Death of the universe?

  • #1
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I don't quite understand this.

Some models say that if the universe keeps expanding indefinitely eventually matter will disolve into radiation, I also read that the particles of matter such as protons wil decay (I am not sure how reliable is this if no one ever deteceted proton decaying). Now it's not as if one day the universe will be empty, right?
 

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  • #2
Ryan_m_b
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  • #3
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But what of the quarks, will they decay as well?

Well doesn't this resemble the start of the universe with soup of particles?

It always leads to the question how can particles be assembled to planets and stars etc...
 
  • #4
Ryan_m_b
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But what of the quarks, will they decay as well?

Well doesn't this resemble the start of the universe with soup of particles?

It always leads to the question how can particles be assembled to planets and stars etc...
I'm not sure about quarks but it's not like the start of the universe because everything is vastly more spread out and entropy has increased to near maximum. There simply isn't that much energy in the entire universe to do work anymore.

Stellar and planetary formation is a product of gravity. Whilst gravity will still remain the sheer size of the universe and the rate of its expansion compared to the star formation era means that you wont be getting anymore stars or planets. Just a vast, cold, mostly empty universe where once an eon two leptons might fly past each other.
 
  • #5
bcrowell
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Some models say that if the universe keeps expanding indefinitely eventually matter will disolve into radiation,[...]
Not true. Although Hawking radiation will convert some matter into radiation, the universe is predicted to have matter in it at all future times. See Adams and Laughlin, http://arxiv.org/abs/astro-ph/9701131 , §VD. More recently, Penrose has had a lot of motivation to poke around and look at mechanisms for complete conversion to photons, and at one time was pushing nonstandard particle-physics mechanisms for this as a prediction of his conformal cyclic cosmology (CCC). The fact that he couldn't find standard mechanisms for it shows that the current state of the art does *not* predict it to happen.

But what of the quarks, will they decay as well?
If a particular quark ends up in a black hole, then the black hole will eventually evaporate, and the evaporation will produce mostly photons. Proton decay is also a possibility: http://en.wikipedia.org/wiki/Proton_decay
 
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  • #6
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How can you disprove proton decay?

I mean I understand how you can prove it exists, you just watch for such an occurence (though I am not sure what do one need to detect in order to be witnessing proton decay), but to disprove it looks tough empirically, isn't it?
 
  • #7
Nabeshin
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How can you disprove proton decay?

I mean I understand how you can prove it exists, you just watch for such an occurence (though I am not sure what do one need to detect in order to be witnessing proton decay), but to disprove it looks tough empirically, isn't it?
That's the difficulty with disproving ANYTHING in science.
 
  • #9
bcrowell
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How can you disprove proton decay?

I mean I understand how you can prove it exists, you just watch for such an occurence (though I am not sure what do one need to detect in order to be witnessing proton decay), but to disprove it looks tough empirically, isn't it?
All you can do is put a lower limit on the lifetime. But in any case it doesn't seem relevant to the current discussion. The proton would decay into leptons, not radiation.
 
  • #10
Chronos
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There is no evidence of proton decay in any experiments conducted to date.
 
  • #11
Chalnoth
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There is no evidence of proton decay in any experiments conducted to date.
Except for the fact that protons had to be produced in the early universe. That fact alone implies that proton decay must be possible.
 
  • #12
bcrowell
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Except for the fact that protons had to be produced in the early universe. That fact alone implies that proton decay must be possible.
Huh? No, that's wrong. There is no proton decay in the standard model. Therefore you seem to be claiming that the existence of protons disproves the standard model.
 
  • #13
Chalnoth
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Huh? No, that's wrong. There is no proton decay in the standard model. Therefore you seem to be claiming that the existence of protons disproves the standard model.
There's also no baryon asymmetry in the standard model. So yes, the existence of protons does disprove the standard model.
 
  • #14
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There's also no baryon asymmetry in the standard model. So yes, the existence of protons does disprove the standard model.
wait, what? I thought the Standard Model was... the standard model. How can it be disproved by something as fundamental as the existence of protons? Are you saying that the Standard Model doesn't explain the existence of protons? And so that would mean that the SM is not complete?
 
  • #15
Chalnoth
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wait, what? I thought the Standard Model was... the standard model. How can it be disproved by something as fundamental as the existence of protons? Are you saying that the Standard Model doesn't explain the existence of protons? And so that would mean that the SM is not complete?
Well, to put it more correctly, there is no way within the standard model to produce the asymmetry of matter and anti-matter in the early universe. So far we don't have enough experimental data to show us which model is the correct one, however (all experimental tests of the standard model have been quite consistent with it).
 
