Is Dark Matter Really Matter?
- 1Korea Astronomy and Space Science Institute
- 2University of Science and Technology
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 Ly α Alcock–Paczynski (AP) information, Dark Energy Survey (DES) supernovae, and two complementary cosmic microwave background (CMB) treatments. When constant w DM and w DE are varied together, we find w DM = 0.00096 8 − 0.000496 + 0.000501 and w DE = − 0.938 0 − 0.0262 + 0.0259 (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are w DM = 0.00087 0 − 0.000410 + 0.000408 and w DE = − 0.935 3 − 0.0254 + 0.0258 . Both standard ΛCDM values are disfavored at approximately 2 σ 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 Padé- 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 ₀ w a model according to both the best-fit χ ² and the deviance information criterion (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. With future high-redshift distance measurements, particularly improved Ly α observations, together with growth and lensing probes, these possibilities may be further clarified.
Versions
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BibTeX
@article{jiang2026dark,
title = {{Is Dark Matter Really Matter?}},
author = {Jun-Qian Jiang and Arman Shafieloo},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aeaa2a},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeaa2a},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Is Dark Matter Really Matter?
AU - Jun-Qian Jiang
AU - Arman Shafieloo
PY - 2026
DA - 2026/10/07
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeaa2a
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeaa2a
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 Ly α Alcock–Paczynski (AP) information, Dark Energy Survey (DES) supernovae, and two complementary cosmic microwave background (CMB) treatments. When constant w DM and w DE are varied together, we find w DM = 0.00096 8 − 0.000496 + 0.000501 and w DE = − 0.938 0 − 0.0262 + 0.0259 (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are w DM = 0.00087 0 − 0.000410 + 0.000408 and w DE = − 0.935 3 − 0.0254 + 0.0258 . Both standard ΛCDM values are disfavored at approximately 2 σ 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 Padé- 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 ₀ w a model according to both the best-fit χ ² and the deviance information criterion (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. With future high-redshift distance measurements, particularly improved Ly α observations, together with growth and lensing probes, these possibilities may be further clarified.
SN - 2041-8205
SN - 2041-8213
ER -