As for backward time travel, it is possible to find solutions in general relativity that allow for it, such as a rotating black hole. Traveling to an arbitrary point in spacetime has very limited support in theoretical physics, and is usually connected only with quantum mechanics or wormholes.

88 miles per hour

Until recently, most studies on time travel are based upon classical general relativity. Coming up with a quantum version of time travel requires physicists to figure out the time evolution equations for density states in the presence of closed timelike curves (CTC).

A wormhole can be visualized as a tunnel with two ends at separate points in spacetime (i.e., different locations, different points in time, or both). Wormholes are consistent with the general theory of relativity by Einstein, but whether wormholes actually exist remains to be seen.

Black holes of stellar mass form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. There is consensus that supermassive black holes exist in the centers of most galaxies.

Assuming that dark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside the Local Supercluster will pass behind the cosmological horizon.

Eternal return (also known as eternal recurrence) is a concept that the universe and all existence and energy has been recurring, and will continue to recur, in a self-similar form an infinite number of times across infinite time or space.

A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to 3.16×1016 seconds (or simply 1,000,000,000). It is sometimes abbreviated Gy, Ga (“giga-annum”), Byr and variants. The abbreviations Gya or bya are for “billion years ago”, i.e. billion years before present.

4.603 billion years

4.503 billion years

225 days

248 years

around 24 hours

12 years