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

  1. John says:

    It’s funny that you posted this today, with the example of explaining gravity in terms of rubber as xkcd posted a comic about this today.

  2. The Physicist The Physicist says:

    Clearly, someone around here is psychic!

  3. christian smith says:

    sorry to bug you guys again, but what other options do we have to define stuff in besides space?

  4. The Physicist The Physicist says:

    “In” implies space. So the options are kinda limited.

  5. I_am_Awesome says:

    well, um there has to be a start of the universe, which means there was an OUTSIDE of the universe right? there can’t just be NOTHING its impossible…

  6. screen says:

    Based on our way of thinking one would surmise that there would have to be an outside to the universe. But at the same time one must wonder what this outside would look or feel like; and how possible is it for this outside to continue on indefinately. The number “0” has a value of nothing, but it is still something… it’s property is still “zero.”

  7. Burnt match, says:

    Yep, whilst this remains a enigma the major Faiths have every right to hold on to the Old testament God.
    The eternal is one,that belief has never changed,but physics moves the goal posts on a whim.?
    Lincoln was right,about intelligent foolers,

  8. Alan McDougall says:

    The universe did not emerge into an empty void , it brought the void with it and we call that void spacetime

  9. Knook says:

    The answers to these and other questions are often overlooked because humans tend to apply rules when seeking answers to the unknown. Who says the unknown follows rules?

  10. John Robert Size says:

    I think Knook has hit it on the head it is very unlikely that the unknown, that we are such a very small part of would follow the rules our scientists logically discover, no matter how great their scientific achievements are because we think and reason logically while the unknown just acts.

  11. Pratik Singh says:

    If the universe came into existence after big bang , where did the big bang took place and what was there before big bang . where is are universe ? this is biggest question in my mind.

  12. Zlat says:

    There was an article recently claiming there might be proof that the universe is a computer simulation and we’re all living in it. Wouldn’t in that case the space be a function of software, an abstract ‘space’ accessible inside the program, while anything outside it would be unknowable (though still abstract)?

  13. Jon Hammarck says:

    If we define the universe as something that contains everything that exists (space, time & matter) the answer to the question of what exists out side of the universe is ´not the universe. Its the same answer as to what exists out side of your self. The notion of ´you contains everything that you are made of, and is limited to the outmost extent of ‘you. Everything out side of that boundary is, in fact, ‘not you.

  14. Rob N says:

    Interesting topic. I think not only should we be studying the forces of physics but also how our minds work and how we perceive the commotion going on around us. What we “feel” to be the truth out there will never be verified. We will only get an idea of how things work by studying what we possibly can observe. Everything else is part of the answer that we can never piece together. For there is no answer. “Laws” change Infinitely as we reach a limit of observation. Imagine how large an atom would be without any one there to observe it. The very fact that we exist and give things “size” and label things is part of the problem.

  15. Liam says:

    There is another theory known as “The big rip” where basically, the expansion of the universe will accelerate to a point where ever atom in it will split and explode. What if this is just the same universe recycling itself over and over and the big bang is just the explosion of the atoms ripping?

    I’m certainly no physicist and I don’t have much knowledge on this sort of thing. Im just a curious 12 year old.

  16. knowitall says:

    This universe or big bang, is nothing more than a local event in an infinitely large universe.

  17. Joshua says:

    So far, I am going with the Simulation. It seems to answer the most while fitting in with the facts.

    Lets pretend that we have a Hard drive. On the hard drive is a bunch of Information.

    -When you play a video game, you may feel that the universe around you is massive.
    -You could make an object as small as you want and the universe as large as you want.

    In the video game, your character is moving. Lets say you are on a quest to go leave and go 600 Galaxies away and talk to some guy on some planet. So You get into a ship and begin flying there. It takes you about 10 hours to get to your location.
    You then think… “Holy crap that was a long trip and this game sucks”.

