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[2001.05874] Can the quasi-molecular mechanism of recombination decrease the Hubble tension?

Posted: April 06 2020
by Cosmo Comments
This paper was commented on through Cosmo Comments. The following comments can also be viewed as annotations on the paper via Hypothesis.

The paper discussed here, 'Can the quasi-molecular mechanism of recombination decrease the Hubble tension?’ by Revaz Beradze and Merab Gogberashvili adds to the enriched discussion on the Hubble tension. Specifically, this is another way of looking at the discrepancy between the high- (i.e. CMB) and low-redshift (i.e. Type IA supernova) probes as a physical manifestation, rather than the alternative discussion platforms such as systematics. The authors point to the standard recombination picture and propose that the so-called ‘quasi-molecular recombination’ mechanism (QMR) discussed in a paper by Kereselidze et al. (2019) could provide a reduction to the Hubble tension by altering atomic constants during recombination according to the recent paper by Liu et al. (2019). There are several points of contention with regards to this claim. Furthermore, this is not going to be a discussion on the validity of the statements made in Kereselidze et al. (2019) or Liu et al. (2019) — simply a reference.

The physical motivation for looking at molecular hydrogen is solid. It has been discussed at length in Kereselidze et al. (2019) and the authors here attempt to introduce the same rate equations for $H_2^+$ and other species. Neither paper has looked at the previous studies on molecular hydrogen, e.g. Dawn of Chemistry by Daniele Galli and Francesco Palla (2012) and Seager et al. (2000). The authors also claim that this idea has not been discussed by the authors of RECFAST; however the molecular abundances have actually been the subject of discussion in Seager et al. (2000, follow up paper to RECFAST paper), explicitly showing how low the abundances of molecular hydrogen are. The authors look to 'estimate the influence’ of the QMR effect, without going further than presenting the rate equations. Furthermore, Kereselidze et al. (2019) do not discuss the impact of the QMR mechanism and its impact on the free electron fraction, simply on the cosmological recombination radiation (CRR). Though this QMR mechanism may be vital for the underlying understanding of the hydrogen and helium recombination lines (I do not have the answer to that question); many of the effects that concern the CRR can at this level alter the full recombination calculation by <1%. The propagation of a variation of <1% from the free electron fraction is going to be very difficult to isolate in the CMB anisotropies.

The lack of calculation for the two-photon decay process and specific ionisation energies of hydrogen is particularly important. The authors claim that QMR can vary these parameters and that this can then provide ample variations of the Hubble constant, referring to the paper 'Can Non-Standard Recombination Resolve the Hubble Tension?’ by Liu. et al (2019). The Liu et al. paper explicitly states that the two photon decay and ionisation energies require some ‘exotic physics’ to disrupt the current robust picture of recombination. That paper also shows that one would need shifts in the ionisation energy by 1% and shifts in the two-photon decay rate by 5% to reduce the tension. This is not consistent with the level of variations arising from any molecular hydrogen abundances, regardless of the QMR. It does not seem apparent how such a small effect within the recombination epoch at all could impact specifically the two-photon decay rate and the ionisation energies of the atoms.

In conclusion, there are not enough details on the interactions and the magnitudes of these variations. We know that smaller corrections to the recombination calculation are important for the CRR and even to 1% level of the ionisation history, but it is a bold claim to suggest that this could alleviate the Hubble tension in a noticeable way. One could implement these variations in the recombination code to quantify these statements. The message of the paper is really fascinating, with the synergy of the Kereselidze et al. (2019) and the Liu et al. (2019) paper. However, since the impacts of molecular species have been studied previously, the magnitude of these variations needs some quantification in order to compete with the negligibility proposed by the original papers.

[These comments were shared with us by a member of the cosmology community. They do not necessarily reflect the opinion of the Cosmo Comments team.]