Accelerating Universe

The accelerating universe is
the observation that the
universe appears to be
expanding at an increasing rate,
which in formal terms means
that the cosmic scale factor a(t)
has a positive second
derivative,[1] implying that the
velocity at which a given galaxy
is receding from us should be
continually increasing over
time[2] (here the recession
velocity is the same one that
appears in Hubble's law;
defining 'velocity' in cosmology
is somewhat subtle, see
Comoving distance#Uses of the
proper distance for a
discussion). In 1998,
observations of Type Ia
supernovae suggested that the
expansion of the universe has
been accelerating[3][4] since
around redshift of z~0.5.[5] The
2006 Shaw Prize in Astronomy
and the 2011 Nobel Prize in
Physics were both awarded to
Saul Perlmutter, Brian P. Schmidt,
and Adam G. Riess for the 1998
discovery of the accelerating
expansion of the Universe
through observations of distant
supernovae.[6][7]
Contents [hide]
1 Corroboration
2 Density drops
3 Explanatory models
4 Divergent expansion
5 Dark energy dominates
6 See also
7 References
Corroboration
In the past few years, these
observations have been
corroborated by several
independent sources: the cosmic
microwave background
radiation and large scale
structure,[8] age of the
universe,[9] as well as improved
measurements of the
supernova,[10][11] and X-ray
properties of galaxy clusters.
Density drops
An expanding universe means
that density drops due to
continual space being added
between all matter. If
acceleration continues,
eventually all galaxies beyond
our own local supercluster will
redshift so far that it will
become hard to detect them,
and the distant universe will
turn dark.
Explanatory models
Models attempting to explain
accelerating expansion include
some form of dark energy,
cosmological constant,
quintessence, dark fluid or
phantom energy. The most
important property of dark
energy is that it has negative
pressure which is distributed
relatively homogeneously in
space.
Divergent expansion
Phantom energy in a scenario
known as the Big Rip causes an
exponentially increasing
divergent expansion, which
overcomes the gravitation of
the local group and tears apart
our Virgo supercluster, it then
tears apart the Milky Way
Galaxy, our solar system, and
finally even atoms.
Measurements of acceleration
are thought crucial to
determining the ultimate fate of
the universe, however we
should expect the implications
of such a major discovery to
develop slowly over many years
in the same way the big bang
model has continued to develop.
Dark energy dominates
As the Universe expands, the
density of dark matter declines
more quickly than the density of
dark energy (see equation of
state) and, eventually, the dark
energy dominates. Specifically,
when the volume of the
universe doubles, the density of
dark matter is halved but the
density of dark energy is nearly
unchanged (it is exactly
constant if the dark energy is a
cosmological constant). In the
cosmological constant models,
the dark energy already
dominates the mass-energy of
matter, including dark matter,
and the expansion of the
universe is approximately
exponential with time. In this
scenario the scale factor
doubling time of the expansion,
in the future, will be
approximately 11.4 billion years.
Pic from wikipedia.org