The Fate of the Universe: Expansion, Matter Decay, and the Ultimate End

[MathematicalPhysicist]

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?

 

[Ryan_m_b]

Not empty but pretty much dead. Some models predict an era where all that is left are photons, neutrinos and electrons. Take a look at this timeline:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Timeline

Doesn't look fun from the Degenerate era onwards.

 

[MathematicalPhysicist]

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...

 

[Ryan_m_b]

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 won't be getting anymore stars or planets. Just a vast, cold, mostly empty universe where once an eon two leptons might fly past each other.

 

[bcrowell]

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

 

[MathematicalPhysicist]

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?

 

[Nabeshin]

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.

 

[George Jones]

Have a look at these articles:

http://math.ucr.edu/home/baez/end.html;

http://arxiv.org/abs/astro-ph/9902189;

http://arxiv.org/abs/0704.0221.

 

[bcrowell]

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.

 

[Chronos]

There is no evidence of proton decay in any experiments conducted to date.

 

[Chalnoth]

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.

 

[bcrowell]

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.

 

[Chalnoth]

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.

 

[SHISHKABOB]

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?

 

[Chalnoth]

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).

 

[bcrowell]

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.

 

[Chalnoth]

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.

 

[PAllen]

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.

 

[Chalnoth]

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.

 

[twofish-quant]

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.

 

[twofish-quant]

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.

 

[Chalnoth]

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.

 

[PAllen]

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.

 

[Chalnoth]

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.

 

[PAllen]

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).

 

[Haelfix]

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).

It is true in a sense, but it depends on the rates and details of the new physics to a certain degree.

In the standard model, proton decay proceeds via dimension 6 operators. This is a nonrenormalizable, baryon number violating interaction that is suppressed by 2 powers of a cutoff scale that is taken to be very high.

Postulating some sort of new physics between the electroweak scale and the Planck scale, will set this cutoff scale, and you must be very careful that these dimension 6 operators don't come dangerously close to violating the proton decay bounds (which are bounded by experiment to something like >10^33 years).

This is a typical and powerful constraint that phenomenologists use, but there are mechanisms or extra symmetries that you can introduce to tame the rates to a certain extent.

However if you violate baryon number, there will be proton decay at some level, it just might be small relative to some other new physics.

So for instance, in the standard model baryon number is an accidental symmetry and is actually violated by nonperturbative physics. Eg instanton physics will yield nonzero tunneling cross sections between degenerate weak SU(2) vacua that imply proton decay on the order of ~10^170 years or something enormous like that. So you see, it is a true statement, but it can be made essentially irrelevant for any sensible physics.

 

[twofish-quant]

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.

CPT has to hold in order for their to be Lorenz covariance.

So I don't think this gets you out of forcing proton decay to exist.

I'm still not understanding how CP-violation *requires* proton decay. What I'm looking for is some mathematical statement that if CP-violation exists and protons don't decay then some fundamental physical constraint (i.e. Lorenz covariance) is violated.

But then you'd still have decays with those heavier particles as intermediate steps.

You can get around that. If the particles are heavy enough then any reactions that get you to the heavy particles would in the process produce at least one new proton for anyone that decays. The mass scales are large enough so that's possible.

 

[twofish-quant]

As far as I am aware, all GUT's yet proposed require proton decay in order to have baryogenesis.

And that's a limitation with GUT's rather than some fundamental constraint of the universe.

Basically the way that GUT's work is to put the SM into some larger group. If you want anything non-trivial then this involves mixing baryon and lepton groups. If you don't mix those groups, then you end up with just the SM and you have something that is non-publishable.

However this is "argument by aesthetics" or "argument by lack of imagination." I don't know of (and would be interesting to hear of) any arguments that say that CP-violation *requires* proton decay because of some fundamental physical constraint. I can think of some mechanisms that would supress proton decay. You'd end up with an ad-hoc and ugly theory, but argument by aesthetic or mathematical simplicity isn't something that i think counts for every much here.

 

[twofish-quant]

However if you violate baryon number, there will be proton decay at some level, it just might be small relative to some other new physics.

And that new physics could very well generate new protons at faster than the rate that protons decay. We are talking about energy scales much larger than the mass of the proton.

 

[Chalnoth]

You can get around that. If the particles are heavy enough then any reactions that get you to the heavy particles would in the process produce at least one new proton for anyone that decays. The mass scales are large enough so that's possible.

Then that wouldn't be a baryon-number violating decay.

