The pulsating universe theory, which is more commonly known as the oscillating or cyclic universe theory, posits that the universe goes through regular cycles of expansion and destruction. This theory is credited to Albert Einstein.
In the cyclic model, the universe begins with a "big bang" and ends with a "big crunch." After the big bang, the universe expands until gravitational forces force it to stop. At this point, the universe contracts until it implodes into a singularity. A gravitational singularity is a point where gravitational forces are infinite. After this contraction occurs, the cycle begins again with a new big bang.
The original cyclic universe theories conflict with the established laws of thermodynamics, which state that entropy in a system can only increase. The collapse of the universe into a big crunch results in a loss of entropy. Newer models solve this problem by factoring in dark energy, a type of energy not yet discovered in Einstein's time.
This theory is not the currently accepted theory about the origin and end of the universe, but does explain some shortcomings of other theories. Most prominently, it solves the thermal state problem: how the universe begins at such a high temperature. In cyclic theories, the implosion of all matter and energy in the universe into a singularity produces the required energy to initiate a big bang.