9 ms·

How big is the entire universe?

188 points by sajid 14y ago

hansbo 14y ago
Interesting. I had no idea that the curvature of the universe was an indication of its size. Does this imply that a flat universe is also infinite?
- calinet6 14y ago
  This article and the science behind it at this point is making some very interesting assumptions, such as that the Universe is closed (like the Earth is in their analogy). But I believe at this point, the real answer is that we don't yet know. We're looking at a universe that looks flat and basically saying "well, it could be flat and finite, or it could be closed and curved, or it could be flat and infinite... but last time we thought our plane of existence was flat we ended up way off, so we better not make that assumption again." Point being: take this conclusion about the size of the Universe with a grain of salt.
- Sharlin 14y ago
  As an example of a flat but finite topology, think of the game "Asteroids" where, if the player's spaceship flies beyond the edge of the screen, it reappears at the opposite edge. This topology actually forms a torus! And, even though a torus definitely seems curved when embedded in 3D, its surface is actually flat - the angles of any triangle drawn on the surface always add up to exactly 180 degrees.
- nessus42 14y ago
  Cosmologist do usually take a flat universe to indicate an infinite universe. They also take "negative curvature" to indicate an infinite universe. Only a universe with "positive curvature" is taken to indicate a finite universe.
lifeisstillgood 14y ago
Well, that blew me away. I would happily have gone for 13billion light years across. This one little blog (that was a few hours work covering decades of hard work by hundreds of people) has reminded me of the vast importance of education, scientific inquiry and just plain old reading. One recent suggestion here in the UK is to make maths study compulsory till age 18. If I can go till age 40 and not know how we know how big the universe is, and I am supposedly in the top 5% of educated people, then yes yes yes. Is there any politican I can vote for who will double the science budget ?
- tosseraccount 14y ago
  Shouldn't it be 13 * 2 = 26 across? 13 light years that way, and 13 light years the opposite way? Is there some "relativity trick" I'm missing? Is some space aliean on the edge of the universe going to see 13 light years in all directions?
  - ed209 14y ago
    that would assume that there is a center to the universe. I'm not sure there is (speaking as the lay-est of laypeople). There are articles on the topic which I won't embarras myself by summarising in a HN comment... http://www.quora.com/The-Universe/Does-the-Universe-have-a-center http://www.quora.com/The-Universe/Does-the-Universe-have-a-c...
    - deleted 14y ago
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  - lifeisstillgood 14y ago
    considering I knew even less than I thought I did 20 minutes ago I am going to wait for someone who actually knows to chime in.
  - goodside 14y ago
    This is a misconception. If you just want a satisfying mental image of the Universe: It's really big, it doesn't have any point defined as its center, and it doesn't have any wall-like borders. Aliens living 14.6 billion LY away from us see a Universe that looks just like what we see in all directions. If it is limited in size at all, it's because it wraps around on itself ("if you go straight long enough you end up where you were"). Nobody has firm evidence that it actually does wrap around on itself, and there's reason to believe that if it does, it's on a scale that's much larger than we could ever hope to observe. We will likely never have any sign inconsistent with it being literally infinite regardless of whether or not it is. If you want a better answer than that, you'll need a very firm grasp of special relativity and a basic understanding of general relativity and the metric expansion of spacetime. In which case, read this: http://arxiv.org/pdf/astro-ph/0310808.pdf http://arxiv.org/pdf/astro-ph/0310808.pdf
xefer 14y ago
Max Tegmark had an interesting article in Scientific American a while back discussing the implications of an infinite universe: http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself.
- hunterjrj 14y ago
  ... Or an infinite number of duplicates of yourself?
- mootothemax 14y ago
  Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself. Wouldn't it also mean that there are an infinite number of duplicates out there? Including one typing this very comment, but who typos and leaves the "s" off "duplicates"? I'm very much not a mathematician nor physicist, can this really be true if the universe is infinite in size? Asking as a layman, at what point when dealing with infinite possibilities and probabilities does something become certain? That's what really terrifies me.
  - andyjohnson0 14y ago
    Including one typing this very comment, but who typos and leaves the "s" off "duplicates" There would be an infinite number of those, too.
  - nessus42 14y ago
    If the universe is flat, and therefore infinite, yes it means that most probably there are an infinite number of identical duplicates of you. It does not mean, however, that there are any duplicates of you that make a particular typo. I should think that there are an infinite number of such duplicates for any given typo, BUT it may also be the case that such worlds are impossible. E.g., physics is deterministic enough and chaotic enough that there is no slight nudge to the initial conditions of a world that will result in such a minor eventual difference to occur. I.e., just because there are an infinite number of parallel worlds, doesn't mean that every imaginable thing occurs in them. What occurs in a parallel world, must be possible.
