Late-time cosmology in (phantom) scalar-tensor theory: dark energy and the cosmic speed-up

TL;DR

This paper explores late-time cosmology using (phantom) scalar-tensor theories with exponential potentials, demonstrating models that align with observations, address the Big Rip singularity, and introduce a novel higher-derivative dark energy model.

Contribution

It presents exact solutions for scalar-tensor cosmologies with phantom behavior and introduces a new higher-derivative scalar-tensor dark energy model with transient acceleration.

Findings

Models can produce current acceleration consistent with observations.

Quantum gravity effects may delay or prevent the Big Rip singularity.

A new higher-derivative scalar-tensor theory admits transient acceleration phases.

Abstract

We consider late-time cosmology in a (phantom) scalar-tensor theory with an exponential potential, as a dark energy model with equation of state parameter close to -1 (a bit above or below this value). Scalar (and also other kinds of) matter can be easily taken into account. An exact spatially-flat FRW cosmology is constructed for such theory, which admits (eternal or transient) acceleration phases for the current universe, in correspondence with observational results. Some remarks on the possible origin of the phantom, starting from a more fundamental theory, are also made. It is shown that quantum gravity effects may prevent (or, at least, delay or soften) the cosmic doomsday catastrophe associated with the phantom, i.e. the otherwise unavoidable finite-time future singularity (Big Rip). A novel dark energy model (higher-derivative scalar-tensor theory) is introduced and it is shown to admit an effective phantom/quintessence description with a transient acceleration phase. In this case, gravity favors that an initially insignificant portion of dark energy becomes dominant over the standard matter/radiation components in the evolution process.