Massless graviton in de Sitter as second sound in two-fluid hydrodynamics

TL;DR

This paper proposes a two-fluid hydrodynamic model in de Sitter spacetime that reveals a collective mode analogous to second sound, interpreted as a massless graviton propagating at light speed.

Contribution

It introduces a novel two-fluid approach to de Sitter thermodynamics, identifying a massless graviton-like mode as a second sound analog.

Findings

Discovered a collective mode in de Sitter that propagates at the speed of light.

Identified this mode as analogous to second sound in two-fluid dynamics.

Proposed this mode as a massless graviton in de Sitter spacetime.

Abstract

The concept of gravitons and their masses, clear in the case of Minkowski spacetime, remains ambiguous for de Sitter spacetime. Here, we used a two-fluid approach to de Sitter thermodynamics and found a collective mode that is analogous to second sound in the two-fluid dynamics of the de Sitter state. This mode is massless and propagates at the speed of light. This suggests that this second-sound analog is a massless graviton propagating in de Sitter spacetime. The type of graviton this mode represents requires further consideration.