goodside 14y ago
The short answer is nobody has a plausible upper bound on the size of the entire Universe, and there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. It's analogous to how if you stood on the ground of featureless planet, the land you could inhabit is finite, but it has no borders and no point where you could stick a flag in the ground and say "this is the center of the planet's land". Most of the time when people give hard numbers for the size of the Universe, they either mean A) the portion of the Universe we can see, B) the portion of the Universe we could ever see in principle, or C) the portion of the Universe which could, in principle, be causally influenced by early particle interactions that could have causally influenced us. If you suspect someone might be talking out of their ass about cosmology, ask them how big the Universe is -- if they give you a clean multiple of 14.6 billion light years, they're an idiot. Technically inclined readers will find this more enlightening than the article above: http://arxiv.org/pdf/astro-ph/0310808.pdf http://arxiv.org/pdf/astro-ph/0310808.pdf
- peterwaller 14y ago
  > [...] there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. My understanding is that for your interpretation here of "finite" to be true, there must be some curvature. If you only consider the geometry of the universe, then what we see with our instruments does allow us to put a bound on the size of the curvature. If you want to say something along the lines of "outside of the observable universe is probably cheese whiz" then that's your perogative, and no-one can refute that. But (from the article) the geometry of the universe that we have measured is consistent with an infinite universe, and gives a minimum bound on its size which is many multiples of the observable universe.
  - Strilanc 14y ago
    It's mathematically simple for a space to loop without being curved locally, although I am under the impression that there are physics reasons to infer we don't exist in such a space. The pac-man universe acts euclidean until you have universe-sized effects. For example, a circle will satisfy circumference = diameter * pi until its diameter is greater than the span of the universe. Basically the universe could 'hide' the fact that it is a torus from pac-man by simply being really large.
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kennon 14y ago
Very cool, but I'm still a little confused. The earth's surface more or less just extends out in two dimensions, but doesn't space extend out in three? How does all of this work when you have take into account the z-axis as well as x and y? How would a three-dimensional object "close" in around itself?
- jmmcd 14y ago
  To imagine a 2d surface closing in on itself, you imagine it in 3d. To imagine a 3d object closing in on itself, you unfortunately have to try to imagine it in 4d.
  - Splines 14y ago
    I read Flatland a long time ago as a student and found it enlightening when thinking about 4d. https://en.wikipedia.org/wiki/Flatland https://en.wikipedia.org/wiki/Flatland
- InclinedPlane 14y ago
  There are different kinds of dimensions. There are simple, Euclidian dimensions where each dimension is perfectly orthogonal to the others, where parallel lines never intersect, etc. But there are other possibilities, non-Euclidean spaces and volumes. This is where the example of the sphere comes in. If you were a 2-dimensional being you could be on a plane or you could be on the surface of a sphere. Similarly, if you are a 3-dimensional being, such as us, you could be within a simple R^3 volume, or you could be on a hyper-sphere or another non-euclidian volume. The math is a little more complex, but similar.
jsmcgd 14y ago
The visible curve of the Earth is due to atmospheric distortion? What? Surely not. Surely the curve you see is the horizon of your view?
- hcarvalhoalves 14y ago
  I believe the author is stretching it. In the iceberg photo case, that's just barrel distortion from the camera lens.
tokenadult 14y ago
There was a talk about this issue at the Midwest Science of Origins Conference on 31 March 2012 by Marco Peloso of the University of Minnesota. Peloso reviewed the evidence available for the condition of the Universe just after the Big Bang. He also mentioned that current observations are consistent with a "flat" geometry of the entire universe, but pointed out other lines of evidence, not mentioned in the blog post submitted here, consistent with a finite (although very, very large) size for the universe. Finite size is consistent with current observations and theories, and an issue that the submitted article doesn't have a lot of space to address. Per a Wikipedia article, http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_contents.2C_structure.2C_and_laws http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_content... consistent with what I heard in the lecture earlier this year, "The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite." Wikipedia cites the Landau and Lifshitz physics textbook from the Soviet Union, Landau, Lev, Lifshitz, E.M. (1975). The Classical Theory of Fields (Course of Theoretical Physics, Vol. 2) (revised 4th English ed.). New York: Pergamon Press. pp. 358–397. ISBN 978-0-08-018176-9, for this statement. Some multiverse theories such as those mentioned in another comment posted before this one can be overlaid on a simpler theory of a single finite "observable" (in principle) universe with the properties we know from human observation. Testing theories like those, or like the "level I multiverse" theory mentioned in the other comment, still needs more work. The current Udacity course in physics http://www.udacity.com/overview/Course/ph100/CourseRev/1 http://www.udacity.com/overview/Course/ph100/CourseRev/1 started off its first unit with the students reproducing the effort of Eratosthenes of Cyrene to measure the circumference of the earth more than 2,200 years ago. On reasonable assumptions known to the ancient Greeks, it was possible to get a surprisingly accurate estimate of the earth's circumference, with a major source of error being simply measuring distances between one city and another a few days' journey away.
