Scalar-tensor theories and cosmology

TL;DR

Scalar-tensor theories extend general relativity by introducing scalar fields with various couplings, and are tested through diverse observations including solar-system, binary-pulsar, and cosmological data to constrain their parameters.

Contribution

This paper reviews the theoretical framework of scalar-tensor theories and emphasizes the importance of combining different observational tests to constrain scalar-Gauss-Bonnet couplings.

Findings

Different observational tests provide complementary constraints.

Solar-system tests limit local scalar effects.

Cosmological data constrains large-scale scalar interactions.

Abstract

Scalar-tensor theories are the best motivated alternatives to general relativity and provide a mathematically consistent framework to test the various observable predictions. They can involve three functions of the scalar field: (i) a potential (as in "quintessence" models), (ii) a matter-scalar coupling function (as in "extended quintessence", where it may also be rewritten as a nonminimal coupling of the scalar field to the scalar curvature), and (iii) a coupling function of the scalar field to the Gauss-Bonnet topological invariant. We recall the main experimental constraints on this class of theories, and underline that solar-system, binary-pulsar, and cosmological observations give qualitatively different tests. We finally show that the combination of these data is necessary to constrain the existence of a scalar-Gauss-Bonnet coupling.