Constraining GREA, an alternative theory accounting for the present cosmic acceleration

Abstract

The origin of the Universe's late-time accelerated expansion remains unknown. The General Relativistic Entropic Acceleration (GREA) theory offers a compelling alternative to $\hat{I}›$CDM, attributing cosmic acceleration to entropy growth associated with cosmic and black hole horizons, without invoking a cosmological constant. We test GREA against the latest DESI DR2 Baryon Acoustic Oscillations (BAO), multiple Type Ia supernova compilations (Union3, Pantheon$\text{+}$, DES-SN5YR), and cosmic microwave background (CMB) distance measurements. While GREA is not nested within $\hat{I}›$CDM, it achieves a comparable goodness-of-fit, highlighting its potential as a theoretically motivated framework that circumvents some of the fine-tuning issues of the standard $\hat{I}›$CDM cosmology. We find that the best-fit model features a transient phantom crossing at $z\lesssim 2$, with $w_a\equiv \mathrm{d}w(a=1)/\mathrm{d}a\simeq -0.3$, in good agreement with observations. However, its present-day value $w_0\equiv w(z=0)$ is tightly constrained to be $w_0\simeq -1$. Upcoming low-redshift (i.e. $z<1$) cosmological probes, from both background and perturbations, will offer promising avenues for further exploring the viability of the GREA theory.

27 pages, 11 figures