tocomment 14y ago
But if the big bang happened 13 billion years ago there shouldn't be any matter or energy outside of a 13 billion light year radius, right?
- brandong 14y ago
  Due to the expansion of space the edge of the observable universe is now about 46 billion light years away: http://en.wikipedia.org/wiki/Metric_expansion_of_space http://en.wikipedia.org/wiki/Metric_expansion_of_space
- Nogwater 14y ago
  No, unfortunately it doesn't work like that. It's not like the big bang took place "within space" -- like a grenade in the center of a stadium, but the bang is the rapid expansion of space(-time) itself. So, the universe could have even been infinite and very dense, and then BOOM!, it's still infinite but less dense. The observable universe is even bigger than you'd expect: http://en.wikipedia.org/wiki/Observable_universe#Size http://en.wikipedia.org/wiki/Observable_universe#Size
  - equalarrow 14y ago
    Yes! Mind blown (again). :) While it's really enjoyable to talk/think about these topics from a purely scientific point of view, there's another side to all this, which is the sheer beauty of such a thing. There's a great show on Discovery called How the Universe Works. Season 1 had an episode called Galaxies that sorta touches on some of this. The graphics for the show, first off, are awesome. When they show the super zoomed out view of all the galaxy clusters and how everything is connected, immediately I thought, "wow, those look like neurons". And that's where the beauty of nature comes in. You can start to see how systems arrange and the patterns used from the micro to the macro. The distances on either level are incomprehensible (strings, atoms, molecules, galaxies, universe), but the patterns are visible. And this is why I love science. While I'm not a physicist, I always appreciated Feynman because you could see he had that same love for the extremely complex and the simple observations - and he had a great way of describing it. I think the same goes for Sagan, although, in a totally different field. But Sagan too just had that love for the beauty of nature, on top of his accomplishments of the Mariner missions and other planetary science. I have a 6mo son right now and I hope to instill that love of the beauty of nature through science. We're each an insignificant speck on an insignificant blue speck in an unfathomable universe. But, our connections to each other and our appreciation for the beauty of it all are a privilege. And posts like this, shows like How the Universe Works will be my way of conveying to him his place in it all. I hope he'll appreciate it someday like I do. I hope, some day all people will.
kapkapkap 14y ago
Relevant -- http://scaleofuniverse.com/ http://scaleofuniverse.com/
- thangalin 14y ago
  Also relevant -- http://www.davidjarvis.ca/dave/gallery/star-sizes/ http://www.davidjarvis.ca/dave/gallery/star-sizes/
acdanger 14y ago
"The universe is a big place, perhaps the biggest." -- Kilgore Trout
nessus42 14y ago
Another factoid that causes the mind to reel is that a flat universe, which is of infinite size, also has zero total energy. This means that it is conceivable that someday we might figure out how to manufacture entire new universes. Who says that there's no such thing as a free lunch!
- Ralith 14y ago
  > zero total energy ... means that it is conceivable that someday we might figure out how to manufacture entire new universes. How does that follow?
  - nessus42 14y ago
    It doesn't follow that we will be able to do it, nor even that it is possible, but before the understanding that flat universes contain zero energy, the claim that you might manufacture a universe would seem to completely absurd. Where would you get all that energy? Especially if energy is conserved? As it turns out, it doesn't matter if energy is conserved, because a flat universe doesn't have any net energy, and consequently they can come into existence without violating known conservation laws. In any case, you don't have to take my word for it. Look up Alan Guth, the MIT cosmologist who invented the inflationary universe theory. This is his claim.
javert 14y ago
Why presume that the universe is a surface, instead of a giant volume? After all, the normal way of looking at it is that we're in space, not on it... This is an especially appalling oversight in an article intended for the intelligent layman.
- laughinghan 14y ago
  It's an analogy, because while it's easy to visualize how a two-dimensional surface can be curved, it's hard to visualize how a three-dimensional surface can be curved, because we think of it as a "volume".
  - javert 14y ago
    There is no such thing as a "three-dimensional surface" in plain English. I don't know why that is even coming up.
    - fragsworth 14y ago
