makes the scalar and vector sectors of the perturbations mix so that, even in a perfectly isotropic background cosmology, preferred direction effects can appear in the perturbations. We also comment on models that give rise to isotropic solutions when averaging over rapid oscillations of the vector fields. The explored models are classified according to distinctive field configurations that provide inequivalent realisations of the Cosmological Principle.