Effective dark matter component presents a robust signature of negative pressure by the DESI observations

Abstract

Comprehensive cosmological analysis of an effective non-standard dark matter(NSDM) model, characterized by an equation of state $w_{\mathrm{dm}} = w_2 a^2$, which allows for mild deviations from the previously assumed pressureless cold dark matter, is elaborated in the present work. This effective description framework is the scenarios that matter contents coupled to three distinct single-parameter dynamical dark energy models: i.e, the thawing scalar field, the Modified Emergent Dark Energy(MEDE) scenario, and the constant-$w$ model. We constrain these frameworks by using the latest cosmological probes, including the Planck 2018 Cosmic Microwave Background(CMB) distance priors, the Baryon Acoustic Oscillation(BAO) measurements from the Data Release 2 of the Dark Energy Spectroscopic Instrument(DESI), and three compilations of Type Ia Supernovae(SN Ia) namely the Dark Energy Survey Year 5 (DESY5) compilation, the Union3 compilation, and the PantheonPlus (PP) sample. Across all three dark energy scenarios and all dataset combinations, we find a consistent preference for negative values of the parameter $w_2$. Furthermore, this result is robust against the choice of dark energy parametrization, suggesting a model-independent deviation from "standard" cold dark matter. This result indicates that the dark matter fluid possesses a small but non-vanishing negative pressure, meaning a non-cold nature. While the inferred Hubble constant $H_0$ remains consistent with the Planck $\Lambda$CDM value and does not fully alleviate the $H_0$ tension with local measurements, the persistent detection of $w_2 < 0$ across a wide range of independent cosmological probes provides compelling evidence for new physics in the dark matter sector -- suggesting that dark matter may be better described as an effective fluid endowed with a mild negative pressure, rather than as a perfectly cold, pressureless substance.