The Effect of Cosmological Evolution on Solar System Constraints and on the Scalarization of Neutron Stars in Massless Scalar-Tensor Theories

TL;DR

This paper investigates whether generalized scalar-tensor theories of gravity can satisfy solar system and cosmological constraints while still allowing for neutron star scalarization, concluding it is not possible within the considered framework.

Contribution

The authors extend the conformal coupling in scalar-tensor theories to explore compatibility with observational constraints and neutron star phenomena.

Findings

Generalized coupling does not reconcile solar system and cosmological constraints with neutron star scalarization.

Scalarization remains incompatible with Big-Bang nucleosynthesis constraints in the extended theory.

The theory cannot simultaneously satisfy all observational constraints and produce neutron star scalarization.

Abstract

Certain scalar-tensor theories of gravity that generalize Jordan-Fierz-Brans-Dicke theory are known to predict non-trivial phenomenology for neutron stars. In these theories, first proposed by Damour and Esposito-Far\`ese, the scalar field has a standard kinetic term, and couples conformally to the matter fields. The weak equivalence principle is therefore satisfied, but scalar effects may arise in strong-field regimes, e.g. allowing for violations of the strong equivalence principle in neutron stars ("spontaneous scalarization") or in sufficiently tight binary neutron-star systems ("dynamical/induced scalarization"). The original scalar-tensor theory proposed by Damour and Esposito-Far\`ese is in tension with solar-system constraints (for couplings that lead to scalarization), if one accounts for cosmological evolution of the scalar field and no mass term is included in the action. We here extend the conformal coupling of that theory, in order to ascertain if, in this way, solar-system tests can be passed, while retaining a non-trivial phenomenology for neutron stars. We find that even with this generalized conformal coupling, it is impossible to construct a theory that passes both Big-Bang nucleosynthesis and solar-system constraints, while simultaneously allowing for scalarization in isolated/binary neutron stars.