# nLab hydrogen line

## Idea

What is called the 21cm hydrogen line or just hydrogen line is the signature of the electromagnetic radiation of wavelength about 21cm which is emitted by hydrogen atoms when the electrons in their shell change quantum state from the “triplet” to the singlet ground state.

## In astrononmy

In astronomy?, the Tully-Fisher relation is a relation between the asymptotic behaviour of galaxy rotation curves, witnessed by the width of their 21cm hydrogen line, and the total visible baryonic mass of the galaxies.

## In cosmology

In the context of cosmology it has been argued (Higan-Rees 79) that observation of a absorption at the 21cm hydrogen line in the cosmic microwave background would be a signal from the time when the first stars? formed and ignited, a cosmological epoch not otherwise accessible to observation. A claim of detection of this signal by the EDGES experiment appeared in BRMMM 18.

## References

### In astrophysics and cosmology

The suggestion that the 21cm hydrogen absorption line in the cosmic microwave background should be an interesting probe for early universe cosmology goes back to

• C. J. Hogan, Martin Rees, Spectral appearance of non-uniform gas at high $z$, Monthly Notices of the Royal Astronomical Society, Volume 188, Issue 4, 1 October 1979, Pages 791–798 (doi:10.1093/mnras/188.4.791)

Review includes

• Jonathan Pritchard, A. Loeb, 21 cm cosmology in the 21st century, Rep. Prog. Phys. 75, 086901 (2012) (arXiv:1109.6012)

• Jonathan Pritchard, Cosmological probes of inflation and the 21 cm line (video recording)

• Saleem Zaroubi, The Epoch of Reionization, in Wiklind T., Mobasher B., Bromm V. (eds) The First Galaxies, Springer (arXiv:1206.0267)

A claim of detection of the cosmological signal by the EDGES experiment is due to

• Judd D. Bowman, Alan E. E. Rogers, Raul A. Monsalve, Thomas J. Mozdzen, Nivedita Mahesh, An absorption profile centred at 78 megahertz in the sky-averaged spectrum, Nature volume 555, pages 67–70 (01 March 2018) (doi:10.1038/nature25792)

• Raul A. Monsalve, Bradley Greig, Judd D. Bowman, Andrei Mesinger, Alan E. E. Rogers, Thomas J. Mozdzen, Nicholas S. Kern, Nivedita Mahesh, Results from EDGES High-Band: II. Constraints on Parameters of Early Galaxies, The Astrophysical Journal, Volume 863, Number 1 (arXiv:1806.07774)

Discussion incluces

Calling this argument into question:

• Richard Hills, Girish Kulkarni, P. Daniel Meerburg, Ewald Puchwein, Concerns about Modelling of the EDGES Data (arXiv:1805.01421)

Arguments that this detection of the 21cm line rules out substantial contributions of fuzzy dark matter:

Last revised on April 10, 2020 at 11:34:55. See the history of this page for a list of all contributions to it.