caharris Posted June 4, 2011 Share Posted June 4, 2011 I consider myself to be fairly up-to-date when it comes to astronomy and cosmology, given that I've never taken an official class on either subjects. I read the popular science books, and I have a few text books about it. So the basic concepts of how the universe works are things that I decently understand. I was having a conversation with a person the other day about the center of the universe. I tried to explain how the universe is isotropic and homogeneous, and how we observe space expanding, and how this suggests that there is no center or middle of the universe. However, he insisted that the expansion of the universe and how the things inside it have nothing to do with whether there is a center of the universe or not. He said that when he means center, he is not talking about a source or origin, but 'middle.' I've used the balloon analogy, I've used FAQs from Sean Carroll and Ned Wright, I used part of a lecture from Lawrence Krauss... Yet, either something isn't clicking with this guy, or I just completely misunderstand the question. So I thought I'd ask you guys (and gals). I'm thinking the reason he might not understand is he is thinking of the universe as if it were some shape and we just live inside of it (e.g. fish living in water). So the question is, What does it mean when physicists say there is no center of the universe? Thank you for your time, caharris Link to comment Share on other sites More sharing options...
caharris Posted June 4, 2011 Author Share Posted June 4, 2011 I consider myself to be fairly up-to-date when it comes to astronomy and cosmology, given that I've never taken an official class on either subjects. I read the popular science books, and I have a few text books about it. So the basic concepts of how the universe works are things that I decently understand. I was having a conversation with a person the other day about the center of the universe. I tried to explain how the universe is isotropic and homogeneous, and how we observe space expanding, and how this suggests that there is no center or middle of the universe. However, he insisted that the expansion of the universe and how the things inside it have nothing to do with whether there is a center of the universe or not. He said that when he means center, he is not talking about a source or origin, but 'middle.' I've used the balloon analogy, I've used FAQs from Sean Carroll and Ned Wright, I used part of a lecture from Lawrence Krauss... Yet, either something isn't clicking with this guy, or I just completely misunderstand the question. So I thought I'd ask you guys (and gals). I'm thinking the reason he might not understand is he is thinking of the universe as if it were some shape and we just live inside of it (e.g. fish living in water). So the question is, What does it mean when physicists say there is no center of the universe? Thank you for your time, caharris Link to comment Share on other sites More sharing options...
Ophiolite Posted June 5, 2011 Share Posted June 5, 2011 I doub my understanding or reading in this area is much different from yours, so let the blind lead the blind. In a sense everywhere is the centre, since the universe is thought to have expanded from an incredibly dense 'thing' that was almost a point. Since it probably has no edge then how can it have a centre. Your balloon analogy holds. Show your friend a square sheet of paper and ask him to point to the centre of the surface of the paper. Then ask him to do the same thing for the surface of the balloon. Now tell him imagine that instead of a two dimesional surface there is a three dimensional space. When he says he can't note that few people can: we didn't need that kind of ability when we roamed the savanna. Link to comment Share on other sites More sharing options...
DrRocket Posted June 5, 2011 Share Posted June 5, 2011 So the question is, What does it mean when physicists say there is no center of the universe? caharris Turn the question around, and ask what a center would mean ? Link to comment Share on other sites More sharing options...
caharris Posted June 5, 2011 Author Share Posted June 5, 2011 Well I think the way that he is picturing the universe is like we were inside of a sphere or something. He thinks that because the universe may be finite, that this means the universe has a center, or middle, like a sphere has a radius. I don't know how much it will help to use the balloon analogy, because he thinks the expansion of the universe is irrelevant to whether there's a center, sort of like if you lengthen the radius of a sphere there is still a center. Link to comment Share on other sites More sharing options...
BJC Posted June 5, 2011 Share Posted June 5, 2011 I consider myself to be fairly up-to-date when it comes to astronomy and cosmology, given that I've never taken an official class on either subjects. I read the popular science books, and I have a few text books about it. So the basic concepts of how the universe works are things that I decently understand. I was having a conversation with a person the other day about the center of the universe. I tried to explain how the universe is isotropic and homogeneous, and how we observe space expanding, and how this suggests that there is no center or middle of the universe. However, he insisted that the expansion of the universe and how the things inside it have nothing to do with whether there is a center of the universe or not. He said that when he means center, he is not talking about a source or origin, but 'middle.' I've used the balloon analogy, I've used FAQs from Sean Carroll and Ned Wright, I used part of a lecture from Lawrence Krauss... Yet, either something isn't clicking with this guy, or I just completely misunderstand the question. So I thought I'd ask you guys (and gals). I'm thinking the reason he might not understand is he is thinking of the universe as if it were some shape and we just live inside of it (e.g. fish living in water). So the question is, What does it mean when physicists say there is no center of the universe? Thank you for your time, caharris There is no center if there is no edge. The balloon analogy is okay providing you realize the balloon is a 3 dimensional object whereas the universe is 4 dimensional. The radius is time, the surface of the balloon 2 dimensions of space, say length and width, the 3rd dimension cannot be shown. In fact it is almost impossible to imagine a 4 dimensional object - the "surface of the balloon" is a 3D volume, not a 2D plane. Another problem with the analogy is "the sphere" The shape of the universe is not strictly known. Link to comment Share on other sites More sharing options...
