Acceleration of the universe in matter dominant era by conformal symmetry breaking

TL;DR

This paper proposes that breaking conformal symmetry through a scalar field coupled to gravity can explain the universe's acceleration during the matter-dominant era, offering a new perspective on cosmic acceleration.

Contribution

It introduces a model where conformal symmetry breaking drives acceleration in the matter era, linking scalar field dynamics to cosmic acceleration and addressing the coincidence problem.

Findings

Universe accelerates in matter era due to symmetry breaking

Scalar field coupling influences cosmic acceleration

Provides a solution to the coincidence problem

Abstract

We introduce a scenario in which the breakdown of conformal symmetry is responsible for the acceleration of universe in the matter dominant era. In this regard, we consider a self interacting scalar field non-minimally coupled to the Ricci scalar and the trace of energy-momentum tensor. For a traceless energy-momentum tensor in radiation dominant era, the coupling to matter vanishes and we are left with a conformal invariant gravitational action of Deser, where the universe may experience a decelerating phase in agreement with observations. In matter dominant era, the coupling to matter no longer vanishes, the conformal symmetry is broken down, and the matter inevitably becomes pressureless. The corresponding field equations are obtained and it is shown that the universe may have an accelerating phase in this era, provided that the value of self interaction coupling constant satisfies an specific lower bound. Moreover, we provide a reasonable solution to the coincidence problem.