With each passing moment in the Universe, we’re constantly stepping forward in time. Each successive instant gives way to the next, with time continuously appearing to flow in the same direction — forward — without fail. And yet, it isn’t particularly clear exactly why this is the case. Still, if we look for it, we can find that a number of things also happen to always move in the same direction, from moment-to-moment, exactly the way time does. Objects move through the Universe proportional to their velocity. They change their motion due to the effects of gravity and the other forces. On large scales, the Universe expands. And everywhere we look, the entropy of the Universe always goes up.
As the story of our cosmic evolution continues, we think all of these things will continue: the laws of physics will still apply just as they do today, dark energy’s presence ensures that the Universe will keep on expanding, and entropy will keep increasing, as dictated by the laws of thermodynamics. Many have speculated — although there is no proof — that the arrow of thermodynamics and the arrow of time may be related. Still others have speculated that dark energy might evolve over time, rather than being a constant, leaving the door open to the possibility that it might someday counteract and reverse our Universe’s expansion. What happens, then, if we put these speculations together?
We’d wind up imagining that perhaps the Universe will cease expanding, that it will instead begin collapsing, and that we’d have to then ask whether this means that entropy could decrease and/or time could even start running backward? It’s a mind-bending possibility, and one for the laws of physics to answer. Let’s see what they have to say about it all!
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