DrRocket Posted June 5, 2011 Share Posted June 5, 2011 (edited) I don't know how much it will help to use the balloon analogy, because he thinks the expansion of the universe is irrelevant to whether there's a center, sort of like if you lengthen the radius of a sphere there is still a center. The expansion of the universe IS irrelevant to whether there is a center. What is relevant is that the model of the universe is a homogeneous and isotropic intrinsic manifold, not embedded in anything larger. In that context the word "center" is meaningless. Edited June 5, 2011 by DrRocket Link to comment Share on other sites More sharing options...
michel123456 Posted June 5, 2011 Share Posted June 5, 2011 (...) since the universe is thought to have expanded from an incredibly dense 'thing' that was almost a point. (...) There is the problem. The word "point" makes him think of something very small embedded into something else because that's the way we learn a point is in geometry. Now you must explain to your friend that this very small thing: _is not embedded into anything _has no edge _has no center _and that we look at the remnants of this very small thing all around us at billions and billions of LY of distance. Good luck. Link to comment Share on other sites More sharing options...
JCP Posted June 5, 2011 Share Posted June 5, 2011 (edited) When physicists say that there is no center of the universe, they mean that there is no point out in space that they can refer to as "the center of expansion". Because all the evidence suggests that all the galaxies are moving away from each other as time goes on. There isn't much that is said about the actual center of the universe, if there is one. Current theories suggest that the universe is flat, and therefore must be infinite in all directions. So, for all intents and purposes, there shouldn't be a center of the universe. A quote from the link: We now know that the universe is flat with only a 0.5% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe. Edited June 5, 2011 by JCP Link to comment Share on other sites More sharing options...
DrRocket Posted June 5, 2011 Share Posted June 5, 2011 When physicists say that there is no center of the universe, they mean that there is no point out in space that they can refer to as "the center of expansion". Because all the evidence suggests that all the galaxies are moving away from each other as time goes on. There isn't much that is said about the actual center of the universe, if there is one. Current theories suggest that the universe is flat, and therefore must be infinite in all directions. So, for all intents and purposes, there shouldn't be a center of the universe. A quote from the link: Faltness "within 0.5%" is inadequate to discriminate between a small positive curvature, a small negative curvature, or zero curvature, and hence tells you nothing about whether the universe is finite or infinite. Under the assumption of homogeneity and isotropy, positive curvature implies a spherical geometry, negative curvature implies a hyperbolic geometry and zero curvature implies a Euclidean topology. The first is finite, the latter two are infinite. If the requirement of global isotropy is relaxed then there are other possibilities. One that is seriously considered is a flat torus (sometimes called "Pac Man space" in popularizations). This is a flat compact (finite) manifold, topologically a 3-dimensional generalization of a donut.. You will find all sorts of incorrect and misleading statements regarding the possibilities for the topology of space, and what that means, in the literature. The reason that it is said that the universe has no "center" is because the universe appears to be homogeneous and because there is no meaning to "center'. Remember that the big bang was not an explosion in space but an explosion of space. Link to comment Share on other sites More sharing options...
michel123456 Posted June 6, 2011 Share Posted June 6, 2011 (edited) What is going on here? Edited June 6, 2011 by michel123456 Link to comment Share on other sites More sharing options...