      Unfortunately, "plain English" is not sufficient to describe all physical phenomena, particularly with scales we aren't exposed to in day-to-day life.
      - javert 14y ago
        If that's truly the case, which I doubt, people should seriously just stop trying to write physics articles for the layman. By the way, scale has nothing to do with any of the objections I'm raising. It's not a problem of scale.
    - laughinghan 14y ago
      I believe you're an reasonable, intelligent layman. Do you believe we're reasonable, intelligent people too? Are you willing to consider the possibility that the article did not make an "appalling oversight", the article merely phrased things confusingly? You and I both have a good idea of what we think of as a two-dimensional surface, and as a three-dimensional space. In particular, we have an idea of how many real numbers uniquely identify any point in the surface or space (2 and 3, respectively), and we call these coordinates. Now comes the important part: we also have an idea of how to measure the length of any path in the surface or space, and as a derivative idea, the idea that the distance between any two points is the length of the shortest path between them. On a flat two-dimensional surface--among the English-speakers who converse often about such things, this is called a "plane"--it turns out that the distance between any two points is exactly the square root of the sum of the squares of the differences between the coordinates of each point (in other words, sqrt( (x1-x0)^2 + (y1-y0)^2 )). There is a similar relationship between distances between points and their coordinates in three-dimensional space, right? I'm sure you've seen all this before. Now, there are two-dimensional surfaces for which that relationship is untrue, for example, the surface of a sphere or a surface that is "saddle-shaped" (like in the diagram, although they don't (and can't) show it extending infinitely). In these two examples, it turns out we can describe such surfaces, rather than as a set of points each uniquely determined by 2 coordinates, instead as a set of points uniquely determined by 3 coordinates and one constraint on what those coordinates can be, where the 3D distance has the relationship with coordinates that we're used to; for example, the surface of a sphere can both be thought of as being uniquely determined by longitude and latitude, and as being uniquely determined by x,y, and z coordinates, subject to the constraint that every point in the surface is the same 3D distance from some point in the 3D space. When described this way, things that would be non-obvious if I told you the relationship between distance and latitude and longitude, such as the fact that the 3D sphere is closed, become very obvious, because we have a good intuition for distances like that. Now consider the following: let's say you're trying to figure out how the world works. An approach that's been quite successful is describing all the possible ways you can think of that are consistent with what we know, and then conducting experiments to try to rule each possibility out. One possibility you're describing involves the points being uniquely identified by 3 coordinates, but the distance between points doesn't have the usual square-root-of-the-sum-of-the-squares-of-the-differences relationship with the coordinates, and is instead just-so-slightly off. Let's say it is off in such a way that by describing the same set of points with 4 coordinates and 1 constraint (similar to how we described the sphere as 3 coordinates and 1 constraint), where the 4D distance between points uniquely described by those 4 coordinates obeys a relationship to the coordinates that is very similar to intuitive distances for the flat 2D surface and 3D space, so similar that almost all the same reasoning applies (continually increasing a coordinate, for example, will eventually continually increase distance in these cases, but not for longitude and latitude of a sphere). Don't you think describing this as a "curved three-dimensional surface" is a sensible and accurate description?
thangalin 14y ago
The article used one of my drawings without my permission. http://davidjarvis.ca/dave/gallery/ http://davidjarvis.ca/dave/gallery/
- scott_s 14y ago
  Email the author, it looks like he thinks the image was made by someone else.
ars 14y ago
The first half of this is from the excellent book: Relativity: The Special and General Theory by Albert Einstein It's written for the interested layman, the article is from part 3 of the book where Einstein explains this.
boredguy8 14y ago
This might be a bit of a nit, but it happens enough: the quote that begins that article is misattributed. It's from Daniel J. Boorstin. http://en.wikiquote.org/wiki/Stephen_Hawking#Misattributed http://en.wikiquote.org/wiki/Stephen_Hawking#Misattributed
ctchocula 14y ago
Fields Medal winner Terence Tao gives a public lecture on this material: http://www.youtube.com/watch?v=7ne0GArfeMs http://www.youtube.com/watch?v=7ne0GArfeMs
nrmehta 14y ago
For the layperson (like me), the below is an intriguing and related TED talk from Brian Greene. A big conclusion is that we now believe the universe is expanding at an accelerating rate based upon the edge of the observable universe. But Greene posits that 1000s of years in the future, this accelerating edge will be too far away for us to observe and the universe will look to future mankind to be more static and small. I'm not physicist so I can't criticize this claim but I found it intriguing. http://www.ted.com/talks/brian_greene_why_is_our_universe_fine_tuned_for_life.html http://www.ted.com/talks/brian_greene_why_is_our_universe_fi...

