Baryogenesis constraints on generalized mass-to-horizon entropy

TL;DR

This work studies baryogenesis in a cosmology where a generalized mass-to-horizon entropy, with $M = \gamma \frac{c^2}{G} L^n$ leading to $S = \gamma \frac{2n}{n+1}\tilde{r}_A^{\,n-1} S_{BH}$, modifies the Friedmann equations via the gravity-thermodynamics correspondence. The modified dynamics yields a nonvanishing $\dot{R}$ during radiation domination, enabling gravitational baryogenesis through the curvature-current coupling $\mathcal{L}_{int} = \frac{1}{M_*^2} J^\mu \partial_\mu R$ and producing a baryon asymmetry $\eta_B$ that depends on the entropic exponent $n$. By matching to the observed BAU, they derive a bound $0 < 1-n \lesssim \mathcal{O}(10^{-2})$ at the decoupling temperature $T_D \simeq 10^{16}\,\text{GeV}$, indicating that sub-extensive corrections to the area law are cosmologically viable. They also compare with late-time and primordial constraints on $n$, finding a tension that may be alleviated by a scale-dependent $n$; this work thus ties microscopic horizon thermodynamics to early-universe baryogenesis and contemporary cosmological data.

Abstract

We investigate the generation of the baryon asymmetry within the cosmological framework based on a generalized mass-to-horizon entropy. This entropy, recently proposed as a power-law extension of the Bekenstein-Hawking area law, arises from a modified mass-horizon relation constructed to ensure consistency with the Clausius relation. By applying the gravity-thermodynamics conjecture, the resulting corrections to the Friedmann equations modify the evolution of the Hubble parameter. Consequently, even the standard supergravity coupling between the Ricci scalar and the baryon current can generate a non-vanishing matter-antimatter asymmetry. Comparison with observational data yields a stringent constraint on the entropic exponent, namely $0 < 1 - n \lesssim \mathcal{O}(10^{-2})$, at the decoupling temperature $T_D \simeq 10^{16}\,\text{GeV}$, corresponding to the current upper limit on tensor-mode fluctuations at the inflationary scale. These findings indicate that minor, subtle, yet physically significant departures, from the standard Bekenstein-Hawking entropy ($n = 1$) may be required to achieve full consistency with present cosmological observations.

Figures (1)

• Figure 1: 3D plot of $\eta_B$ in Eq. \ref{['expan']}. The grey region is excluded by the observational bound $\eta_B \lesssim 10^{-10}$ParticleDataGroup:2018ovx.