Cosmological constraints on mass-varying dark matter

As one of the fundamental unknowns of our Universe, the mass of dark matter remains to be a topic of great interest. We consider the possibility of a time-variation of the dark matter mass. We study the cosmological constraints on a model where the dark matter mass transitions from zero to a finite value in the early Universe. In this model, the matter power spectrum exhibits power suppression below a certain scale that depends on the epoch of transition, and the angular power spectrum of the cosmic microwave background shows a distinctive phase shift and power suppression at small scales. We use the latest cosmic microwave background data and the ${S}_{8}$ priors from weak lensing data to place a lower limit on the transition redshift. We also find that the data from the Atacama Cosmology Telescope (ACT) show a mild preference for the mass-varying dark matter model over $\mathrm{\ensuremath{\Lambda}}$ Cold Dark Matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$).