Also more Universe. It's Universe all the way down. But I know you're going to find that answer unsatisfying, so now I'm going to break your brain. 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. 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. And so, as the Universe expands, it would take you longer and longer to lap the Universe and return to your starting position.
But there's no direction you could travel in that would take you outside or "off" of the Universe. Even if you could move faster than the speed of light, you'd just return to your starting position more quickly.
We see other galaxies moving away from us in all directions just as our ant would see other ants moving away on the surface of the balloon.
A great analogy comes from my Astronomy Cast co-host, Dr. Pamela Gay. Instead of an explosion, imagine the expanding Universe is like a loaf of raisin bread rising in the oven. From the perspective of any raisin, all the other raisins are moving away in all directions. But unlike a loaf of raisin bread, you could travel in any one direction within the bread and eventually return to your starting raisin.
Remember that our entire comprehension is based on 3-dimensions. If we were 4-dimensional creatures, this would make much more sense. For a much deeper explanation, I highly recommend you watch my good friend, Zogg the Alien explain how the Universe has no edge. Imagine the balloon is inflating. In every direction you look, other ants are moving away from you.
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. If you could stare far enough into space, you would be looking at the back of your own head. And so, as the Universe expands, it would take you longer and longer to lap the Universe and return to your starting position.
We see other galaxies moving away from us in all directions just as our ant would see other ants moving away on the surface of the balloon. A great analogy comes from my Astronomy Cast co-host, Dr. Pamela Gay. Instead of an explosion, imagine the expanding Universe is like a loaf of raisin bread rising in the oven. From the perspective of any raisin, all the other raisins are moving away in all directions.
But unlike a loaf of raisin bread, you could travel in any one direction within the bread and eventually return to your starting raisin. Or was there an initial set of conditions that the fabric of the balloon started off with that determined these expansion properties before anyone ever came along to observe it? The complete and honest answer is we don't know , and moreover, we can't know! From our vantage point, we can only see that the balloon is expanding. We can assume it would be illogical not to that there's more balloon out there beyond what we can see; we can track exactly how the balloon is expanding today and how it's been expanding throughout the Universe's history; we can measure the properties of everything observable to us and study how it affect and is affected by the balloon, and so much more.
But as far as what lies beyond the part of the balloon we can observe, including in dimensions beyond the two that make up the surface of the balloon, we simply don't have the information to say. Our Universe is like a three dimensional version of the balloon's surface, where galaxies are like raisins baking in a gigantic loaf of bread.
The bread is the invisible fabric of space; the galaxies are the raisins within. We can measure the raisins within our view -- where "our view" is determined by the speed of light and the amount of time that's passed since the Big Bang -- and we assume that there's more raisins and more bread outside of what we can see, but that's all we can know. We can determine the past expansion history of our Universe, we can find that the expansion is accelerating rather than any of the other options and hence, describe the expansion , but as far as what's happening outside of what we can observe, we have more questions than answers.
Are there four or more spatial dimensions, total? Is there truly a center to this loaf of bread or to our Universe?
Is it infinite, or simply bigger than we can perceive? Does it ever curve back in on itself and reconnect? And is there something bigger and grander than what we can ever hope to observe, that it truly is expanding into? We not only don't know, we have no idea how it would ever be possible to know.
But that's part of the wonder and joy of science: until we know, we have to admit that even the most absurd-sounding explanation that can't be ruled out is actually possible. The Universe doesn't need to be expanding into anything greater than itself; it may simply be expanding, because that's what space does in general relativity. But it could be doing much, much more. If we're lucky, perhaps someday we'll devise a way to find out.
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