Granted, maybe it's possible to come up with a theory which allows baryon number violation in one direction only. My knowledge of high-energy physics isn't sufficient to rule something like this out. But it naively seems massively unlikely.

 

[twofish-quant]

Then that wouldn't be a baryon-number violating decay.

If it's two different processes it would be.

Something like that happens in atomic nuclei. Free neutrons will decay to protons, but when you bind neutrons and protons then any n->p decay is balanced by p->n.

Granted, maybe it's possible to come up with a theory which allows baryon number violation in one direction only. My knowledge of high-energy physics isn't sufficient to rule something like this out. But it naively seems massively unlikely.

I think it's trivial to allow one way baryon number violation. You go into your favorite GUT, and say *God says so* and it is done. You end up with an ugly ad-hoc theory, but one that doesn't contradict any observations or settled physical principles.

Also, what I'm looking for is something "clean" like the Sahkarov arguments. You can easily show that any matter/anti-matter imbalance starting from a symmetry situation requires CP-imbalance, and that's a clean argument based on very firm physical principles.

If there is a similar argument saying CP-imbalance -> proton decay, I'd be interested. Proton decay is a generic feature of GUT's, but you can just argue "GUT's are wrong."

 

[PAllen]

You know, there is just as much an asymmetry in leptons. Why not claim electrons exist implies electrons decay? Whatever mechanism could allow for lepton asymmetry could, in some not yet described theory, allow for baryon asymmetry.

Again, there is a whole chain of assumptions here:

- Universe began with a big bang scenario (of course I don't dispute this, but it is separate from particle physics; in the past, eternal cyclic theories were considered, in which case baryon asymmetry is just a boundary condition of the infinite past).

- Any extension to SM to account for evolution of baryons asymmetry from assumed symmetric big bang state must have the character of known GUTs.

Please note: in no way do I dispute that big bang is highly likely, and GUT type phenomenology is likely in a successful BSM theory, however I prefer to be clear on chain of certainty and reasoning:

- we are confident there is baryon asymmetry (but not certain)
- the most successful cosmology models start with zero baryon number
- thus, there must be a mechanism to explain this

The above are not, IMO ironclad, and further assumptions requiring proton decay are somewhat less certain.

 

[Chronos]

I remain skeptical on the basis of lack of experimental evidence favoring proton decay.

 

[twofish-quant]

You know, there is just as much an asymmetry in leptons. Why not claim electrons exist implies electrons decay?

Not that I disagree with your main point, but for completeness...

One general result of GUT's is that B and L are not conserved but B-L is. The reason that people don't talk about electron decay is that there aren't any particles lighter than the electron that it can decay to without violating some conservation rule.

- Any extension to SM to account for evolution of baryons asymmetry from assumed symmetric big bang state must have the character of known GUTs.

I think that's the big one that I find difficult to swallow. Things could change quickly if we see some strong evidence that GUT's are right, but right now GUT's have enough problems that if someone argues that we are just wrong, that I can't counterargue.

- the most successful cosmology models start with zero baryon number

I wonder about this one. Starting with zero baryon number makes the universe nice and symmetric, but is there any physical reason way we can't start with non-zero baryon number?

 

[Chalnoth]

You know, there is just as much an asymmetry in leptons. Why not claim electrons exist implies electrons decay? Whatever mechanism could allow for lepton asymmetry could, in some not yet described theory, allow for baryon asymmetry.

Electrons are the lightest charged particle. So they can't decay unless there is non-conservation of charge, which we have no reason to believe.

Again, there is a whole chain of assumptions here:

- Universe began with a big bang scenario (of course I don't dispute this, but it is separate from particle physics; in the past, eternal cyclic theories were considered, in which case baryon asymmetry is just a boundary condition of the infinite past).

- Any extension to SM to account for evolution of baryons asymmetry from assumed symmetric big bang state must have the character of known GUTs.

Please note: in no way do I dispute that big bang is highly likely, and GUT type phenomenology is likely in a successful BSM theory, however I prefer to be clear on chain of certainty and reasoning:

- we are confident there is baryon asymmetry (but not certain)
- the most successful cosmology models start with zero baryon number
- thus, there must be a mechanism to explain this

The above are not, IMO ironclad, and further assumptions requiring proton decay are somewhat less certain.

I would say requiring baryogenesis from a hot dense symmetric state is ironclad at present given the evidence. And I have a really, really hard time buying that you can produce a theory which explains baryogenesis from such a state without proton decay that also respects CPT symmetry.