Vacuum Energy
The preceding problems with the big bang
can be alleviated all at once (at least in principle) by
a new kind of cosmology called the inflationary universe.
The Inflationary Epoch
In the
corresponding theory of inflation, the Universe, because of properties of
elementary particles not accounted for in the standard big bang models, expands for
a fleeting instant at its beginning at a much higher rate (precisely, at a rate such that
the Universe expands exponentially)
than that expected for
the big bang. This period, which is called the inflationary epoch, is
a consequence in the simplest inflationary theories of a
phase transition. (An example from everyday life of a phase transition is
the conversion of liquid water to ice.)
Cosmological Constants and Exponential Expansion
Starting about
10-35 seconds after creation and
continuing for some 10-32 seconds,
this phase transition
filled the Universe with a kind of energy called the vacuum energy. As
a consequence of this vacuum energy, the Universe expanded very rapidly
(exponentially) for a short period.
This period of rapid expansion is called cosmic inflation.
As noted in the right panel, a nonzero value of the vacuum energy density is equivalent to a
nonzero cosmological constant, and leads effectively to a kind of antigravity.
It is not clear whether there is any relation between the vacuum energy density that
drove inflation and the tiny vacuum energy density thought to
be present in the current Universe that is causing it to accelerate. In the present
Universe the effect of the vacuum energy (dark energy) competes with the
gravitational effect of mass in the evolution of the Universe, but during inflation
the vacuum energy completely dominated the evolution of the Universe.