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Author Topic:   New Big Bang Cosmology
jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 31 of 48 (109136)
05-18-2004 8:29 PM
Reply to: Message 30 by RingoKid
05-18-2004 8:13 PM


There appear to be things that travel faster than the speed of light.
BUT...
(and there is always that but)
it appears that for those objects, the speed of light is a floor instead of a ceiling. They cannot travel slower than the speed of light.
So far there are no theories that propose an object that can move from slower than the speed of light to faster than the speed of light that I am aware of.

Aslan is not a Tame Lion

This message is a reply to:
 Message 30 by RingoKid, posted 05-18-2004 8:13 PM RingoKid has not replied

Replies to this message:
 Message 33 by Sylas, posted 05-18-2004 9:19 PM jar has not replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 32 of 48 (109138)
05-18-2004 8:57 PM
Reply to: Message 30 by RingoKid
05-18-2004 8:13 PM


RingoKid writes:
I said the bubble membrane was 13.7 billion light years thick as in the distance covered by light from the initial point of the big bang to reach it's current position at the leading edge of the inflating universe...
According to conventional Big Bang cosmology, there is no such thing as an "initial point" in space; and no such thing as a "leading edge" of expansion. These concepts usually arise when people think of the Big Bang as an explosion, which is a misleading analogy.
The Big Bang is an expansion of space itself; not an explosion of something within space.
Caveat: there are models which speak of leading edges in expansion of space, but it is much better to get to grips with the simplest models first. The leading edge in such models is still nothing like the expanding shell of matter from a conventional explosion.
A conventional explosion has a central point, and a rapidly expanding shell of material which pushes out into a surrounding space. The Big Bang, by contrast, says that all of space used to be filled with matter at extreme density. As space expands, the density drops.
This is covered in the website I cited for you previously. See in particular Ned Wright's FAQ answer to How can the Universe be infinite if it was all concentrated into a point at the Big Bang?, and Where was the center of the Big Bang?.
One of the hardest things in getting started with understanding modern cosmology is the number of common preconceptions than need to be unlearned.
On a side note, is it a known fact that nothing can exceed the speed of light if so is that the expansion rate of the universe if not then can we assume the gravitational pull of a black hole is "faster" or does it just accelerate up to speed faster ???
I can't really follow what you are asking here. The speed of light is a limit on the motion of things through space, but it turns out not to be a limit on motions of space itself. Motions of space are something way outside our conventional assumptions about reality, but they are a consequence of general relativity. The distance between two points in a rapidly expanding space can increase many times faster than the speed of light. Of course, two such objects are invisible to each other.
The expansion rate of the universe is not measured as a simple velocity, but with units of km/sec/Mparsec. The current expansion rate is about 71 km/s/MParsec. What that means is that points in space which are 1 MParsec apart will have the distance between them increasing at a rate of 71 km/sec. The speed of light is about 3*105 km/sec, and so objects at a distance of about 4230 MegaParsecs will be separating from us at a rate corresponding to the speed of light. This is often described as moving away from us at that speed, but in general relativity this is not really a good way of putting it. The very concept of distance and speed tends to break on cosmological scales; they are localized approximations that work well on a "flat" space.
One MegaParsec is 3260000 light years, and so 4230 MegaParsecs is about 13.7 billion light years. Sound familiar? This is the limit of the "visible" universe; the edge of what we could ever possibly see; and 13.7 billion years is the age of the universe; the time that it has been expanding.
eg...a rear drive light powered engine pushes a white car up to speed and a front drive black hole powered engine pulls a black car up to speed.
which car would win assuming of course the black car doesn't disappear into it's own engine...
Sorry; I can't parse that into a meaningful question. A black hole involves a severe distortion of space, in such a way that notions of distance and speed get a bit tricky.
Cheers -- Sylas
This message has been edited by Sylas, 05-18-2004 07:59 PM

This message is a reply to:
 Message 30 by RingoKid, posted 05-18-2004 8:13 PM RingoKid has replied

