Papers › Small-scale clumping at recombination and the Hubble tension
Small-scale clumping at recombination and the Hubble tension
Michael Rashkovetskyi, Julian B. Muñoz, Daniel J. Eisenstein, Cora Dvorkin
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Despite the success of the standard ΛCDM model of cosmology, recent data improvements have made tensions emerge between low- and high-redshift observables, most importantly in determinations of the Hubble constant H₀ and the (rescaled) clustering amplitude S₈. The high-redshift data, from the cosmic microwave background (CMB), crucially relies on recombination physics for its interpretation. Here we study how small-scale baryon inhomogeneities (i.e., clumping) can affect recombination and consider whether they can relieve both the H₀ and S₈ tensions. Such small-scale clumping, which may be caused by primordial magnetic fields or baryon isocurvature below kpc scales, enhances the recombination rate even when averaged over larger scales, shifting recombination to earlier times. We introduce a flexible clumping model, parametrized via three spatial zones with free densities and volume fractions, and use it to study the impact of clumping on CMB observables. We find that increasing H₀ decreases both Ωₘ and S₈, which alleviates the S₈ tension. On the other hand, the shift in Ωₘ is disfavored by the low-z baryon-acoustic-oscillations measurements. We find that the clumping parameters that can change the CMB sound horizon enough to explain the H₀ tension also alter the damping tail, so they are disfavored by current Planck 2018 data. We test how the CMB damping-tail information rules out changes to recombination by first removing ℓ>1000 multipoles in Planck data, where we find that clumping could resolve the H₀ tension. Furthermore, we make predictions for future CMB experiments, as their improved damping-tail precision can better constrain departures from standard recombination. Both the Simons Observatory and CMB-S4 will provide decisive evidence for or against clumping as a resolution to the H₀ tension.
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