In space that has size, you can move back and forth, but time space is different. Time is irreversible.
The Big Bang theory states that the universe and time have a beginning, but physicists are divided over whether the universe will end and, more importantly, how.
Theories range from the decay of dark matter to the Big Rip, and astrophysicist Thomas Kitching explained in The Conversation how this might work. He also detailed why time moves forward forever and what the expansion of the universe means for humanity.
While we take it for granted that time has a given direction, will the universe end in a Big Rip? physicists believe that most natural laws are "time reversible," which means they would work just as well if time worked in the opposite direction.
Any universal concept of time must ultimately be based on the evolution of the cosmos itself. For a long time, scientists, including Albert Einstein, thought that the universe was static and infinite.
Observations later showed that in fact it is expanding, and at an ever faster pace. This means that it had to arise from a denser state, which we call the Big Bang, implying that time has a beginning.
But there is a catch, Einstein's special theory of relativity shows that time is relative: the faster you move relative to me, the slower time will pass for you, relative to my perception of time. It follows from this that in our Universe of expanded galaxies and rotating stars and planets, the present and the future are relative.
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Some researchers argue that the Big Bang may not even have been the beginning of the universe, but there are also "parallel universes" where time runs in different directions.
Time has a beginning, but whether it will have an end depends on the nature of dark energy, which causes it to expand with acceleration.
The rate of this expansion can tear the universe apart over time, causing it to end in a Big Rip. Alternatively, dark energy could decay, reshaping the Big Bang, and the universe will end in the Big Bang, or simply expand forever.