Common misconceptions in Cosmology

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Common misconceptions in Cosmology

Postby SpitValve » Sat Jun 16, 2007 11:23 pm UTC

Arxiv article:

http://arxiv.org/abs/astro-ph/0310808v2

I found it fascinating really, actually is making me consider twisting my PhD project into something more general rel-ish.

Summing up:

1. Distant galaxies can recede faster than c
2. We can see galaxies that are receding faster than c
3. "Inflation" is no more superluminal expansion than any other period

Basically, the Hubble constant is decreasing. This is true of any expanding universe, whether accelerating (q<0) or decelerating (q>0) provided q>-1, which is true unless you have fraction of "dark energy" + fraction of dark matter and "normal" matter > 1.

By the Hubble law (to first order), v=H*d, so objects that are further than d=c/H away are receding faster than the speed of light. Everything within this distance is within our "Hubble sphere". Light emitted from objects outside the Hubble sphere is initially receding from us. However, as H decreases, d increases and the Hubble sphere expands to take in light from objects that are moving superluminally.

Anyway, I found it fascinating. Makes me want to dive back into tensors to work out all the maths again...

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Postby gmalivuk » Sun Jun 17, 2007 1:37 am UTC

Very interesting. I only read the first couple pages, but they've gone ahead and done something I've been curious about for awhile but which I didn't know enough equations to do well. (Basically, making the space-time diagrams that plot comoving distance against time.)

I can't decide whether it's depressing or not that, even given infinite time, we'll only ever have information out to about 62Glyr away. Granted, that is a fucking unimaginably huge volume, so I suppose it's nothing to worry about.
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Postby 3.14159265... » Sun Jun 17, 2007 1:55 am UTC

I read very little, and understand very little I suppose of the maths, as I only read a bit, but will read more, however I wanted to say something about this:

I can't decide whether it's depressing or not that, even given infinite time, we'll only ever have information out to about 62Glyr away. Granted, that is a fucking unimaginably huge volume, so I suppose it's nothing to worry about.
remember http://forums.xkcd.com/viewtopic.php?t=4832
What if this is exactly what defines the event horizon for our "universe"/black-hole. It fits my idea of a event horizon quite well. Spitvalve, GM, take it away!
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Postby SpitValve » Sun Jun 17, 2007 5:05 am UTC

3.14159265... wrote:
I can't decide whether it's depressing or not that, even given infinite time, we'll only ever have information out to about 62Glyr away. Granted, that is a fucking unimaginably huge volume, so I suppose it's nothing to worry about.
remember http://forums.xkcd.com/viewtopic.php?t=4832
What if this is exactly what defines the event horizon for our "universe"/black-hole. It fits my idea of a event horizon quite well. Spitvalve, GM, take it away!


Except it works backwards: if it was a black-hole event horizon, it means we live in size a 62 Glyr sphere *outside* the blackhole...

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Postby Bondolon » Sun Jun 17, 2007 5:12 am UTC

SpitValve wrote:
3.14159265... wrote:
I can't decide whether it's depressing or not that, even given infinite time, we'll only ever have information out to about 62Glyr away. Granted, that is a fucking unimaginably huge volume, so I suppose it's nothing to worry about.
remember http://forums.xkcd.com/viewtopic.php?t=4832
What if this is exactly what defines the event horizon for our "universe"/black-hole. It fits my idea of a event horizon quite well. Spitvalve, GM, take it away!


Except it works backwards: if it was a black-hole event horizon, it means we live in size a 62 Glyr sphere *outside* the blackhole...


I thought he was saying that our universe is a black hole, and that this volume is the size to the event horizon. I got the impression that, given our universe "could" be the innards of a black hole, our universe would exist within the singularity of said black hole, such that there's a boundary between our universe and the one we're in...

In any case, it would be pretty weird to have a black hole be inverted like that, with the event horizon being on the inside of a sphere and the 1/singularity being on the outside of the sphere.

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Postby SpitValve » Sun Jun 17, 2007 5:47 am UTC

Bondolon wrote:I thought he was saying that our universe is a black hole, and that this volume is the size to the event horizon. I got the impression that, given our universe "could" be the innards of a black hole, our universe would exist within the singularity of said black hole, such that there's a boundary between our universe and the one we're in...

