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Author Topic: Very basic cosmology question  (Read 35831 times)

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #25 on: 17/04/2005 00:47:46 »
Rosy, that's a pretty good analogy. At each point in space the expansion is uniform (I think) so the further away any 2 objects are the more space there is between them to expand. I've seen it explained as dots on a balloon. Assume the dots are equally spaced when the ballon is flaccid. As the balloon is inflated the surface between each dot expands uniformly. Therefore neighbouring dots will move apart slower than dots that are on opposite sides. *wonders if, as the universe is at least 4-dimensional, time expands too*
 

Offline podboq

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Re: Very basic cosmology question
« Reply #26 on: 17/04/2005 08:21:38 »
I've been reading through posts, and I"m nearly sure someone has said the same thing as I'm about to say....

The universe is as big as it's light sphere, outside which, not even light has reached.  The space outside the material universe is void.  I've heard that the universe is likely 12-14 billon years old, meaning it's exactly  24-28 billion light-years in diameter, and is a perfect sphere.

It might be possible to reach the light-shell of the universe if we were able to travel at the speed of gravity.

newbielink:http://www.ldolphin.org/vanFlandern/gravityspeed.html [nonactive]

conclusion reached and referenced in this page:  The speed of gravity is  2x10(to the 10th) c.
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #27 on: 17/04/2005 14:11:45 »
Here again is the confusion. You are thinking of it as some sort of explosion with light and matter flying away from some central point. That is not the case.

However, there is a limit to what we can see, a.k.a the observable universe. If space were not expanding, the most distant object we could see would be about 14 billion light-years away from us, the distance from which light could have reached us in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon on its way here expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.

There is no reason to expect that the space beyond the edge of our observable universe is any different than the space that we can see.
 

Offline doughnut

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Re: Very basic cosmology question
« Reply #28 on: 17/04/2005 17:10:55 »
Hi Rosy, nice analogy thankyou - I get it!
 

Offline rosy

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Re: Very basic cosmology question
« Reply #29 on: 17/04/2005 19:16:38 »
I was interested by the inflating balloon analogy... my Dad was telling me that one explanation of red shift being given to GCSE students is that red shift is due to the expansion of the universe resulting in the increased wavelength of light.
When I was doing GCSEs, I was given an explanation in terms of the red shift being due to the wavelength increasing as the light source moved away over the period of the radiation.

I think I've convinced myself that the two could be equivalent but thinking about it makes my head hurt, and I don't have time to try to do the maths (which I suspect is either trivial or hideous). If anyone has a short explanation that makes it obvious it'd be nice to see it...
 

Offline neilep

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Re: Very basic cosmology question
« Reply #30 on: 17/04/2005 21:05:05 »
I think we're all inside one inflating paper bag and one day it's going to go ' pop '......but, may I ask ( activates total layman mode !!)...if the Universe is expanding then isn't it in danger of being diluted  ? and could it dilute so far that it just falls apart ?....if it's not being diluted then where is the extra ' space ' coming from to aid the expansion ?

Great thread by the way.

Men are the same as women.... just inside out !!
 

Offline Sandwalker

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Re: Very basic cosmology question
« Reply #31 on: 17/04/2005 22:00:10 »
In m-theory/string theory there is a symmetry/duality (Supersymmetry) between the large and the small that says strange things like the large is equivilant to the small and vise-versa.

Perhaps the same could be said of our universe, it is experienced/exists in our space-time frame in a multi-dimensional superlarge changing state, but to a photon it has neither dimensions nor change as they exist (traveling at c) in null spacetime.

This would mean (classical physics) that it exist at all points in its path at the same time and that all points in its path are the same point! If we add quantum mechanics then all possible points in all possible paths in the universe exist thus.

This sounds like a singularity to me.  

Its all a matter of perception.
 

Offline Quantum cat

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Re: Very basic cosmology question
« Reply #32 on: 18/04/2005 10:29:29 »
I had always imagined the big bang as a balloon inflating. The 2D surface was like our 3D space, and it was expanding into 4D which we couldn't understand much like a stick figure drawn on said balloon can't understand us looking at him. It would mean though that space *was* expanding into something, hyperspace, and as you guys have said, the universe *is* everything and it's not expanding *into* anything. It also means there's a finite number of cubic metres of space like square centimetres on a balloon, and you guys said that space is infinite. Also, there would be a centrepoint, the centre of the balloon, that we couldn't understand because it'd be in hyperspace not on the surface. And it has been repeated that there is no "centre".

