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View Full Version : The shape of the universe (prolly a dum question)



Drbuzz0
2007-Aug-30, 08:57 PM
I'm sure this is going to sound like an idiotic question, especially given it's rather basic and coming from someone who likes to think they're pretty knowledgeable about science in general...

What is the general shape of the universe? (or at least how it is theorized to be since observation can only tell you so much).

I realize this is tough because only so much can be seen and that much of the really deep objects are not in the same place they were when the light which we are not observing was produced. And also, granted that there is no "universal now" and you can't take a realtime snapshot of the universe...


but... if you could step back and look at the whole damn thing (not counting the fact that light speed is an issue) Is there a general shape?

I know there are some concept images of galaxy clusters and such but based on the whole "big bang" it seems it would be basically round.

This is overly simplistic: But if you imagine an explosion that throw matter out in all directions, what you will have is basically a sphere of matter flying apart. Most of the matter would be around the outside of the sphere and then as you move in you have less and less density of material and then something of a void in the center, from which the material was expelled.

Or have I missed something completely

01101001
2007-Aug-30, 09:28 PM
What is the general shape of the universe?

The visible Universe? The 3D geometry? Round as round can be. Spherical. With the tip of your nose very close to the center.


(or at least how it is theorized to be since observation can only tell you so much).

Oh, the entire Universe? The curvature and topology? Nobody knows.

Wikipedia: Shape of the Universe (http://en.wikipedia.org/wiki/Shape_of_the_universe)

http://upload.wikimedia.org/wikipedia/commons/thumb/9/98/End_of_universe.jpg/275px-End_of_universe.jpg (http://en.wikipedia.org/wiki/Shape_of_the_universe)


Considerations of the shape of the universe can be split into two parts; the local geometry relates especially to the curvature of the observable universe, while the global geometry relates especially to the topology of the universe as a whole—which may or may not be within our ability to measure.


But if you imagine an explosion that throw matter out in all directions, what you will have is basically a sphere of matter flying apart. Most of the matter would be around the outside of the sphere and then as you move in you have less and less density of material and then something of a void in the center, from which the material was expelled.

Stop.

Cosmology Primer FAQ (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html)


Does the universe have a center?
No. Our observable universe looks basically the same from the point of view of any observer. We see galaxies moving away from us in all directions, but an astronomer living in any one of those galaxies would also see all the galaxies (including our own) moving away from them. In particular, the Big Bang is not an explosion that happened at some particular point in space; according to the Big Bang model, the entire universe came into existence expanding at every point all at once.

Cougar
2007-Aug-30, 09:45 PM
Oh, the entire Universe? The curvature and topology? Nobody knows.
This is not to say that this is not an area of active research. To get an idea of just how active, you'll want to read Janna Levin's How the Universe Got Its Spots (http://en.wikipedia.org/wiki/Janna_Levin).

Astaro
2007-Aug-30, 10:36 PM
My understanding Is that the global geometry of the universe has 3 properties that we believe to be true.

Its 'flat', omega, as in the diagram above, is equal to 1, which means that, ignoring the influence of gravity, a pair of parallel light beams will remain parallel indefinitely.

It is infinte in extent. those light beams are going to keep going.

and its closed, those infinte extents are wrapped up in themselves.

the simplest geometry that has these properties is a torus, or donut shape.

I'm not sure if this is actually meaningfull however.

01101001
2007-Aug-30, 10:40 PM
My understanding Is that the global geometry of the universe has 3 properties that we believe to be true.

Do you have some citation for what we believe, or is it only your understanding?

mugaliens
2007-Aug-31, 08:12 AM
Due to expansion (whatever the true nature of it's cause), I believe the observable universe (within the Hubble Limit) is shaped like the top of the Superdome (slight curve), upon which any observer always sits in the middle, and all the rest of the marbles are slowly rolling off the edge.

Astaro
2007-Aug-31, 08:43 AM
01101001,

Well, that's what they said on astronomycast.

01101001
2007-Aug-31, 01:34 PM
01101001,

Well, that's what they said on astronomycast.

I don't listen to it. If they said it, I doubt it was with the same conviction as you put it. I think it had to more speculative. I'm not a cosmologist, but a quick scan of accessible literature tells me those are still unknowns.

If you can provide another source of such seeming certainty to read, I'd appreciate it. Did the speakers cite literature? Even the audio file with approximate starting time is welcome so I can understand the context of what you understood to be true beliefs.

