Sunday, 25 September 2022

Astronomy Colloquium (15.6/2022):Can Non-standard Recombination Resolve the Hubble Tension?

06.15.2022 2:00 pm - 3:00 pm


Title: Can Non-standard Recombination Resolve the Hubble Tension?
Speaker:Zhiqi Huang(黄志琦)
Abstract: The inconsistent Hubble constant values derived from cosmic microwave background (CMB) observations and from local distance-ladder measurements may suggest new physics beyond the standard LambdaCDM paradigm. It has been found in earlier works that, at least phenomenologically, non-standard recombination histories can significantly reduce the Hubble tension. Following this path, we vary physical and phenomenological parameters in RECFAST, the standard code to compute ionization history of the universe, to explore possible physics beyond standard recombination. We find that the CMB constraint on the Hubble constant is sensitive to the hydrogen ionization energy and the 2s-to-1s two-photon decay rate, both of which are atomic constants, and is insensitive to other details of recombination. The conclusion drawn from the aforementioned study, that recombination within the standard model of physics cannot resolve the Hubble tension, however, is recently challenged by a series of works on the quasi-molecular mechanism of recombination (QMR) by Kereselidze et al. QMR is a three-body quantum process where an electron and two neighboring protons in the early Universe directly form an ionized hydrogen molecule in a highly excited state, which then descends to lower levels or dissociates. It has been suggested that the increased binding energy due to the participation of a second proton may lead to an earlier cosmic recombination that alleviates the Hubble tension. We revisit the quasi-molecular channel of recombination in more details, and find that the original work significantly overestimated the probability of finding a pair of adjacent protons in the pre-recombination epoch. Our new estimation suggests that QMR cannot be the primary cause of the Hubble tension, and reaffirms the robustness of Hubble tension within the standard model of physics.
Time:14:00-15:00PM, 15/June, Wednesday
Meeting ID: 831 4360 9634
Passcode: 436377
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