Is DESI Seeing Dynamical Dark Energy, or a Cosmic Glitch in Gravity?

Abstract

Combined baryon acoustic oscillation and cosmic microwave background data have begun to mildly favor departures from $\Lambda \mathrm{CDM}$. This is usually framed as 2-parameter dynamical dark energy. However, a 1-parameter "cosmic glitch in gravity" model, whose cosmological gravitational coupling differs from Newton's constant as in Hořava gravity, fits at least as well. $\text{Planck}$+ACT+SPT and DESI~DR2 give $G_{\mathrm{cosmo}}/G_{\mathrm{N}}=0.9920\pm 0.0025$, a $3.3\sigma$ preference for weaker cosmological gravity, robust to adding CMB lensing and supernovae. Fitted to the CMB alone, the glitch predicts DESI distances; $w_0{w}_a$CDM does not.

6+8 pages, 9 figures, 4 tables