Brian Greene continues the WSF Live Forum all month long. Each day, he’ll answer one of your questions for this ongoing series that delves into the fundamental nature of space, time, and reality as we may or may not know it.
Today, Brian Greene considers two related questions:
If the universe is expanding, is it theoretically not infinite? In other words, if the universe is infinite, how can it be a discrete thing that is expanding? And if it is not infinite, is space infinite? Or does “space” only exist inside the universe, and not beyond?
—Michael Pick, New York, NY
It is a truly awesome concept that the universe is expanding. But what is it expanding into? Into what “space” is it expanding and what happens to that “space” when the universe contracts/has contracted?
We don’t know whether the universe is finite or infinite. But in either case, the concept of “space expanding” raises basic questions: (a) what’s the universe expanding “into”? and (b) if the universe is in fact infinite, how can it expand as it seemingly can’t grow larger than the infinite size it already has? Let me address both.
(a) When we speak of space expanding, we generally don’t envision space expanding into a pre-existing container and claiming new territory as it grows. Instead, we envision space stretching, and in the process creating new space that the universe and its contents can then occupy. Space expands, and hence grows, by virtue of creating new space.
(b) If the universe is finite, the expansion just described increases the overall size of the universe. However, a more useful marker of the expansion is to consider any two objects, say galaxies, that are being driven apart by the expansion of space. You can think of them like two poppy seeds in a muffin that’s being cooked and as the dough expands, the poppy seeds move apart from one another. In this way of thinking, the expansion of space is reflected in the distance between two such galaxies growing larger and larger over time. And that characterization of the expansion holds true whether the universe is finite or infinite. So, when we speak of space expanding we mean that the distance between objects in space increases over time, even if there isn’t a useful sense in which the overall size of space is growing (as would be the case for an infinite universe).
Brian Greene is co-founder of the World Science Festival and professor of physics and mathematics at Columbia University. His books include The Elegant Universe, The Fabric of the Cosmos, and The Hidden Reality. NOVA’s miniseries “The Fabric of the Cosmos” airs Wednesday nights on PBS.