Towards Viable Cosmological Models of Disformal Theories of Gravity

TL;DR

This paper analyzes the late-time behavior of disformal gravity models, finding that most are not viable for explaining dark energy, but certain tuned models could match observations if only dark matter is disformally coupled.

Contribution

It provides a dynamical systems analysis of disformal gravity, identifying conditions under which models can or cannot describe viable dark energy scenarios.

Findings

No stable attractors for viable dark energy in general disformal models

Models with singular Jordan frame metrics occur only in the infinite future

A special parameter tuning yields a fixed point matching observed dark energy properties

Abstract

The late-time cosmological dynamics of disformal gravity are investigated using dynamical systems methods. It is shown that in the general case there are no stable attractors that screen fifth-forces locally and simultaneously describe a dark energy dominated universe. Viable scenarios have late-time properties that are independent of the disformal parameters and are identical to the equivalent conformal quintessence model. Our analysis reveals that configurations where the Jordan frame metric becomes singular are only reached in the infinite future, thus explaining the natural pathology resistance observed numerically by several previous works. The viability of models where this can happen is discussed in terms of both the cosmological dynamics and local phenomena. We identify a special parameter tuning such that there is a new fixed point that can match the presently observed dark energy density and equation of state. This model is unviable when the scalar couples to the visible sector but may provide a good candidate model for theories where only dark matter is disformally coupled.