Entropic issues in contemporary cosmology

TL;DR

This paper examines entropic challenges in modern cosmology, analyzing how various models address the universe's initial low entropy state and discussing the implications of inflation, holography, and thermodynamics.

Contribution

It critically evaluates brane cosmology, inflationary models, and holographic principles in explaining the universe's low entropy origin, highlighting limitations and proposing conditions for consistency.

Findings

Pushing the start time to negative infinity worsens the entropy problem.

De Sitter space entropy constraints imply cyclic models face heat death.

High-energy inflation with quantum gravity principles can produce suitable initial conditions.

Abstract

Penrose [1] has emphasized how the initial big bang singularity requires a special low entropy state. We address how recent brane cosmological schemes address this problem and whether they offer any apparent resolution. Pushing the start time back to $t=-\infty$ or utilizing maximally symmetric AdS spaces simply exacerbates or transfers the problem. Because the entropy of de Sitter space is $S\leq 1/\Lambda$, using the present acceleration of the universe as a low energy $(\Lambda\sim 10^{-120}$) inflationary stage, as in cyclic ekpyrotic models, produces a gravitational heat death after one cycle. Only higher energy driven inflation, together with a suitable, quantum gravity holography style, restriction on {\em ab initio} degrees of freedom, gives a suitable low entropy initial state. We question the suggestion that a high energy inflationary stage could be naturally reentered by Poincare recurrence within a finite causal region of an accelerating universe. We further give a heuristic argument that so-called eternal inflation is not consistent with the 2nd law of thermodynamics within a causal patch.