In any case, it would be pretty weird to have a black hole be inverted like that, with the event horizon being on the inside of a sphere and the 1/singularity being on the outside of the sphere.


Sorry, that was exactly the point I was trying to make :)

If we were inside a black hole, we wouldn't see the black hole's event horizon as an event horizon at all, because stuff can quite happily fall into the black hole. We'd observe stuff outside the black hole but not the other way around. So it doesn't make sense for the particle horizon to be the event horizon of a black hole we live inside.

As for living in the singularity of a black hole... I'm not really sure what that would mean really :)

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Postby Bondolon » Sun Jun 17, 2007 5:51 am UTC

SpitValve wrote:
Bondolon wrote:I thought he was saying that our universe is a black hole, and that this volume is the size to the event horizon. I got the impression that, given our universe "could" be the innards of a black hole, our universe would exist within the singularity of said black hole, such that there's a boundary between our universe and the one we're in...

In any case, it would be pretty weird to have a black hole be inverted like that, with the event horizon being on the inside of a sphere and the 1/singularity being on the outside of the sphere.


Sorry, that was exactly the point I was trying to make :)

If we were inside a black hole, we wouldn't see the black hole's event horizon as an event horizon at all, because stuff can quite happily fall into the black hole. We'd observe stuff outside the black hole but not the other way around. So it doesn't make sense for the particle horizon to be the event horizon of a black hole we live inside.

As for living in the singularity of a black hole... I'm not really sure what that would mean really :)


Well, it would mean two things.

1. It would be completely impossible for our universe to interact with the host universe, making our existence in said black hole a moot point, since it could never be proved nor disproved. To say "there's this other universe that we can't detect in any way" would be like saying there's an apple in the room that nobody could EVER see.

2. Our Universe would eventually evaporate. In other words, we wouldn't have a hot death, a cold death, a big crunch or a big fade. We'd just have a big "where the hell did the universe go?"

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Postby gmalivuk » Sun Jun 17, 2007 3:15 pm UTC

3.14159265... wrote:What if this is exactly what defines the event horizon for our "universe"/black-hole. It fits my idea of a event horizon quite well. Spitvalve, GM, take it away!


The event horizon for our universe, as shown in the diagrams on that site, is one that we're on the "outside" of. So yeah, it's like an inverted black hole, sort of, only with no singularity. Though actually living inside a black hole in the normal sense would cause problems, a point I made a couple times in that other thread is that all kinds of things besides black holes cause event horizons, and being on one side or the other of those doesn't cause any problem. (After all, since we will only ever be able to get information from things out to about 62Gly, for any comoving observer beyond that distance, we are on the other side of an event horizon, as nothing we do will ever reach them.)

Bondolon wrote:To say "there's this other universe that we can't detect in any way" would be like saying there's an apple in the room that nobody could EVER see.


Or like saying "so there's this God dude..."
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Postby Bondolon » Sun Jun 17, 2007 4:18 pm UTC

gmalivuk wrote:
Bondolon wrote:To say "there's this other universe that we can't detect in any way" would be like saying there's an apple in the room that nobody could EVER see.


Or like saying "so there's this God dude..."


Well, people who believe in God believe it is possible to interact with God, and it happens often. Of course, they don't think you can actually see God in a personal form, but they believe interaction is possible. I think that's a little different from saying that there is something that could never, every be detected in any conceivable way, as there would be no mechanism by which any type of data could come to us. Again, believers in God think there is a mechanism for that.

Whether they're right or not, though, who's to honestly say?

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Postby gmalivuk » Sun Jun 17, 2007 8:06 pm UTC

Bondolon wrote:Well, people who believe in God believe it is possible to interact with God, and it happens often. Of course, they don't think you can actually see God in a personal form, but they believe interaction is possible. I think that's a little different from saying that there is something that could never, every be detected in any conceivable way, as there would be no mechanism by which any type of data could come to us. Again, believers in God think there is a mechanism for that.

Whether they're right or not, though, who's to honestly say?


