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Watsisname
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24 Nov 2019 05:54

midtskogen wrote:
Source of the post Assuming that the Big Bang produced exactly equal amounts of matter and anti-matter, could the creation of anti-matter black holes be what shifted the balance and prevented everything from just annihilate into pure energy?

Interesting idea.  Unfortunately I don't think this would solve the problem, because if say an equal amount of matter and antimatter collapsed into black holes, then there would still be an equal amount of matter and antimatter not in black holes, which presumably would still annihilate completely.  Then we'd have either a universe full of nothing but black holes, or if those black holes were small enough to evaporate by now, a universe full of Hawking radiation.  So we'd need some mechanism for having more antimatter to collapse into black holes than matter, which only shifts the problem.  We also need this to happen over the right timescales and in such a way as to be consistent with observations.

This problem of why the universe is mostly full of matter rather than purely radiation from equal amount of matter and antimatter annihilating is called baryon asymmetry, and it's not fully understood.  There are also other types of symmetries in particle physics, which were long held to be physical laws, but then discovered to be violated under certain conditions (CP-symmetry for example).
 
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JackDole
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24 Nov 2019 07:03

Honestly, I find it difficult to understand these explanations, especially in English. (The video about Hawking radiation.)
Incidentally, according to the article in German Wikipedia, Hawking himself used the image of matter/antimatter particles to explain the radiation. (And I also think I read it in a book by Hawing, but I can not say for sure. It has been many years ago.)
But what I believe to have been understood is that Hawking radiation has not been proven yet, no one really knows what it is and that it's just a mathematical construct to preserve the entropy!
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Watsisname
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24 Nov 2019 07:45

JackDole wrote:
Source of the post But what I believe to have been understood is that Hawking radiation has not been proven yet, no one really knows what it is and that it's just a mathematical construct to preserve the entropy!


Energy is also just a mathematical construct so that we can write formulas and describe physical processes in terms of a conserved quantity.

That black holes have an entropy associated with their event horizons is not controversial.  We just calculated it!  That this entropy depends on the energy tells us that black holes have a temperature, and therefore they must radiate photons with a blackbody spectrum.  There's really no way around it, and Hawking made the proof even more rigorous.  (He did not use matter-antimatter production, but instead the propagation of modes of the vacuum fields by applying quantum field theory.  Describing this as "matter-antimatter pairs" is just a convenient but oversimplified narrative, almost as sloppy as saying the Big Bang came from nothing.) 

Is there any experimental evidence for concluding Hawking radiation exists?  Well, we do observe it in a variety of analogue black holes in the lab.  But maybe because we don't measure it directly from real astrophysical black holes, one may still be skeptical.  For any reasonably-sized black hole the radiation would be so weak that there is no hope to measure it.  However, we can make an observation with real black holes that supports it indirectly.  Specifically, we can test the other predictions of black hole thermodynamics.  Since the black hole's entropy is proportional to its horizon area, the 2nd law of thermodynamics insists that when two black holes merge together, the total horizon area cannot decrease.

This is a strict limit on how big the resulting black hole can be.  By conservation of mass, it cannot be bigger than the sum of the masses of the two original black holes.  But by law that the entropy and thus area cannot decrease, the square of the resulting mass cannot be less than the sum of the squares of the two original masses.  

Example:  If two 10 solar mass black holes merge, then the greatest possible result is 10+10=20 solar masses, but the minimum possible result is sqrt(102 + 102) = sqrt(200) = 14.14 solar masses.  Whatever difference from 20 solar masses was carried away by gravitational waves.

We can test this by looking at the gravitational waves from black hole mergers with LIGO, and indeed, the gravitational waves never carry more energy than what the laws of black hole thermodynamics allow.

(Aside: what kind of collision would result in the maximum allowed value of just adding the masses together?  That happens if the two black holes collide head-on.  Otherwise, some orbital angular momentum is shed as gravitational waves, and this reduces the total mass of the system.)
 
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midtskogen
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24 Nov 2019 08:46

Watsisname wrote:
Source of the post So we'd need some mechanism for having more antimatter to collapse into black holes than matter,

Yes, that was the idea. Like a tiny asymmetry in the distribution of matter and antimatter.
Watsisname wrote:
Source of the post which only shifts the problem. 

Yes, but perhaps to a context that we understand better.
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A-L-E-X
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24 Nov 2019 10:04

Watsisname wrote:
midtskogen wrote:
Source of the post Assuming that the Big Bang produced exactly equal amounts of matter and anti-matter, could the creation of anti-matter black holes be what shifted the balance and prevented everything from just annihilate into pure energy?

Interesting idea.  Unfortunately I don't think this would solve the problem, because if say an equal amount of matter and antimatter collapsed into black holes, then there would still be an equal amount of matter and antimatter not in black holes, which presumably would still annihilate completely.  Then we'd have either a universe full of nothing but black holes, or if those black holes were small enough to evaporate by now, a universe full of Hawking radiation.  So we'd need some mechanism for having more antimatter to collapse into black holes than matter, which only shifts the problem.  We also need this to happen over the right timescales and in such a way as to be consistent with observations.

This problem of why the universe is mostly full of matter rather than purely radiation from equal amount of matter and antimatter annihilating is called baryon asymmetry, and it's not fully understood.  There are also other types of symmetries in particle physics, which were long held to be physical laws, but then discovered to be violated under certain conditions (CP-symmetry for example).

I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.
 
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Watsisname
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24 Nov 2019 15:14

A-L-E-X wrote:
Source of the post I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.

The problem is that pair production must produce a lot of both matter and antimatter in this universe -- indeed within every tiny volume element of space.  So it's not easy to explain away by supposing that regions of matter and antimatter dominance are just separated by large distances after expansion, either.  There must be something in the particle physics, a type of symmetry breaking, that led to a small fraction of one being produced more than the other.  In fact there are hints that this is the case in particle accelerator experiments, which produce slightly more matter than antimatter, but again the exact nature of it remains unknown.
 
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24 Nov 2019 16:02

Watsisname wrote:
A-L-E-X wrote:
Source of the post I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.

The problem is that pair production must produce a lot of both matter and antimatter in this universe -- indeed within every tiny volume element of space.  So it's not easy to explain away by supposing that regions of matter and antimatter dominance are just separated by large distances after expansion, either.  There must be something in the particle physics, a type of symmetry breaking, that led to a small fraction of one being produced more than the other.  In fact there are hints that this is the case in particle accelerator experiments, which produce slightly more matter than antimatter, but again the exact nature of it remains unknown.

As far as our universe is concerned, wasn't the matter resolved awhile back because experiments showed that certain particles are produced more than their antiparticles are (I believe it was pi mesons or pions?)  So equal amounts of matter and antimatter would have been annihilated, leaving behind an excess of matter.  I dont know if this actually solves the problem, but it does give a way for it to be resolved if this was also the case in the early universe.

And if this is the case, there has to be a reason for it.  So it would mean that matter and antimatter are not quite mirror opposites of each other.....

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