michel123456 Posted June 6, 2011 Share Posted June 6, 2011 (edited) What is the difference with this thread? I consider myself to be fairly up-to-date when it comes to astronomy and cosmology, given that I've never taken an official class on either subjects. I read the popular science books, and I have a few text books about it. So the basic concepts of how the universe works are things that I decently understand. I was having a conversation with a person the other day about the center of the universe. I tried to explain how the universe is isotropic and homogeneous, and how we observe space expanding, and how this suggests that there is no center or middle of the universe. However, he insisted that the expansion of the universe and how the things inside it have nothing to do with whether there is a center of the universe or not. He said that when he means center, he is not talking about a source or origin, but 'middle.' I've used the balloon analogy, I've used FAQs from Sean Carroll and Ned Wright, I used part of a lecture from Lawrence Krauss... Yet, either something isn't clicking with this guy, or I just completely misunderstand the question. So I thought I'd ask you guys (and gals). I'm thinking the reason he might not understand is he is thinking of the universe as if it were some shape and we just live inside of it (e.g. fish living in water). So the question is, What does it mean when physicists say there is no center of the universe? Thank you for your time, caharris Posted Yesterday by Ophiolite, 02:41 AM I doub my understanding or reading in this area is much different from yours, so let the blind lead the blind. In a sense everywhere is the centre, since the universe is thought to have expanded from an incredibly dense 'thing' that was almost a point. Since it probably has no edge then how can it have a centre. Your balloon analogy holds. Show your friend a square sheet of paper and ask him to point to the centre of the surface of the paper. Then ask him to do the same thing for the surface of the balloon. Now tell him imagine that instead of a two dimesional surface there is a three dimensional space. When he says he can't note that few people can: we didn't need that kind of ability when we roamed the savanna. Posted Yesterday by Dr Rocket, 02:51 AM Turn the question around, and ask what a center would mean ? Posted Yesterday by caharris, 03:27 AM Well I think the way that he is picturing the universe is like we were inside of a sphere or something. He thinks that because the universe may be finite, that this means the universe has a center, or middle, like a sphere has a radius. I don't know how much it will help to use the balloon analogy, because he thinks the expansion of the universe is irrelevant to whether there's a center, sort of like if you lengthen the radius of a sphere there is still a center. Posted Yesterday by Dr Rocket, 04:22 AM There is no center if there is no edge. The balloon analogy is okay providing you realize the balloon is a 3 dimensional object whereas the universe is 4 dimensional. The radius is time, the surface of the balloon 2 dimensions of space, say length and width, the 3rd dimension cannot be shown. In fact it is almost impossible to imagine a 4 dimensional object - the "surface of the balloon" is a 3D volume, not a 2D plane. Another problem with the analogy is "the sphere" The shape of the universe is not strictly known. Posted Yesterday, 04:33 AM The expansion of the universe IS irrelevant to whether there is a center. What is relevant is that the model of the universe is a homogeneous and isotropic intrinsic manifold, not embedded in anything larger. In that context the word "center" is meaningless. Posted Yesterday by Michel123456, 07:51 AM There is the problem. The word "point" makes him think of something very small embedded into something else because that's the way we learn a point is in geometry. Now you must explain to your friend that this very small thing: _is not embedded into anything _has no edge _has no center _and that we look at the remnants of this very small thing all around us at billions and billions of LY of distance. Good luck. Edited June 6, 2011 by michel123456 Link to comment Share on other sites More sharing options...
swansont Posted June 6, 2011 Share Posted June 6, 2011 What is going on here? ! Moderator Note Duplicate threads were (presumably inadvertently) started by caharris. They have been merged. Link to comment Share on other sites More sharing options...
michel123456 Posted June 6, 2011 Share Posted June 6, 2011 (edited) Opening parenthesis This was not a duplicated thread started by caharris. This morning, there was a thread named Middle of the universe under "todays posts" and a twin brother thread under "my content" on my pc, with different content. I presume some moderator deleted inadventerly some posts. Parenthesis closed. Edited June 6, 2011 by michel123456 Link to comment Share on other sites More sharing options...
swansont Posted June 6, 2011 Share Posted June 6, 2011 Opening parenthesis This was not a duplicated thread started by caharris. This morning, there was a thread named Middle of the universe under "todays posts" and a twin brother thread under "my content" on my pc, with different content. I presume some moderator deleted inadventerly some posts. Parenthesis closed. ! Moderator Note No, there were two identically titled threads, with identical opening posts. These two posts are now here in this thread — see the first two posts. As they have the same time stamp, I assume it was a glitch that created the second thread. I merged the threads. No posts were deleted. Link to comment Share on other sites More sharing options...
