A Viable Cosmology with a Scalar Field Coupled to the Trace of the Stress-Tensor

TL;DR

This paper explores a scalar field model coupled to the trace of the energy-momentum tensor, which can explain cosmic acceleration and transition to matter-dominated expansion without future event horizons.

Contribution

It introduces a scalar field coupling to the trace of the stress-energy tensor, providing a new mechanism for cosmic acceleration compatible with observational data.

Findings

Scalar field energy density decreases faster with coupling, initiating radiation domination earlier.

The model naturally explains current cosmic acceleration with b7_{\, ext{phi}}=0.7.

The model can avoid future event horizons while matching late-time matter-dominated expansion.

Abstract

We study the cosmological evolution of a scalar field that couples to the trace $T=T^{a}_a$ of energy momentum tensor of all the fields (including itself). In the case of a shallow exponential potential, the presence of coupling to the trace $T$ in the field equation makes the energy density of the scalar field decrease faster thereby hastening the commencement of radiation domination. This effect gradually diminishes at later epochs allowing the scalar field to dominate the energy density again. We interpret this phase as the current epoch of cosmic acceleration with $\Omega_{\phi}=0.7$. A variant of this model can lead to accelerated expansion at the present epoch followed by a $a(t)\propto t^{2/3}$ behaviour as $t\to \infty$, making the model free from future event horizon. The main features of the model are independent of initial conditions. However, fine tuning of parameters is necessary for viable evolution.