Reassessing Dust's Role in Forming the CMB

TL;DR

This paper explores the possibility that dust rethermalization, rather than recombination, could explain the origin of the CMB within the R_h=ct universe, suggesting a different epoch for the surface of last scattering.

Contribution

It proposes a novel dust-based rethermalization scenario for the CMB within the R_h=ct cosmology, challenging the standard recombination paradigm.

Findings

The LSS redshift in this model is about 16, within the Pop III star formation era.

The CMB temperature today could be around 3 K, linking H_0 and baryon-photon ratio.

Dust rethermalization remains a plausible alternative to recombination for CMB origin.

Abstract

The notion that dust might have formed the cosmic microwave background (CMB) has been strongly refuted on the strength of four decades of observation and analysis, in favour of recombination at a redshift z ~ 1080. But tension with the data is growing in several other areas, including measurements of the Hubble constant H(z) and the BAO scale, which directly or indirectly impact the physics at the surface of last scattering (LSS). The R_h=ct universe resolves at least some of this tension. We show in this paper that---if the BAO scale is in fact equal to the acoustic horizon---the redshift of the LSS in this cosmology is z_cmb ~ 16, placing it within the era of Pop III star formation, prior to the epoch of reionization at 15 > z > 6. Quite remarkably, the measured values of z_cmb and H_0 = H(0) in this model are sufficient to argue that the CMB temperature today ought to be ~ 3 K, so H_0 and the baryon to photon ratio are not independent free parameters. This scenario might have resulted from rethermalization of the CMB photons by dust, presumably supplied to the interstellar medium by the ejecta of Pop III stars. Dust rethermalization may therefore yet resurface as a relevant ingredient in the R_h=ct universe. Upcoming high sensitivity instruments should be able to readily distinguish between the recombination and dust scenarios by either (i) detecting recombination lines at z ~ 1080, or (ii) establishing a robust frequency-dependent variation of the CMB power spectrum at the level of ~ 2-4% across the sampled frequency range.