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Light years

by Dagenais
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jjalexand
#19
Mar1-04, 09:23 AM
P: 64
While the big bang theory may have happened 'everywhere at once', maybe that 'everywhere' was a very small region in some enclosing (other dimensional) space?

Don't we talk about the expansion of the universe, the reason for the red-shift?

Don't we have a concept of the 'age' of the universe?

Doesn't that also imply a radius of the universe? (Or at the very least, surely the age in years is equivalent to some length in light years that might possibly have some physical significance in terms of it's size?)

Are we really saying the big bang theory is assoicated with an infinite universe, i.e. everything is rushing away from everything else without a center or a boundary? If it happened every_where_ at once, why not every_when_? If it had a beginning in time, then why not in space as well? If the big-bang happened 'everywhere at once', was that relative to some meta-universe containing ours, or only 'everywhere at once' relative to some (possibly expanding) balloon of local universe.

Russ, as a layman, I would personally be grateful for some further explanation, reference or logical argument clarifying your statement that 'The big-bang happened everywhere at once'.
Phobos
#20
Mar1-04, 11:40 AM
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Originally posted by gunblaze
Then y doesn't the black hole emits light even though its a star before its death??? If it doesn't emits light, y would people noe of its existance(they guessed?)???
Yes, black holes that were created from the collapsed cores of large stars did emit light back when they were functioning stars. But if we're detecting a black hole, then its star-light is history...that light has passed by the Earth already.

So, how do we detect black holes? (1) By their gravitational influence on nearby matter. (2) By the light emitted from stuff falling into the black hole.

Black holes were predicted to exist based on math/physics. Now we see evidences...like (1) and (2) above...that are best explained by a black hole. Based on that, it is believed that black holes are very likely to exist.

Confusing? Consider the example of seeing a star rapidly orbiting what appears to be nothing. Not only that, but given it's orbit and mass, you can calculate the mass that should be in that place where you see nothing. With that calculated mass combined with the volume of empty space being orbited, math/physics tells you that it can only be one thing...a black hole.
Phobos
#21
Mar1-04, 11:44 AM
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jjalexand - Since you asked Russ, I'll be glad to let him respond. But rest assured that the existence of a meta-universe is not ruled out. But at the moment, there's no conclusive evidence for it either. For now, Big Bang Theory doesn't rely on there being a meta-universe.
jjalexand
#22
Mar1-04, 05:12 PM
P: 64
Sorry, I have only just seen all these responses.

Thanks to Marcus, Phobos, Russ Watters, etc for all the excellent help on the 13+13 issue. It looks as if there was a whole area of 'knowledge' about the big-bang that I was not aware of.

I had always assumed the initial singularity was confined to a very small region, and that was why the cosmic background radiation had a very uniform temperature. However, of course it does get complicated that region is the whole universe, introducing an element of self-reference (which always complicates things, e.g. the types of mathematical statement that Godel showed may be unprovable seem to involve self-reference).

I suppose popular science writers may generally gloss over these difficult issues as they may be just too hard to explain in any short text.

I will have a look at the references, thanks.
Nereid
#23
Mar1-04, 06:54 PM
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Phobos wrote: So, how do we detect black holes? (1) By their gravitational influence on nearby matter. (2) By the light emitted from stuff falling into the black hole.
To amplify on this ...
Here's some good observations of stars orbiting the black hole at the centre of the Milky Way.

And this is a very recent observation of light (actually X-rays) from a star being ripped apart as it got near a supermassive black hole (and some of its mass fell into the black hole).
gunblaze
#24
Mar2-04, 02:34 AM
P: 186
Originally posted by Phobos
Yes, black holes that were created from the collapsed cores of large stars did emit light back when they were functioning stars. But if we're detecting a black hole, then its star-light is history...that light has passed by the Earth already.

So, how do we detect black holes? (1) By their gravitational influence on nearby matter. (2) By the light emitted from stuff falling into the black hole.

Black holes were predicted to exist based on math/physics. Now we see evidences...like (1) and (2) above...that are best explained by a black hole. Based on that, it is believed that black holes are very likely to exist.

Confusing? Consider the example of seeing a star rapidly orbiting what appears to be nothing. Not only that, but given it's orbit and mass, you can calculate the mass that should be in that place where you see nothing. With that calculated mass combined with the volume of empty space being orbited, math/physics tells you that it can only be one thing...a black hole.
Thanks, that helps. So what u mean is that if some sort of a star is supposed to be orbiting in space but its light isn't visible means that the the presence of a black hole is near!
jjalexand
#25
Mar2-04, 07:22 AM
P: 64
"Notice that in contrast to special relativity, the redshift does not indicate the velocity, it indicates the distance [12]. That is, the redshift tells us not the velocity of the emitter, but where the emitter sits (at rest locally) in the coordinates of the universe. "

This UNSW paper is so excellent, thank you Marcus, you have the magic touch. I have been puzzling about this for years, it is so well expressed too. (to the extent that I can follow it).
russ_watters
#26
Mar2-04, 10:32 AM
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Originally posted by Phobos
jjalexand - Since you asked Russ, I'll be glad to let him respond. But rest assured that the existence of a meta-universe is not ruled out. But at the moment, there's no conclusive evidence for it either. For now, Big Bang Theory doesn't rely on there being a meta-universe.
By all means, jump in. These types of questions interest me and thats why I answer them, but they do stretch my knowledge of cosmology.
Russ, as a layman, I would personally be grateful for some further explanation, reference or logical argument clarifying your statement that 'The big-bang happened everywhere at once'.
The analogy I prefer is to a balloon with dots on it. Its important to think of the surface of the balloon as your analogy to 3d space. Before you start blowing up the balloon, its very small (in the case of the universe, believed to be infinitely small). All the dots are essentially right on top of each other. When you start blowing it up, the dots move away from each other, but there is no center (on the 2d surface anyway) of expansion and no "edge".

Now, this balloon has a definite surface area. But what if you couldn't see the whole balloon at once? Since we can't see the whole universe, we don't really know if it is infinite or finite in scale.
Nereid
#27
Mar2-04, 10:44 AM
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Another analogy commonly used is raisin bread - in the oven it expands in all directions, and the raisins get further apart; we (galaxies, planets, etc) are the raisins; the dough is space.

Both balloon and raisin bread analogies break down when you take them beyond what they are intended to show. For example, on a balloon you can go round and return to your starting position (not possible in many real universes); bread has an outer crust (yummy!), but real universes don't (at least, not in 3D).


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