StringJunky Posted June 6, 2011 Share Posted June 6, 2011 There is the problem. The word "point" makes him think of something very small embedded into something else because that's the way we learn a point is in geometry. Now you must explain to your friend that this very small thing: _is not embedded into anything _has no edge _has no center _and that we look at the remnants of this very small thing all around us at billions and billions of LY of distance. Good luck. Probably the notion of the universe being a very small point should be got rid of and just say it was much smaller and denser than it is now...it's better to express its earlier state in a relative way. The idea of it being an infinitely small and dense point signifies a breakdown in GR apparently anyway. Link to comment Share on other sites More sharing options...
caharris Posted June 6, 2011 Author Share Posted June 6, 2011 ! Moderator Note No, there were two identically titled threads, with identical opening posts. These two posts are now here in this thread — see the first two posts. As they have the same time stamp, I assume it was a glitch that created the second thread. I merged the threads. No posts were deleted. Yes I think it was a glitch. I was unaware that two threads were created, sorry for the inconvenience. As for everyone else, thank you for the explanations, and I will let you know how any further explanation goes. Link to comment Share on other sites More sharing options...
Airbrush Posted June 6, 2011 Share Posted June 6, 2011 Probably the notion of the universe being a very small point should be got rid of and just say it was much smaller and denser than it is now...it's better to express its earlier state in a relative way. The idea of it being an infinitely small and dense point signifies a breakdown in GR apparently anyway. That's what I had been thinking. Why do the experts suppose the universe originated from a tiny point smaller than a proton? I think it would make more sense to say the universe originated from a region of indeterminable size and kept expanding. Also, how can they be so sure that inflation happened only once for only a tiny fraction of a second, before the universe was larger than a grapefruit? Link to comment Share on other sites More sharing options...
caharris Posted June 7, 2011 Author Share Posted June 7, 2011 Also, how can they be so sure that inflation happened only once for only a tiny fraction of a second, before the universe was larger than a grapefruit? I might be able to answer this, actually. (And if not, I have Guth's book, so I could look it up upon further inquiry.) The universe would have started in a false vacuum in the Higgs field, which is where it got the negative pressure, and thus repulsive gravity. After a certain point, though, the Higgs field would have changed values (to the current energy density, that of the vacuum), and so inflation wouldn't occur again. Of course, I could have just explained that terribly, so who knows if that makes any sense to you. Link to comment Share on other sites More sharing options...
DrRocket Posted June 7, 2011 Share Posted June 7, 2011 I might be able to answer this, actually. (And if not, I have Guth's book, so I could look it up upon further inquiry.) The universe would have started in a false vacuum in the Higgs field, which is where it got the negative pressure, and thus repulsive gravity. After a certain point, though, the Higgs field would have changed values (to the current energy density, that of the vacuum), and so inflation wouldn't occur again. Of course, I could have just explained that terribly, so who knows if that makes any sense to you. In Guth's original theory of inflation he thought that the inflation field was the Higgs field. It was later determined by others that the Higgs field cannot fulfill the role of the inflation field. As of now the inflation field is a separate scalar field of unknown origin, assuming that it exists at all. Link to comment Share on other sites More sharing options...
zapatos Posted June 7, 2011 Share Posted June 7, 2011 Turn the question around, and ask what a center would mean ? Perhaps the 'center' means the center of gravity of a grouping? I imagine that the center of our solar system would be considered the sun. And I think we have a good idea where the center of our galaxy is. That makes me think we can point to the location of the center of our local group and the supercluster we are part of. Is it wrong to think that we could continue the process (assuming we had enough data to do so) of picking the center of ever larger groupings until we have identified the center of the universe? At what point does this process break down? Is my confusion over understanding why the 'center' of the universe is not meaningful likely because I don't understand things like isotropic intrinsic manifolds? Link to comment Share on other sites More sharing options...
csmyth3025 Posted June 7, 2011 Share Posted June 7, 2011 Perhaps the 'center' means the center of gravity of a grouping? I imagine that the center of our solar system would be considered the sun. And I think we have a good idea where the center of our galaxy is. That makes me think we can point to the location of the center of our local group and the supercluster we are part of. Is it wrong to think that we could continue the process (assuming we had enough data to do so) of picking the center of ever larger groupings until we have identified the center of the universe? At what point does this process break down? Is my confusion over understanding why the 'center' of the universe is not meaningful likely because I don't understand things like isotropic intrinsic manifolds? Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction—that is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the universe as a whole. Every location in the universe has its own observable universe which may or may not overlap with the one centered on the Earth... (ref. http://en.wikipedia....rvable_universe ) Please note that we can't tell how far the universe extends beyond our own observable universe. The main point is that even in our own observable universe we can go a couple of hundred galaxies away in any direction and, from that vantage point, everything would appear to be moving away just as we see it here. From that distant galaxy they would see part of our own observable universe and part of the universe that is not observable by us. If every place you go it appears that you are in the center of your observable universe, how do you tell where the center of the universe is? (I know that this is a pretty unscientific explanation) Chris Link to comment Share on other sites More sharing options...
