Relieving the Hubble Tension with Primordial Magnetic Fields

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 ${\mathrm{\Omega }}_m$ 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 $S_8\text{−}{\mathrm{\Omega }}_m$ 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.

Figure 1

The marginalized $H_0$ PDF for the Planck best fit $\mathrm{\Lambda }\mathrm{CDM}$ model and the two baryon clumping models, M1 and M2, fit to Planck combined with H3. The bottom panel shows the PDF of the clumping parameter $b$. The shaded regions show the 68% and 95% C.L. of $H_0$ from SH0ES [3].

Figure 2

The marginalized PDF contours for $S_8\text{−}{\mathrm{\Omega }}_m$ (left) showing the mild tension between Planck and DES Y1 when interpreted within $\mathrm{\Lambda }\mathrm{CDM}$ and its resolution after accounting for clumping. The right panel shows how M1 and M2 simultaneously alleviate the $H_0$ and $S_8$ tensions. The shaded vertical regions show the 68% and 95% C.L. bands of $H_0$ from SH0ES [3].