Modifying $Λ$CDM dynamics via out-of-equilibrium axions: reconciling SH0ES and DESI $H_0$ values
We investigate late-Universe dynamics in which the dark matter component is described by axion particles. The proposed framework departs from the standard $Λ$CDM paradigm due to a small fraction of axions driving the system away from thermal equilibrium. We analyze the evolution of the axion energy density using both a kinetic and a classical field approach, yielding an identical macroscopic evolution equation for the dark matter density. We emphasize that the BGK parameter is introduced phenomenologically at the kinetic level and this does not supply an independent microscopic derivation. The present work therefore explores the phenomenological consequences of late-time, out-of-equilibrium axion production rather than claiming a completed microphysical model. The resulting scenario modifies $Λ$CDM dynamics in the late Universe (specifically at $z \lesssim 1$), while asymptotically recovering the standard baseline at earlier cosmic epochs. We compare the theoretical predictions of our formulation against a comprehensive suite of late-Universe datasets. Our statistical analysis reveals that when the SH0ES local calibration is included, the collisional axion model becomes significantly favored over $Λ$CDM, yielding a best-fit Hubble constant of $H_0 \simeq 73~{\rm km\,s^{-1}\,Mpc^{-1}}$. Ultimately, this cosmological scenario successfully accommodates local distance-ladder measurements while maintaining excellent agreement with Baryon Acoustic Oscillation data from the DESI Collaboration.