Effective phantom dark energy in scalar-tensor gravity

TL;DR

This paper examines scalar-tensor gravity models exhibiting phantom dark energy behavior, finding such models require fine-tuning and recent onset, thus making their physical plausibility questionable.

Contribution

It analyzes the conditions under which scalar-tensor models can produce phantom dark energy, highlighting the need for contrived functions and fine-tuned initial data.

Findings

Models with present phantom behavior are possible but require contrived functions.

The phantom phase must have started recently, at redshift z ≲ 1.

Such models are disfavored due to fine-tuning and recent onset constraints.

Abstract

We revisit the problem of phantom behaviour of effective dark energy in scalar-tensor gravity. The main focus is on the properties of the functions defining the model. We find that models with the present phantom behavior can be made consistent with all constraints, but one of these functions must have rather contrived shape, and the initial data must be strongly fine-tuned. Also, the phantom stage must have begun fairly recently, at $z\lesssim 1$. All this disfavors the effective phantom behaviour in the scalar-tensor gravity.