From dawn to dusk: helium chemistry in the primordial and present-day universe

Well before the discovery of the cosmological microwave background, the abundance pattern of hydrogen, helium, and of the primordial trace elements on one hand and cosmic expansion on the other shared the role of a signpost to big bang cosmology. In the era of recombination, a simple chemistry was setting in, which differs from the present interstellar medium in many respects, as e.g., a UV radiation field mainly originating from recombination history, and the absence of dust grains. Unlike the fingerprint of the elemental abundances, as yet no direct observational signature exists for this "dawn of chemistry" (Galli \& Palla 2013), playing an eminent role in the early universe: Producing the first efficient coolants, it paved the way to the formation of gravitationally bound structures, leading over cosmic history to the rich chemistry and, ultimately, life in the present-day universe. The recent detection of the HeH$^+$ molecule in the Galactic planetary nebula NGC 7027 with SOFIA (Güsten et al. 2019) and its confirmation at the IRTF (Neufeld et al. 2020) is about to revitalize the interest in helium chemistry, assisted by recent laboratory experiments. The contribution at hand introduces the framework of primordial chemistry, reviews the current situation, and suggests future observations, more generally directed towards over-pressurized HII regions in various environments. An excursion will be made to the impact of the fine-tuning of fundamental constants on early chemistry. References:

Galli, D. & Palla, F., 2013, Annu.Rev.Astron.Astrophys 51, 163

Güsten, R. et al., 2019, Nature 568, 357

Neufeld, D.A. et al., 2020, ApJ in press, arXiv:2001.11344