zapatos Posted June 7, 2011 Share Posted June 7, 2011 (ref. http://en.wikipedia....rvable_universe ) Please note that we can't tell how far the universe extends beyond our own observable universe. The main point is that even in our own observable universe we can go a couple of hundred galaxies away in any direction and, from that vantage point, everything would appear to be moving away just as we see it here. From that distant galaxy they would see part of our own observable universe and part of the universe that is not observable by us. If every place you go it appears that you are in the center of your observable universe, how do you tell where the center of the universe is? (I know that this is a pretty unscientific explanation) Chris Your description sounds to me as if the only reason we cannot tell where the center of the universe is located, is because we do not have enough data (i.e. the location of all galaxies in the universe.) When the OP said: "this suggests that there is no center or middle of the universe", it sounded as if he meant that a center of the universe did not exist, or that it was a meaningless question, and could not be answered with additional data. I am not sure if you two are saying the same thing and I misunderstood, or if there is disagreement on whether or not a center of the universe exists, regardless of our ability to identify it. Link to comment Share on other sites More sharing options...
csmyth3025 Posted June 7, 2011 Share Posted June 7, 2011 (edited) Your description sounds to me as if the only reason we cannot tell where the center of the universe is located, is because we do not have enough data (i.e. the location of all galaxies in the universe.) When the OP said: "this suggests that there is no center or middle of the universe", it sounded as if he meant that a center of the universe did not exist, or that it was a meaningless question, and could not be answered with additional data. I am not sure if you two are saying the same thing and I misunderstood, or if there is disagreement on whether or not a center of the universe exists, regardless of our ability to identify it. We are saying the same thing: There is no "center" of the universe. This stems from the assumption (supported by many observations) that the universe is homogenous and isotropic: The two testable structural consequences of the cosmological principle are homogeneity and isotropy. Homogeneity means that the same observational evidence is available to observers at different locations in the universe ("the part of the Universe which we can see is a fair sample"). Isotropy means that the same observational evidence is available by looking in any direction in the universe ("the same physical laws apply throughout"). The principles are distinct but closely related, because a universe that appears isotropic from any two (for a spherical geometry, three) locations must also be homogeneous. The cosmological principle is consistent with the observed isotropy... (ref. http://en.wikipedia....gical_Principle ) Basically, no matter where you go the universe looks the same. The large scale structure of the universe looks the same to someone in a galaxy that's 40 billion light years away as it does to us. It also looks the same to someone that's ten, twenty, or thirty billion light years away. The large scale structure of the universe looks the same to everyone - no matter where they are (it's homogeneous). Likewise, all these widely separated observers would see that the large scale structure of the universe looks about the same no matter which direction they look (it's isotropic). When you think of the well-worn (some say worn out) analogy of a balloon, you can't point to one spot on the balloon and say "That's the center of the surface." Someone else can pick another spot and say with equal confidence "That's the center of the surface." The universe is a sort of 3-D version of the 2-D surface of the baloon. You can't point to any one spot and say "That's the center." The problem that most people have is making the mental leap from a 2-D surface of a sphere, for example, that has no "center" and the 3-D space in which we live - that we think must have a center because that's our everyday experience. As DrR pointed out, the universe might have positive curvature (like a sphere) or it might not. Either way, it doesn't need to have a center and, in fact, it can't have a center according to the cosmological principle. Chris Edited to correct spelling errors Edited June 7, 2011 by csmyth3025 1 Link to comment Share on other sites More sharing options...
StringJunky Posted June 7, 2011 Share Posted June 7, 2011 (edited) When you think of the well-worn (some say worn out) analogy of a balloon, you can't point to one spot on the balloon and say "That's the center of the surface." Someone else can pick another spot and say with equal confidence "That's the center of the surface." The universe is a sort of 3-D version of the 2-D surface of the baloon. You can't point to any one spot and say "That's the center." The problem that most people have is making the mental leap from a 2-D surface of a sphere, for example, that has no "center" and the 3-D space in which we live - that we think must have a center because that's our everyday experience. As DrR pointed out, the universe might have positive curvature (like a sphere) or it might not. Either way, it doesn't need to have a center and, in fact, it can't have a center according to the cosmological principle. Chris Edited to correct spelling errors Let's get rid of the "Balloon Analogy" and start using "The Doughnut Analogy" or, even better, "The Inflatable Ring Analogy" then we can get rid of the middle! http://www.cosmosmagazine.com/node/1566 The link goes into the differences between topology and geometry amongst other things Edited June 7, 2011 by StringJunky Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now