188 points by sajid 14y ago

Interesting. I had no idea that the curvature of the universe was an indication of its size. Does this imply that a flat universe is also infinite?

calinet6 14y ago
This article and the science behind it at this point is making some very interesting assumptions, such as that the Universe is closed (like the Earth is in their analogy). But I believe at this point, the real answer is that we don't yet know. We're looking at a universe that looks flat and basically saying "well, it could be flat and finite, or it could be closed and curved, or it could be flat and infinite... but last time we thought our plane of existence was flat we ended up way off, so we better not make that assumption again." Point being: take this conclusion about the size of the Universe with a grain of salt.
Sharlin 14y ago
As an example of a flat but finite topology, think of the game "Asteroids" where, if the player's spaceship flies beyond the edge of the screen, it reappears at the opposite edge. This topology actually forms a torus! And, even though a torus definitely seems curved when embedded in 3D, its surface is actually flat - the angles of any triangle drawn on the surface always add up to exactly 180 degrees.
nessus42 14y ago
Cosmologist do usually take a flat universe to indicate an infinite universe. They also take "negative curvature" to indicate an infinite universe. Only a universe with "positive curvature" is taken to indicate a finite universe.

lifeisstillgood 14y ago

Well, that blew me away. I would happily have gone for 13billion light years across. This one little blog (that was a few hours work covering decades of hard work by hundreds of people) has reminded me of the vast importance of education, scientific inquiry and just plain old reading. One recent suggestion here in the UK is to make maths study compulsory till age 18. If I can go till age 40 and not know how we know how big the universe is, and I am supposedly in the top 5% of educated people, then yes yes yes. Is there any politican I can vote for who will double the science budget ?

tosseraccount 14y ago
Shouldn't it be 13 * 2 = 26 across? 13 light years that way, and 13 light years the opposite way? Is there some "relativity trick" I'm missing? Is some space aliean on the edge of the universe going to see 13 light years in all directions?
- ed209 14y ago
  that would assume that there is a center to the universe. I'm not sure there is (speaking as the lay-est of laypeople). There are articles on the topic which I won't embarras myself by summarising in a HN comment... http://www.quora.com/The-Universe/Does-the-Universe-have-a-center http://www.quora.com/The-Universe/Does-the-Universe-have-a-c...
  - deleted 14y ago
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- lifeisstillgood 14y ago
  considering I knew even less than I thought I did 20 minutes ago I am going to wait for someone who actually knows to chime in.
- goodside 14y ago
  This is a misconception. If you just want a satisfying mental image of the Universe: It's really big, it doesn't have any point defined as its center, and it doesn't have any wall-like borders. Aliens living 14.6 billion LY away from us see a Universe that looks just like what we see in all directions. If it is limited in size at all, it's because it wraps around on itself ("if you go straight long enough you end up where you were"). Nobody has firm evidence that it actually does wrap around on itself, and there's reason to believe that if it does, it's on a scale that's much larger than we could ever hope to observe. We will likely never have any sign inconsistent with it being literally infinite regardless of whether or not it is. If you want a better answer than that, you'll need a very firm grasp of special relativity and a basic understanding of general relativity and the metric expansion of spacetime. In which case, read this: http://arxiv.org/pdf/astro-ph/0310808.pdf http://arxiv.org/pdf/astro-ph/0310808.pdf

xefer 14y ago

Max Tegmark had an interesting article in Scientific American a while back discussing the implications of an infinite universe: http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself.

hunterjrj 14y ago
... Or an infinite number of duplicates of yourself?
mootothemax 14y ago
Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself. Wouldn't it also mean that there are an infinite number of duplicates out there? Including one typing this very comment, but who typos and leaves the "s" off "duplicates"? I'm very much not a mathematician nor physicist, can this really be true if the universe is infinite in size? Asking as a layman, at what point when dealing with infinite possibilities and probabilities does something become certain? That's what really terrifies me.
- andyjohnson0 14y ago
  Including one typing this very comment, but who typos and leaves the "s" off "duplicates" There would be an infinite number of those, too.
- nessus42 14y ago
  If the universe is flat, and therefore infinite, yes it means that most probably there are an infinite number of identical duplicates of you. It does not mean, however, that there are any duplicates of you that make a particular typo. I should think that there are an infinite number of such duplicates for any given typo, BUT it may also be the case that such worlds are impossible. E.g., physics is deterministic enough and chaotic enough that there is no slight nudge to the initial conditions of a world that will result in such a minor eventual difference to occur. I.e., just because there are an infinite number of parallel worlds, doesn't mean that every imaginable thing occurs in them. What occurs in a parallel world, must be possible.

