Graceful exit problem (cosmology)

The graceful exit problem in cosmology was the need to explain how the universe slowed from its initial inflation to its later rate of expansion.

Cosmological Inflation was a brief period (in Big Bang model) of the primordial universe, during which the universe expanded at an accelerating rate. After inflation, which was dominated by either radiation or matter, the universe expanded at a decelerating rate. Thus inflation could not continue indefinitely. To overcome this problem, every proposed model of inflation must provide a condition of smooth exit to a normal Friedman-Robertson-Walker universe, so that the universe becomes similar to the present one.

Even in the case of ‘old inflation’, in Alan Guth's first paper proposing inflation, has some serious issues (some problems about reheating) to end and thus suffered the graceful exit problem. In 1982 the collaboration of Paul Steinhardt and Andreas Albrecht along with the independent effort of Andrei Linde, solved this problem.

...
Wikipedia