Addressing the $H_0$ tension through matter with pressure and no early dark energy

TL;DR

This paper proposes a new matter with pressure component to resolve the Hubble tension, which modifies the universe's expansion without early dark energy, supported by MCMC analysis favoring this model over standard cosmology.

Contribution

Introduces the $ ext{Lambda}_{ ext{omega}_s} ext{CDM}$ model with a pressureful matter component that alleviates the Hubble tension, distinct from early dark energy approaches.

Findings

The fluid behaves similarly to radiation with $ ext{omega}_s extless= ext{omega}_ ext{gamma}$.

Best-fit model has $ ext{Omega}_s / ext{Omega}_ ext{gamma} ext{approx} 0.45$.

Statistical analysis favors the new model over standard $ ext{Lambda}$CDM.

Abstract

We propose that the Hubble tension arises due to an unaccounted additional component, that behaves as \emph{matter with pressure}. We demonstrate that this fluid remains subdominant compared to both dust and radiation throughout nearly the entire universe expansion history. Specifically, the additional fluid satisfies the Zel'dovic limit with a constant equation of state, $\omega_s > 0$, and a quite small normalized energy density, $\Omega_s$. Accordingly, this component modifies both the sound horizon and the background expansion rate, \emph{acting quite differently from early dark energy models}, without significantly affecting the other cosmological parameters. To show this, we perform a Monte Carlo Markov chain analysis of our model, hereafter dubbed $\Lambda_{\omega_s}$CDM paradigm, using the publicly available \texttt{CLASS} Boltzmann code. Our results confirm the presence of this fluid, with properties that closely resemble those of radiation. We find best-fit values that satisfy $\omega_s \lesssim \omega_\gamma$ and a relative energy density $\Omega_s / \Omega_\gamma = 0.45$, with $\omega_r$ and $\Omega_r$ the equation of state and density of photons, respectively. The effective fluid may be associated with generalized K-essence models or, alternatively, with Proca-type vector fields, albeit we do not exclude \emph{a priori} more exotic possibilities, i.e., dark radiation, axions, and so on. Physical implications of our results are analyzed in detail, indicating a statistical preference for the $\Lambda_{\omega_s}$CDM scenario over the conventional $\Lambda$CDM background.