Second-order peculiar velocity field as a novel probe of scalar-tensor theories

TL;DR

This paper proposes using the second-order velocity field in galaxy bispectrum measurements as a new, unbiased way to test and constrain degenerate higher-order scalar-tensor theories of gravity.

Contribution

It introduces a novel approach focusing on second-order velocity kernels and develops phenomenological functions to trace higher-order growth in DHOST theories.

Findings

Second-order velocity kernels provide unbiased gravity modification signals.

New phenomenological functions effectively trace higher-order growth.

Derived formulae impose constraints on DHOST theory parameters.

Abstract

We investigate the galaxy bispectrum induced by the nonlinear gravitational evolution as a possible probe to constrain degenerate higher-order scalar tensor (DHOST) theories. We find that the signal obtained from the leading kernel of second-order density fluctuations is partially hidden by the uncertainty in the nonlinear galaxy bias, and that the kernel of second-order velocity fields instead provides unbiased information on the modification of gravity theory. Based on this fact, we propose new phenomenological time-dependent functions, written as a combination of the coefficients of the second-order kernels, which is expected to trace the higher-order growth history. We then present approximate expressions for these variables in terms of parameters that characterize the DHOST theories. We also show that the resultant formulae provides new constraints on the parameter space of the DHOST theories.