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The other parameter which we can define at the present epoch is the
dimensionless decelaration parameter 
. This is simply the present
deceleration of the Universe, 
, written in the form
There are three different regions were the value of 
is important for
the evaluation of our universe.
-
The total energy of the universe is positive and
the selfgravitation stays weaker than the expansion forces. As a result of this
the universe keeps expanding for ever. This expansion goes faster if 
is bigger. We say that this universe is open.
-
The total energy is zero and selfgravity keeps
(at 
) the expansion in balance. After infinite time the
expansion is stopped (static universe). This universe is called flat.
-
The total energy is negative now and selfgravity
will conquer the expansion. This means that the expansion is stopped after a
limited time and changes into a contraction (catastrophic gravitational
collaps). This is a closed universe.