Is Dark Matter Really Matter?

2026/08/05 by Jun-Qian Jiang, Arman Shafieloo
Physics and Astronomy · #astro-ph.CO #gr-qc #hep-th

paper · pdf

7+6 pages, 6 figures

arxiv created 2026/08/05 · arxiv updated 2026/08/06

Abstract

In the standard model of cosmology, it is assumed that dark matter is pressureless with equation of state w=0 and dark energy has w=-1. We test these assumptions jointly using DESI DR2 distance measurements, including the recent Lyman-α full-shape Alcock-Paczynski (AP) information, DES supernovae, and two complementary CMB treatment. When constant w_dm and w_de are varied together, we find w_dm=0.000968^+0.000501_-0.000496 and w_de=-0.9380^+0.0259_-0.0262 (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are w_dm=0.000870^+0.000408_-0.000410 and w_de=-0.9353^+0.0258_-0.0254. Both standard ΛCDM values are disfavored at approximately 2σ in the joint extension. Neither parameter departs significantly from its standard value when only that parameter is varied. This behavior arises because late-time distances favor w_de>-1, while maintaining the early-Universe physical matter density requires a compensating positive w_dm, which changes the mapping to the matter density today. Allowing dynamical dark energy clarify further on complexity of the situation: phantom crossing for dark energy makes w_dm=0 consistent with the data, whereas a positive w_dm preference persists when crossing is forbidden. Interestingly, the Pad'e-w parameterization that provides a flexible description of a class of quintessence models (with no phantom crossing), along with w_dm free, is even mildly favored over the phantom-crossing w₀w_a model according to both the best-fit χ² and the DIC under both CMB treatments. One can conclude that the apparent preference for phantom crossing may instead reflect deviations in the dark-matter sector rather than dark-energy dynamics alone. [abridged]

Citations

• The Dark Energy Survey supernova program: a reanalysis of cosmology results and evidence for evolving dark energy with an updated Type Ia supernova calibration
• The choice of Planck CMB likelihood in cosmological analyses
• Unified dark fluid with null sound speed as an alternative to phantom dark energy
• Exotic Dark Matter and the DESI Anomaly
• Exploring non-cold dark matter in a scenario of dynamical dark energy with DESI DR2 data
• SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field
• Revisiting the phenomenologically emergent dark energy model: is non-zero equation of state of dark matter favored by DESI DR2?
• The matter with(in) CPL
• Unified and consistent structure growth measurements from joint ACT, SPT and Planck CMB lensing
• Extended Dark Energy analysis using DESI DR2 BAO measurements
• Cosmology From CMB Lensing and Delensed EE Power Spectra Using 2019-2020 SPT-3G Polarization Data
• Observational constraints on phenomenological emergent dark energy and barotropic dark matter characterized by a constant equation of state parameter
• DESI 2024: Constraints on Physics-Focused Aspects of Dark Energy using DESI DR1 BAO Data
• DESI 2024 VI: cosmological constraints from the measurements of baryon acoustic oscillations
• Union Through UNITY: Cosmology with 2,000 SNe Using a Unified Bayesian Framework
• The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and its Implications for Structure Growth
• Observational constraints on noncold dark matter and phenomenological emergent dark energy
• Cobaya: Code for Bayesian Analysis of hierarchical physical models
• Spherical collapse in Generalized Dark Matter models
• Improving the Flexibility and Robustness of Model-Based Derivative-Free Optimization Solvers
• The Dark Matter equation of state through cosmic history
• Equation of State of Dark Matter after Planck Data
• Observational constraints on variable equation of state parameters of dark matter and dark energy after Planck
• The Cosmic Linear Anisotropy Solving System (CLASS) II: Approximation schemes
• Crossing the phantom divide with parametrized post-Friedmann dark energy
• Dark-Energy Evolution Across the Cosmological-Constant Boundary
• Can dark energy evolve to the Phantom?
• The Atacama Cosmology Telescope
• Bayesian Measures of Model Complexity and Fit
• Exploring the Expansion History of the Universe
• Halo Models of Large Scale Structure
• Cosmic Microwave Background Anisotropies
• Accelerating Universes with Scaling Dark Matter
• Structure Formation with Generalized Dark Matter
• Small Scale Cosmological Perturbations: An Analytic Approach
• Cosmological Perturbation Theory in the Synchronous and Conformal Newtonian Gauges
• Inference from Iterative Simulation Using Multiple Sequences