I guess this has been asked many times, but, if the universe isn't expanding into anything, (no reference point) how can we possibly tell that it is expanding? We have to have some sort of thing to compare to say if something is bigger or smaller, don't we? If I doubled in size, but my ruler doubled too, I wouldn't be able to tell that I had grown. Come to think of it, on a balloon the pen-dots are increasing in diameter too, so they wouldn't realise the distance between them was growing bigger. So if the space between two galaxies doubled, they wouldn't care because they would double in size too.

Oh wait a sec, I've just realised something, duh, light doesn't change! That's our reference point. :-)

Hey maybe now I can make a sketchy explanation about elecromagnetic radiation. All other waves are movements of the drawings on the balloon, so they require lines on the balloon to propagate. But light doesn't because its medium is the balloon itself!! As the balloon gets bigger, the wave is slows down (not really still same distance per time, but distance is increasing) and is stretched which makes the redshift, which is what someone else said here with space increasing... Maybe now I can get a sketchy explanation of magnetic and electric fields and why they work.. one is up/down movement of ballon-space, other is left-right (on a 2D balloon, just up/down). We can't "see" these movements because we're stuck in 3D like a paperman can't tell he's being folded... maybe charges are attracted in an electric field because the paper is being tilted? I can't imagine how moving charges make magnetic fields and moving magnets generate electric fields but that must be a special 3D/4D thing that we can't make analogies with 2D/3D for. Can anyone think of an analogy for magnetic and electric fields? I have ALWAYS wanted to understand them but so far none of my science teachers have been able to explain to me why they happen.
 

Offline Ultima

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Re: Very basic cosmology question
« Reply #33 on: 18/04/2005 11:19:55 »
What are your teachers doing??? You can "see" a magnetic field if you get some iron filings, then you don't need to imagine you can see it in 3D. You can sort of do the same with electric fields by measuring the potential difference with a probe to get an idea of what it looks like. You get magnetic fields when any current flows, but electric fields are present even if there is no flow of current, but if there is any difference in potential caused by more positive or negative charge. Imagine electric fields doing to charged stuff as the same as what gravity does to mass, but instead it can be attractive or repulsive.


Wish I found this link for A Level Physics it pretty much sums up classical stuff about electro magnetic fields:
http://lchc.ucsd.edu/fleetu/10_3_01.pdf


wOw the world spins?
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #34 on: 18/04/2005 18:13:00 »
This is an interesting thread, and here is an interesting fact to add. The cosmic background radiation, CBR, is the most ancient radiation we can see. When we look at the CBR with microwave equipment, we see a dipolar moment in the radiation, that equates to a Dopler shift cause by earth's movement through the CBR. The speed of this movement is ~178 miles per second, more or less. Look it up if you want the exact figure. The point is that it's an entirely pedestrian speed by red-shift-recession standards, of 0.9 C for distant galaxies. I had to wonder if we had not finally found the one true fixed reference that Newton looked for.

Then I wondered what scientists on those distant galaxies would see when they measured their speed through the CBR. 0.9C? I think not. I rather think they will measure ~178 miles/sec. They will also see us receding from them at 0.9 C.
 

Offline Quantum cat

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Re: Very basic cosmology question
« Reply #35 on: 18/04/2005 22:47:32 »
Yes I know what electric and magnetic fields do ... but why do they do that? Where does the energy to make the force come from?
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #36 on: 19/04/2005 02:07:42 »
gsmolin - can you explain a bit more about that 178mph thing? It does seem ridiculously slow. I'm aware of CBR, what it is & that we are moving relative to it. But surely, the CBR must be moving too, in line with universal expansion? Therefore it cannot be an absolute. Or am I misunderstanding CBR?
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #37 on: 19/04/2005 13:07:32 »
That's miles per second, and its probably not the right number anyway. I found a number here: http://pdg.lbl.gov/2002/microwaverpp.pdf
They claim 371 +/- 0.5 km/s.
This number is not a measure of the recession-speed of the CBR, but rather the earth's movement relative to it. There is a solar system dipole moment, and a galactic dipole moment, also a galactic radiation which tends to obscure the CBR.

