Gravity, Entropy, and Cosmology: In Search of Clarity

TL;DR

This paper clarifies the statistical mechanics of gravitating systems and their cosmological implications, emphasizing distinctions between gravity's role in entropy and the entropy of gravity itself, challenging conventional views.

Contribution

It offers a clearer account of gravitational entropy, distinguishing its role in entropy calculations from the entropy of gravity, and clarifies misconceptions about gravitational collapse and thermodynamics.

Findings

Gravity's role in entropy must be distinguished from the entropy of gravity.

Gravitational collapse increases entropy mainly through matter, not gravity itself.

The Second Law does not depend on the statistical mechanics of gravitational collapse.

Abstract

I discuss the statistical mechanics of gravitating systems and in particular its cosmological implications, and argue that many conventional views on this subject in the foundations of statistical mechanics embody significant confusion; I attempt to provide a clearer and more accurate account. In particular, I observe that (i) the role of gravity \emph{in} entropy calculations must be distinguished from the entropy \emph{of} gravity, that (ii) although gravitational collapse is entropy-increasing, this is not usually because the collapsing matter itself increases in entropy, and that (iii) the Second Law of Thermodynamics does not owe its validity to the statistical mechanics of gravitational collapse.