Reassessing dust’s role in forming the CMB

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≡ H(0) in this model are sufficient to argue that the CMB temperature today ought to be ∼ 3 K, so H 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.