Kavli Affiliate: Joshua Frieman
| First 5 Authors: John Zhang, Joshua Frieman, , ,
| Summary:
We explore a model introduced by Cyr-Racine, Ge, and Knox
(arXiv:2107.13000(2)) that resolves the Hubble tension by invoking a “mirror
world" dark sector with energy density a fixed fraction of the “ordinary"
sector of Lambda-CDM. Although it reconciles cosmic microwave background and
large-scale structure observations with local measurements of the Hubble
constant, the model requires a value of the primordial Helium mass fraction
that is discrepant with observations and with the predictions of Big Bang
Nucleosynthesis (BBN). We consider a variant of the model with standard Helium
mass fraction but with the value of the electromagnetic fine-structure constant
slightly different during photon decoupling from its present value. If $alpha$
at that epoch is lower than its current value by $Delta alpha simeq -2times
10^{-5}$, then we can achieve the same Hubble tension resolution as in
Cyr-Racine, et al. but with consistent Helium abundance. As an example of such
time-evolution, we consider a toy model of an ultra-light scalar field, with
mass $m <4times 10^{-29}$ eV, coupled to electromagnetism, which evolves after
photon decoupling and that appears to be consistent with late-time constraints
on $alpha$ variation and the weak equivalence principle.
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