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The dark side of the universe may be more harmonic than we thought

Yan Su, Zhiqi Huang, Yanhong Yao, Junchao Wang, Jianqi Liu

TL;DR

The paper investigates whether a single unified dark fluid (PUDF), built on the PAge framework, can replace the separate dark matter and dark energy components of ΛCDM. It develops theoretical mappings between PUDF and ΛCDM, derives matching conditions for the primary CMB and late-universe observables, and updates parameter constraints using Planck 2018, DESI DR2, DES5YR, and cosmic chronometers. The results show PUDF can reproduce the primary CMB, background expansion, and linear growth similar to ΛCDM when carefully tuned, but current data prefer ΛCDM with strong Bayesian evidence; however, this preference may be influenced by inconsistencies between datasets. The study suggests PUDF remains a viable alternative within present observational limits and emphasizes the need for nonlinear-scale tests and a fundamental PUDF theory to robustly falsify or confirm it.

Abstract

The standard paradigm of cosmology assumes two distinct dark components, namely dark matter and dark energy. However, the necessity of splitting the dark-side world into two sectors has not been experimentally or theoretically proven. Unified dark fluid models provide an alternative in which a single fluid accounts for both phenomena. It is shown in Wang et al. 2024 that a PAge-like unified dark fluid (PUDF) can explain both the cosmic microwave background (CMB) and late-universe data, with the fitting quality not much worse than the standard Lambda cold dark matter ($Λ$CDM) model. Using the Planck 2018 CMB, baryon acoustic oscillations measurement from the dark energy spectroscopic instrument (DESI) data release 2, dark energy survey 5-year supernova data, and cosmic-chronometer data, we update the constraints on PUDF and clarify its physical implications. We show that PUDF can reproduce the primary CMB anisotropies, the background expansion history, and linear growth that are very close to the $Λ$CDM prediction. Nevertheless, the combined datasets still favor $Λ$CDM, largely due to the significant tension between CMB and DESI + SNe data, which exceeds the $4σ$ level in PUDF and remains non-negligible in the $w$CDM framework. Using mock data generated from the Planck best-fit $Λ$CDM model, we find that PUDF and $Λ$CDM cannot be statistically distinguished, indicating that the precision of current data is insufficient to separate the two models. Overall, the apparent preference for $Λ$CDM may be driven by dataset inconsistencies rather than a genuine physical difference, leaving unified dark fluid models as viable alternatives within current observational limits.

The dark side of the universe may be more harmonic than we thought

TL;DR

The paper investigates whether a single unified dark fluid (PUDF), built on the PAge framework, can replace the separate dark matter and dark energy components of ΛCDM. It develops theoretical mappings between PUDF and ΛCDM, derives matching conditions for the primary CMB and late-universe observables, and updates parameter constraints using Planck 2018, DESI DR2, DES5YR, and cosmic chronometers. The results show PUDF can reproduce the primary CMB, background expansion, and linear growth similar to ΛCDM when carefully tuned, but current data prefer ΛCDM with strong Bayesian evidence; however, this preference may be influenced by inconsistencies between datasets. The study suggests PUDF remains a viable alternative within present observational limits and emphasizes the need for nonlinear-scale tests and a fundamental PUDF theory to robustly falsify or confirm it.

Abstract

The standard paradigm of cosmology assumes two distinct dark components, namely dark matter and dark energy. However, the necessity of splitting the dark-side world into two sectors has not been experimentally or theoretically proven. Unified dark fluid models provide an alternative in which a single fluid accounts for both phenomena. It is shown in Wang et al. 2024 that a PAge-like unified dark fluid (PUDF) can explain both the cosmic microwave background (CMB) and late-universe data, with the fitting quality not much worse than the standard Lambda cold dark matter (CDM) model. Using the Planck 2018 CMB, baryon acoustic oscillations measurement from the dark energy spectroscopic instrument (DESI) data release 2, dark energy survey 5-year supernova data, and cosmic-chronometer data, we update the constraints on PUDF and clarify its physical implications. We show that PUDF can reproduce the primary CMB anisotropies, the background expansion history, and linear growth that are very close to the CDM prediction. Nevertheless, the combined datasets still favor CDM, largely due to the significant tension between CMB and DESI + SNe data, which exceeds the level in PUDF and remains non-negligible in the CDM framework. Using mock data generated from the Planck best-fit CDM model, we find that PUDF and CDM cannot be statistically distinguished, indicating that the precision of current data is insufficient to separate the two models. Overall, the apparent preference for CDM may be driven by dataset inconsistencies rather than a genuine physical difference, leaving unified dark fluid models as viable alternatives within current observational limits.

Paper Structure

This paper contains 7 sections, 16 equations, 4 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Theoretical Comparison between PUDF and $\Lambda$CDM
  3. PUDF basics
  4. Matching the primary CMB
  5. Matching late-universe observables
  6. Parameter Inference
  7. Discussion and Conclusions

Figures (4)

  • Figure 1: Comparison of the primary CMB $TT$ and $EE$ power spectra of PUDF and $\Lambda$CDM when the matching condition \ref{['eq:CMBmatchexact']} is applied. The lower panels give the relative difference ($\Delta C_\ell^\mathrm{TT} = C^\mathrm{TT}_{\ell, \mathrm{PUDF}}-C^\mathrm{TT}_{\ell, \Lambda\mathrm{CDM}},\, \Delta C_\ell^\mathrm{EE}=C^\mathrm{EE}_{\ell, \mathrm{PUDF}}-C^\mathrm{EE}_{\ell, \Lambda\mathrm{CDM}}$).
  • Figure 2: The primary-CMB matching condition \ref{['eq:CMBmatch']}, $q_0$ matching condition \ref{['eq:q0match']} and age matching condition \ref{['eq:agematch']} for $\Omega_m=0.28$ (left panel), $\Omega_m=0.315$ (middle panel) and $\Omega_m=0.35$ (right panel), respectively.
  • Figure 3: Comparison of CMB deflection power spectrum $C_{\ell}^{dd}$ of PUDF and $\Lambda$CDM when the primary-CMB matching condition \ref{['eq:CMBmatchexact']} and the $q_0$ matching condition \ref{['eq:q0match']} are applied. The lower panel gives the relative difference($\Delta C_\ell^\mathrm{dd} = C^\mathrm{dd}_{\ell, \mathrm{PUDF}}-C^\mathrm{dd}_{\ell, \Lambda\mathrm{CDM}}$).
  • Figure 4: Two-dimensional marginalized contours at the $1\sigma$ and $2\sigma$ confidence levels for (a) the {$\eta, p_{\rm age}$} parameter space in PUDF and (b) the {$\omega_{\rm cdm}\equiv \Omega_c h^2, w$} parameter space in $w$CDM, derived from the CMB and DESI+DES5YR datasets.