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Exploring the interplay of late-time dynamical dark energy and new physics before recombination

Alex González-Fuentes, Adrià Gómez-Valent

Abstract

Cosmological models exhibiting crossing of the phantom divide improve the fit to current data, suggesting late-time dark energy (DE) dynamics at $\sim3σ$ CL. However, they favor low values of $H_0$, in tension with SH0ES. This may point to the presence of new physics prior to the decoupling era. In this work, we reconstruct the background DE functions using the Weighted Function Regression (WFR) method, introducing three main improvements compared to our previous JCAP 12 (2025) 049. First, we adopt the Frequentist-Bayesian approach for the weights. Second, we combine CMB and BAO with the DES-Dovekie SNIa sample and compare our findings with those derived from Pantheon+, still assuming standard recombination. Third, we study in a model-independent manner the viability of early-time ``solutions'' to the Hubble tension and how they affect the evidence for dynamical DE at late times, under the influence of the SH0ES and the more conservative CCHP calibration of the cosmic ladders, separately. We find that, if the physics prior to decoupling is unmodified, the probability of phantom crossing is $\sim 96.7\text{--}98.5\%$, with $Λ$CDM excluded at $\sim 2.5σ$ and $\sim 3σ$ CL. New physics before recombination can alleviate the Hubble tension, but requires extremely large values of the reduced matter density parameter when the SH0ES calibration is employed, in strong tension with those inferred from full CMB analyses. This raises serious concerns about the actual viability of these models to explain the SH0ES measurement. We find that phantom crossing, while not excluded, is no longer required, with only a very mild preference for quintessence. Nevertheless, given the aforesaid tension in $ω_m$, it would be rash to draw firm conclusions about how the dynamical DE signal is affected in these scenarios. [abridged]

Exploring the interplay of late-time dynamical dark energy and new physics before recombination

Abstract

Cosmological models exhibiting crossing of the phantom divide improve the fit to current data, suggesting late-time dark energy (DE) dynamics at CL. However, they favor low values of , in tension with SH0ES. This may point to the presence of new physics prior to the decoupling era. In this work, we reconstruct the background DE functions using the Weighted Function Regression (WFR) method, introducing three main improvements compared to our previous JCAP 12 (2025) 049. First, we adopt the Frequentist-Bayesian approach for the weights. Second, we combine CMB and BAO with the DES-Dovekie SNIa sample and compare our findings with those derived from Pantheon+, still assuming standard recombination. Third, we study in a model-independent manner the viability of early-time ``solutions'' to the Hubble tension and how they affect the evidence for dynamical DE at late times, under the influence of the SH0ES and the more conservative CCHP calibration of the cosmic ladders, separately. We find that, if the physics prior to decoupling is unmodified, the probability of phantom crossing is , with CDM excluded at and CL. New physics before recombination can alleviate the Hubble tension, but requires extremely large values of the reduced matter density parameter when the SH0ES calibration is employed, in strong tension with those inferred from full CMB analyses. This raises serious concerns about the actual viability of these models to explain the SH0ES measurement. We find that phantom crossing, while not excluded, is no longer required, with only a very mild preference for quintessence. Nevertheless, given the aforesaid tension in , it would be rash to draw firm conclusions about how the dynamical DE signal is affected in these scenarios. [abridged]

Paper Structure

This paper contains 11 sections, 36 equations, 6 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. The Frequentist-Bayesian method
  4. Analytical distribution for the Bayes factor
  5. The Weighted Function Regression method
  6. Cosmological data
  7. Results
  8. Evidence for dynamical DE within CPL in the light of the FB method
  9. DE reconstruction with standard pre-recombination physics
  10. DE reconstruction with new physics before recombination
  11. Conclusions

Figures (6)

  • Figure 1: Comparison of the contour plots from MCMC (solid) and Fisher approximation (dashed) with data from CMB, DESI DR2 BAO and DES-Dovekie SNIa for both the $\Lambda$CDM and CPL models. The posterior distribution is Gaussian in very good approximation. See the main text for the corresponding discussion.
  • Figure 2: Logarithm of the Bayes factor with data from CMB + DESI DR2 + DES Dovekie comparing $\Lambda$CDM and CPL as a function of the extra prior volume in parameter space. This is defined for uniform priors as $\pi_{i}=\mathcal{V}_{{\rm extra},i}^{-1}$ for each parameter beyond $\Lambda$CDM and the total $\mathcal{V}_{\rm extra}=\Pi_{i=1}^{n_\mathrm{extra}}\mathcal{V}_{{\rm extra},i}$. For reference, this has been normalized to the priors used by DESI DR2: $w_0\sim \mathcal{U}[-3,1]$, $w_a\sim \mathcal{U}[-3,2]$, without imposing $w_0+w_a<0$. We also display the Jeffreys' scale Trotta:2008qt and information criteria AIC, DIC, BIC. The black point on the right corresponds to the result obtained using the prior constructed from the marginalized $3\sigma$ limits of the extra parameters, whereas the white point on the left is obtained using the DESI DR2 prior.
  • Figure 3: Distribution of the difference of $\chi^2_{\rm min}$ obtained from $N\sim 6000$ mock realizations assuming the validity of $\Lambda$CDM, and the corresponding theoretical curve, described by a $\chi^2$ with 2 degrees of freedom, cf. section \ref{['sec:distBF']}. The red vertical dashed line corresponds to the value obtained with real data, with an associated $p-$value=0.00299. Therefore, the $\Lambda$CDM is excluded at 99.70% CL in front of the CPL, i.e., at $2.97\sigma$ CL.
  • Figure 4: Reconstruction of the EoS parameter, normalized DE density, Hubble rate, deceleration parameter and jerk obtained with the datasets and WFR-bases analyzed in section \ref{['sec:standardphysics']}. For the datasets containing DES-Dovekie we have used $n_J=3$ in both $w-$ and $\rho-$basis, whereas for Pantheon+ we have employed the $\rho-$basis with $n_J=4$, see the main text for details. We show the Hubble rate normalized to the PlanckPR4 best-fit $\Lambda$CDM model, which sets $\Omega_{\rm m}^0 = 0.315$ and $H_0=67.26\,$km/s/Mpc Rosenberg:2022sdy. In each of the plots, gray dashed lines correspond to $\Lambda$CDM and the black dash-dotted line in the $q(z)$ plot sets the border between deceleration and acceleration.
  • Figure 5: Contour plots obtained for the CPL parametrization making use of the uncalibrated combination of data and the corresponding ones obtained upon applying the calibration from SH0ES or CCHP. The corresponding fitting results are displayed in Tables \ref{['tab:tab_sh0es_w']} and \ref{['tab:tab_cchp_w']}, respectively. In the plane $(\omega_m,H_0)$, we also display in a blue vertical band the $1\sigma$ EDE constraint $\omega_m=0.1468\pm0.0026$ from the profile likelihood analysis of Ref. Gomez-Valent:2022hkb, obtained with the full Planck 2018 likelihood in combination with SNIa and BAO, and without including the SH0ES prior. This is to illustrate the existing tension between the required values of $\omega_m$ that would be needed to explain the SH0ES measurement in scenarios with new physics before the decoupling and the one obtained from a full CMB analysis. Similar constraints on $\omega_m$ are also obtained in models that speed up the recombination process, see e.g. Table II of Mirpoorian:2025rfp.
  • ...and 1 more figures