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Ask Brian Greene: What Exactly Is the Universe Expanding Into?


bg bwBrian 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).

See other questions from this 2011 series

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.



  1. 6461orange says

    I had a similar question, except to say that, therefore the properties of space itself are crated by the process contained within the existing processes of how the universe expands to begin with, an secondly therefore the space contained within the universe is “elastic” and this elasticity, is it possible then, this elasticity is what helps to explain the existence of gravity and how gravity works within the given space of the universe?

  2. Clyde Sherman says

    All of this is still talking about what happens inside a closed system, or should I say what the closed system looks like, without addressing whats outside the closed system. 2-D, 3-D, 4-D or even 10-D doesn’t explain anything but the shape of the closed system or how we PERCEIVE it. Matter must have some mass that takes up some space at a some time. In order for any matter with mass to exist, there has to be space at a certain time. If the universe is expanding,or stretching, (TAKING) up more space at a certain time, the space had to exist outside of the matter, i.e, the universe, for it to TAKE UP.The muffin as it cooks, does “expand” and “take up” more space.
    If at the core of everything is energy and ACCORDING to the 3rd law of THERMODYNAMICS, energy will eventually be evenly distributed and the UNIVERSE will reach ABSOLUTE zero, doesn’t that imply a finite universe at some point.
    Also, Could what it is expanding into be the Anti-Matter that we have been looking for? Matter is “chasing” Anti-matter and when they meet they are obliterated into energy, speeding up the expansion?? Still doesn’t explain what the Anti-matter is expanding or stretching into into.

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