Replies to this message:
 Message 34 by RingoKid, posted 05-18-2004 10:36 PM Sylas has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 33 of 48 (109140)
05-18-2004 9:19 PM
Reply to: Message 31 by jar
05-18-2004 8:29 PM


jar writes:
There appear to be things that travel faster than the speed of light.
Tachyons. However, this is not an "appearance", but a speculative notion lacking any empirical support. That is, tachyons might exist; and theoretical predictions can be made. Attempts to test the predictions, however, have turned up a universal negative. There is nothing that "appears" to be tachyons, so far; and no particular reason to think they actually exist.
The rest of your comments on their theoretical properties are fair enough.
Cheers -- Sylas

This message is a reply to:
 Message 31 by jar, posted 05-18-2004 8:29 PM jar has not replied

  
RingoKid
Inactive Member


Message 34 of 48 (109152)
05-18-2004 10:36 PM
Reply to: Message 32 by Sylas
05-18-2004 8:57 PM


...about that last question
If you shone a light out from inside a black hole it would be trying to escape but it couldn't because the acccelerative pull on it would keep it inside the hole...
...think of a person on a bungy leash running away. They can't escape the pull of the bungy cord unless they hit a critical speed and develop a force that exceeds that of the bungy's ability to hold on to you and it snaps then you are away racing (escape velocity ???)
so if you attach yourself to the bungy which is at maximum extension and tried to run away at light speed you would still get pulled backwards at a rate faster than you are trying to escape...y/n
does that make sense ???...or is it dependent on your weight/size
Next question...do all black holes exert the same amount of pull no matter how big they are ???

This message is a reply to:
 Message 32 by Sylas, posted 05-18-2004 8:57 PM Sylas has replied

Replies to this message:
 Message 35 by NosyNed, posted 05-18-2004 11:08 PM RingoKid has not replied
 Message 36 by Sylas, posted 05-18-2004 11:23 PM RingoKid has not replied

  
NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 35 of 48 (109156)
05-18-2004 11:08 PM
Reply to: Message 34 by RingoKid
05-18-2004 10:36 PM


Amount of pull
Next question...do all black holes exert the same amount of pull no matter how big they are ???
No.
The black holes gravitational pull is, like all massive objects, proportional to it's mass. So a bigger black hole has more mass and "pulls harder".
But at the "edge" of a black hole which is called the "event horizon" they all pull with the same force. Just enough so that the escape velocity is light speed. But the event horizon is of a greater diameter for a more massive black hole than for a smaller one.
So a very massive black hole would be much greater in diamter (at the event horizon) than a less massive black hole. If you outside the event horizon the pull of a more massive one would be greater than a for a less massive one.
Inside the black hole all bets are off. Darned if I know if you can even talk sensibly about the inside. As I understand it the calculations say that all the mass is concentrated at a point at the center. Inside the event horizon I presume the "pull" is greater for a more massive one also and goes up faster as you move deeper. But I don't know.
does that make sense ???...or is it dependent on your weight/size
I don't think the bungy analogy is all that useful but ok if it helps you. It doesn't depend on your weight, size etc. Just like to objects of different mass (size?) are accelerated at the same rate falling to earth so too will they be pulled by a black hole.

This message is a reply to:
 Message 34 by RingoKid, posted 05-18-2004 10:36 PM RingoKid has not replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 36 of 48 (109158)
05-18-2004 11:23 PM
Reply to: Message 34 by RingoKid
05-18-2004 10:36 PM


