The Observational Future of Cosmological Scalar-Tensor Theories

TL;DR

Future cosmological surveys will significantly improve constraints on deviations from General Relativity using a broad class of scalar-tensor theories, achieving precision comparable to Solar System tests by combining multiple observational probes.

Contribution

This paper forecasts how upcoming surveys will constrain Horndeski scalar-tensor theories without relying on quasi-static approximations or screening assumptions.

Findings

Constraints on deviations from General Relativity will improve by an order of magnitude.

Combining different survey types enhances the precision of gravity tests.

Future experiments can constrain the Brans-Dicke parameter to Solar System levels.

Abstract

The next generation of surveys will greatly improve our knowledge of cosmological gravity. In this paper we focus on how Stage IV photometric redshift surveys, including weak lensing and multiple tracers of the matter distribution and radio experiments combined with measurements of the cosmic microwave background will lead to precision constraints on deviations from General Relativity. We use a broad subclass of Horndeski scalar-tensor theories to forecast the accuracy with which we will be able to determine these deviations and their degeneracies with other cosmological parameters. Our analysis includes relativistic effects, does not rely on the quasi-static evolution and makes conservative assumptions about the effect of screening on small scales. We define a figure of merit for cosmological tests of gravity and show how the combination of different types of surveys, probing different length scales and redshifts, can be used to pin down constraints on the gravitational physics to better than a few percent, roughly an order of magnitude better than present probes. Future cosmological experiments will be able to constrain the Brans-Dicke parameter at a level comparable to Solar System and astrophysical tests