A New Census of the Universe's Entropy
Stefano Profumo, Liam Colombo-Murphy, Gabriela Huckabee, Maya Diaz Svensson, Stuti Garg, Ishan Kollipara, and Alison Weber

TL;DR
This paper updates the calculation of the universe's total entropy across various components, including black holes and dark sectors, revealing that primordial black holes could dominate the entropy budget, approaching the maximal entropy of de Sitter space.
Contribution
It provides the first comprehensive update of the universe's entropy estimates, incorporating recent observations and constraints on black holes, dark matter, and gravitational waves.
Findings
Primordial black holes could dominate the universe's entropy.
Dark sector models with many degrees of freedom also contribute significantly.
Entropy estimates now include updated black hole and gravitational wave background data.
Abstract
The question of what is the total entropy of the universe, how it compares to the maximal entropy of de Sitter space, and how it is distributed across the universe's components, bears considerable importance for a number of reasons. Here, we first update the computation of the entropy associated with various sectors of the observed universe, including in the diffuse cosmic and late-time gamma-ray and neutrino backgrounds, in baryonic matter both in diffuse components, in stars and stellar remnants, and in cosmic rays; we then update, crucially, the estimate of entropy in stellar-mass and super-massive black holes, whose abundance and mass function has come into increasingly sharp definition with recent observations and with the rapidly growing statistics of black-hole-black-hole mergers observed with gravity wave detectors. We also provide a new, corrected estimate of the potential…
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Taxonomy
TopicsCosmology and Gravitation Theories
