Contents

# Contents

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## Properties

### Hydrogen line

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

### General

Lecture notes includes

• 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)