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View Full Version : Can universal expansion produce hawking radiation?



tommac
2010-Apr-23, 11:35 PM
from:
http://www.bautforum.com/showthread.php/102898-universal-expansion-can-produce-hawking-radiation?p=1721682#post1721682

To be honest this was more of a question, that eventually got a q and a locked. I threw it here. The math is beyond me, so I will ask it as a question. If this is not allowed ... please reclose. I have been suspended for 10 days because I asked this question so I already did my time and I will abandon the thread if this is not allowed.

a) If the big rip existed, I read that eventually the unvierse would be expanding at such a rate that atoms would be ripped apart.

b) would an event horizon exist in a big rip scenerio ( I assume yes and I assume that the event horizon would close in on any and all points )

c) if atoms can be ripped apart, then would virtual particles also be ripped apart ( I realize that this is using an analogy ) I have heard that it has something to do with quantum tunnelling ... if there is an event horizon and/or if the expansion of the universe is expanding local space time at the speed of light, then would the same properties exist as a black hole or unruh radiation in the end stages of the big rip?

gzhpcu
2010-Apr-25, 05:54 PM
c) if atoms can be ripped apart, then would virtual particles also be ripped apart ( I realize that this is using an analogy ) I have heard that it has something to do with quantum tunnelling ... if there is an event horizon and/or if the expansion of the universe is expanding local space time at the speed of light, then would the same properties exist as a black hole or unruh radiation in the end stages of the big rip?
Quantum tunneling has been proposed as a mechanism which allows one of the virtual particles in the vicinity of a Black Hole's event horizon to escape. (using the Schrödinger Wave equation for the probability cloud of where the particle could actually be, it might happen that - despite low probability -escapes a strong gravitational barrier).
I would think it depends on how "the universe" is defined. If it is encased in something larger and has an event horizon into which one of the virtual particles can escape into, then perhaps yes. In this case, the universe would be leaking energy and radiation would be escaping into a larger universe.
Spacetime can expand faster than the speed of light, as is proposed by the Gluth inflationary universe model.

tommac
2010-Apr-26, 01:57 AM
Quantum tunneling has been proposed as a mechanism which allows one of the virtual particles in the vicinity of a Black Hole's event horizon to escape. (using the Schrödinger Wave equation for the probability cloud of where the particle could actually be, it might happen that - despite low probability -escapes a strong gravitational barrier).
I would think it depends on how "the universe" is defined. If it is encased in something larger and has an event horizon into which one of the virtual particles can escape into, then perhaps yes. In this case, the universe would be leaking energy and radiation would be escaping into a larger universe.
Spacetime can expand faster than the speed of light, as is proposed by the Gluth inflationary universe model.

Wouldn't it be leaking inward and everywhere? This is the part I dont get.

gzhpcu
2010-Apr-26, 05:48 AM
Wouldn't it be leaking inward and everywhere? This is the part I dont get.
Leaking means escaping the event horizon, where nothing is supposed to escape. In this case, escaping the event horizon of the universe, instead of that of a black hole.

Incomplete
2010-Apr-30, 09:28 PM
Yes. It can and does.


from:
http://www.bautforum.com/showthread.php/102898-universal-expansion-can-produce-hawking-radiation?p=1721682#post1721682

To be honest this was more of a question, that eventually got a q and a locked. I threw it here. The math is beyond me, so I will ask it as a question. If this is not allowed ... please reclose. I have been suspended for 10 days because I asked this question so I already did my time and I will abandon the thread if this is not allowed.

a) If the big rip existed, I read that eventually the unvierse would be expanding at such a rate that atoms would be ripped apart.

b) would an event horizon exist in a big rip scenerio ( I assume yes and I assume that the event horizon would close in on any and all points )

Yes. But you don't even need a big rip for that.

c
) if atoms can be ripped apart, then would virtual particles also be ripped apart ( I realize that this is using an analogy ) I have heard that it has something to do with quantum tunnelling ... if there is an event horizon and/or if the expansion of the universe is expanding local space time at the speed of light, then would the same properties exist as a black hole or unruh radiation in the end stages of the big rip?

Yes, more or less. But again, there's no need for a big rip. Accelerated observers see particles for a very similar reason. In the early universe the expansion accelerated exponentially, there was a horizon, and particles were created.

In fact, the stars you see in the night sky are a direct result of the process you're asking about.

tommac
2010-May-04, 09:07 PM
Leaking means escaping the event horizon, where nothing is supposed to escape. In this case, escaping the event horizon of the universe, instead of that of a black hole.

In a sufficiently increasingly accelerating universe , there should be no leak right? There would only be leak if we could eventually travel to the same place that the photons could travel to right? Sorry about wording.

speedfreek
2010-May-04, 11:30 PM
I think I see what you are getting at. You are asking if, in a Big-Rip Scenario, virtual particle pairs would be ripped apart, rather than cancelling each other out, filling the universe with individual virtual particles that can never meet, due the accelerating expansion causing an event horizon at the smallest scales. Or something like that? :)

WayneFrancis
2010-May-05, 12:42 AM
Isn't the issue here a question of the the probability of a particle tunnelling the distance needed to pass the EH? The smaller the black hole the shorter the distance that needs to be tunnelled, the higher the probability of success and thus the higher the temperature? This is why bigger black holes are colder then smaller ones. Then for universal expansion the EH is effectively ~27 billion ly in diameter which would amount to a very low, but still non zero, probability of HR being produced because of it.

I'm talking about now and not at the "end of the universe" when everything is expanding away. Then I'd admit that the EH could be very small and I'm not sure what would really happen here and if it would make a difference. This is because unlike a BH emitting HR in which the negative particle always falls into the black hole and the positive escape reducing the mass of the BH in universal expansion the tunnelling would happen in both directions and should end up, on average, the same.

If this did occur wouldn't we expect to see lots of gamma ray bursts from the annihilations but without a loss of the amount of energy we'd expect just from HR?


This isn't much different to the question I had asking if cosmic inflation could rip apart quark pairs, which should require much less energy levels then producing particles from virtual particles.

tommac
2010-May-05, 02:25 AM
I think I see what you are getting at. You are asking if, in a Big-Rip Scenario, virtual particle pairs would be ripped apart, rather than cancelling each other out, filling the universe with individual virtual particles that can never meet, due the accelerating expansion causing an event horizon at the smallest scales. Or something like that? :)

Exactly ...