The Bispectrum of f(R) Cosmologies

TL;DR

This study investigates the bispectrum of dark matter in f(R) modified gravity models through simulations, revealing its potential to distinguish gravitational effects from galaxy bias despite similarities in power spectra.

Contribution

It demonstrates that the dark matter bispectrum's dependence on gravity is weak, offering a new way to break degeneracies caused by galaxy bias in cosmological tests.

Findings

Bispectrum differences are significant when power spectra differ.

Similar final power spectra lead to less than 4% bispectrum differences.

Bispectrum mainly depends on the power spectrum, not gravity.

Abstract

In this paper we analyze a suite of cosmological simulations of modified gravitational action f(R) models, where cosmic acceleration is induced by a scalar field that acts as a fifth force on all forms of matter. In particular, we focus on the bispectrum of the dark matter density field on mildly non-linear scales. For models with the same initial power spectrum, the dark matter bispectrum shows significant differences for cases where the final dark matter power spectrum also differs. Given the different dependence on bias of the galaxy power spectrum and bispectrum, bispectrum measurements can close the loophole of galaxy bias hiding differences in the power spectrum. Alternatively, changes in the initial power spectrum can also hide differences. By constructing LCDM models with very similar final non-linear power spectra, we show that the differences in the bispectrum are reduced (<4%) and are comparable with differences in the imperfectly matched power spectra. These results indicate that the bispectrum depends mainly on the power spectrum and less sensitively on the gravitational signatures of the f(R) model. This weak dependence of the matter bispectrum on gravity makes it useful for breaking degeneracies associated with galaxy bias, even for models beyond general relativity.