[DoctorBeaver (OP)]

DrPhil, you seem to be a very knowledgable person whether or not you consider yourself over the hill.
I was being a bit facetious when I mentioned about them going phut. But can I bring another dimension (no pun intended) to this? If there is enough matter in the universe to cause it to be 1 giant black hole, wouldn't gravity gradually slow the photons down and eventually pull them back?

[DrPhil]

The answer to that question hinges on knowing the density of the Universe. To determine that I'd need to solve the problem of the "dark matter."  If I knew that, I wouldn't be here posting at The Naked Scientists Forum, I'd be on my way to Stockholm  to collect my Nobel Prize.

[DrPhil]

The big bang was not an explosion in space; it was more like an explosion of space. There are no fragments of a big bang bomb flying around. Matter was not ejected at varying speeds. The galaxies are not traveling through space away from us. Individual galaxies move around at random within clusters, but the clusters of galaxies are essentially at rest. Instead the space between the galaxies and us is expanding.

There is no center. It did not go off at a particular location and spread out from there into some imagined preexisting void. It occurred everywhere at once.

The redshift is not really a doppler redshift like you hear with a moving train whistle. It is a cosmic redshift which is a bit different than a doppler shift. As space expands, light waves get stretched. If the space doubles in size during the waves' journey, their wavelengths double and their energy is halved.

Unfortunately the terms "explosion", "receding" and "moving away" are used a bit too casually.  The distances to remote galaxies is increasing because space itself is expanding.

(because I'm lazy parts of this reply were copied from: http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=2&catID=2  ) []

[doughnut]

Thanks for the reply - the article is excellent just what I was looking for! []

But now I have a problem... so the theory is saying space is expanding between (say) galaxies, but it doesn't have within it anything to explain why the further away they are the faster they are receding, it just says that's the way it is, v=Hd right?

[rosy]

Well, I think my understanding of it can be explained in terms of lamp-posts.
If you think of a row of lamp-posts 10m apart and the space between any two lamp-posts is increasing at a rate of 1m/s then after 1s the nearest lamp-post is 11m away (1m further) but the 10th lampost down is 110m away (as each gap has increased by 1m) and so the gap between you and that lamp-post is increasing by 10m/s as opposed to 1m/s for the nearest one. And the same will apply if you look the other way down the street, or are in a carpark with lamp-posts in all directions... generalising lamp-posts to 3D may be a challenge, I'd best not try and push the analogy too far. But you get the idea (I hope).
I'm sure that misses all sorts of subtlties but it seems to provide at least a plausible answer to your problem.

[DoctorBeaver (OP)]

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*

[podboq]

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.

[DrPhil]

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.

[doughnut]

Hi Rosy, nice analogy thankyou - I get it!

[rosy]

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...

[neilep]

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 !!

[Sandwalker]

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.

[Quantum cat]

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.

[Ultima]

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?

[gsmollin]

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.

[Quantum cat]

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?

[DoctorBeaver (OP)]

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?

[gsmollin]

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.

[chimera]

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. []

http://frontwheeldrive.com/joao_magueijo.html

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

[gsmollin]

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.