The point is, both claims are, scientifically speaking, untestable. You can make testable God claims, but theists often weasel their way out of those when they're arguing about it.
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Postby SpitValve » Sun Jun 17, 2007 8:31 pm UTC

gmalivuk wrote:The point is, both claims are, scientifically speaking, untestable. You can make testable God claims, but theists often weasel their way out of those when they're arguing about it.


I'm interested in what examples of testable God claims you can think of.

Edit: Also it's interesting looking at the extensive list of publications that have made one of these misconceptions. 1st on the list the the Feynman lectures.... It also includes Halliday-Resnick-Walker, my main 1st year textbook and someone and Wheeler's Spacetime Physics, which is a really good special rel book which one of my undergrad courses was based on. So these misconceptions are in some pretty authorative places :)

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Re: Common misconceptions in Cosmology

Postby yy2bggggs » Sun Jun 17, 2007 9:57 pm UTC

SpitValve wrote:Summing up:
1. Distant galaxies can recede faster than c
2. We can see galaxies that are receding faster than c
3. "Inflation" is no more superluminal expansion than any other period

Or summing up even more, space itself expands.

Here's another interesting reference:
http://www.astro.ucla.edu/~wright/cosmology_faq.html

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Postby gmalivuk » Mon Jun 18, 2007 12:39 am UTC

SpitValve wrote:I'm interested in what examples of testable God claims you can think of.


"God caused the Flood, and that's what killed all the dinosaurs and made the Grand Canyon."
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Postby 3.14159265... » Mon Jun 18, 2007 1:12 am UTC

If we were inside a black hole, we wouldn't see the black hole's event horizon as an event horizon at all, because stuff can quite happily fall into the black hole. We'd observe stuff outside the black hole but not the other way around. So it doesn't make sense for the particle horizon to be the event horizon of a black hole we live inside.


Well, how do we know if things actually go through the event horizon?

I just thought that all we knew was that, nothing not even something traveling at c (light) could get through the event horizon.

Now is that not also true in this case?

So all that has to happen for what I said to be taken litterally is a transform of one observer seeing very dense mass (that doesn't allow light to come out), and another something traveling away at the speed of light.

Seeing as how these observers have NO way of interacting, then their observations can be QUITE different. no? I go study actual physics?
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Postby Bondolon » Mon Jun 18, 2007 1:26 am UTC

3.14159265... wrote:
If we were inside a black hole, we wouldn't see the black hole's event horizon as an event horizon at all, because stuff can quite happily fall into the black hole. We'd observe stuff outside the black hole but not the other way around. So it doesn't make sense for the particle horizon to be the event horizon of a black hole we live inside.


Well, how do we know if things actually go through the event horizon?

I just thought that all we knew was that, nothing not even something traveling at c (light) could get through the event horizon.

Now is that not also true in this case?

So all that has to happen for what I said to be taken litterally is a transform of one observer seeing very dense mass (that doesn't allow light to come out), and another something traveling away at the speed of light.

Seeing as how these observers have NO way of interacting, then their observations can be QUITE different. no? I go study actual physics?


The event horizon (as just a spherical map of points) is permeable in the direction of the black hole. If we're inside the black hole, light is going to have no problem reaching us, as light regularly enters black holes (and never comes back out). In other words, we'd be able to see everything outside of the black hole. I don't know if that's what you're saying, but them's the facts.

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Postby 3.14159265... » Mon Jun 18, 2007 3:03 am UTC

The event horizon (as just a spherical map of points) is permeable in the direction of the black hole. If we're inside the black hole, light is going to have no problem reaching us, as light regularly enters black holes (and never comes back out). In other words, we'd be able to see everything outside of the black hole. I don't know if that's what you're saying, but them's the facts.

Lets think about only measurable things. Now can we measure light "entering" a black hole? so what makes us think it actually does enter.

If there is a black hole, it could distort observations such that something appearing to be entering from the outsider's view, will never actually "reach" us, due to relativistic effects on space, and more importantly time.
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Postby SpitValve » Mon Jun 18, 2007 3:31 am UTC

3.14159265... wrote:
The event horizon (as just a spherical map of points) is permeable in the direction of the black hole. If we're inside the black hole, light is going to have no problem reaching us, as light regularly enters black holes (and never comes back out). In other words, we'd be able to see everything outside of the black hole. I don't know if that's what you're saying, but them's the facts.

