Lawrence Berkeley National Laboratory

The origin of the Universe's late-time accelerated expansion remains unknown. The General Relativistic Entropic Acceleration (GREA) theory offers a compelling alternative to ΛCDM, 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+, DES-SN5YR), and cosmic microwave background (CMB) distance measurements. While GREA is not nested within ΛCDM, 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 ΛCDM cosmology. We find that the best-fit model features a transient phantom crossing at z ≲ 2, with wa ≡ dw(a = 1)/da ≃ -0.3, in good agreement with observations. However, its present-day value w 0 ≡ w(z = 0) is tightly constrained to be w 0 ≃ -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.