2-2
g
Bullet Cluster
If dark
matter does not exist, then the next most likely explanation must
be general relativity – the prevailing theory of gravity
– is incorrect and
should be modified. The Bullet Cluster, the result of a recent
collision of
two galaxy clusters, provides a challenge for modified gravity
theories because
its apparent center of mass is far displaced from the baryonic
center
of mass. Standard dark matter models can easily explain this observation,
but modified gravity has a much harder time, especially since
the observational
evidence is model-independent.
2-2
h
Type Ia supernova distance measurements
Type Ia
supernovae can be used as standard candles to measure extragalactic
distances, which can in turn be used to measure how fast the universe
has expanded in the past. Data indicates the universe is expanding
at an
accelerating rate, the cause of which is usually ascribed to dark
energy.
Since observations indicate the universe is almost flat, it is
expected the
total energy density of everything in the universe should sum
to 1 (Otot
˜ 1). The measured dark energy density is O? ˜ 0.690;
the observed
ordinary (baryonic) matter energy density is Ob ˜ 0.0482
and the energy
density of radiation is negligible. This leaves a missing Odm
˜ 0.258
which nonetheless behaves like matter (see technical definition
section
above) – dark matter.
2-2
i
Sky surveys and baryon acoustic oscillations
Baryon
acoustic oscillations (BAO) are fluctuations in the density of
the
visible baryonic matter (normal matter) of the universe on large
scales.
These are predicted to arise in the Lambda-CDM model due to acoustic
oscillations in the photon–baryon fluid of the early universe,
and can be
observed in the cosmic microwave background angular power spectrum.
BAOs set up a preferred length scale for baryons. As the dark
matter and
baryons clumped together after recombination, the effect is much
weaker in
the galaxy distribution in the nearby universe, but is detectable
as a subtle
(˜1 percent) preference for pairs of galaxies to be separated
by 147 Mpc,
compared to those separated by 130–160 Mpc. This feature
was predicted
theoretically in the 1990s and then discovered in 2005, in two
large galaxy
redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy
Redshift
Survey. Combining the CMB observations with BAO measurements from
galaxy redshift surveys provides a precise estimate of the Hubble
constant
and the average matter density in the Universe. The results support
the
Lambda-CDM model.
