📝 SummarXiv

Abstract

The first year results of DESI (DR1) provide evidence that dark energy may not be quantum vacuum energy ($\Lambda$). If true, this would be an extraordinary development in the 25-year quest to understand cosmic acceleration. The best-fit DESI $w_0w_a$ models for dark energy, which underpin the claim, have strange behavior. They achieve a maximum energy density around $z\simeq 0.5 $ and rapidly decrease before and after. We explore physics-based models where the dark energy is a rolling scalar-field. Our four scalar-field models are characterized by one dimensionless parameter $\beta$, which in the limit of $\beta \rightarrow 0$ reduces to $\Lambda$CDM. While none of our models fit the DESI data significantly better than $\Lambda$CDM, for values of $\beta$ of order unity, they fit about as well as $\Lambda$CDM. Alternatives to vacuum energy make different predictions for the age of the Universe and are potential discriminators. We also consider the second data release from DESI (DR2), CMB data and supernovae data. The DR2 results are consistent with the DR1, and the combination of DESI, CMB and SNe favor a 95\% credible interval $\beta = 0.22 - 0.95$, providing modest evidence for a scalar-field explanation for dark energy. While the DESI data prefer $w_0w_a$ to a scalar field, the SNe data prefer a scalar field to $w_0w_a$. The robust DESI preference for sharply-peaked dark energy may be telling us something important about dark energy or the DESI data themselves.