  • #16
bcrowell
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There's also no baryon asymmetry in the standard model. So yes, the existence of protons does disprove the standard model.
Oh, please.

First off, the existence of protons does not require baryon asymmetry. If (a) protons exist, and (b) an equal number of antiprotons does not exist, and (c) the initial conditions of the universe had zero baryon number, then (d) baryon asymmetry is required. The logic here is a & b & c -> d, not a -> d.

Second, this is a distraction from your mistake in claiming that the existence of protons implies proton decay. It does not.

When someone points out to you that you've made a mistake, please just admit it and move on rather than trying to make some new claim that distracts attention from the mistake.
 
  • #17
Chalnoth
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Second, this is a distraction from your mistake in claiming that the existence of protons implies proton decay. It does not.
When combined with the fact that protons had to be produced in the early universe, it does.
 
  • #18
PAllen
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When combined with the fact that protons had to be produced in the early universe, it does.
Even if you assume (reasonably) that the existing universe almost all matter, how on earth do you favor proton decay over differential productions rates? I'll buy that, with reasonable cosmological assumptions, SM cannot explain existence of matter in current quantity, but I find differential productions rates via unknown CP violations more plausible than proton decay.
 
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  • #19
Chalnoth
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Even if you assume (reasonably) that the existing universe almost all matter, how on earth do you favor proton decay over differential productions rates? I'll buy that, with reasonable cosmological assumptions, SM cannot explain existence of matter in current quantity, but I find differential productions rates via unknown CP violations more plausible than proton decay.
Such CP violations imply non-conservation of baryon number. And if baryon number is not conserved, then protons can decay.
 
  • #20
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How can you disprove proton decay?
You can't prove that protons decay at all, but you can set limits.

http://hep.bu.edu/~kearns/pub/kearns-pdk-snowmass.pdf

I mean I understand how you can prove it exists, you just watch for such an occurence (though I am not sure what do one need to detect in order to be witnessing proton decay), but to disprove it looks tough empirically, isn't it?
You disprove it within certain limits. The limits that we have right now have already falsified a number of theories.
 
  • #21
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Such CP violations imply non-conservation of baryon number. And if baryon number is not conserved, then protons can decay.
If CP is violated then so is T, and if T is violated then you can have processes that create baryons violate baryon number conservation while requiring that the reverse processes conserve baryon number.

You can also have a situation in which you create X/anti-X bosons through pair production. The decay of X/anti-X bosons into protons would not conserve baryon number, but any processes involving protons decaying into lighter particles could.

If you have any literature that says that those scenarios are impossible, I'd be interested in seeing them.
 
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  • #22
Chalnoth
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If CP is violated then so is T, and if T is violated then you can have processes that create baryons violate baryon number conservation while requiring that the reverse processes conserve baryon number.
This line of reasoning presumes that CPT is a perfect symmetry, which means that you can perform the time-reversal of all processes, just with the positions of matter and anti-matter switched. So I don't think this gets you out of forcing proton decay to exist.

You can also have a situation in which you create X/anti-X bosons through pair production. The decay of X/anti-X bosons into protons would not conserve baryon number, but any processes involving protons decaying into lighter particles could.
But then you'd still have decays with those heavier particles as intermediate steps.
 
  • #23
PAllen
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This line of reasoning presumes that CPT is a perfect symmetry, which means that you can perform the time-reversal of all processes, just with the positions of matter and anti-matter switched. So I don't think this gets you out of forcing proton decay to exist.

.
What if anti-protons can decay (slowly) but not protons? That seems to meet the requirements.
 
  • #24
Chalnoth
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What if anti-protons can decay (slowly) but not protons? That seems to meet the requirements.
Honestly, I'm not entirely sure. It just seems incredibly unlikely.

As far as I am aware, all GUT's yet proposed require proton decay in order to have baryogenesis.
 
  • #25
PAllen
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Honestly, I'm not entirely sure. It just seems incredibly unlikely.

As far as I am aware, all GUT's yet proposed require proton decay in order to have baryogenesis.
That is also true of what I know, but I don't see that as equivalent to the statement that any modification of SM to allow for baryon asymmetry must entail proton decay. It is an assumption that a GUT is true of our universe. Further, I've certainly seen proposed mechanisms for the origin of baryon asymmetry that don't entail proton decay (irrespective whether the model contains proton decay as one of its other predictions).
 

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