Will all matter be converted to photons?

[bcrowell]

For the ultimate photon fate of the universe we need to have:
Black holes as singularities, which is not the best of models, gravastar being a nice contender (purely personal opinion).
Hawking radiation, no physical evidence thus far.
Dark energy, which accelerates stuff, yet all our observations are of horribly past events.

You're certainly right that all of this stuff requires brave extrapolations that may be wrong. However, the FAQ is not "What is the ultimate fate of the universe?" but "Will all matter be converted to photons?," which is much more restricted. The answer is no, and I don't think any of the things you list have any effect on that. Whether or not a black hole has a singularity inside it has no effect on the outside universe. If Hawking radiation doesn't exist (or if black hole event horizons don't exist), that would eliminate one of the possible mechanisms for converting matter into photons, and the answer would be an even more conclusive no. Although the evidence for accelerating cosmological expansion and dark energy is in fact much stronger than you appear to believe, if it turned out to be wrong, that wouldn't change the result; in fact, the paper by Adams and Laughlin was written before cosmological acceleration was discovered, but they conclude the same thing.

I didn't think there was any experimental evidence for proton decay. What mechanisms turn baryonic matter into something else?

Right, there's some pretty strong theoretical motivation, but no empirical evidence. As noted in #1, this only changes the detailed composition of the matter that we end up with. It doesn't change the 'no" answer to the question posed. IIRC scenarios with and without proton decay are discussed in some detail in the Adams-Laughlin paper.

 

[Jon Richfield]

This process of disintegration is not restricted to galaxies. In the long run, all material objects are expected to disintegrate into subatomic particles through a variety of thermal and quantum-mechanical fluctuations, although some of the processes are expected to take extremely long times.[Baez 2004]
...
So if we look at the cosmic cereal box a gajillion years from now, what will be on the list of ingredients? It will contain stable, massive subatomic particles such as electrons.[Baez 2004] (The exact roster of particles depends on unknown particle physics such as dark matter and proton decay.)
...

Please clarify a point concerning matter-antimatter. Does that scenario not assume that the net universal count of leptons, quarks etc is non-zero? And if so, do we have any basis for assuming this is so? (I am not asserting or hoping anything in particular; this is just a question without baggage, either emotional or theoretical.)

 

[mfb]

We know it is, the amount of antimatter is tiny in our universe.
But even if it wouldn't, expansion of the universe spreads out the different particles so fast that there would not be a complete annihilation any more.

 

[AlabamaCajun]

Photons are but one of the force carriers, IMO, I don't see why matter would decay to wandering photons.
How far would the universe have to expand to allow all particles to just spread out even to decay just to helium or sub particles.
As long as an attractive force is in place particles will be attracted to other particles forming matter/anti-matter or energy.
The opposite affect is the total collapse into one dark object, it may have already happened (insert big bank text here).

 

[Drakkith]

Photons are but one of the force carriers, IMO, I don't see why matter would decay to wandering photons.

Essentially all decay mechanisms would eventually lead to the creation of photons and leptons. They are the least massive, stable particles and have nothing else they can decay to and still obey conservation laws.

As long as an attractive force is in place particles will be attracted to other particles forming matter/anti-matter or energy.

That might be true if it weren't for the expansion of the universe. Below a certain density expansion would keep pretty much any two particles from being pulled together under gravity.

The opposite affect is the total collapse into one dark object, it may have already happened (insert big bank text here).

Are you referring to the Big Crunch?

 

[bcrowell]

Photons are but one of the force carriers, IMO, I don't see why matter would decay to wandering photons.

The only thing I would point out in addition to what Drakkith said in #35 is the following. People who believe, incorrectly, that all matter will be converted to photons seem to believe that the mechanism of conversion is that all matter gets eaten by black holes and recycled via Hawking radiation. Hawking radiation does in principle include all kinds of particles other than photons. However, these other particles have nonvanishing mass $m$, and the Hawking radiation's temperature is normally much less than $mc^2/k$, so the probability of emitting these particles is ordinarily very small. So if you believed, incorrectly, that all matter gets recycled, then it would make sense to believe that almost all of the resulting stuff would be photons.

 

[Greg Bernhardt]

Great FAQ!