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How big is the Universe?
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| lexrazor | Date: Thursday, 28.02.2013, 19:13 | Message # 1 |
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Astronaut
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| Hey there guys i came across this fun lil video bout the size of the Universe which i think you'd enjoy too. Maybe some of you have seen it, maybe you havent but i thoguht it's be nice to see some opinions 
https://www.youtube.com/watch?v=5NU2t5zlxQQ
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| midtskogen | Date: Thursday, 28.02.2013, 20:34 | Message # 2 |
 Star Engineer
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| It asks "does the universe have a centre?" I think the answer is "no" or "possibly", depending on what is meant by "the universe".
If we ask "does the earth have a centre?", you might answer "yes", but if you're then given a world atlas and asked to point where it is, you can't. The same thing with the universe. You can't point at a specific place in space and call it the centre. But if the extra dimensional geometry of the universe was know, then, depending on the geometry, the answer could be yes and you could explain why.
NIL DIFFICILE VOLENTI
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| lexrazor | Date: Thursday, 28.02.2013, 20:57 | Message # 3 |
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Astronaut
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| I think the world atlas concept you're talking about may be wrong because we do know that the center is the Earth's core. But if you're talking only about the surface then the answer is yes and no. Yes because every point on the surface is a center relative to the points around it and no because the spherical shape of the atlas doesnt have a start and an end. Unless you count the circles of latitude and the meridians as your starts and ends.
As far as the center of the universe goes i have to agree with it not having one or at least not having one so far cuz we dont know a whole lot about it.
Edited by lexrazor - Thursday, 28.02.2013, 21:02 |
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| Watsisname | Date: Thursday, 28.02.2013, 22:38 | Message # 4 |
 Galaxy Architect
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| Very good video! His figures are correct and he does a very good job of explaining the concepts in a way that is easily understandable.
The observable universe indeed does have an edge (by definition), and the observer is at the center of their own observable universe. However there is no center or edge (spatially) to the universe as whole. For one thing this would contradict the principles of isotropy and homogeneity for observers for which the edge is within their Hubble volume. There's also a point of semantics, which is that the universe is defined as containing everything that exists, yet the existence of an edge means there is some external property to the universe.
Some may find it conceptually useful to think of the expansion of the universe as extra-dimensional, analogous to the two-dimensional surface of a balloon expanding in the third dimension, but we should not take it too literally because it is neither testable or necessary to model the universe this way. Mathematically, the expanding universe is described by the FLRW metric, where the spatial component of the metric is a function of time.
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| Tim | Date: Thursday, 28.02.2013, 23:01 | Message # 5 |
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| One thing I don't get.
How could the universe be growing, if it's infinite?
I prefer the bubblebath multiverse theory, personally.
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| lexrazor | Date: Thursday, 28.02.2013, 23:03 | Message # 6 |
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Astronaut
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| Im familiar with the baloon principal it does explain the expansion pretty well. In any case tho the answer to the whole "how the universe works" thing is either extremely complicated somewhere in the quantum level or extremely simple.
I agree that the video is indeed really good and simple and pretty much a lil kid will probably have no trouble understanding. Also to be honest it never really came to my mind how the size of the observable universe was expanding too and that the iconic 13,7 bilion ly could now be doubled or tripled or that they were just the radius of the observable "bubble". And the video made it so simple i slaped my forehead going "omg how can i be this stupid not to notice that" lol
In my opinion to think of the universe as infinate, without edges or any other ends makes it a bit more simple to comprehand than to think that something exsists beyond the universe.
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| midtskogen | Date: Thursday, 28.02.2013, 23:15 | Message # 7 |
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| The globe analogy is fine for explaining why the sum of angles of a triangle isn't 180 degrees in large scales in the universe, like a triangle drawn on a globe wont have a sum of 180, but things aren't just as simple as our universe being a 3D "plane" in a 4D (or 5D, etc) universe with similar properties in the extra dimension, like the 2D surface of a globe is in a 3D world. We can imagine objects flying through the globe, which in the globe plane suddenly will appear and disappear. Since we don't experience 3D objects suddenly appearing out of nothing and then disappear again, at least not at the atom scale and up, we obviously isn't living in a 3D "plane" of a 4D world where 4D objects move freely around.
It's a creepy thought that if we live in a 3D "plane", then someone with the ability to leave the 3D plane into the 4D world and to look down on us on the 3D "plane" from a 4D perspective would be able to see pretty much everything, through walls, inside our bodies, etc. Added (01.03.2013, 02:15)
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There's infinite, and there's infinite. You can go in one direction infinitely on Earth without reaching an edge, but does it make Earth infinite?
I look forward to the day someone can explain the true geometry of the universe.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Thursday, 28.02.2013, 23:28 | Message # 8 |
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Galaxy Architect
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| Quote (Tim) One thing I don't get.
How could the universe be growing, if it's infinite?
Imagine an infinite grid for which the grid squares grow larger with time. (No, you can't actually imagine such a thing, but it's probably the best one can do).
