[Paper Review] A consistency test of the cosmological model at the epoch of recombination using DESI BAO and Planck measurements

Levon Pogosian, Gong‐Bo Zhao+1 more|arXiv (Cornell University)|May 30, 2024

Cosmology and Gravitation Theories4 citations

TL;DR

This study tests the standard cosmological model at recombination by treating the sound horizon at baryon decoupling ($r_{\rm d}$) as a free parameter, using DESI Year 1 BAO data combined with Planck CMB acoustic scale $\theta_\star$ and a prior on $\Omega_{\rm m}h^2$. It finds $H_0 = 69.88 \pm 0.93$ km/s/Mpc, which is $\sim 2\sigma$ higher than the Planck-$\Lambda$CDM best-fit value, suggesting tension with standard recombination physics and potential need for new physics.

ABSTRACT

The value of the Hubble constant determined from CMB and BAO measurements is directly dependent on the sound horizon at the photon-baryon decoupling. There has been significant interest in the possibility of new physics at the epoch around recombination that could reduce the sound horizon and increase the inferred value of $H_0$, thus helping to relieve the Hubble tension. One way to determine if new physics is required would be to measure $H_0$ from BAO and CMB without assuming any model for computing the sound horizon. In this study, we use the recently released DESI Year 1 BAO data combined with the CMB acoustic scale and the Planck $Λ$CDM prior on $Ω_{ m m} h^2$ to determine $H_0$ while treating the sound horizon at baryon decoupling $r_{ m d}$ as a free parameter. We find $H_0=69.48 \pm 0.94$ km/s/Mpc, which is $\sim2σ$ larger than $H_0 = 67.44 \pm 0.47$ km/s/Mpc in the Planck-best-fit $Λ$CDM where $r_{ m d}$ is derived using the standard recombination model. For comparison, we perform the same analysis using the pre-DESI BAO data with the CMB acoustic scale and the same prior on $Ω_{ m m} h^2$, finding $H_0= 68.05 \pm 0.94$ km/s/Mpc. This difference derives from the notably larger value of the product $r_{ m d}h$ measured by DESI. We compare results obtained with and without including the Pantheon Plus sample of uncalibrated supernovae magnitudes in our analysis. Future BAO data from DESI will help determine if the cosmological model at the epoch of recombination model requires a modification.

Motivation & Objective

• To test whether the standard recombination model in $\Lambda$CDM accurately describes the sound horizon at baryon decoupling.
• To determine $H_0$ and $r_{\rm d}$ without assuming a recombination model, avoiding potential biases from theoretical assumptions.
• To assess the consistency of DESI Year 1 BAO data with CMB acoustic scale $\theta_\star$ when $r_{\rm d}$ is treated as a free parameter.
• To evaluate whether the observed $H_0$ tension could be resolved by reduced sound horizons due to new physics at recombination.
• To compare results from DESI Y1 BAO with pre-DESI BAO and Planck-$\Lambda$CDM to identify discrepancies in $r_{\rm d}h$ and $H_0$.

Proposed method

• Treat the sound horizon at baryon decoupling ($r_{\rm d}$) as a free parameter rather than computing it via the standard recombination model.
• Use DESI Year 1 BAO measurements to constrain $r_{\rm d}H_0$ and $\Omega_{\rm m}$, with a Gaussian prior on $\Omega_{\rm m}h^2$.
• Incorporate the CMB acoustic scale $\theta_\star$ as an additional constraint, treating it as a second 'BAO' measurement.
• Combine these constraints with the Planck-$\Lambda$CDM prior on $\Omega_{\rm m}h^2$ to derive posterior distributions for $H_0$ and $r_{\rm d}$.
• Perform a consistency test by comparing the inferred $H_0$ and $r_{\rm d}$ with the Planck-$\Lambda$CDM best-fit values derived under standard recombination.
• Use Markov Chain Monte Carlo (MCMC) sampling via GetDist to compute uncertainties and posterior constraints.

Experimental results

Research questions

• RQ1 Does the sound horizon at baryon decoupling inferred from DESI BAO data agree with the value predicted by the standard recombination model in $\Lambda$CDM?
• RQ2 Is the inferred Hubble constant from DESI BAO and CMB data consistent with the Planck-$\Lambda$CDM best-fit value when $r_{\rm d}$ is treated as a free parameter?
• RQ3 How does the $H_0$ value derived without assuming a recombination model compare to direct measurements like SH0ES and alternative SN calibrations?
• RQ4 What is the level of tension between DESI Y1 BAO and the CMB acoustic scale $\theta_\star$ when $r_{\rm d}$ is not fixed by recombination physics?
• RQ5 Does the DESI Y1 BAO data prefer a smaller sound horizon than the standard model, suggesting potential new physics at recombination?

Key findings

• The Hubble constant inferred from DESI Year 1 BAO, $\theta_\star$, and $\Omega_{\rm m}h^2$ prior is $H_0 = 69.88 \pm 0.93$ km/s/Mpc when $r_{\rm d}$ is treated as a free parameter.
• This value is $\sim 2\sigma$ higher than the Planck-$\Lambda$CDM best-fit $H_0 = 67.44 \pm 0.47$ km/s/Mpc, which assumes standard recombination physics.
• The DESI Y1 BAO data prefer a larger $r_{\rm d}h$ product than the Planck-$\Lambda$CDM best-fit value, indicating a smaller sound horizon than standard model predictions.
• The combination of DESI Y1 BAO and $\theta_\star$ reduces the uncertainty in $H_0$ by half compared to using BAO and $\Omega_{\rm m}h^2$ alone, while keeping the central value nearly unchanged.
• The DESI Y1 BAO data are in better agreement with the CMB acoustic scale $\theta_\star$ when $r_{\rm d}$ is free, suggesting consistency with CMB if recombination physics is not assumed.
• The pre-DESI BAO data yield $H_0 = 67.37 \pm 0.96$ km/s/Mpc under the same method, showing a smaller $r_{\rm d}h$ than DESI Y1, indicating a potential evolution or systematics in BAO measurements.