Hints of noncold dark matter? Observational constraints on barotropic dark matter with a constant equation of state parameter

Abstract

This study investigates the potential of a cosmological model termed $\Lambda w$DM, in which a cosmological constant play the role of dark energy and dark matter is barotropic and has a constant equation of state parameter ($w_{\mathrm{dm}}$), to address the $S_8$ tension between early- and late-universe observations. By incorporating the latest cosmological datasets — including Planck Cosmic Microwave Background (CMB), Baryon Acoustic Oscillation (BAO), Ia supernovae (SNe Ia), Redshift Space Distortions (RSD), and weak lensing (WL) — we constrain the $\Lambda w$DM compared to $\Lambda$CDM. Our analysis reveals a marginal preference for a non-zero $w_{\mathrm{dm}}=2.7_{-1.9}^{+2.0}\times 10^{-7}$ (at 95% confidence level) when combining CMB, SDSS BAO, SNe Ia, RSD, and WL data, and a marginal preference for a non-zero $w_{\mathrm{dm}}=2.29_{-2.0}^{+1.9}\times 10^{-7}$(at 95% confidence level) when combining CMB, DESI Y1 BAO, SNe Ia, RSD, and WL data. In addition, we find that, compared to $\Lambda$CDM, $\Lambda w$DM can alleviate the $S_8$ tension from $>3\sigma$ to $<1\sigma$. Furthermore, we find that, for CMB+SDSS+PP+RSD+WL datasets, the $\Lambda w$DM model is close to being positively preferred over the $\Lambda$CDM model.