Is Dark Matter Really Matter?
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 wdm and wde are varied together, we find wdm=0.000968+0.000501−0.000496 and wde=−0.9380+0.0259−0.0262 (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are wdm=0.000870+0.000408−0.000410 and wde=−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 wde>−1, while maintaining the early-Universe physical matter density requires a compensating positive wdm, which changes the mapping to the matter density today. Allowing dynamical dark energy further highlights the complexity of the situation: phantom crossing for dark energy makes wdm=0 consistent with the data, whereas a positive wdm 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 wdm free, is even mildly favored over the phantom-crossing w0wa 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]
Versions
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BibTeX
@misc{jiang2026dark,
title = {{Is Dark Matter Really Matter?}},
author = {Jun-Qian Jiang and Arman Shafieloo},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.04763},
eprint = {2608.04763},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.04763},
}
RIS
TY - UNPB
TI - Is Dark Matter Really Matter?
AU - Jun-Qian Jiang
AU - Arman Shafieloo
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2608.04763
UR - https://arxiv.org/abs/2608.04763
AB - 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 further highlights the 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 w_0w_a model according to both the best-fit χ^2 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]
ER -