Exotic dark matter and the DESI anomaly
Matteo Braglia, Xingang Chen, Abraham Loeb
Abstract
Abstract Exotic dark matter (EDM) refers to a dark matter species whose equation of state deviates from zero at late times. This behavior enables it to model a variety of non-standard late-time cosmologies, offering alternatives to various dark energy (DE) models, especially when the DE sector violates the null energy condition. In this work, by fitting to a compressed CMB likelihood, BAO, and Supernovae (SNe) data and comparing models in a Bayesian approach, we show that simple models of exotic dark matter are statistically comparable to the w 0 w a CDM DE model in explaining the recent anomaly in the late-time cosmological evolution suggested by DESI and supernova observations, although in both classes of models the evidence against the ΛCDM model only appears when the DES-Y5 or Union3 SNe dataset is included. The value of H 0 remains similar to that in the DE model, except in the no-SNe case, where the DE model predicts lower values than ΛCDM, thereby worsening the Hubble tension, whereas the EDM models yield values closer to that of ΛCDM, albeit with larger uncertainty. In addition, the EDM models predict a drastically different energy budget for the present-day universe compared to the standard model, and provide an explanation for a coincidence problem in the DE-model explanation of the DESI anomaly.