Delvo
2007-Aug-31, 02:47 PM
Funny thing about the idea of an infinite universe... it would mean the pre-Big-Bang "point" was also already infinite, since it couldn't ever expand fast enough or long enough to reach infinity from a finite starting point.

Cougar
2007-Aug-31, 03:37 PM
If you can provide another source of such seeming certainty to read, I'd appreciate it.
The CMB and WMAP's 3rd year results (http://map.gsfc.nasa.gov/m_uni/uni_101shape.html) provide a pretty tight constraint on the flatness of the universe.....



The WMAP spacecraft can measure the basic parameters of the Big Bang theory including the geometry of the universe. If the universe were open, the brightest microwave background fluctuations (or "spots") would be about half a degree across. If the universe were flat, the spots would be about 1 degree across. While if the universe were closed, the brightest spots would be about 1.5 degrees across.

Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 2% margin of error.


The "infinite" or "expanding forever" seemed pretty well established (due to lack of enough matter to adequately slow the expansion), even before the expansion was seen to be accelerating. I'm not a big fan or authority on infinity.... One can circle the Earth forever, yet the surface of the Earth is finite.... :think:

As to "closed".... um, I should know this, but is there a difference between geometrically closed and topologically closed?

01101001
2007-Aug-31, 03:49 PM
The CMB and WMAP's 3rd year results (http://map.gsfc.nasa.gov/m_uni/uni_101shape.html) provide a pretty tight constraint on the flatness of the universe.....



We now know that the universe is flat with only a 2% margin of error.

Yeah, but we're not certain, which is the flavor I got from Astaro.

It is still unknown, right? It's certainly close to flat. But, to me that is a long ways from flat.

Cougar
2007-Aug-31, 04:16 PM
Well, here's an interesting paper by some crazy Russians.... :)


Title: Estimation of the size of the Universe from a topological viewpoint
Creator/Author: Sokolov, D.D. ; Shvartsman, V.F.
Publication Date: 1974 Feb 01
OSTI Identifier OSTI ID: 4342946
Other Number(s) CODEN: ZETFA
Resource Type: Journal Article
Resource Relation: Zh. Eksp. Teor. Fiz., v. 66, no. 2, pp. 412-420
Subject: N56600 --Physics (Astrophysics & Cosmology)--Cosmology; UNIVERSE-- TOPOLOGY;SHAPE
Description/Abstract: An analysis of the observational data shows that the data are consistent with the hypothesis that the space of the Universe is a topologically closed structure of small size (irrespective of the curvature sign) which is glued together''.A lower limit on the modern size of the Universe is obtained.The maximal and minimal gluing parameters'' of the world are not less than 400 and 10 Mps respectively.Thus it is possible that the maximal diameter of the Universe is 10 times smaller than the distances to distant quasars usually assumed. If the true gluing parameters are really of the magnitude mentioned above then some of the sources with a redshift z > 0.0025 and all sources with z> 0.1 are ghosts'' of long extinct and also of existing objects which are close to us. Several dozens of ghosts may correspond to a single object on the celestial sphere.(auth)
Language: Russian
System Entry Date: 2001 Jun 03

I believe Janna Levin et al. have ruled out the values of "gluing parameters" cited above. I believe a careful inventory and comparison of distant objects has ruled out a topologically finite universe with a diameter much less than that of our visible universe.

Cougar
2007-Aug-31, 04:31 PM
Yeah, but we're not certain, which is the flavor I got from Astaro.
Well, right. Astaro said....

My understanding Is that the global geometry of the universe has 3 properties that we believe to be true.
I suppose scientists might "believe" something to be "true," but science is not really in the business of "Truth."


It is still unknown, right? It's certainly close to flat. But, to me that is a long ways from flat.
Well, a little context is in order. Ten or 20 years ago, we had no idea whether the universe was open, closed, or flat. All the textbooks said, "The universe is either open, closed, or flat," but there were no solid observations to hang one's hat on. Now, with observed variations in the CMB we're pretty sure to an accuracy of 2%. This is a significant advance. I'd hate to be a writer of Astronomy textbooks, which are likely going to be out of date by the time they're published.

BobbyGene
2007-Aug-31, 05:15 PM
I'm sure this is going to sound like an idiotic question, especially given it's rather basic and coming from someone who likes to think they're pretty knowledgeable about science in general...