The point is that nobody is moving a very great speed through the universe. This has been known for some time, long before the CBR missions measured it so well, and was used as an argument for the steady-state theory. I remember the steady-state argument describing the ridiculous requirements in velocity changes that the big-bang theory required, and how there was no way that could have happened. Then Alan Guth discovered inflation, and that's exactly what happened.
 

Offline chimera

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Re: Very basic cosmology question
« Reply #38 on: 20/04/2005 02:34:47 »
quote:
Originally posted by gsmollin
Then Alan Guth discovered inflation, and that's exactly what happened.



Unless Joao Magueijo's Variable Speed of Light theories pan out, of course. 8)

http://frontwheeldrive.com/joao_magueijo.html

Currently reading his book, and must say he sounds like a pretty sane fellow for a crank.
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #39 on: 21/04/2005 01:31:02 »
If light speed were dependent upon the phase of the space-time, it would solve the horizon problem. If this theory can solve the flatness problem, and produce the correct spectrum in the CBR as inflation does, then it could be viable.

Inflation certainly requires a paradigm shift in thinking. Variable c is actually easier to swallow. I would expect that such a theory would reduce to SR and GR at this epoch.
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #40 on: 24/04/2005 14:13:30 »
Now I'm totally lost. I think I'll go back to contemplating my navel!
 

Offline chimera

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Re: Very basic cosmology question
« Reply #41 on: 24/04/2005 23:45:03 »
Oh, don't be, DoctorBeaver, no need, I think. It just means that some people, not only Joao, think that in the beginning (ahem) the speed of light may have been different to what it is now. A lot different, as in numbers with lots of zeros in the power dept.

And he makes a decent case, solving indeed both 'horizon' and 'flatness' problems, gsmollin.

(These two problems arise from our normal big bang model, and are explained by Alan Guth's inflation model - although in all honesty the current accepted model is not the same as his original, far from it.)

Quite elegantly even. Also, fortunately, at some point of developing his theories he decided that sticking within the standard SR and GR framework as much as possible was not only a somewhat smarter 'career move', it actually made things easier. BTW Einstein had his own VSL in 1919, did you know that? Different from Magueijo's, and totally ignored as an aberration these days, if remembered at all.

The first part of the book does not mention his theories at all, btw - it is by far the most brilliant general introduction to get up to speed with current theory I've seen so far. His own stuff in part two is much less clearly written, more blog-style, but still comes out, discarded version after discarded version, which is a bit much, after a while.

Yet, as gsmollin also remarked, c having been/being variable is actually even easier to digest than inflation alone. Especially the 'flatness' problem becomes self-regulating, like a thermostat, turning a very improbable scenario into something almost inevitable.

Yep, recommended read, all in all.

 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #42 on: 25/04/2005 17:31:14 »
Could the speed of light have depended on the folding up of certain dimensions in the early life of the universe? I've heard it mooted that certain constants could be the values they are as a result of the size of these other dimensions. I've also heard the theory that these values could leak into our universe from another universe. Any thoughts on that?
 

Offline chimera

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Re: Very basic cosmology question
« Reply #43 on: 26/04/2005 22:10:37 »
[scratches head]

Hardest thing to answer. Dimensions, normally 3 spatial and 1 time, can also be folded as infinitesemal small loops (Calabi-Yau Manifolds), but essentially that's sweeping unwanted stuff under the rug.

Scientists like their stuff as 'tensor-free' as possible, but they do not always know exactly what to do with certain parameters or constants.

You cannot really say whether those values 'leak' or 'create' our universe, or that saying it all comes from some other 'universe' is a meaningful statement. It's unfalsifiable, and out of our reach, except to speculate.

