Cosmological Implications of Thermodynamic Split Conjecture

TL;DR

This work introduces the Thermodynamic Split Conjecture (TSC), arguing that black hole horizon thermodynamics cannot be naively applied to cosmological horizons, and that cosmology requires its own intrinsic thermodynamic rules defined by $S_{ m cos}(H)$ and $T_{ m eff}(H)$. By replacing the BH-derived entropy and temperature with cosmology-native forms, the paper shows that key early-Universe phenomena—eternal inflation, vacuum transitions, and PBH production—and even late-time dynamics can differ substantially, parameterized by $T_{ m eff}(H)=\chi(H)\frac{H}{2\pi}$, $S_{ m cos}(H)=\alpha H^{-p}$, and $D_{ m eff}(H)=\beta(H)\frac{H^3}{8\pi^2}$. These deformations offer a controlled route to potentially alleviate $H_0$ and $S_8$ tensions while preserving GR in the BH/GH limit, and they motivate empirical efforts to determine the correct horizon thermodynamics via cosmological observations and connections to quantum gravity. The results underscore that the link between horizon thermodynamics and cosmology is not fixed but testable, with implications spanning stochastic inflation, vacuum stability, entropy bounds, and the thermodynamic interpretation of cosmological dynamics.

Abstract

Building on initial work on the Thermodynamic Split Conjecture (TSC), which posits that black hole and cosmological horizon thermodynamics are generically inequivalent, we examine the consequences of that split for the Gibbons Hawking temperature and its role across cosmology. We consider many key results in both early and late universe cosmology and show that many important results such as those governing eternal inflation, vacuum tunneling, quantum breaking and primordial black holes can change. The analysis further reveals that small, TSC motivated corrections to horizon thermodynamics can subtly modify Friedmann dynamics, potentially helping to address the $H_0$ and $S_8$ tensions. The work thus provides a unified route from quantum gravity motivated thermodynamics to observational cosmology and motivates dedicated tests of the thermal laws governing the Universe itself.