Lets think about only measurable things. Now can we measure light "entering" a black hole? so what makes us think it actually does enter.

If there is a black hole, it could distort observations such that something appearing to be entering from the outsider's view, will never actually "reach" us, due to relativistic effects on space, and more importantly time.


We think it enters because that's what General Relativity says it does, and the observations seem to fit it. If you don't believe relativity is a good description of reality, feel free to make up your own theory and we'll ignore it: because Real Black Holes Do Not Work That Way!

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Postby 3.14159265... » Mon Jun 18, 2007 3:51 am UTC

ahah, I don't understand general relativity at ALL! thats why I wasn't sure how we KNOW light enters a black hole.
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Postby SpitValve » Mon Jun 18, 2007 4:01 am UTC

3.14159265... wrote:ahah, I don't understand general relativity at ALL! thats why I wasn't sure how we KNOW light enters a black hole.


?

We know that in General Relativity that light enters a black hole. It's just a natural outcome of the Swartzchild solution. Null geodesics go in but not out.

If light can't enter a black hole, then General Relativity is wrong, and we have no idea what a black hole is.

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Postby Bondolon » Mon Jun 18, 2007 4:03 am UTC

SpitValve wrote:
3.14159265... wrote:ahah, I don't understand general relativity at ALL! thats why I wasn't sure how we KNOW light enters a black hole.


?

We know that in General Relativity that light enters a black hole. It's just a natural outcome of the Swartzchild solution. Null geodesics go in but not out.

If light can't enter a black hole, then General Relativity is wrong, and we have no idea what a black hole is.


And where would the light even go if not in? Energy has to be conserved.

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Postby 3.14159265... » Mon Jun 18, 2007 4:16 am UTC

We know that in General Relativity that light enters a black hole. It's just a natural outcome of the Swartzchild solution. Null geodesics go in but not out.
I will take your word for it.

And where would the light even go if not in? Energy has to be conserved.
Well this is what I was day dreaming about.

observer 1: outside
observer 2: inside
observer 3: with the light

Observer 1, turns on a light from one side of where there is an even horizon, and waits to get it from the other side, and doesn't.

Observer 2, sees nothing.

Observer 3, moves towards something dense, the closer it gets, the longer distances become, and thus it never reaches the event horizon.

Though from what spitvalve said, I will stop talking lol
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Postby Bondolon » Mon Jun 18, 2007 4:20 am UTC

3.14159265... wrote:
We know that in General Relativity that light enters a black hole. It's just a natural outcome of the Swartzchild solution. Null geodesics go in but not out.
I will take your word for it.

And where would the light even go if not in? Energy has to be conserved.
Well this is what I was day dreaming about.

observer 1: outside
observer 2: inside
observer 3: with the light

Observer 1, turns on a light from one side of where there is an even horizon, and waits to get it from the other side, and doesn't.

Observer 2, sees nothing.

Observer 3, moves towards something dense, the closer it gets, the longer distances become, and thus it never reaches the event horizon.

Though from what spitvalve said, I will stop talking lol


Well, again, the event horizon is just a measuremental thing. It's the point at which the gravitational forces become so great that anything going in won't be coming back out. However, it is certainly something that gets passed. What is never reached is the black hole/singularity entity itself. A neat trick someone taught me to understanding a singularity was to think about it like the end-point of an asymptote. The end point is of course infinity. Whenever the black hole is formed, then, space stretches to a point such as that. Even matter that is relatively near to the singularity when it is formed will never reach it.

Main points here: Event horizon is the point of no return, Singularity is the point of no reaching.

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Postby SpitValve » Mon Jun 18, 2007 4:54 am UTC

Bondolon wrote:Main points here: Event horizon is the point of no return, Singularity is the point of no reaching.


I thought that once you're inside the black hole, you always end up in the singularity fairly quickly? Perhaps not in one piece though.

To an outside observer you appear to increasingly slow down (but never quite stop) and redden as you approach the event horizon though. But remember the equivalence principle: basically everything feels pretty normal to the person actually falling into the black hole (neglecting spagghettification). So from the point of view of someone falling into a black hole, you just float along through the event horizon unawares, then fall into the singularity without knowing what hit you.

