Observational evidence for Early Dark Energy as a unified explanation for Cosmic Birefringence and the Hubble tension

TL;DR

This work tests an axion-like Early Dark Energy model with $n=3$ as a unified explanation for the Hubble tension and cosmic birefringence by jointly fitting CMB $EB$ data and local $H_0$ measurements. Using lensed CMB spectra, CLASS_EDE, and MCMC (Cobaya), the authors show that allowing the background cosmology to vary significantly changes the predicted $EB$ spectrum, enabling good agreement with observations when $f_{EDE}$, $z_c$, $g_{EDE}$, and other standard parameters are constrained together. The analysis yields a consistent $H_0$ around the local value and a rotation angle-related birefringence signal, with EB data favoring a nonzero $\beta$ that can be accommodated within the model ($\beta \approx 0.059^{\circ}$). The results suggest EDE remains a viable candidate for addressing both tensions, though future precise measurements (e.g., LiteBIRD) will be decisive in confirming or refuting this unified scenario, particularly through any redshift evolution of $\beta$.

Abstract

We test the $n$=3 Ultralight Axion-like model of Early Dark Energy (EDE) with the observationsof the $EB$ mode of the cosmic microwave background (CMB) radiation, and local expansion rate measurements. Our results show that the shape of the CMB $EB$ angular power spectrum is sensitive to the background cosmological parameters. We run Markov chain Monte Carlo (MCMC) simulations to fit the $Λ$CDM + EDE parameters simultaneously, and find that the EDE model with $n$=3 can provide a good fit to the observed CMB $EB$ spectra, consistent with the locally measured value of the Hubble constant. Our result is the first to show that axion-like EDE can provide a unified explanation for the observed cosmic birefringence and the Hubble tension.

Figures (5)

• Figure 1: The PDF of $g_{EDE}$ from the CMB $EB$ power spectrum using the best-fit parameter values for the remaining parameters from Poulin:2019 (blue line), and from Eskilt:2023 (green line). The corresponding best fit values are $g_{EDE}$=0.3472 and $g_{EDE}$=0.539, respectively.
• Figure 2: Comparison of the theoretical CMB $EB$ power spectrum by fitting only $g_{EDE}$ keeping all the other parameters fixed to the best-fit results of Eskilt:2023 (green line), and by fitting all 10 EDE+$\Lambda$CDM parameters with the CMB $TT$, $EE$, $EB$, lensing data, BAO and SH0ES data (orange line). The black points with error bars denote the observed CMB $EB$ power spectrum found in Eskilt:2022.
• Figure 3: Comparison of the CMB $EB$ power spectra corresponding to a 10% change in each of the cosmological parameters.
• Figure 4: The 1D marginalised PDFs of the 10 fitted EDE+$\Lambda$CDM parameters, along with $H_0$ (derived), from the MCMC fitting of CMB, BAO and SNIa.
• Figure 5: The 2D posteriors of the 10 fitted EDE+$\Lambda$CDM parameters, along with $H_0$ (derived), from the MCMC fitting of CMB, BAO and SNIa.