The Swampland and Screened Modified Gravity

TL;DR

This paper examines how swampland conjectures constrain scalar-tensor theories with screening mechanisms, finding that certain models like chameleon and symmetron can be consistent with these conjectures, while the dilaton behaves differently.

Contribution

It analyzes the compatibility of screened modified gravity models with swampland conjectures, providing conditions under which they are not in the swampland and exploring their cosmological implications.

Findings

Chameleon models are not in the swampland if coupling > 1 and scalar mass >> Hubble rate.

Viable dilaton models satisfy solar system tests and are not in the swampland.

Dark energy in chameleon and symmetron models is transient, unlike the slowly evolving dilaton.

Abstract

We consider the implications of the swampland conjectures on scalar-tensor theories defined in the Einstein frame in which the scalar interaction is screened. We show that chameleon models are not in the swampland provided the coupling to matter is larger than unity and the mass of the scalar field is much larger than the Hubble rate. We apply these conditions to the inverse power law chameleon and the symmetron. We then focus on the dilaton of string theory in the strong coupling limit, as defined in the string frame. We show that solar system tests of gravity imply that viable dilaton models are not in the swampland. In the future of the Universe, if the low energy description with a single scalar is still valid and the coupling to matter remains finite, we find that the scalar field energy density must vanish for models with the chameleon and symmetron mechanisms. Hence in these models dark energy is only a transient phenomenon. This is not the case for the strongly coupled dilaton, which keeps evolving slowly, leading to a quasi de Sitter space-time.