I think that when you're within the black hole all timelike wordlines (i.e. paths particles can follow) terminate with the black hole, but I'd have to check my course notes to remind myself what that looks like for the observer...

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Postby Bondolon » Mon Jun 18, 2007 5:00 am UTC

SpitValve wrote:
Bondolon wrote:Main points here: Event horizon is the point of no return, Singularity is the point of no reaching.


I thought that once you're inside the black hole, you always end up in the singularity fairly quickly? Perhaps not in one piece though.

To an outside observer you appear to increasingly slow down (but never quite stop) and redden as you approach the event horizon though. But remember the equivalence principle: basically everything feels pretty normal to the person actually falling into the black hole (neglecting spagghettification). So from the point of view of someone falling into a black hole, you just float along through the event horizon unawares, then fall into the singularity without knowing what hit you.

I think that when you're within the black hole all timelike wordlines (i.e. paths particles can follow) terminate with the black hole, but I'd have to check my course notes to remind myself what that looks like for the observer...


I was under the impression that there wasn't a definitive "this is what happens" set up, but that also one of the prevalent ideas was as such: as the gravity approaches the infinity, so does the space itself, such that the singularity is almost literally outside of space, and that anything approaching it continues to approach it eternally (or until the black hole evaporates). Again, that was just the impression that I got, but it seems pretty logical.

edit: to kinda ultra-clarify, in this model there would be an enormous, ultra-massive blob of primordial matter heading toward the singularity at just about the speed of light, but never reaching it.

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Postby SpitValve » Mon Jun 18, 2007 5:15 am UTC

hmm...

Makes me want to sit more GR courses to get this straight :)

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Postby Bondolon » Mon Jun 18, 2007 5:19 am UTC

SpitValve wrote:hmm...

Makes me want to sit more GR courses to get this straight :)


Well, GR has to be "kinda" wrong by default, since it predicts two things: a zero-dimension point for a non-spinning singularity, and a flat (zero width perpendicular to the axis of rotation) ring-shaped singularity. It's impossible in quantum physics to have anything have a zero-dimension in any direction, much less all of them.

The whole infinitely stretched space thing is one good-sounding explanation I've heard to explain that problem away. If it is non-spinning, everything is just heading toward a single, unreachable point, while if it is spinning, the angular momentum we would measure would be the matter spinning infinitely around and toward it.

edit: See, Hawking predicts that singularities eventually evaporate. Rolling it over in my head just now, I just toyed with the notion that maybe there's not enough gravity to stretch it infinitely, just near-infinitely, such that the first matter that finally reaches the singularity is exactly what destabilizes it, causing it to evaporate. Just a random thought, though.

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Postby Vaniver » Mon Jun 18, 2007 3:10 pm UTC

See, Hawking predicts that singularities eventually evaporate.
I never liked this idea. Is Hawking radiation really of such a large scale that it'll outweigh the amount of junk falling in? And, even if you talk about some hypothetical situation where you have multiple black holes and nothing else, won't most of the Hawking radiation that leaves a black hole just enter another black hole?
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Postby evilbeanfiend » Mon Jun 18, 2007 3:22 pm UTC

Vaniver wrote:
See, Hawking predicts that singularities eventually evaporate.
I never liked this idea. Is Hawking radiation really of such a large scale that it'll outweigh the amount of junk falling in? And, even if you talk about some hypothetical situation where you have multiple black holes and nothing else, won't most of the Hawking radiation that leaves a black hole just enter another black hole?


iirc, with an expanding universe its possible for black holes to 'starve' and evaporate (as stuff gets further apart it will be less likely that hawking radiation will fall into a black hole). obviously in big crunch universe everything ends up in the same black hole by definition.
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Postby gmalivuk » Mon Jun 18, 2007 3:24 pm UTC

SpitValve wrote:
Bondolon wrote:Main points here: Event horizon is the point of no return, Singularity is the point of no reaching.


I thought that once you're inside the black hole, you always end up in the singularity fairly quickly? Perhaps not in one piece though.


Yes, as you measure time, it's fininte before you reach the singularity.