    But did you know, that every single atom and everything in existence in that video game is actually just some information stored on the hard drive? In fact… None of the information on the Hardrive actually goes anywhere. All of it is right next to each other. Nothing is moving… It is all just a n illusion based off your experience and your limitations in the video game.

    Right now, you may think all of these letters in this sentence is spaced out, but actually the information that creates all of this is technically in the same spot on the harddrive and the info is simply taken and used to portray an illusion of distance.

    So how does this help knowing this exactly? Well, this would explain how protons are able to somehow communicate with each other despite their distance from each other. This would explain how the universe was created, even from the start of it. This would even support the idea of an expanding universe. The video game illusion of an expanding universe is only a fraction of the space that it actually takes up on a harddrive. You could have a Harddrive the size of 9999999999999999 and the Expanding universe would simply only take up 0000.1% of that every second. (assuming the Harddrive in this scenario has a limited space)

    This would even support Religious ideas. Like, how come god does not help anyone or how come god does not fight his own battles, or how come we should not be afraid of death constantly? If you think about it, this may just simply be a simulation preparing people for the real universe. Why? My guess would be that you could be immortal and they cannot afford to just have untrained immortal running about. Who knows, any way, that is off topic. lol

    So thinking about it, you could technically jump from one space (earth) to another space on the other side of the universe immediately since the information on the the Harddrive is right next to each other. But sadly, we are still limited to the game’s rules and would need to find a way of how to manipulate the program to bend our limitations and figure out how to “map change”. ;)

    Cons? Well… there is a couple I thought off.
    #1. This in no way explains the same on going questions about the “real” universe.

    #2. Thinking about an infinite harddrive is the same concept of thinking about an expanding universe which would make no sense.. (unless the universe is not infinite. Just large enough for the simulation. maybe it loops? lol)

    #3. Everything is too easy to explain with a computer simulation.

    #4. Proving we live in a computer simulation would be tough… Unless we can prove it by creating our own universe and showing how it is doen. :D THEN we will have an infinite loop of THOSE people making another computer universe just to prove they live in a computer simulation which drives THOSE simulated people to create a simulation to prove they live in computer simulation—->>>>

    But the meaning of life?… Well.. The first beings made a computer simulation and some troll wanted to see how many computer simulated universe would keep going and going and going. So there is no meaning.. just some dam Troll.


  18. Lucy says:

    Will we ever see past the current observable universe if the universe is expanding and everything is moving away from everything else?

  19. The Physicist The Physicist says:

    As light has a chance to travel farther the visible universe becomes larger, and the expansion itself means that the farthest (and oldest) things are farther away now then they were.

  20. Elise says:

    If a person were to cross the boundary between the observable universe and…. whatever is beyond, would that person simply be a continuation of the universe, or simply cease to exist due to the lack of… well anything? If the observable universe is all there is and possibly all there ever will be, should the “space” outside even be considered “space”? Theoretically, would time even exist outside of the known universe?

  21. Vangrab says:

    What you don’t realize is that universe and the big bang cannot be created out of nothing, and universe cannot expand in nothing-what you’re trying to say is that the 2d universe is created from dimensionlessness-that is absolutely impossible, since universe has size, and something that has size and volume cannot exist in nothing-where size and diameter is 0!
    Do you have any idea what are you talking about?
    This is not science, it’s more like pseudo-science.
    Universe and everything in it all the energy mass and everything else, cannot expand if there is not some kind of space outside of the universe.
    The energy of the big bang also came from something that requires space, because that energy has size and volume, this is not some kind of energy with zero dimension, zero diameter, and zero size.
    Space does exist even without objects, because let’s suppose you start to move, you would get the feeling you’re not moving anywhere/nowhere-but the fact remains you’re moving, if there was no space around where you can actually move there is no way will be able to move at all-physicists really don’t get this do they…
    Your interpretation is about the universe and all those models surely show the universe so beautiful and everything, but whenever I ask what’s outside of this, they say nothing, if that was the case there would never be any big bang or the universe or the model of the universe-it is some kind of space-that is 100& sure and 100% proven.
    The more precise statement is that the big bang was created in the universe and created everything else, we have today-if you want to go this way.
    But the problem is the big bang hypothesis is totally unprovable.

