Challenges in Constraining Gravity with Cosmic Voids

TL;DR

This paper investigates how cosmic void statistics can be used to distinguish between modified gravity models and general relativity, emphasizing the importance of realistic mock galaxy catalogs and analyzing velocity and clustering signals.

Contribution

It demonstrates that void size and velocity statistics can differentiate gravity models when using mock galaxy tracers, and shows the insensitivity of the void quadrupole to gravity models.

Findings

Void size functions differ between models when using dark matter halos but not with galaxy tracers.

Void velocities and dispersions are enhanced in modified gravity models.

Void quadrupole moments are similar across models and can unbiasedly estimate growth rate.

Abstract

We compare void size and clustering statistics for nDGP and $f(R)$ gravity models and GR using N-body simulations. We show how it is critical to consider the statistics derived from mock galaxy catalogs rather than the dark matter halos alone. Marked differences between the void size functions for GR and $f(R)$ models which present when voids are identified using dark matter halos are removed when voids are identified, more realistically, from mock galaxy tracers of the halos. The void radial velocities and velocity dispersions in the $f(R)$ and nDGP models are enhanced relative to GR in both halos and mock galaxy identified voids. Despite this, we find that the redshift space void quadrupole moments derived from the mock galaxy tracers are strikingly similar across the three gravity models. The Gaussian Streaming Model (GSM) is shown to accurately reconstruct $\xi_2$ in modified gravity models and we employ the GSM, using a functional derivative approach, to analyze the insensitivity of $\xi_2$ to the gravity model. Assuming linear theory, we show the void quadrupole to be an unbiased estimator of the redshift space growth rate parameter $\beta=f/b$ in the modified gravity theories.