New cosmological bounds on axions in the XENON1T window

Abstract: Motivated by a possible $\sim$ eV-mass solar axion explanation to excess events recently detected by the XENON1T experiment, I revisit and update cosmological constraints on axions in this mass range. I find that of the allowed XENON1T mass window (0.1 - 4.1 eV for DFSZ axions and 46 - 56 eV for KSVZ axions), only 0.1 - 0.35 eV remains viable at 95% confidence given current cosmological probes. If a 0.35 eV DFSZ axion existed, it would be detectable at ${\sim}7\sigma$ via two independent physical effects with the next-generation CMB-S4 experiment. Conversely, even a combination of CMB-S4 with future DESI measurements falls just short of guaranteeing a 0.1 eV-mass axion can be detected or ruled out. A future limit of $\Delta N_{\rm eff}<0.027$ could rule out any generic axion-like particle across a wide range of masses as long as the reheating temperature is not too low, or alternatively, a future cosmological detection of such an axion-like particle could become the tightest existing observational lower bound on the reheating temperature.