  22. Vangrab says:

    Outside the universe is space, empty or not.

  23. T.G. says:

    I always liked the idea of what Einstein seemed to propose which was that the universe is unbounded but finite. By which I think he meant there’s no inside or outside to space. But, at the same time, being a physical entity and not a “nothingness” it cannot be infinite.

    Therefore, space, which is everywhere, is also limited by it’s curvature bending it back upon itself. So a traveler starting out in straight line would one day return to their starting position while never encountering a boundary or wall anywhere.

    But today, now that the microwave background has been imaged, scientists are saying the universe is probably flat, and therefore is infinite. To which I’d say the only way for me to imagine that would be an eternally expanding universe that by definition would be infinite. But without it, can it be infinite?

  24. Mike says:

    Could it be that our universe is in orbit, along with other universe’s, around a colossal black hole?

  25. Dr. VARUN S. RAJ(mensa and torr member) says:

    Let me begin by saying that “expanding” isn’t really the best word to describe what is happening to the universe, although that is the word that is often used – a word choice which I think leads to a lot of unnecessary confusion regarding what is already a difficult topic! A more accurate word for what the universe is doing might be “stretching”.Think of a big black balloon with white spots on it.

    The difference between “expanding” and “stretching”, for me at least, is that an “expanding universe” conjures up an image where there is a bunch of galaxies floating through space, all of which started at some center point and are now moving away from that point at very fast speeds. Therefore, the collection of galaxies (which we call the “universe”) is expanding, and it is certainly fair to ask what it is expanding into.

    The current theories of the universe, however, tell us that this is not the picture we should have in mind at all. Instead, the galaxies are in some sense stationary – they do not move through space the way that a ball moves through the air. The galaxies simply sit there. However, as time goes on, the space between the galaxies “stretches”, sort of like what happens when you take a sheet of rubber and pull at it on both ends. Although the galaxies haven’t moved through space at all, they get farther away from each other as time goes on because the space in between them has been stretched.

    Of course, when we think of space in everyday life, we don’t think of it as something which is capable of stretching. Space, to us, just seems like something which is there, and which everything else in the universe exists within. But according to Einstein’s theory of general relativity, space isn’t really as simple as our common sense tells us. If we want to understand the actual way that the universe functions, we need to find some way to incorporate Einstein’s ideas into our mental picture and imagine space as a more complicated entity which is capable of doing things like “bending” and “stretching”.

    To help us imagine this, a lot of people have come up with analogies for the universe in which space is represented by something more tangible. For example, there is the analogy with a sheet of rubber (or sometimes a balloon) that I mentioned above. My favorite analogy, though, involves imagining the universe as a gigantic blob of dough. Embedded in the dough are a bunch of raisins, spread throughout. The dough represents space, and the raisins represent the galaxies. (To the best of my knowledge, this analogy was originally proposed by Martin Gardner in his 1962 book Relativity for the Million.) We have no idea how big the dough is at this point – all we know is that it is very big, and we, sitting on some raisin somewhere inside it, are so far away from the “edge” that the edge can’t possibly have any effect on us or on what we see.

    Now, someone puts the dough in the oven and it begins to expand. The raisins move apart from each other, but relative to the dough they don’t move at all – the same particles of dough that start off near a particular raisin will always be next to that raisin. That is what I meant when I said that the galaxies aren’t really moving through space as the universe expands – here, the raisins aren’t moving through the dough, but the distance between the raisins is still getting larger.

