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What is at the end of the Universe? My question is what is at the end of the universe if it really does end? because if the big bang theory is correct then then when the whole universe was one atom or very small what did it expand into because if there was nothing there it couldn’t have expanded. For example if you have a room and you say that the walls are the end of the universe then you build on to the room to make it bigger (the universe expanding) there has to be room on the other side of
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  What is at the end of the Universe?My question is what is at the end of the universe if it really does end? because if the big bang theory is correct then then when the whole universe was one atom or very small what did it expand into because if there was nothing there it couldn’t have expanded. or example if you have a room and you say that the walls are the end of the universe then you build on to the room to ma!e it bigger the universe expanding# there has to be room on the other side of the wall for you to build into. There is nothing called the end of the Universe. There are three possibilities of the shape of the Universe. First, the Universe might have what we call positive curvature like a sphere. In this case, the Universe is called closed and it has a finite size but without a boundary, ust like a balloon. In a closed Universe, you could, in principle, fly a spaceship far enough in one direction and get back to where you started from. The second possibility is that the Universe is flat. This kind of Universe can be imagined by cutting out a piece of a balloon material and stretching it with your hands. The surface of the material is flat and not curved. !ou can epand and contract it by tugging on either end. Flat Universes are infinite in etent and have no boundaries. Finally, the Universe might be open or have negative curvature. #uch Universes are also infinitein spatial etent and have no boundaries. Thus whatever be the shape of the Universe, there is nothing called a boundary and hence nothing called the edge or end of the Universe. $egarding the second %uestion of epansion, remember that space eists only I& the Universe and there is no meaning to the term outside the Universe . 'hat happens in epansion is that the space itself is epanding. 'ith respect to your room analogy, it is not that the walls of your room are pushing against something but that the space in the room is epanding( there is nothingto push against. Thus, when we talk of galaies receding from us due to the epanion, it is not that the galaies are moving, but the space in between us and the galaies is epanding. Why is it difficult to understand the expansion of the universe? )ne of the most fre%uently asked %uestions at *sk and *stronomer++and one of the most infamously tough to answer++has to do with the epansion of the universe what is at the end of the universe- ecause if the universe is epanding, it must be epanding into  something, right- &o. * number of previous pages /see related %uestions 0 address this %uestion, but I would like to stop for a minute to talk about why it is so tough for us to get our minds around this issue, and why answers that are technically correct can still be unsatisfying. Trying to understand the epansion of the universe is a bit of a parado++because really understanding it involves giving up on ever comprehending it in the same way that you comprehend your everyday physical world. I might tell you to picture the universe as an epanding raisin+cake or a four dimensional balloon /some favorite mental models0, but it ust isn1t 1  possible to grasp the universe in the same intuitive way that you understand, say, the shape of aball in your hand. That doesn1t mean that it is impossible to understand the epansion of the universe, but it does mean that, for most of us, that understanding will be of a different %uality than what we1re used to++it will be grounded in mathematics rather than in physical eperience. $ am very confused about things my science boo! says about the expanding universe. %very boo! $ have seen has defined the universe as everything . $f the universe is expanding what is it expanding into? $t would have to expand into even more universe. $ understand that the red spectrum indicates that things are moving away from us but that is drifting not expanding& right? $f you could help me to understand this& it would be appreciated. 'han! you for your time. This is a very good %uestion which is not at all easy to give a satisfactory answer to2 The first timeI tried to write an answer to this, we got so many follow+up %uestions from people who were still confused that I decided to try to answer it again, this time much more comprehensively. The long eplanation is below. 3owever, if you ust want a short answer, I1ll say this if the universe is infinitely big, then the answer is simply that it isn1t epanding into anything  ( instead, what is happening is that every region of the universe, every distance between every pair of galaies, is being stretched , but the overall size of the universe was infinitely big to begin with and continues to remain infinitely big as time goes on, so the universe1s size doesn1t change, and therefore it doesn1t epand into anything. If, on the other hand, the universe has a finite size, then it may be legitimate to claim that there is something outside of the universe that the universe is epanding into. 3owever, because we are, by definition, stuck within the space that makes up our universe and have no way to observe anything outside of it, this ceases to be a %uestion that can be answered scientifically. #o the answer in that case is that we really don1t know what, if anything, the universe is epanding into. &ow, for those of you who want a more comprehensive discussion 4et me begin by saying that epanding isn1t 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 topic2 * more accurate word for what the universe is doing might be stretching . The difference between epanding and stretching , for me at least, is that an epanding universe conures up an image where there is a bunch of galaies 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 galaies /which we call the universe 0 is epanding, and it is certainlyfair to ask what it is epanding 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 galaies are in some sense stationary + they do not move through space the way that a ball moves through the air. The galaies simply sit there. 