Re: ...about that last question
RingoKid writes:
If you shone a light out from inside a black hole it would be trying to escape but it couldn't because the acccelerative pull on it would keep it inside the hole...
Yes... all light paths inside the Schwarzschild radius (the event horizon) are directed inwards, and light or anything else can only move closer to the central singularity. Anything at all will reach that central singularity within a small finite time, no matter what it does with engines or propulsion devices.
I believe that another way of expressing this is to regard space itself as flowing into a black hole, in some metrics; but I'd have to defer to Eta Carinae on such details.
...think of a person on a bungy leash running away. They can't escape the pull of the bungy cord unless they hit a critical speed and develop a force that exceeds that of the bungy's ability to hold on to you and it snaps then you are away racing (escape velocity ???)
so if you attach yourself to the bungy which is at maximum extension and tried to run away at light speed you would still get pulled backwards at a rate faster than you are trying to escape...y/n
I guess so. The escape velocity at the event horizon is equal to the speed of light. One way to think of this is that an outwards directed photon at that exact distance is moving outwards at the same speed as space is flowing inwards, and so it does not actually move. A better way to think of it is to imagine a light source falling into the hole. The closer it gets to the horizon, the longer it takes photons to get back out to an external observer. The photons also get redshifted to invisibility. An external observer thus "sees" an increasingly redshifted object, but never sees it cross the horizon. We had a discussion on this recently, in which someone thought this meant that the object does not ever cross the event horizon. But it does, and quickly. You just can't observe the crossing from outside.
Inside the event horizon, there is no escape velocity.
does that make sense ???...or is it dependent on your weight/size
It does not depend on your weight or size. The Schwarzschild radius is a point of no return for anything, regardless of composition.
Next question...do all black holes exert the same amount of pull no matter how big they are ???
If you are a given distance from a black hole (say, one million kilometres) then the force depends on the mass of the hole. In fact, the force depends only on the mass involved; regardless of whether it is compressed into a hole or not. What makes a hole special is that you can get very close to it without hitting any surface. For example, a black hole with the mass of the Earth would have a Schwarzschild radius of just under a centimetre. But you can't get that close to the Earth, because ground level gets in the way. A hole the mass of the Sun has a Schwarzschild radius of about 3 kilometres.
Cheers -- Sylas

This message is a reply to:
 Message 34 by RingoKid, posted 05-18-2004 10:36 PM RingoKid has not replied

  
RingoKid
Inactive Member


Message 37 of 48 (109405)
05-20-2004 7:30 AM


OK try this...
Can the gavitational pull of a large black hole accelerate something to exceed light speed once it has crossed the event horizon ???
quote:
Caveat: there are models which speak of leading edges in expansion of space, but it is much better to get to grips with the simplest models first. The leading edge in such models is still nothing like the expanding shell of matter from a conventional explosion
I didn't actually say explosion of matter...
so do these caveat models contradict conventional big bang models if one says leading edge and middle and one says no ???
...also if a light particle initiated by the big bang started it's travels thru space at T=0 and it travelled for 13.7 billiion years where would it be now ???

Replies to this message:
 Message 38 by jar, posted 05-20-2004 8:34 AM RingoKid has not replied
 Message 39 by Sylas, posted 05-20-2004 9:28 AM RingoKid has not replied

  
jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 38 of 48 (109414)
05-20-2004 8:34 AM
Reply to: Message 37 by RingoKid
05-20-2004 7:30 AM


Re: OK try this...
Frankly, inside the event horizon just about all bets are off.

Aslan is not a Tame Lion

This message is a reply to:
 Message 37 by RingoKid, posted 05-20-2004 7:30 AM RingoKid has not replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 39 of 48 (109421)
05-20-2004 9:28 AM
Reply to: Message 37 by RingoKid
05-20-2004 7:30 AM