goodside 14y ago

The short answer is nobody has a plausible upper bound on the size of the entire Universe, and there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. It's analogous to how if you stood on the ground of featureless planet, the land you could inhabit is finite, but it has no borders and no point where you could stick a flag in the ground and say "this is the center of the planet's land". Most of the time when people give hard numbers for the size of the Universe, they either mean A) the portion of the Universe we can see, B) the portion of the Universe we could ever see in principle, or C) the portion of the Universe which could, in principle, be causally influenced by early particle interactions that could have causally influenced us. If you suspect someone might be talking out of their ass about cosmology, ask them how big the Universe is -- if they give you a clean multiple of 14.6 billion light years, they're an idiot. Technically inclined readers will find this more enlightening than the article above: http://arxiv.org/pdf/astro-ph/0310808.pdf http://arxiv.org/pdf/astro-ph/0310808.pdf

peterwaller 14y ago
> [...] there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. My understanding is that for your interpretation here of "finite" to be true, there must be some curvature. If you only consider the geometry of the universe, then what we see with our instruments does allow us to put a bound on the size of the curvature. If you want to say something along the lines of "outside of the observable universe is probably cheese whiz" then that's your perogative, and no-one can refute that. But (from the article) the geometry of the universe that we have measured is consistent with an infinite universe, and gives a minimum bound on its size which is many multiples of the observable universe.
- Strilanc 14y ago
  It's mathematically simple for a space to loop without being curved locally, although I am under the impression that there are physics reasons to infer we don't exist in such a space. The pac-man universe acts euclidean until you have universe-sized effects. For example, a circle will satisfy circumference = diameter * pi until its diameter is greater than the span of the universe. Basically the universe could 'hide' the fact that it is a torus from pac-man by simply being really large.

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kennon 14y ago

Very cool, but I'm still a little confused. The earth's surface more or less just extends out in two dimensions, but doesn't space extend out in three? How does all of this work when you have take into account the z-axis as well as x and y? How would a three-dimensional object "close" in around itself?

jmmcd 14y ago
To imagine a 2d surface closing in on itself, you imagine it in 3d. To imagine a 3d object closing in on itself, you unfortunately have to try to imagine it in 4d.
- Splines 14y ago
  I read Flatland a long time ago as a student and found it enlightening when thinking about 4d. https://en.wikipedia.org/wiki/Flatland https://en.wikipedia.org/wiki/Flatland
InclinedPlane 14y ago
There are different kinds of dimensions. There are simple, Euclidian dimensions where each dimension is perfectly orthogonal to the others, where parallel lines never intersect, etc. But there are other possibilities, non-Euclidean spaces and volumes. This is where the example of the sphere comes in. If you were a 2-dimensional being you could be on a plane or you could be on the surface of a sphere. Similarly, if you are a 3-dimensional being, such as us, you could be within a simple R^3 volume, or you could be on a hyper-sphere or another non-euclidian volume. The math is a little more complex, but similar.

jsmcgd 14y ago

The visible curve of the Earth is due to atmospheric distortion? What? Surely not. Surely the curve you see is the horizon of your view?

hcarvalhoalves 14y ago
I believe the author is stretching it. In the iceberg photo case, that's just barrel distortion from the camera lens.

tokenadult 14y ago

There was a talk about this issue at the Midwest Science of Origins Conference on 31 March 2012 by Marco Peloso of the University of Minnesota. Peloso reviewed the evidence available for the condition of the Universe just after the Big Bang. He also mentioned that current observations are consistent with a "flat" geometry of the entire universe, but pointed out other lines of evidence, not mentioned in the blog post submitted here, consistent with a finite (although very, very large) size for the universe. Finite size is consistent with current observations and theories, and an issue that the submitted article doesn't have a lot of space to address. Per a Wikipedia article, http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_contents.2C_structure.2C_and_laws http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_content... consistent with what I heard in the lecture earlier this year, "The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite." Wikipedia cites the Landau and Lifshitz physics textbook from the Soviet Union, Landau, Lev, Lifshitz, E.M. (1975). The Classical Theory of Fields (Course of Theoretical Physics, Vol. 2) (revised 4th English ed.). New York: Pergamon Press. pp. 358–397. ISBN 978-0-08-018176-9, for this statement. Some multiverse theories such as those mentioned in another comment posted before this one can be overlaid on a simpler theory of a single finite "observable" (in principle) universe with the properties we know from human observation. Testing theories like those, or like the "level I multiverse" theory mentioned in the other comment, still needs more work. The current Udacity course in physics http://www.udacity.com/overview/Course/ph100/CourseRev/1 http://www.udacity.com/overview/Course/ph100/CourseRev/1 started off its first unit with the students reproducing the effort of Eratosthenes of Cyrene to measure the circumference of the earth more than 2,200 years ago. On reasonable assumptions known to the ancient Greeks, it was possible to get a surprisingly accurate estimate of the earth's circumference, with a major source of error being simply measuring distances between one city and another a few days' journey away.

