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e
Cosmic microwave background
Although
both dark matter and ordinary matter are matter, they do not behave
in the same way. In particular, in the early universe, ordinary
matter
was ionized and interacted strongly with radiation via Thomson
scattering.
Dark matter does not interact directly with radiation, but it
does affect the
CMB by its gravitational potential (mainly on large scales), and
by its effects
on the density and velocity of ordinary matter. Ordinary and dark
matter
perturbations, therefore, evolve differently with time and leave
different
imprints on the cosmic microwave background (CMB).
The cosmic microwave background is very close to a perfect blackbody
but
contains very small temperature anisotropies of a few parts in
100,000. A
sky map of anisotropies can be decomposed into an angular power
spectrum,
which is observed to contain a series of acoustic peaks at near-equal
spacing but different heights. The series of peaks can be predicted
for any
assumed set of cosmological parameters by modern computer codes
such
as CMBFAST and CAMB, and matching theory to data, therefore, constrains
cosmological parameters. The first peak mostly shows the density
of
baryonic matter, while the third peak relates mostly to the density
of dark
matter, measuring the density of matter and the density of atoms.
The CMB anisotropy was first discovered by COBE in 1992, though
this
had too coarse resolution to detect the acoustic peaks. After
the discovery
of the first acoustic peak by the balloon-borne BOOMERanG experiment
in
2000, the power spectrum was precisely observed by WMAP in 2003–2012,
and even more precisely by the Planck spacecraft in 2013–2015.
The results
support the Lambda-CDM model.
The observed CMB angular power spectrum provides powerful evidence
in
support of dark matter, as its precise structure is well fitted
by the Lambda43
CDM model, but difficult to reproduce with any competing model
such as
modified Newtonian dynamics (MOND).
2-2
f
Structure formation
Structure
formation refers to the period after the Big Bang when density
perturbations collapsed to form stars, galaxies, and clusters.
Prior to structure
formation, the Friedmann solutions to general relativity describe
a
homogeneous universe. Later, small anisotropies gradually grew
and condensed
the homogeneous universe into stars, galaxies and larger structures.
Ordinary matter is affected by radiation, which is the dominant
element
of the universe at very early times. As a result, its density
perturbations
are washed out and unable to condense into structure. If there
were only
ordinary matter in the universe, there would not have been enough
time for
density perturbations to grow into the galaxies and clusters currently
seen.
Dark matter provides a solution to this problem because it is
unaffected by
radiation. Therefore, its density perturbations can grow first.
The resulting
gravitational potential acts as an attractive potential well for
ordinary matter
collapsing later, speeding up the structure formation process.
