# Breaking cosmic degeneracies: Disentangling neutrinos and modified gravity with kinematic information

## TL;DR

This paper demonstrates that velocity-based measurements, such as growth rate and virial velocities, can effectively distinguish between effects of neutrinos and modified gravity on cosmic structure formation, overcoming degeneracies in power spectrum analyses.

## Contribution

It introduces the use of kinematic information from simulations to break the degeneracy between neutrino mass effects and modified gravity in large-scale structure studies.

## Key findings

- Velocity divergence distinguishes neutrino effects from modified gravity.
- Growth rate is enhanced in models with modified gravity and similar power spectra.
- Virial velocities constrain deviations from general relativity independently of neutrino mass uncertainties.

## Abstract

Searches for modified gravity in the large-scale structure try to detect the enhanced amplitude of density fluctuations caused by the fifth force present in many of these theories. Neutrinos, on the other hand, suppress structure growth below their free-streaming length. Both effects take place on comparable scales, and uncertainty in the neutrino mass leads to a degeneracy with modified gravity parameters for probes that are measuring the amplitude of the matter power spectrum. We explore the possibility to break the degeneracy between modified gravity and neutrino effects in the growth of structures by considering kinematic information related to either the growth rate on large scales or the virial velocities inside of collapsed structures. In order to study the degeneracy up to fully non-linear scales, we employ a suite of $N$-body simulations including both $f(R)$ modified gravity and massive neutrinos. Our results indicate that velocity information provides an excellent tool to distinguish massive neutrinos from modified gravity. Models with different values of neutrino masses and modified gravity parameters possessing a comparable matter power spectrum at a given time have different growth rates. This leaves imprints in the velocity divergence, which is therefore better suited than the amplitude of density fluctuations to tell the models apart. In such models with a power spectrum comparable to $\Lambda$CDM today, the growth rate is strictly enhanced. We also find the velocity dispersion of virialised clusters to be well suited to constrain deviations from general relativity without being affected by the uncertainty in the sum of neutrino masses.