tocomment 14y ago

But if the big bang happened 13 billion years ago there shouldn't be any matter or energy outside of a 13 billion light year radius, right?

brandong 14y ago
Due to the expansion of space the edge of the observable universe is now about 46 billion light years away: http://en.wikipedia.org/wiki/Metric_expansion_of_space http://en.wikipedia.org/wiki/Metric_expansion_of_space
Nogwater 14y ago
No, unfortunately it doesn't work like that. It's not like the big bang took place "within space" -- like a grenade in the center of a stadium, but the bang is the rapid expansion of space(-time) itself. So, the universe could have even been infinite and very dense, and then BOOM!, it's still infinite but less dense. The observable universe is even bigger than you'd expect: http://en.wikipedia.org/wiki/Observable_universe#Size http://en.wikipedia.org/wiki/Observable_universe#Size
- equalarrow 14y ago
  Yes! Mind blown (again). :) While it's really enjoyable to talk/think about these topics from a purely scientific point of view, there's another side to all this, which is the sheer beauty of such a thing. There's a great show on Discovery called How the Universe Works. Season 1 had an episode called Galaxies that sorta touches on some of this. The graphics for the show, first off, are awesome. When they show the super zoomed out view of all the galaxy clusters and how everything is connected, immediately I thought, "wow, those look like neurons". And that's where the beauty of nature comes in. You can start to see how systems arrange and the patterns used from the micro to the macro. The distances on either level are incomprehensible (strings, atoms, molecules, galaxies, universe), but the patterns are visible. And this is why I love science. While I'm not a physicist, I always appreciated Feynman because you could see he had that same love for the extremely complex and the simple observations - and he had a great way of describing it. I think the same goes for Sagan, although, in a totally different field. But Sagan too just had that love for the beauty of nature, on top of his accomplishments of the Mariner missions and other planetary science. I have a 6mo son right now and I hope to instill that love of the beauty of nature through science. We're each an insignificant speck on an insignificant blue speck in an unfathomable universe. But, our connections to each other and our appreciation for the beauty of it all are a privilege. And posts like this, shows like How the Universe Works will be my way of conveying to him his place in it all. I hope he'll appreciate it someday like I do. I hope, some day all people will.

kapkapkap 14y ago

Relevant -- http://scaleofuniverse.com/ http://scaleofuniverse.com/

thangalin 14y ago
Also relevant -- http://www.davidjarvis.ca/dave/gallery/star-sizes/ http://www.davidjarvis.ca/dave/gallery/star-sizes/

acdanger 14y ago

"The universe is a big place, perhaps the biggest." -- Kilgore Trout

nessus42 14y ago

Another factoid that causes the mind to reel is that a flat universe, which is of infinite size, also has zero total energy. This means that it is conceivable that someday we might figure out how to manufacture entire new universes. Who says that there's no such thing as a free lunch!

Ralith 14y ago
> zero total energy ... means that it is conceivable that someday we might figure out how to manufacture entire new universes. How does that follow?
- nessus42 14y ago
  It doesn't follow that we will be able to do it, nor even that it is possible, but before the understanding that flat universes contain zero energy, the claim that you might manufacture a universe would seem to completely absurd. Where would you get all that energy? Especially if energy is conserved? As it turns out, it doesn't matter if energy is conserved, because a flat universe doesn't have any net energy, and consequently they can come into existence without violating known conservation laws. In any case, you don't have to take my word for it. Look up Alan Guth, the MIT cosmologist who invented the inflationary universe theory. This is his claim.

javert 14y ago

Why presume that the universe is a surface, instead of a giant volume? After all, the normal way of looking at it is that we're in space, not on it... This is an especially appalling oversight in an article intended for the intelligent layman.