3owever, as time goes on, the space between the galaies stretches , sort of like what happens when you take a sheet of rubber and pull at it on both ends. *lthough the galaies haven1t 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. )f course, when we think of space in everyday life, we don1t think of it as something which is capable of stretching. #pace, to us, ust seems like something which is there , and which everything else in the universe eists within. ut according to 5instein1s theory of general relativity, space isn1t really as simple as our common sense tells us. If we want to understand the 2  actual way that the universe functions, we need to find some way to incorporate 5instein1s ideas into our mental picture and imagine space as a more complicated entity which is capable of doingthings 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 eample, there is the analogy with a sheet of rubber /or sometimes a balloon0 that I mentioned above. 6y favorite analogy, though, involves imagining the universe as a gigantic blob of dough. 5mbedded in the dough are a bunch of raisins, spread throughout. The dough represents space, and the raisins represent the galaies. /To the best of my knowledge, this analogy was srcinally proposed by 6artin 7ardner in his 89:;book Relativity for the Million .0 'e have no idea how big the dough is at this point + all we know isthat it is very   big, and we, sitting on some raisin somewhere inside it, are so far away from the edge that the edge can1t possibly have any effect on us or on what we see. &ow, someone puts the dough in the oven and it begins to epand. The raisins move apart from each other, but relative to the dough  they don1t move at all + the same particles of dough that startoff near a particular raisin will always be net to that raisin. That is what I meant when I said that the galaies aren1t really moving through space as the universe epands + here, the raisins aren1t 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 galaies are all moving through space away from some point at the center. * lot of concepts and definitions that seem simple to us in the old picture are much more complicated now. For eample What is the distance between two galaxies?  In the old picture, this is an easy %uestion to answer theoretically /though not necessarily in practice20. <ust get yourself a giant tape measure and clip it to a faraway galay, then come back to our galay and hold on tight. *s the galay 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 epands, because the galaies are not actually moving with respect to each other2 Instead, it will read one billion light+years the whole time. !ou could be perfectly ustified in saying that the distance between the galaies has not changed as time goes on. 'hen 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 allthe 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 galay is two billion light+years away, even though the first tape measure said it was one billion light+years away. *s you can see, the concept of distance in this new picture of the universe is somewhat more complicated than in the old picture2 It is unclear whether the universe as a whole is really epanding + all that we really measure is a stretching of the space between each pair of galaies. /&ote that we might have to have an imaginary tape measure whose atoms aren1t actually being held together by intermolecular forces in order for the scenario described above to actually take place as described.0 /y the way, this analogy of the tape measure is pretty similar to what actually happens to light when it travels between galaies. 'hen light is emitted from one galay and travels through space to another galay, during its trip through space it also  will be stretched, causing it to have alonger wavelength and therefore causing its color to appear more towards the red end of the 3  spectrum. This is what leads us to see red shifted light when we look at faraway galaies, and it ismeasurements of this redshift that allow us to estimate the distances to these galaies.0 Where is the center of the universe?  In the old picture, it is easy to say where the center of the universe is + it1s the point in space that all the galaies are moving away from. In the new picture, though, this isn1t so clear. $emember, the galaies aren1t actually moving away from each other + they1re sitting still2 4et1s go back to the dough analogy + sure, you can imagine that even if the dough is really really big, it has some pointwithin it which is the geometric center. ut this definition is not very useful. #ince the dough represents the space that we live in, we have no way to see outside of the dough to get a senseof the entire shape and figure out where the center is. #o if you are stuck inside the dough, and have no way to see anything ecept the dough, and if you are so far from the edge of the doughthat you can1t see it and it can1t have any effect on you, then what difference do you notice between the point where you1re 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 don1t even think about it. In fact, we can go a step further and imagine that the center isn1t even there at all2 3ow- 'ell, what if instead of ust 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 that1s e%ually in between all of them. #o 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 srcinal %uestion. In our old picture of the universe, the answer would be simple, although very unsatisfying. The collection of galaies that make up the universe is moving through space( therefore, the universe is epanding into even more space than it already encompassed. In our new picture, though, the galaies are ust raisins spread throughout the dough + their presence is largely irrelevant to the %uestion of the universe1s epansion. 'hat 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 epands into . ut for our universe, that is a very complicated %uestion to ask2 The boundary at the edge of the dough represents the edge of space. y definition, we eist within space and have no way to leave it2 #o we don1t think there is any way to observe or measure what is beyond, unless it had some effect on us that we currently don1t know about. It would be really weird to imagine reaching the end of space. 'hat would it look like, for eample- These are %uestions that we have no way to give a scientific answer to, so the simple answer is that we don1t know2 *ll 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. )bservations 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 can1t currently see it and it doesn1t have any effect on us. If the universe is indeed infinite, then the simple answer to the srcinal %uestion is that the universe doesn1t have anything   to epand into. Thinking about infinity is always complicated, but a good analogy can be made with simple math. Imagine you have a list of numbers 8, ;, =, etc., all the way up to infinity. Then you multiply every number in this list by ;, so that you now have ;,>,:,etc., all the way up to infinity. The distance between adacent number in your list has stretched /it is now ; instead of 80, but can you really say that the total etent of all your numbers has epanded - !ou started off with numbers that went up to infinity, and you finished 4
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