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what part does the expansion of the Universe play in determining whether it is an open or a closed one?
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Re: closed or open universe
Wed, October 7, 2009 - 11:54 PMThis is a hotly debated issue in some circles. From one point of view the periphery of the sphere we consider to be our universe is expanding at the speed of light, at least from our reference in the near field. That being so there's obviously no way to probe on the other side of the expanding edge because the light from objects at this theoretical edge would never reach us. Some think the universe, our little niche, is an infinitesimally small spec in what has been termed a mega-verse, potentially expanding indefinitely.
I'd say it's a matter of belief as to which concept you adhere to because there's really no way to get a definitive answer with our current technology. -
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Re: closed or open universe
Thu, October 8, 2009 - 12:36 AMwould it be fair to assume that it is in no way logical to assert this be a closed system unless:
a) it has been entirely explored
b) the trajectory of expansion (??) has been conclusively proven
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Re: closed or open universe
Thu, October 8, 2009 - 1:40 AMWell, I don't think we have to worry about the universe being explored entirely.
I think the traditional view is we dwell within a closed and expanding system. Personally I'm not so sure. It has been shown by the red shift that the farther objects are from us, the more rapidly they're receding. -
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Re: closed or open universe
Thu, October 8, 2009 - 8:23 AMThe 3 dimensional Universe may be finite with no boundaries analogous to a two-dimensional spherical surface. It is finite (i.e. of finite volume), yet has no bounds. All points have an equivalence
Even if the Universe is finite without boundaries, it's so fricken vast it might as well be infinite for all intents & purposes.
Weirdly enough non-Euclidean geometry casts doubt on the infiniteness of space-time without coming into conflict with the machinations of thought or with the lessons of experience.
Also, there is yet another possibility - “elliptical space.” It can be regarded as curved space in which the two “counter-points” are identical (indistinguishable from each other). An elliptical universe can thus be considered to some extent as a curved universe possessing central symmetry. -
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Re: closed or open universe
Thu, October 8, 2009 - 8:27 AM<<“counter-points>>
i.e. "lobes" of the ellipse
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Re: closed or open universe
Thu, October 8, 2009 - 9:24 AMFor this issue, closed is roughly the same as finite like a sphere (no edges but finite surface area) whereas open is more like an infinitely large sheet of paper (no edges but infinite area). There is another possibility: semi-closed (for mathematics this isn't a valid option, something is either open or closed, but is a great one for the general public). To be semi-closed one needs an object like an infinitely long hose - closed (finite) in one or more directions but open (infinite) in the other.
What does all this mean:
In a closed universe both space and time are finite (there is a maximum size with a definite beginning and end).
In an open universe neither space nor time are finite (we expand forever).
In a semi-open universe we most likely would have a finite size but could evolve forever (infinite time).
Cosmological theories predict certain properties of the cosmic microwave background for each type of universe, and those predictions are testable. So far it looks like we are almost flat with no signs of being finite. -
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Re: closed or open universe
Thu, October 8, 2009 - 9:46 AMi don't know the first thing about it, but i agree that the Universe is flat AND takes human form. but anyway back to the topic..
am i correct in saying a closed Universe is not infinite (OR is it the immeasurable act of expansion that classifies a closed Universe as infinite?)
and regarding the closed Universe: how are boundaries verified? -
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Re: closed or open universe
Thu, October 8, 2009 - 12:09 PM"and regarding the closed Universe: how are boundaries verified? "
Being closed is not the same as having boundaries. A sphere has no boundaries along its surface (there is one perpendicular to the surface but that direction is meaningless to one living in the surface) but is closed.
"am i correct in saying a closed Universe is not infinite (OR is it the immeasurable act of expansion that classifies a closed Universe as infinite?) "
Yes, a closed universe is finite. However immeasurable is not the same infinite - one can have something that is immeasurable (counting the grains of sand on a beach) but still finite. -
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Re: closed or open universe
Thu, October 8, 2009 - 12:13 PM>Being closed is not the same as having boundaries. A sphere has no boundaries along its surface (there is one perpendicular to the surface but that direction is meaningless to one living in the surface) but is closed. <
but we are *inside* the container not upon a sphere.. -
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Re: closed or open universe
Thu, October 8, 2009 - 1:32 PMIt's far too easy to take these analogies a little too literally (including the famous balloon painted with dots analogy).
Nothing in general relativity says that a two-dimensional closed universe would have to exist as a sphere inside a three-dimensional space. It only only says that such a universe would have certain properties (e.g. periodicity) in common with such a sphere. I think as useful as these analogies are, it's ultimately best to think of the "closed" (as an example) universe as a three-dimensional space with the strange property that things which go off to the right eventually come back again from the left.
There's the issue of critical density also. Per Einstein's equations a finite universe must have a larger density of matter and energy inside it than an infinite universe would have. There is a certain critical density that determines the overall structure of the universe. If the density of the universe is lower than this value, the universe must be infinite, whereas a greater density would indicate a finite universe. "Open" and "closed" respectively.
