Revisiting the phenomenologically emergent dark energy model: is non-zero equation of state of dark matter favored by DESI DR2?

TL;DR

The paper tests whether a non-zero dark matter EoS $w_{ m dm}$ is favored within the PEDE model as a way to address the Hubble tension. It constrains $w_{ m dm}$ using a data combination of Planck CMB, DESI DR2 BAO, and SN data from DESY5 and PantheonPlus, employing a PEDE+$w_{ m dm}$ framework with $H^2(z)/H_0^2$ given by the standard components plus a PEDE term $f(z)$ and a PEDE EoS $w_{ m de}(z)$. The analysis finds a negative $w_{ m dm}$ favored by some datasets (e.g., $w_{ m dm}=-0.00093 \\pm 0.00032$ with CMB+DESI+DESY5, ≈$3\sigma$ from zero), but PantheonPlus reduces the significance to roughly $2\sigma$, and $w_{ m dm}$ is positively correlated with $H_0$, so a more negative $w_{ m dm}$ worsens the Hubble tension. Bayesian evidence strongly disfavors PEDE and PEDE+$w_{ m dm}$ relative to $\Lambda$CDM when DESI, DESY5, and PantheonPlus are included, indicating that current data do not support a non-CDM component within PEDE while also alleviating the Hubble tension. The results suggest that future work should explore non-CDM scenarios in other frameworks or address potential SN systematics to fully assess this possibility.

Abstract

The nature of dark matter remains one of the most fundamental and unresolved questions in modern cosmology. In most cosmological models, dark matter is typically modeled as pressureless dust with an equation of state (EoS) parameter $w_{\rm dm} = 0$. However, there is no fundamental theoretical reason to exclude the possibility of a non-zero dark matter EoS parameter. In this work, we explore the possibility of a non-zero dark matter EoS within the phenomenologically emergent dark energy (PEDE) model, given its simplicity and proven ability to alleviate the Hubble tension. We perform observational constraints by using the latest baryon acoustic oscillation data from DESI DR2, the cosmic microwave background (CMB) data from Planck, and the type Ia supernova data from DESY5 and PantheonPlus. From our analysis, we observe that a negative dark matter EoS parameter is preferred in all scenarios. Specifically, the CMB+DESI+DESY5 data yields $w_{\mathrm{dm}} = -0.00093 \pm 0.00032$, deviating from zero at approximately the $3σ$ level. However, this deviation is likely driven by unidentified systematics or inconsistencies in the DESY5 data, with the deviation decreasing to $2σ$ when using PantheonPlus data. Meanwhile, a negative $w_{\rm dm}$ would increase the Hubble tension due to the positive degeneracy between $w_{\rm dm}$ and $H_0$. Furthermore, Bayesian evidence suggests that the $Λ$CDM model is strongly preferred over the PEDE+$w_{\rm dm}$ model. These analyses illustrate that it is not possible to both support a non-cold dark matter component within the PEDE model and alleviate the Hubble tension simultaneously.

Figures (6)

• Figure 1: Constraints on the cosmological parameters using the CMB, DESI, DESY5, and PantheonPlus data in the PEDE model. Left panel: Two-dimensional marginalized contours ($1\sigma$ and $2\sigma$ confidence levels) in the $H_0$--$\Omega_{\rm m}$ plane by using the CMB, CMB+DESI, CMB+DESI+DESY5, and CMB+DESI+PantheonPlus data in the PEDE model. Right panel: The marginalized one-dimensional posteriors on $H_0$ in PEDE model, from CMB, CMB+DESI, CMB+DESI+DESY5, and CMB+DESI+PantheonPlus as labelled. The light pink vertical band corresponds to the value of $H_0$ from the SH0ES team Riess:2021jrx.
• Figure 2: Constraints on the cosmological parameters using the CMB, CMB+DESI, CMB+DESI+DESY5, and CMB+DESI+PantheonPlus data in the PEDE+$w_{\rm dm}$ model. The light pink vertical band corresponds to the value of $H_0$ from the SH0ES team Riess:2021jrx.
• Figure 3: Constraints on the cosmological parameters using the CMB, DESI, and DESY5 data in the PEDE+$w_{\rm dm}$ model.
• Figure 4: Comparison of the temperature power spectrum for the $\Lambda$CDM (green) and PEDE+$w_{\rm dm}$ (red) models, using the best-fit values inferred from the CMB alone analysis, over the Planck temperature power spectrum data points (black vertical bars with blue dots). We also present the fractional difference between the $\Lambda$CDM and PEDE+$w_{\rm dm}$ models.
• Figure 5: Best-fit predictions for distance-redshift relations for the $\Lambda$CDM and PEDE+$w_{\rm dm}$ models using CMB and DESI data. Upper panel: Best-fit predictions for (rescaled) distance-redshift relations for $\Lambda$CDM (solid line) and PEDE+$w_{\rm dm}$ (dotted-dashed line) obtained from the analysis of CMB+DESI data. The predictions encompass the three distinct types of distances probed by DESI BAO measurements, including $D_{\mathrm{V}}$ (green), $D_{\mathrm{M}}$ (orange), and $D_{\mathrm{H}}$ (light blue). The error bars represent $1\sigma$ uncertainties. Lower panel: Difference between the model prediction and data-point for each BAO measurement, normalized by the observational uncertainties. The predictions for $\Lambda$CDM and PEDE+$w_{\rm dm}$ are shown by filled and cross-shaped markers, respectively. The shaded region highlights the $±1\sigma$ range for reference.