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Author Topic: Photons & the end of the universe  (Read 4114 times)

Offline DoctorBeaver

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Photons & the end of the universe
« on: 17/06/2005 12:14:02 »
Photons travel at C & nothing with mass can do that. It follows, therefore, that in the expansion of the universe photons will always outrun matter. If, as has been hypothesised, the universe may one day collapse back in on itself, what would happen to the photons that were furthest away? Would the collapse suck the whole of spacetime back in on itself or just the ordinary matter in the universe?
Also, if current theories are correct and there is such a thing as dark matter which is immune from gravitational effects (e.g. WIMPS), what would happen to that if the universe collapsed? Wouldn't you be left with a universe consisting solely of photons & non-gravitational (sorry, I don't know the correct terminology) dark matter with everything else collapsed back into a singularity? And would that dark matter universe continue to expand?


 

Offline DrPhil

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Re: Photons & the end of the universe
« Reply #1 on: 19/06/2005 14:15:28 »
>> dark matter which is immune from gravitational effects
How can dark matter be immune from gravitational effects? I thought the whole point of the dark matter theory was to explain the gravitational motions of galaxies that can't be attributed to all the visible matter. Doesn't the theory predict that it will be the gravitational force of the dark matter that will cause the universe's expansion to reverse, leading to an eventual Big Crunch? If dark matter is immune to gravitational effects then doesn't the whole idea of a big crunch go away?

Aren't WIMPs called "weakly interacting" because they feel only the weak force and gravity... i.e. they are hard to detect because they are not sensitive to electromagnetic forces.

And since when did photons cease to be affected by gravitational fields?
« Last Edit: 19/06/2005 17:43:12 by DrPhil »
 

Offline realmswalker

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Re: Photons & the end of the universe
« Reply #2 on: 20/06/2005 05:16:45 »
actually i read some where dark matter is just the outline or somethin of another dimension...or somethin (im probably wrong)
 

Offline DoctorBeaver

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Re: Photons & the end of the universe
« Reply #3 on: 20/06/2005 09:14:19 »
Dr Phil - I wasn't suggesting that photons aren't affected by gravity but that by the time the matter in the universe begins to collapse back in on itself the photons may be far enough away not to be affected by the gravitational forces.
As for dark matter, maybe I misunderstood what I read. I took it to mean that WIMPS are either not affected by gravity or are affected to such a small extent that it's negligible.
 

Offline DrPhil

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Re: Photons & the end of the universe
« Reply #4 on: 20/06/2005 11:56:29 »
DrBeaver - I didn't mean to come across as being so critical. Just that your theory didn't jive with my limited and somewhat dated knowledge of dark matter. I posted those questions with the hope that someone might drop by and clear it up.

It would seem to me that if space itself stops stretching and begins to spring back on itself then that means that the whole universe will shrink.  The photons, dark matter and everything else will get closer and closer together as the space between them shrinks. In the end, the whole universe and everything in it will be concentrated at a single point.
« Last Edit: 20/06/2005 11:57:20 by DrPhil »
 

Offline gsmollin

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Re: Photons & the end of the universe
« Reply #5 on: 20/06/2005 13:42:31 »
DrB is referring to some sort of gravitational collapse that does not involve the universe. A "universal" gravitational collapse takes everything- by definition. For the sake of argument, lets assume DrB is writing about a collapse as large as the observable universe. What would collapse, and what would escape?

I suppose you can answer that by saying that gravitationally bound matter would collapse. Matter and energy not gravitationally bound would escape. Electromagnetic energy moves at c and has low mass, so it is most likely to escape. Next comes elementary particles such as neutrinos, which move either at c, or nearly so, and have zero to very small rest mass. After that comes baryonic matter that has been accelerated to a large fraction of c by high energy processes, such as cosmic rays. Least likely to escape would be the baryonic matter bound into stable structures such as galaxies, and the galactic haloes made of dark matter.

At some point in this collapse, the density of matter would reach a critical point, and the escape velocity of the collapse would reach c. After that, nothing would escape, and the matter and energy would become crushed to a singularity.

I have omitted dark energy from the above hypothetical scenario. I don't know what to say. Dark energy is reputed to be repulsed by gravity, at least at long distances, so I suppose the dark energy would escape. Actually, dark energy is reputed to make a large gravitational collapse impossible, and this scenario will never happen.
 

Offline DoctorBeaver

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Re: Photons & the end of the universe
« Reply #6 on: 21/06/2005 08:30:53 »
GSMolin - that was indeed my point... Is there anything that says a collapse would shrink the whole universe rather than just pulling the matter back in?
 

Offline qpan

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Re: Photons & the end of the universe
« Reply #7 on: 28/06/2005 16:33:10 »
Photons are not affected by gravity, else the speed of light would change with distance.
Photons are, however, affected the the warping of space-time due to gravity, thus causing their path to curve when passing near massive objects.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
 

Offline DrPhil

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Re: Photons & the end of the universe
« Reply #8 on: 03/07/2005 12:44:54 »
quote:
Originally posted by qpan

Photons are not affected by gravity...
Photons are affected by the warping of space-time and the warping of space-time is due to gravity.

Therefore photons ARE affected by gravity.
 

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Re: Photons & the end of the universe
« Reply #8 on: 03/07/2005 12:44:54 »

 

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