Q: What is the universe expanding into? What’s outside the universe?

Physicist: Probably nothing.  We do know that the universe doesn’t need anything to expand into, and we haven’t seen any evidence that there is anything outside of the universe.  But there has still been some speculation.

In the last hundred years physics has gotten pretty weird, and defining “universe” has become a little tricky.  So, in what follows I’m defining the universe as “all the places that could be connected to one another by a sufficiently long rope” (never mind how the rope got there).

Having never been outside of the universe, it’s hard to discuss it with any certainty.  Most of the theories about the outside of the universe fall into the “I can’t say you’re wrong for sure” category.

We can say that space isn’t “made of anything”, and that it doesn’t need any kind of “higher space” to exist in.  If space did need some other kind of space to live in, you find that the question just gets pushed back.  After all, what’s outside of that space?

In a conversation about spacetime often as not you’ll have some unpleasant and unrepentant jackass drawing parallels to rubber or sheets or something else material.  It’s not that these metaphors are misleading (although they are a little) it’s that they reinforce the quiet, underlying assumption that space is made of something, and that it needs somewhere to be.  About the best definition of space is “Space is nothing more and nothing less than what rulers measure”.  If you think about some of the properties of space, as described in relativity (both general and special), you find that it has all kinds of properties that a material can’t have.

For example, there’s no difference whatsoever between moving and being stationary.  So, it’s impossible to meaningfully talk about “moving through space”, when you may as well be motionless.  Even worse, you can fit an arbitrarily large amount of space within any volume (as measured from outside that volume).  Think: Dr. Who’s TARDIS.  Or, if that’s not your thing: the diameter of a circle drawn around the volume can be arbitrarily great, while the circumference stays finite.

Point is: space isn’t stuff.  And the universe doesn’t need anything to expand into.

When you picture the universe as a whole it’s almost impossible not to think of a fish bowl or a bubble.

The Universe: Nothing like this.

Implicit in that picture of the universe is an outside.  However, that outside is defined in terms of space, and all of space should be inside the universe.  When you try to talk about the outside of the universe you find yourself asking questions like “okay, where are you?” or “how far from the universe are you?”, you know, the types of questions that really rely on some notion of position and space.

That all being said, there are some theories that do talk about things outside of the universe.  There are some proponents of M-theory who claim that the universe could be a sheet floating in a higher dimensional space, and that there are other universe-sheets floating along side us, just a tiny distance away.  Although the other sheets act exactly like what almost everybody would call “other universes”, it’s would be slightly more accurate to say that the collection of sheets and the higher space they float around in are all part of the same “super-universe”.  They’re still at least a little connected to each other.

Aside: Btw, when physicists want to talk about more than three dimensions they (being born and bred here) like to knock off dimensions to help picture things.  So, if you want to imagine the universe in a higher dimensional space, just get rid of a dimension.  The universe goes from a 3-D volume to a 2-D “sheet”.

The universe may also have “bubbled off” of some other larger universe, or spontaneously started, or who knows.  If there are truly other universes, or any other stuff outside of our universe, it’ll be “causally separated” from everything going on here (or that has or ever will go on here).  Rather than thinking of other universes as being “somewhere else” it’s better to think of them as “in every way independent”.  You can’t even sensibly talk about “going there”.

Science (and more generally: everything we can know), being based on observation, inference, and experience, can’t say much about things entirely outside of the universe.  We can infer, and make some spectacular guesses, but that’s about all.  In another 20 years or so our entire approach to the nature of the universe will have completely changed (it always seems too).

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59 Responses to Q: What is the universe expanding into? What’s outside the universe?

  1. Mushtaq says:

    We human beings are just like ants and ants can not know the milkyway…

  2. David says:

    Having read the above, I can now see why astrophysicists are having problems with dark matter. Space is made of nothing? Why can you not have a perfect vacuum.

  3. Luke Coughlan says:

    Thank you Mister Physicist,

    Very thought proving…
    They talk of of something in the Universe called Dark Matter or Anti Gravity Stuff 🙂
    But may be there are FORCES OUTSIDE OF OUR UNIVERSE that are acting
    on every thing in our Universe, we just can not measure it, YET !


  4. JG Collins says:

    Space like time expands into nothing.

    Yesterday is history tomorrow just a concept.

  5. innocent says:

    Science is now becoming a belief. the space dimensions you observe cannot be changed such as expand or contract it for it is not matter. the term expansion is relative to something else e.g matter expands in space. into what is space expanding? is space contained in something else?
    cosmologists and theoretical physicists formulate their theories based on observations and or abstract using terms which are in contradiction even with common sensation. do not take realism for theories or abstractions. space is x y z cant expand.

  6. Ramesh Verma says:

    Yes, we are definitely ants, but something little bit more superior or sometime less(inferior) that we can understand each other and comments over anything like the ants a tiny creature and about a giant universe because everything within our approach, understanding and power to thing and right to write. We are sometime superior because we can understand the universe, nature’s nature and either we can follow the order or may create chaos, everything is in our approach in our minds but they can’t ..They can do one thing that human can’t do anyway , what this nature exhibit them they follow and they are in a state of order ,perhaps they know the admirable order , perfection of this nature which we human will never know. I have more than that but i can not express my views mere in words but in my feelings.
    thank you all.

  7. Luiz Henriques Alfenas says:

    I think that the universe is infinite, so there is nothing outside it.

  8. omega_sh says:

    Was the older universe similar to ours? I wonder sometimes.

  9. omega_sh says:

    Who am I?
    Why am I here?

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