Question: What is a Lyman-Alpha Forest

David Robertson answered on 23 Jun 2016:

Neutral hydrogen atoms will absorb off specific wavelengths of light when excited. The Lyman alpha line is the first of these lines (it is actually two lines very close together). So if light from a far away star goes through a cloud of hydrogen between the star and us, we will see a specific “line” missing from the spectrum of wavelengths we receive at the wavelength of the Lyman alpha line. If the could of hydrogen is moving then the absorption line is changed in wavelength by the Doppler effect and we can tell how fast the cloud is moving. If the light comes through lots of different clouds, all moving at different speeds then we see lots of different, but close together, absorption lines. This is the Lyman-alpha forest.