What is the general shape of the universe? (or at least how it is theorized to be since observation can only tell you so much).

I realize this is tough because only so much can be seen and that much of the really deep objects are not in the same place they were when the light which we are not observing was produced. And also, granted that there is no "universal now" and you can't take a realtime snapshot of the universe...


but... if you could step back and look at the whole damn thing (not counting the fact that light speed is an issue) Is there a general shape?

I know there are some concept images of galaxy clusters and such but based on the whole "big bang" it seems it would be basically round.

This is overly simplistic: But if you imagine an explosion that throw matter out in all directions, what you will have is basically a sphere of matter flying apart. Most of the matter would be around the outside of the sphere and then as you move in you have less and less density of material and then something of a void in the center, from which the material was expelled.

Or have I missed something completely



You probably haven't missed much other than an agreement on a few ideas. Generally we think of shape as some kind of visual geometrical shape. This doesn't seem to fit very well with the idea of the Universe having a shape.

The Universe is generally said to include everything in it. This includes, of course, light. When we say that the Universe is about 12 billion light years old, we mean that light from the Big Bang was emitted 12 billion years ago. This implies that the shape of the Universe is determined by the distance light has traveled since the Big Bang.

The shape of the Universe really becomes murky when you imagine that you are standing at one edge of the Universe and looking outward. What you are seeing, of course, is not what the Universe is now, but is what the Universe looked like in the past. Of course, the further out you look, the further back into the past you are looking.

I don't think light is a very good way to provide a shape to the Universe. Light moves a little then is curved by gravity, moves a little further, is curved by gravity again, and continues this rather round-about journey. It seems to me that we aren't going to get very far with a visual description of the shape of the Universe.

Einstein's field equations are generally used to provide answers to questions about the shape of the Universe. You don't have to be Einstein to get a general idea of these equations. In their simplest form, the equations consider how the Universe changes over time. The things they consider are how mass, energy, and gravity interact. This seems to imply that gravity is the primary player. If so, then the Universe probably shouldn't be looked at as having a visual shape, instead the shape of the Universe should be looked at as an interplay between mass, energy and gravity.
:lol:

BobbyGene
2007-Aug-31, 05:42 PM
Funny thing about the idea of an infinite universe... it would mean the pre-Big-Bang "point" was also already infinite, since it couldn't ever expand fast enough or long enough to reach infinity from a finite starting point.

Now don't go getting picky. :lol:

The Law of Conservation of Energy states that energy can neither be created nor destroyed. If this is so, then there can have been no beginning else energy would have been created. Neither can there be an end else energy will be destroyed. This implies that there can be no limit to the amount of energy.

This is not to say that time and energy are restricted to our Universe. I can speculate on where our Universe exists within a larger "place", but that is not the question.

It is difficult to conceive of something that has always existed. It is probably as difficult to conceive of something that has no limits. But if the Law of Conservation of Energy is true, what other explanation can there be?

As to singularities. Planck found that energy come in little packets as defined by Planck's constant. If this is true then there can be no amount of energy smaller that that defined by the constant. Energy does not have time or distance, but the effect is described in terms of these factors. Since a force, and energy is a force, is described by time and distance, you can vary either as long as you make a corresponding change to the other. As an example, you can make the time smaller as long as you make the distance larger. Physicists usually define the packet of energy as found by Plank using a second as the time factor. The distance factor can then be computed.

The development of the early Universe seems to be governed by both Quantum Mechanics and Gravity (usually referred to as Relativity). The idea is that the Laws of Quantum Mechanics best describe the earliest instants of the Universe and the Laws of Relativity were minimal. As the Universe expanded, the Laws of Quantum Mechanics began to lessen and the Laws of Relativity began to grow stronger. Some books put the beginning of our Universe as the point where the Laws of Quantum Mechanics and Relativity were equal. The general idea is before this point, when the Universe was governed by Quantum Mechanics, the only descriptions that could be made were probabilities. After Relativity took over, descriptions such as time and distance were better descriptions.
:lol:

John Mendenhall
2007-Aug-31, 05:49 PM
Making full use of the 'weasel' words (see WIKI) the universe is probably:

1. Spherical, in the sense that we see the same distance in all directions.

2. Flat, in three space dimensions.

3. Infinite, in the sense of unbounded.

Now, you can have a lot of fun bandying other ideas about. Many good physicists have. But for simplicity, a la Occam, the above will do fine.