# The Maxwell-Chern-Simons gravity and its cosmological implications

## TL;DR

This paper explores a modified gravity theory with vector fields and Chern-Simons coupling, analyzing its cosmological solutions, degrees of freedom, and evolution of anisotropic universes, revealing a tendency toward isotropic, accelerating expansion.

## Contribution

It introduces a gravity model with two vector fields and a Chern-Simons term, analyzing its perturbations, degrees of freedom, and cosmological evolution, including anisotropic universe behavior.

## Key findings

- The theory admits a de Sitter solution with healthy perturbations.
- Six degrees of freedom propagate on de Sitter space, including tensor and vector modes.
- The anisotropic Bianchi I universe evolves towards isotropic, accelerating expansion.

## Abstract

We consider the cosmological implications of a gravitational theory containing two vector fields coupled minimally to gravity as well as a generalized Chern-Simons term that couples the two vector fields. One of the vector fields is the usual Maxwell field, while the other is a constrained vector field with constant norm included in the action via a Lagrange multiplier. The theory admits a de Sitter type solution, with healthy cosmological perturbations. We will show that there is 6 degrees of freedom propagate on top of de Sitter space-time, two tensor polarizations and four degrees of freedom related to two massless vector fields interacting with each other via Chern-Simons interaction term. We also investigate in detail the behavior of the geometric and physical parameters of a homogeneous and anisotropic Bianchi type I Universe, by using both analytical and numerical methods, by assuming that the matter content of the Universe can be described by the stiff causal and pressureless dust fluid equations of state. The time evolution of the Bianchi type I Universe strongly depends on the initial conditions of the physical and geometrical quantities, as well as on the numerical values of the model parameters. Two important observational parameters, the mean anisotropy parameter, and the deceleration parameter, are also studied in detail, and we show that independently of the matter equation of state the cosmological evolution of the Bianchi type I Universe always ends in an isotropic and exponentially accelerating, de Sitter type, phase.