# Big Bang Nucleosynthesis Hunts Chameleon Dark Matter

## TL;DR

This paper investigates the behavior of chameleon-like scalaron dark matter in $F(R)$ gravity during Big Bang Nucleosynthesis, establishing universal bounds on its mass and showing it can evade current constraints while remaining detectable with future measurements.

## Contribution

It demonstrates the model-independent dynamics of scalaron in $F(R)$ gravity during BBN and derives universal bounds on its mass and coupling, improving understanding of chameleon dark matter constraints.

## Key findings

- Scalaron dynamics governed by $R^{2}$ term and chameleon mechanism.
- Universal bound on scalaron mass during BBN.
- Scalaron can evade current BBN constraints and be detectable in future measurements.

## Abstract

We study the chameleon field dark matter, dubbed \textit{scalaron}, in $F(R)$ gravity in the Big Bang Nucleosynthesis (BBN) epoch. With an $R^{2}$-correction term required to solve the singularity problem for $F(R)$ gravity, we first find that the scalaron dynamics is governed by the $R^{2}$ term and the chameleon mechanism in the early universe, which makes the scalaron physics model-independent regarding the low-energy scale modification. In viable $F(R)$ dark energy models including the $R^{2}$ correction, our analysis suggests the scalaron universally evolves in a way with a bouncing oscillation irrespective of the low-energy modification for the late-time cosmic acceleration. Consequently, we find a universal bound on the scalaron mass in the BBN epoch, to be reflected on the constraint for the coupling strength of the $R^2$ term, which turns out to be more stringent than the one coming from the fifth force experiments. It is then shown that the scalaron naturally develops a small enough fluctuation in the BBN epoch, hence can avoid the current BBN constraint placed by the latest Planck 2018 data, and can also have a large enough sensitivity to be hunted by the BBN, with more accurate measurements for light element abundances as well as the baryon number density fraction.