The
Future According to the Big Bang TheoryBefore
observations of dark energy, cosmologists considered two scenarios
for the future of the universe. If the mass density of the universe
is above the critical density, then the universe would reach a maximum
size and then begin to collapse. It would become denser and hotter
again, ending with a state that was similar to that in which it started—a
Big Crunch. Alternatively, if the density in the universe is equal
to or below the critical density, the expansion would slow down,
but never stop. Star formation would cease as the universe grows
less dense. The average temperature of the universe would asymptotically
approach absolute zero. Black holes would evaporate. The entropy
of the universe would increase to the point where no organized form
of energy could be extracted from it, a scenario known as heat death.
Moreover, if proton decay exists, then hydrogen, the predominant
form of baryonic matter in the universe today, would disappear, leaving
only radiation.
Modern
observations of accelerated expansion imply that more and more
of the currently visible universe will pass beyond our event
horizon and out of contact with us. The eventual result is not
known. The Lambda-CDM model of the universe contains dark energy
in the form of a cosmological constant. This theory suggests that
only gravitationally bound systems, such as galaxies, would remain
together, and they too would be subject to heat death, as the universe
cools and expands. Other explanations of dark energy—so-called
phantom energy theories—suggest that ultimately galaxy clusters
and eventually galaxies themselves will be torn apart by the ever-increasing
expansion in a so-called Big Rip.

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