Big Bang Nucleosynthesis Constraint on Baryonic Isocurvature Perturbations

TL;DR

This paper investigates how large baryonic isocurvature perturbations during big bang nucleosynthesis affect deuterium abundance, deriving constraints on their amplitude and applying these to inflationary leptogenesis scenarios.

Contribution

It provides the first stringent constraint on baryonic isocurvature perturbations at small scales using BBN deuterium measurements, and applies this to inflationary leptogenesis models.

Findings

Constraint on perturbation amplitude: -2.016 (2c3) for scale k^{-1}  0.0025 pc.

Most stringent constraint for scales 0.1 Mpc^{-1}  k  4  10^8 Mpc^{-1}.

Application of constraints to relaxation leptogenesis scenario, limiting the number of last e-folds.

Abstract

We study the effect of large baryonic isocurvature perturbations on the abundance of deuterium (D) synthesized in big bang nucleosynthesis (BBN). We found that large baryonic isocurvature perturbations existing at the BBN epoch ($T\sim 0.1\,$MeV) change the D abundance by the second order effect, which, together with the recent precise D measurement, leads to a constraint on the amplitude of the power spectrum of the baryon isocurvature perturbations. The obtained constraint on the amplitude is $\lesssim 0.016 \,(2\sigma)$ for scale $k^{-1} \gtrsim 0.0025\,\text{pc}$. This gives the most stringent one for $0.1\,\text{Mpc}^{-1} \lesssim k \lesssim 4\times 10^8\, \text{Mpc}^{-1}$. We apply the BBN constraint to the relaxation leptogenesis scenario, where large baryon isocurvature perturbations are produced in the last $N_\text{last}$ $e$-fold of inflation, and we obtain a constraint on $N_\text{last}$.