Re: OK try this...
RingoKid writes:
Can the gavitational pull of a large black hole accelerate something to exceed light speed once it has crossed the event horizon ???
No.
RingoKid writes:
so do these caveat models contradict conventional big bang models if one says leading edge and middle and one says no ???
Not really. The big bang is a very general concept; and refers to the idea that space expands, and that this expansion has spread out the entire visible universe (everything we can see) from what was once a very tiny region of space filled with unbelievably hot and dense matter and energy.
The simplest mathematical models capture the entire universe within one expanding space. There are a number of ways that the simple model can be made more complicated. There are a range of ideas relating to how expansion got started, and to the possibility of other independently expanding spaces (multiverse), and to discontinuities (or edges) in space. There are variations in the topology and the geometry of the visible universe as well. Models without edges, and models which have discontinuities or edges, and many other variations even more strange, are all still big bang models. They all still involve the expansion of everything we now see from a very small region within an extremely hot and dense state of the universe.
Many of the different speculative cosmological models are about different ideas relating to how the expansion got started, and to what might be going on beyond the limits of the visible universe.
...also if a light particle initiated by the big bang started it's travels thru space at T=0 and it travelled for 13.7 billion years where would it be now ???
That is an excellent question, and grasping the answer to this was for me the key that suddenly let me grasp what the big bang is all about. I can still recall the sudden flash of recognition when it finally hit me just how background radiation relates to the big bang.
The answer is that is that such photons as you describe do exist. They are everywhere. All of space, as far as we can see, is filled with such photons, and they are moving in all directions. They are called the cosmic microwave background radiation.
13.7 billion years ago, a tiny fraction of second after the initial singularity, you can imagine that the universe was filled with hot dense energy. One way to think of this is to imagine that everything we can see now, out to the furthest reaches of our most powerful telescopes, is filled with matter looking a bit like the centre of the Sun.
Even that is not really correct, in several respects. In fact, the universe was far denser and hotter than the Sun in the earliest moments. Second, the Sun is a particle, with a surface and edge. The state I am describing has no edge; all of space is like the centre of the Sun. The universe might be infinite, in which case this mass extends forever without bound. Or it might be finite, in which case you should think not of a edge to matter, but of space turning back in on itself like a four dimensional balloon, so that if you go off in one direction you find yourself back where you started, without having turned around. This is a bit like sailing around the world; travel in one direction and you get back where you started. Except, of course, that an ocean surface is two dimensional but the universe is three dimensional. You might call it a hypersphere.
The difficulty is that space is also expanding, so if you set out on a trip around the universe, the expansion might prevent you from ever completing the journey.
Anyhow, when the universe was this hot and dense, it was also opaque. Light could not travel freely though space, because it keeps hitting things. In fact, it took about 300,000 years before space had thinned out enough to be transparent to light. When that occurred, all of space was filled with very hot energetic photons, which were suddenly free to travel through space without impediment.
Those photons continued to travel through space in straight lines, and they are still travelling today, 13.7 billion years later. Of course, all this time space has been expanding, and this has the effect of "stretching" photons; so now they have a much longer wavelength. They are very cool; no longer hot and energetic. We see them now as microwave radiation, coming to the Earth from every corner of the sky.
Cheers -- Sylas

This message is a reply to:
 Message 37 by RingoKid, posted 05-20-2004 7:30 AM RingoKid has not replied

  
usncahill
Inactive Member


Message 40 of 48 (109530)
05-20-2004 9:46 PM


First off, this thread has been most enlightening for me. I have a few questions similar to Ringokid's. My questions are based on current ideas on the matter, not that anyone 'knows' the answers exactly. Opinions will do.
Did space start expanding from a point? And was everything, particles and such, 'created' instantly afterwards?
This also brings to mind that space, three dimensions, if expanding, should have a center point. Is space able to expand because it is infinite?
One more. How could all of the dense matter be 'created' simultaneously? I guess I have a hard time getting past it starting at a point; similar, maybe, to quantum fluctuations allowing particles to appear at some place near each other and then be annihilated.

Replies to this message:
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NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 41 of 48 (109532)
05-20-2004 10:04 PM
Reply to: Message 40 by usncahill
05-20-2004 9:46 PM


Expanding Space
This also brings to mind that space, three dimensions, if expanding, should have a center point. Is space able to expand because it is infinite?
It has no center that is within our space-time. That is the whole thing is expanding. Every point is moving away from every other one in exactly the same way. If there was a center that point would be unique in some way. There isn't one unique point.
I'm not sure I know enough to describe it correctly. When you suggest there is a center it's because you see it expanding within the 3 dimensions we deal with. In this case the expansion is through a higher dimension. (at least I think that is what the math says). So the center is not anywhere we can point to. It is in another dimesion (??).
The space that we can see isn't infinite. It could be a little pocket set in an infinite large cosmos. But if we can't see out of this we don't know that.
All the particles and such came from the enomously dense energy in the original starting point. As the whole thing cooled the energy turned into particles.
I guess I have a hard time getting past it starting at a point; similar, maybe, to quantum fluctuations allowing particles to appear at some place near each other and then be annihilated.
Yea, me too. In fact, I don't think that this is well explained by physics yet. After the first tiny fraction of a second of the big bang it seems the physics is good and well understood (even if not by me). Before that I think we just don't understand it very well at all.
This message has been edited by NosyNed, 05-20-2004 09:05 PM

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 Message 40 by usncahill, posted 05-20-2004 9:46 PM usncahill has not replied

  
RingoKid
Inactive Member


Message 42 of 48 (109603)
05-21-2004 3:43 AM


so let me get this straight...
...the event horizon is the point at which something hits lightspeed and disappears off the radar so to speak...???
beyond that it instantly hits the singularity or does it continue accelerating for who knows how long depending on the mass of the hole and eventually hits a singular nothing or does it continue til infinity as time means nothing and therefore distance is irrelevent...
and Nosy that's exactly what I'm trying to get at !!!
...the centre is unique and exists in a different dimension, something that exists outside of spacetime.

