# The splashback radius in symmetron gravity

## TL;DR

This paper investigates how the splashback radius in dark matter haloes is affected by symmetron gravity, revealing potential observable deviations from General Relativity that could constrain modified gravity theories.

## Contribution

It introduces a semi-analytical model of spherical collapse in symmetron gravity, showing how screening impacts the splashback radius and predicting measurable deviations.

## Key findings

- Splashback radius can differ by up to 10% from GR predictions in symmetron models.
- Screening effects become significant as overdensities grow, affecting the collapse dynamics.
- Current experiments can potentially constrain symmetron parameters based on splashback measurements.

## Abstract

The splashback radius $r_\mathrm{sp}$ has been identified in cosmological $N$-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted material. We employ a semi-analytical approach to study the spherical collapse of dark matter haloes in symmetron gravity and provide insights into how the phenomenology of splashback is affected. The symmetron is a scalar-tensor theory of gravity which exhibits a screening mechanism whereby higher-density regions are screened from the effects of a fifth force. In this model, we find that, as over-densities grow over cosmic time, the inner region becomes heavily screened. In particular, we identify a sector of the parameter space for which material currently sitting at $r_\mathrm{sp}$ has followed, during the collapse, the formation of this screened region. As a result, we find that for this part of the parameter space the splashback radius is maximally affected by the symmetron force and we predict changes in $r_\mathrm{sp}$ up to around $10\%$ compared to its General Relativity value. Because this margin is within the precision of present splashback experiments, we expect this feature to soon provide constraints for symmetron gravity on previously unexplored scales.