Buzz
Time cannot reverse, but in the eyes of physicists, the arrow of time is not uni-directional; under certain special circumstances, time can begin to flow backwards.
Modern cosmological models suggest that our universe originated from a big bang 13.82 billion years ago; before that, there was no time, space, matter, physical laws, and everything came into existence only after that big bang.
In a very brief moment after the big bang, the universe expanded from a point of singularity to a significant scale; this expansion rate surpasses the speed of light, and in the current expansion of the universe, the expansion rate of space-time is also faster than the speed of light.
That's why distant galaxies are receding from Earth faster than the speed of light, and it's also why there exists an observable universe with a diameter of 93 billion light-years.
Guided by Einstein's notion of integrating space-time, many physicists believe that the unidirectional nature of present time is caused by the expansion of the universe, meaning that once the universe transitions from expansion to contraction, the direction of the universe will change, and the arrow of time will also reverse.
The currently observed universe is in a state of accelerating expansion, driven by dark energy, which constitutes 70% of the total mass and energy of the universe. It provides additional thrust and helps propel the universe's expansion process forward.
If future physicists discover that the assessment of dark energy ratio is too high or that dark energy is depleted, the entire universe will transition from accelerating expansion to decelerating expansion, then cease expansion altogether, ultimately transitioning from a halted state to a collapsed or contracted state.
If a major collapse occurs at a certain point in the future, it will not only shorten distances in space but also involve a reversal of time. In such a scenario, the distances between galaxies will progressively shrink. If there are indeed numerous advanced intelligent civilizations in the universe, humanity will soon discover their existence as the universe contracts.
As ordinary individuals, we might perceive the universe's collapse akin to a movie played in reverse, but there's a fundamental difference between the collapse of the universe itself and a movie played in reverse, and that is entropy.
In thermodynamics, entropy refers to the level of disorder in a closed system, and its core attribute is that it only increases, never decreases, meaning the level of disorder in a closed system will only rise.
In the case of our universe being a large closed system, the entropy value of the universe will continue to increase due to the big bang and the expansion of space and time, but when the collapse of the universe begins, the entropy value will not reverse; instead, it will continue to increase.
While time reversal may seem unbelievable to many, it is indeed possible, albeit with very low probability.