    This new picture of the universe which I am asking you to imagine is, on a practical level, much different from the old picture in which the galaxies are all moving through space away from some point at the center. A lot of concepts and definitions that seem simple to us in the old picture are much more complicated now. For example:

    What is the distance between two galaxies? In the old picture, this is an easy question to answer theoretically (though not necessarily in practice!). Just get yourself a giant tape measure and clip it to a faraway galaxy, then come back to our galaxy and hold on tight. As the galaxy moves away, it will pull on the tape measure, and you will easily be able to read off the distance as the tape measure unwinds… one billion light-years, one and half billion light-years, two billion light-years, etc.

    In our new picture of the universe, however, with the raisins and the dough, the tape measure will not unwind at all as the universe expands, because the galaxies are not actually moving with respect to each other! Instead, it will read one billion light-years the whole time. You could be perfectly justified in saying that the distance between the galaxies has not changed as time goes on. When you bring the tape measure back in, however, you will notice something unusual; due to the stretching of space, your tape measure will have stretched as well, and if you compare it to an identical tape measure which you had sitting in your pocket the entire time, you will see that all the tick marks on it are twice as far apart as they used to be. Using the tape measure from your pocket as a reference, you would now say that the galaxy is two billion light-years away, even though the first tape measure said it was one billion light-years away. As you can see, the concept of “distance” in this new picture of the universe is somewhat more complicated than in the old picture! It is unclear whether the universe as a whole is really “expanding” – all that we really measure is a stretching of the space between each pair of galaxies. (Note that we might have to have an “imaginary” tape measure whose atoms aren’t actually being held together by intermolecular forces in order for the scenario described above to actually take place as described.)

    (By the way, this analogy of the tape measure is pretty similar to what actually happens to light when it travels between galaxies. When light is emitted from one galaxy and travels through space to another galaxy, during its trip through space it also will be stretched, causing it to have a longer wavelength and therefore causing its color to appear more towards the red end of the spectrum. This is what leads us to see redshifted light when we look at faraway galaxies, and it is measurements of this redshift that allow us to estimate the distances to these galaxies.)

    Where is the center of the universe? In the old picture, it is easy to say where the center of the universe is – it’s the point in space that all the galaxies are moving away from. In the new picture, though, this isn’t so clear. Remember, the galaxies aren’t actually moving away from each other – they’re sitting still! Let’s go back to the dough analogy – sure, you can imagine that even if the dough is really really big, it has some point within it which is the geometric center. But this definition is not very useful. Since the dough represents the space that we live in, we have no way to see “outside” of the dough to get a sense of the entire shape and figure out where the center is. So if you are stuck inside the dough, and have no way to see anything except the dough, and if you are so far from the “edge” of the dough that you can’t see it and it can’t have any effect on you, then what difference do you notice between the point where you’re at and the point that is actually at the geometric center of the entire blob of dough? The answer is that there is no difference, absolutely none. The concept of the “center of the universe” loses all meaning, so we don’t even think about it.

    In fact, we can go a step further and imagine that the center isn’t even there at all! How? Well, what if instead of just being really really big, the dough were infinitely big – that is, you could walk forever in a straight line and never reach a place where the dough ends. In that case, there really would be no center of the universe – the only way you can define the center is to mark out the edges and find the point that’s equally in between all of them. So if the universe is infinitely big and has no edges, then it also has no center, not even on a theoretical level.

    What does the universe expand into? Finally, we can return to the original question. In our old picture of the universe, the answer would be simple, although very unsatisfying. The collection of galaxies that make up the universe is moving through space; therefore, the universe is expanding into even more space than it already encompassed. In our new picture, though, the galaxies are just raisins spread throughout the dough – their presence is largely irrelevant to the question of the universe’s expansion. What we really care about is the dough, and whether or not it has a boundary.

