death of the universe?!

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_...verse#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

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.

bcrowell

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.

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

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

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

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

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

There's also no baryon asymmetry in the standard model. So yes, the existence of protons does disprove the standard model.

SHISHKABOB

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

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

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

When combined with the fact that protons had to be produced in the early universe, it does.