I think the most difficult hurdle one must overcome with this is that you must not think of the universe as some sphere of space which is embedded in some external "something" and expanding into it. Rather, the universe is everything there is -- there is no outside. It is challenging to conceive of an infinite expanding space, but mathematically it is perfectly valid and very powerful.
Quote I prefer the bubblebath multiverse theory, personally.
It is a compelling idea, but it is not currently testable.
edit:
Quote (midtskogen)
It's a creepy thought that if we live in a 3D "plane", then someone with the ability to leave the 3D plane into the 4D world and to look down on us on the 3D "plane" from a 4D perspective would be able to see pretty much everything, through walls, inside our bodies, etc.
Ooh, have you read Flatland? 
Edited by Watsisname - Thursday, 28.02.2013, 23:55 |
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| Voekoevaka | Date: Friday, 01.03.2013, 01:04 | Message # 9 |
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| When I was young, I had another understanding of the observable universe :
When we observe farther, we see older things, and the limit of our observable universe is the light that cames from the big bang.
But, when we look in the past, distances are smaller, because the universe is growing. So, in the image, distance AB is smaller than distance CD., and the circle "Big bang" is just a point. Whatever the direction we look, the farthest we can see point is the same : the Big bang. So I imagine a representation where distance are accurate :
The (observable) universe is a sphere, us and the big bang are opposite points. The "big bang" point is creating new galaxies (blue arrows), because when we observe the edge of the observable universe, we see matter forming galaxies.
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| Watsisname | Date: Friday, 01.03.2013, 02:45 | Message # 10 |
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Galaxy Architect
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| Yes, the scale factor of the universe goes to zero as t goes to zero (the Big Bang), so your understanding is correct, but I would hesitate to use your second figure to show that for pedagogical purposes. It's too easy for a viewer to interpret it incorrectly, and in my opinion it doesn't provide very much insight that cannot be more easily gained from a simpler diagram.
To show the appearance of the universe from our perspective, I would use what you have in the first figure. The observable universe is a sphere with us at the center and the particle horizon at the edges. The figure represents this by removing the third spatial dimension without altering the actual geometry. Then all the observed consequences of the universe's evolution become intuitive when you consider the finite speed of light combined with the expansion of space. The figure clearly shows that we can look out in all directions and see the CMB, and the average separation of galaxy clusters decreases with z. The look-back time is as easy to read as the radial distance from the Earth, which is exactly how it works in reality.
To show the evolution of scale factor and composition of the universe with time and irrespective of the observer, I would use a diagram such as this:
Edited by Watsisname - Friday, 01.03.2013, 02:50 |
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| Voekoevaka | Date: Friday, 01.03.2013, 02:56 | Message # 11 |
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| I think the only thing that change between our two diagrams is the manner we represent the 4 dimensions. In your's, there are two pure space dimension and one pure time dimension. On mine's, there are two space-time dimensions (when you are moving in a meridian of the sphere, space is increasing while time is decreasing, these are relativistic coordinates x-ct).
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| midtskogen | Date: Friday, 01.03.2013, 07:53 | Message # 12 |
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Star Engineer
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| Quote (Watsisname) Ooh, have you read Flatland?
No, but, yes, precisely.
NIL DIFFICILE VOLENTI
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| lexrazor | Date: Friday, 01.03.2013, 14:38 | Message # 13 |
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Astronaut
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| What about the expansion theory? The whole expanding baloon, as they've shown it, pictures every object moving away from every other object. But when we think about it it doesnt make any sense because we see events such as galaxy collisions. Now if all the galaxies were moving away from each other how can they still collide? We dont see points on the surface of the baloon colliding with each other as it expands. So that would make the theory wrong, wouldnt it?
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| HarbingerDawn | Date: Friday, 01.03.2013, 15:05 | Message # 14 |
 Cosmic Curator
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| Quote (lexrazor) So that would make the theory wrong, wouldnt it?
No, it just means that you don't understand it; to say that everything is moving away from everything else is a gross oversimplification.
Quote (lexrazor) But when we think about it it doesnt make any sense because we see events such as galaxy collisions. Now if all the galaxies were moving away from each other how can they still collide?
Because the universe is expanding (which it is) does not mean that every single thing is moving away from every single other thing. That would only be true if the universe were perfectly homogenous, in which case it would be a uniform haze of atoms and other particles and we wouldn't be here to talk about it. Objects that are local to each other can still come together through mutual gravitational attraction. But two galaxies billions of light years apart will never meet, only move farther away.
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| lexrazor | Date: Friday, 01.03.2013, 17:27 | Message # 15 |
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Astronaut
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| Quote (HarbingerDawn) Because the universe is expanding (which it is) does not mean that every single thing is moving away from every single other thing. That would only be true if the universe were perfectly homogenous, in which case it would be a uniform haze of atoms and other particles and we wouldn't be here to talk about it. Objects that are local to each other can still come together through mutual gravitational attraction. But two galaxies billions of light years apart will never meet, only move farther away.
Noted. Boy do i feel uneducated now lol ^^"
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