Relieving the Hubble tension with primordial magnetic fields

Abstract: The standard cosmological model determined from the accurate cosmic microwave background measurements made by the Planck satellite implies a value of the Hubble constant $H_0$ that is $4.2$ standard deviations lower than the one determined from Type Ia supernovae. The Planck best fit model also predicts higher values of the matter density fraction $\Omega_m$ and clustering amplitude $S_8$ compared to those obtained from the Dark Energy Survey Year 1 data. Here we show that accounting for the enhanced recombination rate due to additional small-scale inhomogeneities in the baryon density may solve both the $H_0$ and the $S_8-\Omega_m$ tensions. The additional baryon inhomogeneities can be induced by primordial magnetic fields present in the plasma prior to recombination. The required field strength to solve the Hubble tension is just what is needed to explain the existence of galactic, cluster, and extragalactic magnetic fields without relying on dynamo amplification. Our results show clear evidence for this effect and motivate further detailed studies of primordial magnetic fields, setting several well-defined targets for future observations.

• The paper demonstrates that incorporating primordial magnetic fields to induce baryon inhomogeneities alleviates the Hubble constant tension by reducing the sound horizon at recombination.
• The paper modifies CAMB and CosmoMC to include baryon clumping models (M1 and M2) that yield H0 values of approximately 71 km/s/Mpc and 70 km/s/Mpc respectively.
• The study also shows that this approach mitigates the S8-Ωm tension, aligning observations with large-scale surveys and highlighting early-universe magnetism as a key factor.

Relieving the Hubble Tension with Primordial Magnetic Fields: A Study of Small-Scale Baryon Inhomogeneities

In recent cosmological studies, increasing questions have been raised about the discrepancies in measurements of the Hubble constant ($H_0$) from different observational sources. This paper by Jedamzik and Pogosian investigates potential solutions to these tensions by considering the influence of primordial magnetic fields (PMFs) on the evolution of the universe before recombination. This novel approach not only addresses the well-known $H_0$ tension but also considers the secondary less-discussed tension between measurements of the matter density fraction ($\Omega_m$) and clustering amplitude ($S_8$).

Astrophysicists observe a notable discrepancy between the $H_0$ values inferred from the cosmic microwave background (CMB) as measured by the Planck satellite, and local measurements such as those employed by the SH0ES collaboration using Type Ia supernovae (SNIa). This discrepancy, quantified at approximately $4.2\sigma$, suggests the need for new physical insights beyond the standard $\Lambda$CDM cosmological model. The work proposes accounting for enhanced recombination rates driven by small-scale baryon inhomogeneities, potentially introduced by PMFs, to reconcile these observations.

The core hypothesis is that primordial magnetic fields, which existed in the plasma before recombination, induce modest inhomogeneities in baryon densities. The authors' analysis suggests that such baryon density variations can increase the recombination rate, subsequently reducing the sound horizon at recombination. This reduction in the sound horizon could allow for the inferred cosmic scales to be compatible with a higher $H_0$ value, aligning CMB-based estimates closer to local measurements.

The quantitative analysis in the paper includes modifying the established CAMB and CosmoMC tools to include clumping factors associated with baryon densities. The authors present two models, M1 and M2, which are fit against CMB and $H_0$ data (Planck and H3 datasets, respectively). M1 and M2 incorporate different assumptions about the distribution of baryon overdensities. A significant finding is that with clumping included, the best-fit model provides $H_0$ values of $71.03 \pm 0.74$ km s$^{-1}$ Mpc$^{-1}$ for M1 and $69.81 \pm 0.62$ km s$^{-1}$ Mpc$^{-1}$ for M2, reducing the $H_0$ tension to less than $2\sigma$ and $3\sigma$ for M1 and M2, respectively.

Moreover, this framework provides a simultaneous amelioration of the $S_8-\Omega_m$ tension. The inferred parameters from the clumping models are in better agreement with larger scale structure surveys, such as the DES-Y1. The physical plausibility of the paper's model is underpinned by the natural existence of PMFs with strengths of $\sim 0.1$ nG, which are sufficient to explain observed galactic and extragalactic magnetic fields without the necessity of dynamo amplification.

The implications of this research are significant. Not only does it provide a new perspective on reconciling current cosmological tensions, but it also suggests a deeper probe into the role of magnetic fields in the early universe. Should PMFs of required strength be confirmed through collaboration with future observational campaigns like PIXIE or CMB-S4, this could revolutionize our understanding of early cosmic conditions with respect to magnetic phenomena.

Yet, before this hypothesis is universally accepted, further studies must validate the proposed baryon clumping mechanism through more precise measurements and complementary large-scale simulations to refine the baryon distribution model. If realized, this work paves the way for theoretical advancements, engendering a closer coherence between cosmological theory and empirical observations, guiding the paradigm of cosmological models toward including magneto-hydrodynamic phenomena as a fundamental component. Future research avenues could explore the broader implications on early universe dynamics, structure formation, and potentially a reevaluation of early phase transitions or inflationary epochs.

In summary, this paper forms a vital bridge between observed astrophysical phenomena and theoretical cosmology, offering a compelling pathway to resolve significant inconsistencies in the current cosmological model. Its implications, both theoretical and observational, merit the attention of researchers aiming to converge on a more unified understanding of the universe's expansion and structure.