Eternal return (also known as "eternal recurrence") is a concept which posits that the universe has been recurring, and will continue to recur in the exact same self-similar form an incomprehensible and infathomable number of times. The concept has roots in ancient Egypt, and was subsequently taken up by the Pythagoreans and Stoics. With the decline of antiquity and the spread of Christianity, the concept fell into disuse, though Fredrich Nietzsche briefly resurrected it.
In addition, the philosophical concept of eternal recurrence was addressed by Arthur Schopenhauer. It is a purely physical concept, involving no "reincarnation," but the return of beings in the same bodies. Time is viewed as being not linear but cyclical.
The basic premise is that the universe is limited in extent and contains a finite amount of matter, while time is viewed as being infinite. The universe has no starting or ending state, while the matter comprising it is constantly changing its state. The number of possible changes is finite, and so sooner or later the same state will recur. At least one mathematical proof has been developed to disprove this rationale for eternal return.
Physicists such as Stephen Hawking and J. Richard Gott have proposed models by which the (or a) universe could undergo time travel, provided the balance between mass and energy created the appropriate cosmological geometry. More philosophical concepts from physics, such as Hawking's arrow of time for example, discusses cosmology as proceeding up to a certain point, whereafter it undergoes some form of time reversal, (which due to T-symmetry is thought to bring about a chaotic state due to thermodynamic entropy).