Unified formalism for the emergence of space from the first law of thermodynamics

TL;DR

This paper derives a general expansion law for the universe from thermodynamics, unifying various gravity theories and entropy forms, and clarifies the emergence of space through entropic evolution on the apparent horizon.

Contribution

It introduces a unified formalism for cosmic expansion laws applicable to multiple gravity theories and entropy models, advancing the understanding of space emergence from thermodynamics.

Findings

Derived a general expansion law from the first law of thermodynamics.

Extended the law to various gravity theories like Einstein, Gauss-Bonnet, and Lovelock.

Included non-extensive entropy models such as Tsallis entropy.

Abstract

We derive a unified expansion law for our universe from the first law of thermodynamics on the apparent horizon, where entropic evolution depicts the emergence of cosmic space. The derivation advances a general form for degrees of freedom on the surface and bulk, which provides a natural generalization for the expansion law proposed by Padmanabhan. The general expression for the surface degrees of freedom differs from the natural expectation, $ N_{sur }= 4S $ in the emergent gravity paradigm for general theories of gravity. The derivation also provides justification for the selection of Gibbons-Hawking temperature in the original expansion law and for the use of areal volume in the non-flat FRW universe. Since the unified expansion law exclusively depends on the form of entropy, the method is applicable to obtain the expansion law in any gravity theory without any additional ad hoc assumptions. From the general expansion law, we have obtained the expansion law corresponding to different theories of gravity like (n+1) Einstein, Gauss-Bonnet, Lovelock, and Horava-Lifshitz. We also obtained the expansion law for non-extensive entropy like Tsallis entropy from the unified expansion law.