Bondolon wrote:I was under the impression that there wasn't a definitive "this is what happens" set up, but that also one of the prevalent ideas was as such: as the gravity approaches the infinity, so does the space itself, such that the singularity is almost literally outside of space, and that anything approaching it continues to approach it eternally (or until the black hole evaporates). Again, that was just the impression that I got, but it seems pretty logical.


The thing is, your apparent velocity also approaches infinity. Here is a pretty good description of what (we think) happens from the POV of someone falling in. Supposing they don't get ripped to shreds first. (Which really isn't too difficult. Just throw yourself into a massive galactic black hole, and you can fall quite a ways inside the event horizon before the tidal forces become too much.)
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Postby Xayma » Fri Jun 22, 2007 2:21 am UTC

Vaniver wrote:
See, Hawking predicts that singularities eventually evaporate.
I never liked this idea. Is Hawking radiation really of such a large scale that it'll outweigh the amount of junk falling in? And, even if you talk about some hypothetical situation where you have multiple black holes and nothing else, won't most of the Hawking radiation that leaves a black hole just enter another black hole?
Hawking radition is quite weak for most black holes. The fact that any black hole we can infer from observations will be absorbing more of the CMBR then the black hole is emitting in Hawking radiation. According to wikipedia the break even point would be about the mass of the moon which if I remember correctly would be on the order of a cm radius.

One of the reasons why creating microscopic black holes in particle accelerators would be interesting is because they would evaporate due to the Hawking radiation almost instantly giving lots of particles off.

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Postby ossicle » Fri Jun 22, 2007 3:34 pm UTC

I can't understand anything in that paper.

Does anyone feel like simplifying some of what it says and/or commenting on how/whether it pertains to the stuff in this layman's article (which I believe someone posted somewhere on the forum a few weeks ago)?

http://www.nytimes.com/2007/06/05/scien ... 3c&ei=5070

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Postby SpitValve » Fri Jun 22, 2007 10:31 pm UTC

ossicle wrote:I can't understand anything in that paper.

Does anyone feel like simplifying some of what it says and/or commenting on how/whether it pertains to the stuff in this layman's article (which I believe someone posted somewhere on the forum a few weeks ago)?

http://www.nytimes.com/2007/06/05/scien ... 3c&ei=5070


Well, the energy for dark energy isn't _entirely_ conclusive yet, but it's "more likely than not" that the universe is accelerating, so that bit's ok.

There is still a maximum distance you can see, it's just larger than some thought. I get the impression it's increasing rather than decreasing though? So I'm not sure if they're right or not actually :)

Plus there are definitely more than 6 galaxies in the local group :)

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Postby Xanthir » Sat Jun 23, 2007 1:42 am UTC

All right, so there's something that I've always wondered about with Hawking radiation, and maybe someone can explain it (or point me to where I can have it explained).

Basically, Hawking radiation is caused by pairs of positive/negative energy particles that constantly appear and annihilate in extremely short time spans. These are, in turn, caused by the fact that the Uncertainty Principle prevents us from absolutely measuring both the energy and the rate of change of a field. Specifically, we can't ever say that a field is at zero energy, because that exactly pins down both the energy (0) and the rate of change(0). So, we get tiny random fluctuations in the various quantum fields that permeate the universe, producing particles.

Okay, now, if the pair appears *just* on the event horizon, so that one is outside of it and one is inside, they'll be torn apart. Unable to mutually annihilate, they stop hiding behind Heisenburg and must become real particles rather than just virtual particles.

Here's the part that gets me. Looking at it in a naive manner, there'd be no way to guarantee *which* one of the pair fell into the black hole. It could be the positive one, which would increase the hole's mass, or the negative one, which would annihilate something in there and decrease the mass. The latter is what makes it seem like the black hole is radiating energy, while the former would make the black hole radiate negative energy.

The way I've had it explained to me, though, the structure of our universe in non-degenerate spatial conditions prevents negative energy from existing. Thus, when the particle pair reifies the one that exits the black hole is *forced* to become positive. On the other hand, inside a black hole (even if space is still relatively normal, like just inside the horizon of Sag A*, the black hole in the center of our galaxy) it doesn't matter what the hell happens, because it's hidden from the rest of the universe. Thus, even though space is almost the exact same on either side of the horizon, negative energy is allowed to exist inside of it but not outside.

