Can black holes explode?

ML1
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Very good summary there,

Message 99398 in response to message 99397

Very good summary there, thanks.

One thought amongst many:

Might the expansion of our universe be driven or otherwise be caused by some expansion 'pressure' due to energy density/distribution?

For example in an analogous way to how a gas cloud will expand in a vacuum... For that we would measure a reduction in temperature of the gas...

Is there an 'energy density' to be measured for space-time?

Keep searchin',
Martin

ps: Wow... Is that a scientific hypothesis? Theory and a test to prove if true :-)

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Mike Hewson
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RE: Very good summary

Message 99399 in response to message 99398

Quote:
Very good summary there, thanks.


A pleasure ...

Quote:

One thought amongst many:

Might the expansion of our universe be driven or otherwise be caused by some expansion 'pressure' due to energy density/distribution?

For example in an analogous way to how a gas cloud will expand in a vacuum... For that we would measure a reduction in temperature of the gas...


Quite right! Apart from re-labelling a variable or three, that's pretty much the case. If you're familiar with the general/ideal gas law, PV = nRT, then for a given fixed total mass/energy ( ie. an isolated system ) : pressure and temperature go like density. The cosmic analogy is that we are uncertain of the 'equation of state' that applies early on.

Quote:
Is there an 'energy density' to be measured for space-time?


Correct again! The energy/momentum tensor in Einstein's main GR equation ( the 'right-hand' side ) encapsulates that.

Quote:
ps: Wow... Is that a scientific hypothesis? Theory and a test to prove if true :-)


Your score : 3 for 3! :-) :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

ML1
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RE: Your score : 3 for 3!

Message 99400 in response to message 99399

Quote:
Your score : 3 for 3! :-) :-)

7 more to go and I solve the Universe! :-p :-) :-)

That still leaves the real biggie of how our universe 'exists' and why...

More beer needed!

Cheers,
Martin

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RE: That still leaves the

Message 99401 in response to message 99400

Quote:
That still leaves the real biggie of how our universe 'exists' and why...

I've been beaten to working that one out. It's all due to gravity:

Stephen Hawking: God did not create Universe

... "Because there is a law such as gravity, the universe can and will create itself from nothing.

"Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist. ..."

Keep searchin',
Martin

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tullio
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I think that P.S. de Laplace

I think that P.S. de Laplace had preceded him, in a famous answer to Napoleon: Je n'ai pas besoin de cette hypothese la (diacritical omitted). Nihil sub sole novi.
Tullio

MAGIC Quantum Mechanic
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there is nothing new under

Message 99403 in response to message 99402

there is nothing new under the Sun......in more ways than one.

I just came in from my nightly staring up into the night sky thinking about how we look at things that happened many years and millions and billions of years ago.

But then most people look up there and think it is current happenings.

And everything we see is not even close to being new and our Sun is just a tiny star among trillions.

.........goodnight!

Quote:
I think that P.S. de Laplace had preceded him, in a famous answer to Napoleon: Je n'ai pas besoin de cette hypothese la (diacritical omitted). Nihil sub sole novi.
Tullio


 

Mike Hewson
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RE: 7 more to go and I

Message 99404 in response to message 99400

Quote:

7 more to go and I solve the Universe! :-p :-) :-)

That still leaves the real biggie of how our universe 'exists' and why...

More beer needed!


Easy. This is the Anthropic Beer Principle : the universe exists in order to have beings who consume beer.

Cheers, Mike.

( edit ) We've known this DownUnda for many years. It's even been peer reviewed at Wollongong University. By chaps called Bruce actually ..... :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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I've been a bit too quick &

Message 99405 in response to message 99399

I've been a bit too quick & sloppy here ....

Quote:
If you're familiar with the general/ideal gas law, PV = nRT, then for a given fixed total mass/energy ( ie. an isolated system ) : pressure and temperature go like density. The cosmic analogy is that we are uncertain of the 'equation of state' that applies early on.


I should have said something like 'familiar with the concepts leading to the general/ideal gas law .... '

The total energy [ being fundamentally kinetic for an ideal gas as we deliberately ignore potentials b/w molecules - model them as point particles having no volume, and which interact with the container's walls but not each other. Best approximated when dilute. ] if constant means PV = const = nRT. Temperature here is a measure of average kinetic energy per particle, so with nR = Nk ( n is number of moles and R is the general gas constant - a mole's worth of Boltzman's, N being the actual number of molecules and k is Boltzman's constant ), then kT is a molecule's worth of energy. Hence NkT is the energy of all the molecules.

