and Cosmology

8.5 Origin of the Lyman-α Forest
Fig. 8.20. From the analysis of the data in Fig. 8.19 and comparison
with model predictions, constraints on cosmological
parameters can be derived. Here, confidence contours in the
Ω m –σ 8 parameter plane are shown, where Ω Λ = 0 was assumed.
The effect of Ω Λ on the prediction of the shear is
relatively small. The data suggest a Universe of low density.
Presently the largest uncertainty in the quantitative analysis
of shear data is the insufficiently known redshift distribution
of the faint galaxies
8.5 Origin of the Lyman-α Forest
We have seen in Sect. 5.6.3 that in the spectrum of
any QSO a large number of absorption lines at wavelengths
shorter than the Lyα emission line of the QSO
are found. The major fraction of these absorption lines
originate from the Lyα transition of neutral hydrogen
located along the line-of-sight to the source. Since
the absorption is found in the form of a line spectrum,
the absorbing hydrogen cannot be distributed
homogeneously. A homogeneous intergalactic medium
containing neutral hydrogen would be visible in continuum
absorption. In this section, we will first examine
this continuum absorption. We will then summarize
some observational results on the Lyα forest and explain
why studying this provides us with valuable information
about the cosmological parameters.
8.5.1 The Homogeneous Intergalactic Medium
We first ask whether part of the baryons in the Universe
may be contained in a homogeneous intergalactic
medium. This question can be answered by means
of the Gunn–Peterson test. Neutral hydrogen absorbs
photons at a rest wavelength of λ = λ Lyα = 1216 Å.
Photons from a QSO at redshift z QSO attain this wavelength
λ Lyα somewhere along the line-of-sight between
us and
(
the QSO,
)
if they are emitted by the QSO at
−1
λ Lyα 1 + zQSO