The cosmological constant was first proposed by Einstein as a mechanism to obtain a stable solution of the gravitational field equation
that would lead to a static universe, effectively using dark energy to
balance gravity. Not only was the mechanism an inelegant example of fine-tuning,
it was soon realized that Einstein's static universe would actually be
unstable because local inhomogeneities would ultimately lead to either
the runaway expansion or contraction of the universe.
The equilibrium
is unstable: if the universe expands slightly, then the expansion
releases vacuum energy, which causes yet more expansion. Likewise, a
universe which contracts slightly will continue contracting. These sorts
of disturbances are inevitable, due to the uneven distribution of
matter throughout the universe.
More importantly, observations made by Edwin Hubble
showed that the universe appears to be expanding and not static at all.
Einstein famously referred to his failure to predict the idea of a
dynamic universe, in contrast to a static universe, as his greatest
blunder. Following this realization, the cosmological constant was
largely ignored as a historical curiosity.
Alan Guth proposed in the 1970s that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation
in the very early universe. Inflation postulates that some repulsive
force, qualitatively similar to dark energy, resulted in an enormous and
exponential expansion of the universe slightly after the Big Bang.
Such expansion is an essential feature of most current models of the
Big Bang.
However, inflation must have occurred at a much higher energy
density than the dark energy we observe today and is thought to have
completely ended when the universe was just a fraction of a second old.
It is unclear what relation, if any, exists between dark energy and
inflation. Even after inflationary models became accepted, the
cosmological constant was thought to be irrelevant to the current
universe.
The term "dark energy" was coined by Michael Turner in 1998. By that time, the missing mass problem of big bang nucleosynthesis and large scale structure
was established, and some cosmologists had started to theorize that
there was an additional component to our universe.
The first direct
evidence for dark energy came from supernova observations of accelerated expansion, in Riess et al and later confirmed in Perlmutter et al. This resulted in the Lambda-CDM model,
which as of 2006 is consistent with a series of increasingly rigorous
cosmological observations, the latest being the 2005 Supernova Legacy
Survey.
First results from the SNLS reveal that the average behavior
(i.e., equation of state) of dark energy behaves like Einstein's
cosmological constant to a precision of 10 per cent.
Recent results from the Hubble Space Telescope Higher-Z Team indicate
that dark energy has been present for at least 9 billion years and
during the period preceding cosmic acceleration.
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