What's the deal there? Why is that? Just because it's necessary for things to work out right?

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Postby SpitValve » Sat Jun 23, 2007 2:02 am UTC

Xanthir wrote:What's the deal there? Why is that? Just because it's necessary for things to work out right?


"because it's necessary for things to work out right" is actually the same as saying "because any other way leads to a contradiction", which is not a bad argument.

If I give something a push, I have to fall backwards in order for momentum to be conserved: i.e. it's necessary for things to work out.

Anything more than that, I can't help you, I've only done a little GR and almost no crazy particle physics stuff.

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Postby Xayma » Sat Jun 23, 2007 11:19 am UTC

Xanthir wrote:Here's the part that gets me. Looking at it in a naive manner, there'd be no way to guarantee *which* one of the pair fell into the black hole. It could be the positive one, which would increase the hole's mass, or the negative one, which would annihilate something in there and decrease the mass. The latter is what makes it seem like the black hole is radiating energy, while the former would make the black hole radiate negative energy.
If anti matter (as we understand it) was in the black hole and destroyed matter that would be fine a) we would have no way of knowing it connected with matter b) it wouldn't make a difference if we did.

If an anti-particle falls in and annihilates a particle then the resulting energy cannot escape as it is beyond the event horizon, so the black hole won't actually be losing mass from this collision. As energy is equivalent to mass anyway, the mass of the blackhole would still increase by the mass of energy. (To put it another way if I give you a closed, non-absorbing box and you see via its gravitational energy that it has a mass of 1 kg, you couldn't tell if I have a 1 kg block of lead in there or have stored 9 * 10^16 Joules of light or some mixture of that in there.)

The idea of Hawking energy is that in both cases as the particle becomes "real" it must have energy, which means that in order to have zero energy the other particle must have had negative energy (and consequently negative mass) and fallen in. Ie it couldn't have been either matter or antimatter as we know it and has to be some form of exotic matter.

Of course if anti-matter has negative mass then we have a few more problems because then we couldn't just define a black hole by its mass, as photons would be absorbed by bother matter and anti-matter black holes increasing their energy, but it would then increase their mass in opposite directions.

While this isn't actually bad in itself, what would happen if you have a black hole, increase its mass using photons to say twice that then send in enough anti-matter to remove all the matter, would the black hole then be attracted towards matter or anti-matter? Of course if that was the case the anti matter would be repealed so strongly that you wouldn't be able to put any in there, but then you have issues of creating anti-particles in the middle of two black holes and them accelerating out with no apparent source of energy unless the two black holes are losing mass.

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Postby Xanthir » Tue Jun 26, 2007 4:13 pm UTC

Just as a quick comment, antimatter /= exotic matter. Hawking radiation depends on pos/neg particle pairs, not matter/antimatter pairs. I full understand that antimatter still has positive energy.

The idea of Hawking energy is that in both cases as the particle becomes "real" it must have energy, which means that in order to have zero energy the other particle must have had negative energy (and consequently negative mass) and fallen in. Ie it couldn't have been either matter or antimatter as we know it and has to be some form of exotic matter.

There's the crux of my confusion. It's said that the reason the escaping virtual particle becomes positive energy is because exotic matter can't exist in our universe under normal circumstances. But the event horizon isn't some physical barrier radically demarcating our universe into two halves where the laws of physics are completely different. It's just the arbitrary point where gravity is strong enough that light can't escape.

In my mind, you'd have to have some variation of the Cosmic Censorship Hypothesis that said, basically, "What happens in black holes, stays in black holes." In other words, the Universe simply doesn't care what goes on inside of an event horizon because the inside isn't causally connected to the outside. That just seems, I dunno, Universe-centric to me, like we're the only area of space that requires these special rules, and other, degenerate areas can do whatever the hell they want.

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Postby NathanielK » Tue Jun 26, 2007 7:03 pm UTC

The way I understand it from Kip Thorne's Black Holes and Time Warps, the particles gain enough energy to become "real" from tidal forces, and therefore from the black hole. If one of the particles then escapes, the hole only regains half of that energy.
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