So we should be able to answer the question of the universe being infinite or finite by measuring the density of everything around us and seeing whether it is above or below the critical value. This is true in principle, and measuring the average density of the universe is a very active field of research right now. The problem is that the measured density turns out to be pretty close to the critical density. Troy is right, presently the evidence seems to favor an infinite universe, but it is not yet conclusive. -
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Re: closed or open universe
Thu, October 8, 2009 - 1:42 PM"Lorraine: You're my density!" - George McFly Back To The Future -
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This is the maximum depth. Additional responses will not be threaded.
to James
Thu, October 8, 2009 - 3:42 PM<<"Lorraine: You're my density!" - George McFly Back To The Future>>
"If you can't have fun, you can't have anything." - James
Amen, brother.
Sincerely,
Serge
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Re: closed or open universe
Thu, October 8, 2009 - 10:02 PMWe said James.
As I'm rediscovering (I'm TAing General Chemistry Labs), there are many problems with presenting "advanced" material to the general audience - one may either oversimplify or undersimplify, both causing problems. -
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This is the maximum depth. Additional responses will not be threaded.
Re: closed or open universe
Fri, October 9, 2009 - 2:26 AMyea totally like yer all using terminology i'm not really familiar with.. but it's a discourse that saves you a lot of time when talking amongst those in your field.. anyway hope it's not botherin anyone to dumb it down a little while i grapple with the concepts here. just lemme know.
and thanks for the lesson so far :D
btw Serge why can't i join your astrophysics tribe? -
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Re: closed or open universe
Fri, October 9, 2009 - 9:17 AM"anyway hope it's not botherin anyone to dumb it down a little while i grapple with the concepts here. just lemme know."
The point of the tribe is to discuss science (normally QP) with everyone, so no it does bother us (or at least me).
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Re: closed or open universe
Sat, October 10, 2009 - 10:45 PMCali, sometimes it's not all that easy to 'dumb it down' - especially since many of the concepts are derived from complex math. Though, a few people, who are really good writers, are able to make some very complex concepts available to the general reader.
You should also remember that nobody really knows what is going on. In Feynman's QED - The Strange Theory of Light and Matter, he says, "It is my task to convince you *not* to turn away because you don't understand it. You see, my physics students don't understand it either. That is because *I* don't understand it. Nobody does." And though Feynman is specifically referring to quantum electrodynamics, I think his lead can be grafted upon many of the concepts that are discussed on this tribe.
The fact is that there are opposing points of view on the majority of concepts that fall under the umbrellas of quantum physics and cosmology, and you have to be patient, do a lot of reading, and be willing to listen to and evaluate opposing points of view for quite a while before you can even begin to get a handle about what people are talking about, let alone for yourself being able to determine correct and incorrect. And even then, you have to remember that Feynman said that nobody really understands.
One book I recently read that's aimed at the general reader is:
The 'Cosmic Landscape' by Leonard Susskind. He's very good at taking a large number of the type of concepts we're talking about and making them accessible to the general reader.
Also, tribes like this have been quite helpful as a complement to my personal studies. -
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Re: Feynman
Sun, October 11, 2009 - 12:02 PM<<... and you have to be patient, do a lot of reading, and be willing to listen to and evaluate opposing points of view for quite a while before you can even begin to get a handle about what people are talking about, let alone for yourself being able to determine correct and incorrect. And even then, you have to remember that Feynman said that nobody really understands. >>
Though after these words, I would like to say, "thank you Charles, it is a pleasure to have you around here", I must comment on one thing though. This is another trap people fall in - "… remember, Feynman said that nobody understands."
THIS is exactly what rams people, countries, generations, “mankinds” into the oblivion of dogma and fanaticism.
Now, has it been even a few hundred years ago, some of those who have no idea what they are talking about, would take those words LITERALLY, and it would send them into another spin of say, 2-3 thousand years, just like it happened to our civilization. And no reason, fire, or pain would prove them that what was originally meant is otherwise. It took almost 2000(!) years to shake off the stupidity of Greeks and Romans(!), and look how high we shot up in just a couple of centuries.
The important thing to understand/remember when quoting somebody is to realize WHEN those words were said.
At the point of time when HE said it, nobody understood it CORRECTLY/FULLY it is true; however, since then, a lot of time has passed, and by now, though still NOT-FULLY, but we DO understand it MUCH MORE, than we did BACK THEN when he was saying those words, and to add more to this – we are getting exceedingly more and more efficient at it, (discovery), (apply the Law of Accelerated Returns).
Science is A PROCESS, and the PROCESS has INTERMEDIATE stops, (read, achievements) - when we definitively find out that this or that is indeed that, and the way we have understood its inner workings IS indeed the correct way it really does. And as the result, we see the fruits and pleasures of our hard work of discovery, and intellectual inquiry into the world and Universe around us.
...
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Re: Feynman
Sun, October 11, 2009 - 10:48 PMYes, I agree with you with respect to QED. We understand more now than we did, yet the more we do understand (or think we understand), and the deeper that science penetrates, the more the new questions will present themselves. There will always be an edge; that's what's so fascinating about science. -
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Re: Feynman
Mon, October 12, 2009 - 11:40 AM<< ... that's what's so fascinating about science. >>
Exactomundo!..
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Re: closed or open universe
Fri, October 9, 2009 - 2:18 AM>things which go off to the right eventually come back again from the left<
is this how it goes?
>a finite universe must have a larger density of matter and energy inside it than an infinite universe would have<
but there is no basis for comparison.. right?
and how do they go about measuring this density?
and what is the difference between critical density and run of the mill density?
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