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Black holes and redshift (2 questions)

by grichblix
Tags: black, holes, redshift
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grichblix
#1
May28-10, 11:39 AM
P: 2
I'm new to the Physics forum and thought I'd start with a couple of stupid questions.

1) How do we know that black holes are not stellar nuclei that have just collapsed to a volume less than their mass dependent Schwarzschild radius (the radius at which the speed of light is the escape velocity) rather than actual singularities? In terms of physics the two types of objects would behave identically, but the former could actually be quite large in physical volume...

2) For cosmic redshift - we attribute it to an expanding universe, but to me it seems there is another equally plausible explanation. what if the redshift of distant objects, which are also far back in time, refers to the relative velocity of the objects at the time the light was emmitted? It would imply a much larger universe, but one that has gradually slowed in expansion to an almost equilibrium point at present rather than an accelerating expansion.

If there are already answers to these questions could you please point me in the right directions?

Thank you
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brother time
#2
May28-10, 11:59 AM
P: 44
I'm new too and I don't know the first one, but I think the second could be correct. could you clarify one thing about it? How would the object lose it's velocity without interaction with other bodies?
cheers, BT
Calimero
#3
May28-10, 04:40 PM
P: 256
Quote Quote by grichblix View Post
1) How do we know that black holes are not stellar nuclei that have just collapsed to a volume less than their mass dependent Schwarzschild radius (the radius at which the speed of light is the escape velocity) rather than actual singularities? In terms of physics the two types of objects would behave identically, but the former could actually be quite large in physical volume...
We don't know that. Event horizon acts like censor. We can't see what is going on inside.

Quote Quote by grichblix View Post
2) For cosmic redshift - we attribute it to an expanding universe, but to me it seems there is another equally plausible explanation. what if the redshift of distant objects, which are also far back in time, refers to the relative velocity of the objects at the time the light was emmitted? It would imply a much larger universe, but one that has gradually slowed in expansion to an almost equilibrium point at present rather than an accelerating expansion.
Why don't you ask first: How do we know that expansion of universe is accelerating?

http://en.wikipedia.org/wiki/Accelerating_universe

grichblix
#4
May28-10, 06:12 PM
P: 2
Black holes and redshift (2 questions)

I've looked at all the wikipedia articles on the accelerating universe, etc., as well as some other reading. From what I understand, the evidence that the expansion of the universe is accelerating are based on similar redshift measures as the expansion in the first place. Unless there is some evidence that the redshift for individual type 1A supernovas is changing, which would imply a change in velocity, I don't see how else they can address it. Do you have a better suggestion for where I can look up the acceleration of the expansion that details the evidence?

Thanks
Calimero
#5
May28-10, 07:07 PM
P: 256
Quote Quote by grichblix View Post
Unless there is some evidence that the redshift for individual type 1A supernovas is changing, which would imply a change in velocity, I don't see how else they can address it.

Thanks

We don't need evidence that redshift is changing.


Quote Quote by grichblix View Post
Do you have a better suggestion for where I can look up the acceleration of the expansion that details the evidence?
http://www.pnas.org/content/96/8/4224.full
http://map.gsfc.nasa.gov

Hope it helps.
stevebd1
#6
May29-10, 02:20 AM
P: 608
Quote Quote by grichblix View Post
1) How do we know that black holes are not stellar nuclei that have just collapsed to a volume less than their mass dependent Schwarzschild radius (the radius at which the speed of light is the escape velocity) rather than actual singularities? In terms of physics the two types of objects would behave identically, but the former could actually be quite large in physical volume...
According to Schwarzschild metric, outside 2M, dt is the greater quantity and proper t is temporal, once r becomes smaller than 2M, dr becomes the greater quantity and distance becomes temporal. Holding a stable radius within 2M would be akin to holding at 3 pm in time-like geodesics. The only way collapse is going to stop within a static black hole is if the energy that distinguishes space and matter combine and become indistinguishable which is predicted to occur at very small scales.

In a rotating or charged black hole, things are a little different, time-like geodesics are supposed to be reinstated within the deep region of the black hole so there may be an argument that a singularity in the strict sense doesn't occur.
prakash kumar
#7
May30-10, 05:44 AM
P: 16
If not the red shift then perhaps microwave background radiation which is the solid evidence of the Big Bang (or something like that) will show some light on your question of accelerating universe.
To me it seems that the expanding of universe is true in the highlight of redshift and the idea that it is from the relative velocity of bodies is not so good because i can't see why these bodies stopped to accelerate and maintain a constant velocity?


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