    If the dough does have a boundary, then it is legitimate to ask what is beyond the boundary that the dough expands “into”. But for our universe, that is a very complicated question to ask! The boundary at the edge of the dough represents the “edge” of space. By definition, we exist within space and have no way to leave it! So we don’t think there is any way to observe or measure what is beyond, unless it had some effect on us that we currently don’t know about. It would be really weird to imagine reaching the “end” of space. What would it look like, for example? These are questions that we have no way to give a scientific answer to, so the simple answer is that we don’t know! All we do know is that based on our current understanding of theoretical cosmology, the universe does not have a boundary – it is either infinite or it wraps around itself in some way. Observations seem to agree with these predictions in the sense that if the universe does have a boundary, we know that the boundary is so far away from us that we can’t currently see it and it doesn’t have any effect on us.

    If the universe is indeed infinite, then the simple answer to the original question is that the universe doesn’t have anything to expand into. Thinking about infinity is always complicated, but a good analogy can be made with simple math. Imagine you have a list of numbers: 1,2,3,etc., all the way up to infinity. Then you multiply every number in this list by 2, so that you now have 2,4,6,etc., all the way up to infinity. The distance between adjacent number in your list has “stretched” (it is now 2 instead of 1), but can you really say that the total extent of all your numbers has “expanded”? You started off with numbers that went up to infinity, and you finished with numbers that went up to infinity. So the total size is the same! If these numbers represent the distances between galaxies in an infinite universe, then it is a good analogy for why the universe does not necessarily expand even though it stretches.

    Finally, I should point out that not everything in the universe is “stretching” or “expanding” in the way that the spaces between faraway galaxies stretch. For example, you and I aren’t expanding, the Earth isn’t expanding, the sun isn’t expanding, even the entire Milky Way galaxy isn’t expanding. That’s because on these relatively small scales, the effect of the universe’s stretching is completely overwhelmed by other forces (i.e. the galaxy’s gravity, the sun’s gravity, the Earth’s gravity, and the atomic forces which hold people’s bodies together). It is only when we look across far enough distances in the universe that the effect of the universe’s stretching becomes noticeable above the effects of local gravity and other forces which tend to hold things together. (That is why, in the analogy of the tape measure I discussed above, the tape measure that you keep in your pocket does not get stretched, while the one that goes between two galaxies does get stretched. I bet some people were wondering about that!)

  26. slick rick says:

    we all know an object takes up space but what is space exactly? is space infinite? Furthermore where did the big bang come from etc. It seems to me the universe came out of nowhere but it exists. Therefore could nothingness be everything. like if you had a infinite set of positive numbers and negative numbers and you added them together, 1+ (-1)=0 and 2+(-2)=0 and as you tended to negative and positive infinity you would get an infinite number of zeros. I think i went a bit off topic but existence itself seemed to just come out of nowhere. I think our universe and existence itself could just be a product of spontaneous generation don’t you think?

  27. Joey says:

    Remember what the people said god is infinite god must have created the big bang but the real question is why is there nothing outside the universe blank ” 0?

  28. Pingback: Q: How can the universe expand faster than the speed of light? | Ask a Mathematician / Ask a Physicist

  29. Peter says:

    If the universe and everything with it is expanding does this mean we our earth are expanding as well? Can we measure this as our tools are also expanding under the same influence.

  30. The Physicist The Physicist says:

    Nope! The distance between things increases, but things themselves don’t. There’s an old post that talks about it here.

  31. Colin says:

    Does it make sense to say that, beyond the furthest particles from the Big Bang, there is no mass, no space, no time? So “universe” is perpetually coming into being at these “edges” ?

  32. Jon says:

    You Guys are all talking about,the universe,limits,space etc. In English.
    If one uses the actual language of mathematics ( which is Not 2+2=4 but concepts)
    one is immediately struck by the impossibility and real meaninglessness of translation of observed or theorized reality (in the discussed subject) to spoken language.
    Try understanding Wittgenstein’s Tractatus before contributing to more noise on the web. “The limits of my language are the limits of my world”.
    Try defining silence without breaking it.

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