DESI Dark Secrets

Abstract

We critically examine the results of the Dark Energy Spectroscopic Instrument (DESI), which provide evidence that dark energy may not be quantum vacuum energy. We find that the best-fit $w_0w_a$ models for dark energy, which underpin the claim, have unusual behavior. They achieve a maximum energy density around $z\simeq 0.5 $ and rapidly decrease before and after (this redshift is also the pivot point for the DESI results). 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 -- and $w_a \not= 0$ and $w=-1$ can only be achieved at a minimum or maximum of the dark energy. Further, $w_0w_a$ is a one-parameter characterization of scalar-field models and it cannot represent them to the precision needed for the DESI results. We explore scalar-field models characterized by one dimensionless parameter $β$, which for $β\rightarrow 0$, reduce to $Λ$CDM. None of these models fit the DESI data significantly better than $Λ$CDM or as well as the best-fit DESI $w_0w_a$ models. We also examine the CMB and SN data that strengthen the DESI case for evolving dark energy. The combination of DESI, CMB, and SN data favors a 95\% credible interval $β= 0.33 - 0.96$, providing weak evidence for a scalar-field explanation for dark energy. While the DESI data prefer $w_0w_a$, the SN data prefer a scalar field, and together they favor a $w_0w_a$ model. Finally, 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. In sum, the evidence for evolving dark energy is intriguing but not conclusive, and at this point, the DESI results raise more questions than they answer.