"Since astronomers know that the Universe is expanding, what's it expanding into? What's outside of the Universe?"
Ask any astronomer and you'll get an unsatisfying answer. We give you the same unsatisfying answer, but really explain it, so your unsatisfaction doesn't haunt you any more.
The short answer is that this is a nonsense question, the Universe isn't expanding into anything, it's just expanding.
The definition of the Universe is that it contains everything. If something was outside the Universe, it would also be part of the Universe too.
Outside of that? Still Universe. Out side of THAT? Also more Universe. It's Universe all the way down.
Either the Universe is infinite, going on forever, or its finite, with a limited volume. In either case, the Universe has no edge.
When we imagine the Universe expanding after the Big Bang, we imagine an explosion, with a spray of matter coming from a single point. But this analogy isn't accurate.
A better analogy is the surface of an expanding balloon. Not the 3 dimensional balloon, just its 2 dimensional surface.
If you were an ant crawling around the surface of a huge balloon, and the balloon was your whole universe, you would see the balloon as essentially flat under your feet.
Imagine the balloon is inflating. In every direction you look, other ants are moving away from you. The further they are, the faster away they're moving.
Even though it feels like a flat surface, walk in any direction long enough and you'd return to your starting point.
Credit: NASA /WMAP Science Team.
You might imagine a growing circle and wonder what it's expanding into. But that's a nonsense question.
There's no direction you could crawl that would get you outside the surface.
Your 2-dimensional ant brain can't comprehend an expanding 3-dimensional object.
There may be a center to the balloon, but there's no center to the surface. Just a shape that extends in all directions and wraps in upon itself and yet, your journey to make one lap around the balloon takes longer and longer as the balloon gets more inflated.
To better understand how this relates to our Universe, we need to scale things up by one dimension, from a 2-d surface embedded in a 3-d world, to a 3-d volume embedded within a 4-d universe.
Astronomers think that if you travel in any direction far enough, you'll return to your starting position. If you could stare far enough into space, you would be looking at the back of your own head.
Watch the video for the full story.
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