Replies to this message:
 Message 43 by Sylas, posted 05-21-2004 4:53 AM RingoKid has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 43 of 48 (109615)
05-21-2004 4:53 AM
Reply to: Message 42 by RingoKid
05-21-2004 3:43 AM


Re: so let me get this straight...
An object never hits light speed as it falls into a hole; not at the event horizon, and not at any point inside the horizon either. The horizon is a point where events go off our radar, in the sense that signals cannot get back to the outside world from inside the horizon.
An object continues accelerating for a short finite time until it reaches the centeral singularity, at which time we can't say what happens. But it never reaches light speed.
The lifetime of any object inside the horizon is very short; but time does have meaning because we can speak of how long it lasts before reaching the singularity.
At the singularity, the object no longer exists as an object.
Notions of time and distance still have meaning inside the event horizon; but at the point like central singularity descriptions break down.
You can't say that the central singulaity exists in another dimension; that is not a valid inference of what we know, and it is not correct use of the term dimension.
Cheers -- Sylas

This message is a reply to:
 Message 42 by RingoKid, posted 05-21-2004 3:43 AM RingoKid has replied

Replies to this message:
 Message 44 by RingoKid, posted 05-21-2004 5:50 AM Sylas has replied

  
RingoKid
Inactive Member


Message 44 of 48 (109618)
05-21-2004 5:50 AM
Reply to: Message 43 by Sylas
05-21-2004 4:53 AM


Re: so let me get this straight...
sorry Sylas the centre I meant was in regard to Nosy saying this...
quote:
If there was a center that point would be unique in some way. So the center is not anywhere we can point to. It is in another dimension (??).
...and still based on the universe being between the outer surface and inner surface of the bubble skin premise

This message is a reply to:
 Message 43 by Sylas, posted 05-21-2004 4:53 AM Sylas has replied

Replies to this message:
 Message 45 by Sylas, posted 05-21-2004 6:28 AM RingoKid has not replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 45 of 48 (109621)
05-21-2004 6:28 AM
Reply to: Message 44 by RingoKid
05-21-2004 5:50 AM


Re: so let me get this straight...
Ned was wise enough to put the question marks there. There are speculative ideas about blackholes incorporating "wormholes" to another region in space, being a "whitehole"; but they are games with strange solutions to the relativistic equations, and not predictions of what happens in black holes in real life. It is not correct to say that the central singularity is in another dimension.
There is a really excellent web site you should check out; Falling Into a Black Hole by Andrew Hamilton. It shows simulated movies of what you see as you approach a black hole; orbit a black hole, drop probes into a black hole, and finally fall into to a black hole yourself, all the way through the event horizon and right down to the central singularity, along with theoretical discussions explaining details we are discussing here.
...and still based on the universe being between the outer surface and inner surface of the bubble skin premise
That is not a meaningful premise.
We could speculate that the universe is like the skin of a hyperspherical bubble of some kind, but there is no such thing as "inner" or "outer" surfaces. You rather have a geometry in which our three dimensional space is treated as a hypersurface embedded in a higher dimensional space. However, this is not really a good idea; the geometry works without needing to require embedding into a higher dimensional space at all. It can be a useful aid to start thinking about what it means for a space to expand; by analogy with a balloon or something like that; but as soon as you start making concrete suggestions based on conventional intuitions uninformed by a thorough comprehension of Riemannian geometry and general releativity and other horrible things like that, you end up with nonsense.
Cheers -- Sylas

This message is a reply to:
 Message 44 by RingoKid, posted 05-21-2004 5:50 AM RingoKid has not replied

  
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