Cosmic entropy and the Universe’s thermodynamic status

Establish a consistent definition of entropy for the Universe as a whole and ascertain whether the Universe constitutes a closed (isolated) physical system.

Background

The text revisits longstanding difficulties in applying thermodynamics to the Universe as a whole, specifically the challenge of defining cosmic entropy and clarifying whether the Universe can be treated as an isolated system. These issues are central to understanding the arrow of time and initial conditions in cosmology.

References

However, at this point it is necessary to recall the already mentioned problem related to the formulation of the entropy notion for the Universe as a whole and the fundamental question related with the character of the Universe as a physical system, i.e. is the Universe a closed system? So far we do not know the answer to these questions.

— Did the Big Bang and cosmic inflation really happen? (A tale of alternative cosmological models)  (2404.18503 - Postolak, 2024) in Conformal Cyclic Cosmology — Thermodynamics and the Big Bang subsection