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Redshift-space distortions
Large
galaxy redshift surveys may be used to make a three-dimensional
map of the galaxy distribution. These maps are slightly distorted
because
distances are estimated from observed redshifts; the redshift
contains a
contribution from the galaxy’s so-called peculiar velocity
in addition to
the dominant Hubble expansion term. On average, superclusters
are expanding
more slowly than the cosmic mean due to their gravity, while voids
are expanding faster than average. In a redshift map, galaxies
in front of
a supercluster have excess radial velocities towards it and have
redshifts
slightly higher than their distance would imply, while galaxies
behind the
supercluster have redshifts slightly low for their distance. This
effect causes
superclusters to appear squashed in the radial direction, and
likewise
voids are stretched. Their angular positions are unaffected. This
effect is
not detectable for any one structure since the true shape is not
known,
but can be measured by averaging over many structures. It was
predicted
quantitatively by Nick Kaiser in 1987, and first decisively measured
in
2001 by the 2dF Galaxy Redshift Survey. Results are in agreement
with
the Lambda-CDM model.
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Lyman-alpha forest
In astronomical
spectroscopy, the Lyman-alpha forest is the sum of the
absorption lines arising from the Lyman-alpha transition of neutral
hydrogen
in the spectra of distant galaxies and quasars. Lyman-alpha forest
observations can also constrain cosmological models. These constraints
agree with those obtained from WMAP data.
