Roughly 13.7 billion years ago there was an incredible event. Commonly called the Big Bang, this dramatic expansion has lead to what we now call the universe. But what is the universe? I suppose we should get that out of the way first. There are two things that the universe can be. The actual answer would be “everything”.
“The Universe is commonly defined as the totality of existence, including planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy.” – Wikipedia
Looks familiar right? Except that’s not the universe. This is merely the visible universe. We don’t know what lies outside of this region and for most people the answer is “nothing”. There is nowhere outside to speak of, the universe has no edge. Nothing stops there being anything outside of the visible universe, in fact most major Astrophysicists (that I’ve heard talk about the topic) suggest that if you were to move a hundred thousand light years to the left or right (for what directions are worth in space) you’d still see 13.7 billion years out in all directions. But your “edge” opposite the direction you came would be shallower and the “edge” sharing the direction you’ve come would be deeper.
The size of the “universe” as we so commonly know wouldn’t have changed for you. I would see 13.7 billion years into the past and you would too, we’d both be seeing different complete pictures. Both correct and yet incongruous.
What’s more this universe of ours hasn’t stopped expanding. You might be saying “you just answered the question you asked in the header” but bear with me. The universe isn’t just expanding but it is expanding faster and faster. In roughly a trillion years the expansion will be so dramatic that the cosmic microwave background we use to date the universe will be gone. But that isn’t where the oddities end. There is no reason to believe that this expansion will ever slow down currently. That expansion also isn’t limited by special relativity either, so it can (and likely will) someday expand faster than the speed of light.
What happens when the universe is expanding faster than the speed of light? At the micro level, which on the cosmic scale would be entire galaxies, not much. Our solar system won’t fall apart, the galaxy won’t be ripped asunder (though its collision with Andromeda will be something else, apparently the name will become Milkomeda which I find hilarious). The four forces of nature will likely always have control over themselves and the expansion of the universe won’t change that. Entropy will, to some degree, but not expansion.
On the macro level however something big will happen. Galaxies will start spreading apart faster than their light can travel away from them. Every great celestial body in the universe will stop shining in our skies. We will see the Milky Way and nothing more. If the visible universe now is so commonly associated as “everything there is and ever will be” what becomes of that great creature when all that remains is the Milky Way? I wonder what will become of humanity if we manage to survive for so many billions (and likely trillions) of years. Will we regress in our beliefs? Will people begin arguing against anything more than the Milky Way?
There was a time when humans thought there was nothing outside of their land. Then nothing outside of their nation. Then nothing outside of their world. Then nothing outside of their solar system. Many now would argue there is nothing outside of our Universe. But will we someday take a step back and argue there is nothing outside of the Milky Way?
Imagine watching the skies on a cosmic timescale. Seeing every galaxy blink out of existence one after another until the last shimmering body vanishes. Leaving nothing behind but the billions and billions of stars that we call our galactic home. Presuming that many are still around by the time this great event actually transpires.
On that day the universe, as we know it, will have shrunk, and that’ll be a strange day indeed.