General Relativistic Entropic Acceleration at the perturbation level: a CLASS implementation and first Boltzmann-code constraints

arXiv:2607.25841 · gr-qc, astro-ph.CO · Submitted 2026-07-28 · Read on arXiv

Simone D'Onofrio, Dong Ha Lee, Eleonora Di Valentino, Juan García-Bellido

gr-qc, astro-ph.CO

Submitted: 2026-07-28

Comments: 21 pages, 15 figures

Code: https://github.com/zmonneee/CLASS_GREA

License: http://creativecommons.org/licenses/by/4.0/

The gist: General Relativistic Entropic Acceleration (GREA) attributes the late-time acceleration of the Universe to the entropy growth of the causal cosmological horizon, without a cosmological constant, with

Terminology

Abstract

General Relativistic Entropic Acceleration (GREA) attributes the late-time acceleration of the Universe to the entropy growth of the causal cosmological horizon, without a cosmological constant, with a phenomenology fixed by the single O(1) parameter alpha. The model has so far been confronted with data only at the background level. We present its first implementation within an Einstein-Boltzmann solver: the GREA background is integrated directly into CLASS, while the entropic component is evolved as an effective fluid regulated by the parametrized-post-Friedmann scheme, giving access to the full CMB and matter power spectra. A Markov-chain Monte Carlo analysis with COBAYA against the full primary-CMB likelihoods, DESI DR2 BAO and Type Ia supernovae constrains the coupling alpha about 1, in excellent agreement with the theoretical prediction, with a fit matching CDM to within chi squared 6 despite the addition of a single free parameter. The equation of state inferred from the data agrees with binned, model-independent reconstructions and exhibits a second crossing of the phantom divide at z 2, a distinctive prediction of the thermodynamic dynamics rather than of an imposed parametrization.

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