Abstract
Context. We propose that the Hubble tension arises due to an unaccounted-for additional component that behaves as matter with pressure. Aims. We aim to demonstrate that this fluid remains subdominant compared to both dust and radiation throughout the entire expansion history of the Universe. Specifically, the additional fluid satisfies the Zel’dovic limit with a constant equation of state, ω s > 0, and quite a small normalized energy density, Ω s . Methods. This component modifies both the sound horizon and the background expansion rate, acting quite differently from early dark-energy models, without significantly affecting the other cosmological parameters. To show this, we performed a Markov chain Monte Carlo analysis of our model, hereafter dubbed the Λ ω s cold dark matter (Λ ω s CDM) paradigm, using the publicly available CLASS Boltzmann code. Results. Our results confirm the presence of this fluid, with properties that closely resemble those of radiation. We find best-fit values that satisfy ω s ≲ ω γ and a relative energy density of Ω s /Ω γ = 0.33, with ω γ and Ω γ being the equation of state and density of photons, respectively. The additional fluid may be interpreted either as a thermalized scalar field, plausibly associated with the quasi-quintessence model, or as Proca-type vector fields, albeit we did not exclude a priori more exotic possibilities, i.e., dark radiation, axions, and so on. Physical implications of our results were analyzed in detail, indicating a statistical preference for the Λ ω s CDM scenario over the conventional ΛCDM background.