DESI Dark Secrets

The first and second year results of DESI provide consistent 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. We find that the best-fit DESI $w_0w_a$ models for dark energy, which underpin the DESI claim, have unusual behavior: they achieve a maximum energy density around $z\simeq 0.5$ and rapidly decrease before and after. We show that this could be explained by the fact that the $w_0w_a$ parameterization is limited in its ability to model dark energy as it only allows four generic behaviors: monotonically increasing or decreasing, or with a maximum or minimum. In turn, $w=-1$ can only be achieved at a minimum or maximum of the dark energy (for $1+w_0, w_a \not= 0$). $w_0w_a$ is a one-parameter characterization of scalar-field models, and cannot represent them to the precision needed for the DESI results. We explore models where the dark energy is a rolling scalar-field characterized by one dimensionless parameter $\beta$, which, in the limit of $\beta \rightarrow 0$ reduces to $\Lambda$CDM. None of these models fit the DESI data significantly better than $\Lambda$CDM or as well as the best-fit DESI $w_0w_a$ models. We also examine the supernovae data from Pantheon+ that strengthen the DESI claims for evolving dark energy. The combination of DESI, CMB (Planck) and SNe data favor a 95% credible interval $\beta = 0.27 - 1.03$, providing some 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 $w_0w_a$, and together they favor a $w_0w_a$ model. We also point out that the unusual behavior of the best-fit DESI $w_0w_a$ models could arise due to the matter density not varying as expected or an unaccounted for component of energy density in the Universe.