laughinghan 14y ago
It's an analogy, because while it's easy to visualize how a two-dimensional surface can be curved, it's hard to visualize how a three-dimensional surface can be curved, because we think of it as a "volume".
- javert 14y ago
  There is no such thing as a "three-dimensional surface" in plain English. I don't know why that is even coming up.
  - fragsworth 14y ago
    Unfortunately, "plain English" is not sufficient to describe all physical phenomena, particularly with scales we aren't exposed to in day-to-day life.
    - javert 14y ago
      If that's truly the case, which I doubt, people should seriously just stop trying to write physics articles for the layman. By the way, scale has nothing to do with any of the objections I'm raising. It's not a problem of scale.
  - laughinghan 14y ago
    I believe you're an reasonable, intelligent layman. Do you believe we're reasonable, intelligent people too? Are you willing to consider the possibility that the article did not make an "appalling oversight", the article merely phrased things confusingly? You and I both have a good idea of what we think of as a two-dimensional surface, and as a three-dimensional space. In particular, we have an idea of how many real numbers uniquely identify any point in the surface or space (2 and 3, respectively), and we call these coordinates. Now comes the important part: we also have an idea of how to measure the length of any path in the surface or space, and as a derivative idea, the idea that the distance between any two points is the length of the shortest path between them. On a flat two-dimensional surface--among the English-speakers who converse often about such things, this is called a "plane"--it turns out that the distance between any two points is exactly the square root of the sum of the squares of the differences between the coordinates of each point (in other words, sqrt( (x1-x0)^2 + (y1-y0)^2 )). There is a similar relationship between distances between points and their coordinates in three-dimensional space, right? I'm sure you've seen all this before. Now, there are two-dimensional surfaces for which that relationship is untrue, for example, the surface of a sphere or a surface that is "saddle-shaped" (like in the diagram, although they don't (and can't) show it extending infinitely). In these two examples, it turns out we can describe such surfaces, rather than as a set of points each uniquely determined by 2 coordinates, instead as a set of points uniquely determined by 3 coordinates and one constraint on what those coordinates can be, where the 3D distance has the relationship with coordinates that we're used to; for example, the surface of a sphere can both be thought of as being uniquely determined by longitude and latitude, and as being uniquely determined by x,y, and z coordinates, subject to the constraint that every point in the surface is the same 3D distance from some point in the 3D space. When described this way, things that would be non-obvious if I told you the relationship between distance and latitude and longitude, such as the fact that the 3D sphere is closed, become very obvious, because we have a good intuition for distances like that. Now consider the following: let's say you're trying to figure out how the world works. An approach that's been quite successful is describing all the possible ways you can think of that are consistent with what we know, and then conducting experiments to try to rule each possibility out. One possibility you're describing involves the points being uniquely identified by 3 coordinates, but the distance between points doesn't have the usual square-root-of-the-sum-of-the-squares-of-the-differences relationship with the coordinates, and is instead just-so-slightly off. Let's say it is off in such a way that by describing the same set of points with 4 coordinates and 1 constraint (similar to how we described the sphere as 3 coordinates and 1 constraint), where the 4D distance between points uniquely described by those 4 coordinates obeys a relationship to the coordinates that is very similar to intuitive distances for the flat 2D surface and 3D space, so similar that almost all the same reasoning applies (continually increasing a coordinate, for example, will eventually continually increase distance in these cases, but not for longitude and latitude of a sphere). Don't you think describing this as a "curved three-dimensional surface" is a sensible and accurate description?

thangalin 14y ago

The article used one of my drawings without my permission. http://davidjarvis.ca/dave/gallery/ http://davidjarvis.ca/dave/gallery/

scott_s 14y ago
Email the author, it looks like he thinks the image was made by someone else.

ars 14y ago

The first half of this is from the excellent book: Relativity: The Special and General Theory by Albert Einstein It's written for the interested layman, the article is from part 3 of the book where Einstein explains this.

boredguy8 14y ago

This might be a bit of a nit, but it happens enough: the quote that begins that article is misattributed. It's from Daniel J. Boorstin. http://en.wikiquote.org/wiki/Stephen_Hawking#Misattributed http://en.wikiquote.org/wiki/Stephen_Hawking#Misattributed

ctchocula 14y ago

Fields Medal winner Terence Tao gives a public lecture on this material: http://www.youtube.com/watch?v=7ne0GArfeMs http://www.youtube.com/watch?v=7ne0GArfeMs

nrmehta 14y ago

For the layperson (like me), the below is an intriguing and related TED talk from Brian Greene. A big conclusion is that we now believe the universe is expanding at an accelerating rate based upon the edge of the observable universe. But Greene posits that 1000s of years in the future, this accelerating edge will be too far away for us to observe and the universe will look to future mankind to be more static and small. I'm not physicist so I can't criticize this claim but I found it intriguing. http://www.ted.com/talks/brian_greene_why_is_our_universe_fine_tuned_for_life.html http://www.ted.com/talks/brian_greene_why_is_our_universe_fi...