John Barrow (also mentioned in book in the previous posting btw) would be the person who can be seen as our most dedicated thinker maybe as to what constants of nature are, and how they influence what you can do - electromagnetics works in three dimensions, but also in only 1, interestingly. Stuff like that.

http://www.isepp.org/Pages/03-04%20Pages/Barrow.html

[rant]
Personally I suspect we do not live in a strictly 3-dimensional world, with time slapped on as some ghostly fourth, but in a universe with Pi dimensions, 'decohering' (for lack of a better word) continuously, and imperfectly, to  a 3 dimensional one. Hence the illusion of time, and change. Never liked time. Out with it.
[/rant]
« Last Edit: 26/04/2005 22:12:34 by chimera »
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #44 on: 01/05/2005 21:51:37 »
Ah, dear John. As far as I'm concerned he may as well be talking a foreign language. That's the trouble with those Cambridge University academics (myself & Dr Chris excluded, of course!)
 

Offline chimera

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Re: Very basic cosmology question
« Reply #45 on: 02/05/2005 09:21:28 »
Strangely, I find JB extremely easy to read. He's quite an author, keeps the math digestible, and has the best science quotes in the business. He also tells you up-front when he is as clueless as the next guy.

What more could one want? A brain transplant? It's heady stuff, but certainly not intended only for those 'slightly-out-of-touch-with-RL'.

Compared to some of my ideas JB's are totally mundane, parochial even. And those, in turn, pale in utter weirdness to some coming from other quite respectable and accredited sources, believe me.

What worries me is whether they are crazy ENOUGH.
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #46 on: 02/05/2005 13:34:15 »
quote:
Originally posted by chimera

...And he makes a decent case, solving indeed both 'horizon' and 'flatness' problems, gsmollin.

(These two problems arise from our normal big bang model, and are explained by Alan Guth's inflation model - although in all honesty the current accepted model is not the same as his original, far from it.)...

... BTW Einstein had his own VSL in 1919, did you know that? Different from Magueijo's, and totally ignored as an aberration these days, if remembered at all....




I knew that. I think it is most interesting that inflation was "invented" during a panicky time in Guth's post-doc career, when he discovered his GUT was not working. It's been said that necessity is the mother of invention.

I didn't know that. Do you have a source for this?
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #47 on: 02/05/2005 14:15:46 »
I have to agree. His work is certainly more readily understandable than Hawking
 

Offline chimera

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Re: Very basic cosmology question
« Reply #48 on: 02/05/2005 14:32:07 »
quote:
Originally posted by gsmollin
[1] I knew that. I think it is most interesting that inflation was "invented" during a panicky time in Guth's post-doc career, when he discovered his GUT was not working. It's been said that necessity is the mother of invention.

[2] I didn't know that. Do you have a source for this?



[1] If you read Magueijo, you'll see he was quite a Don Quichote with his inflation theory, taking incredible risks that could have ended his career in utter ignomy. He was very, very fortunate to convince just the right people at just the right time.

[2] It was 1911, sorry, the year he spent lecturing in Prague.

(mentioned btw in this review as well)
http://www.thegreatdebate.org.uk/VSLReview1.html

He published a paper and all, which described his VSL. I checked Einsteins publications list, but none of them from that year rings a bell, so you'll have to check the book (I already returned it to the Library).

Einsteins complete list of publications (German, pdf):

http://www.einstein-website.de/z_physics/AEWisPub-04.pdf

addition: the name of the paper is not mentioned in Magueijo's book, alas, but it is described in the works of Banesh Hoffmann, Einstein's collegue and biographer. He called it: 'Heresy! By Einstein himself!'
(Just looked it up. Ofcourse the book was in, noone but us cranks reads stuff like that, after all...:-)
« Last Edit: 02/05/2005 19:35:21 by chimera »
 

Offline chimera

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Re: Very basic cosmology question
« Reply #49 on: 02/05/2005 14:41:46 »
quote:
Originally posted by DoctorBeaver

I have to agree. His work is certainly more readily understandable than Hawking



Lee Smolin said it best: GR describes that what moves, QM that which exists.

Therefore, using GR for describing non-moving things like black holes does not only seem inappropriate, but could indeed take some mathematical gymnastics that NOONE can follow - quite possibly because it is utter balderdash, ofcourse.

(This is not directed at RP btw, only SH...)
 

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Re: Very basic cosmology question
« Reply #49 on: 02/05/2005 14:41:46 »

 

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