LAUSR

We study the chameleon field dark matter, dubbed 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.