So what happens if there is no container ? What do I assert as being the pressure? Think of a small flat area of some hard material I place within the gas. On average it will receive an equal amount of force(s) on one side due to collisions with rebounding gas molecules as compared with it's other side. The force divided by the area is the pressure. For a container wall we consider impacts from one side, the 'inside', having the pressure. This is either equalised by some outside pressure or constrained by the strength of the wall material. Hence a human needs a space suit to constrain their volume when in a vacuum.

Now back to the universe, as the volume goes up the pressure goes down, the density goes down and the so does the temperature. Did the energy go anywhere? The kinetic energy did : as each particle had to spend kinetic energy to get out of the gravitational potential well produce by every other particle. Thus the average kinetic energy goes down which is what we are allocating to the concept of temperature. Each particle is now higher up the gravity well than before and so, in theory, could slip back down later on .... speed up, get hotter etc.

Cheers, Mike.

( edit ) So R = Avagadro's number x Boltzman's constant

( edit ) And for photons one can consider their 'kinetic energy' to be E = hv, which acts in a dynamic sense for collisions and also reduces ( by frequency shift ) when climbing out of a gravity well. Also very early on there is significant interactions occurring due to other forces ( nuclear, electroweak ) that rapidly become negligible with increasing separation. Hence 'what is the equation of state?' back then .....

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

tullio
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Some scientists are believers

Some scientists are believers in God, some are not. I have personally known both kinds and I do not want to be a name dropper. The existence of God cannot be either proven or disproven by science. To believe in God is a pari, like Pascal said.We shall know the truth only after our death.
Tullio

Mike Hewson
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RE: And for photons one can

Message 99407 in response to message 99405

Quote:
And for photons one can consider their 'kinetic energy' to be E = hv, which acts in a dynamic sense for collisions and also reduces ( by frequency shift ) when climbing out of a gravity well. Also very early on there is significant interactions occurring due to other forces ( nuclear, electroweak ) that rapidly become negligible with increasing separation. Hence 'what is the equation of state?' back then .....


Another important, and of current interest regarding the Planck mission, is the Cosmic Microwave Background temperature pattern.

About 300,000 years after the Big Bang things cooled to the point where electrons could combine with protons ( previously too energetic to remain bound to each other ). So if a hydrogen atom ( proton bound with an electron ) has a given ionisation energy ( how much to you put in to completely separate them ), then once the average energy goes down ( temperature ) atoms form and stay formed. Well generally, it's a statistical thing, with an inexact transition from most being separate to most being bound. What was providing the energy to keep them separate was collisions and/or absorption of photons. So if the average energy of the photons goes lower than the ionisation energy of the hydrogen atom, plus the electron energy level jumps within the hydrogen atom becoming discrete ( and not a continuum ) then one has the circumstance where alot of photons no longer interact with the available matter. The parlance is that the radiation 'decouples' from matter.

So if we view this decoupled group of photons many years later, and yes they have cooled and hence frequency shifted downwards, we can see on the sky the temperature of these photons. This is the CMB. However with one thing and another that temperature is not the same every direction you look. There's a bit of a Doppler shift related to the direction that the Earth is travelling with respect to this radiation pattern. There are more recent ( ie. since 300,000 years after the decoupling ) sources to account for. But all in all it is still a pretty constant pattern despite small variations around and about.

One effect is that at 300,000 years after the Big Bang the density of the universe is not uniform. Denser clumps here and more dilute volumes there, with the effect that the strength of gravity varies due to the variable concentration of mass ( plus energy too, to be fully GR compliant ). The key point is that the photons emerging will be variably red-shifted ( due to gravity now ) as they come towards us and be measured today. So one effectively has a mass distribution atlas for the universe on or about 300,000 years after the big bang.

If one makes the additional assumption, or modelling, that relates that pattern at 300,000 years to rather earlier times then : one can peer back to such earlier times. This is not a direct measurement now, perhaps a bit like deducing a boat's hull shape from it's water wake. If you know the wake pattern and how water waves propagate then you might be able to reconstruct the boat's structure !!

So this 'cosmic imprint' is what some go on eloquently about - seeing the mind of God etc. BTW the obvious non-atheist reply to Mr Hawking's hypothesis is that God made it that way. It's orthogonal really, or what Stephen J Gould called 'non overlapping magisteria' .....

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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