FAQ: In the very distant future, will all matter be converted into black holes and then photons?
No. There is a misconception that a lot of laypeople seem to be picking up recently from popularizations, which is that all the matter in the universe will first be sucked into black holes, and then recycled into photons in the distant future through Hawking radiation, so that the only thing left will be photons. This is wrong.
First let’s talk about the black hole part. Physicists have done detailed calculations of this kind of thing. Black holes are not super-powerful cosmic vacuum cleaners that can suck in everything there is. When a black hole forms through gravitational collapse, such as the collapse of a dying star, its mass stays the same, and that means that at some fixed distance such as a billion kilometers, its gravity is no stronger than it used to be. A black hole is in fact a very small object, typically about the size of a city. This makes it a small target, and the space between the stars is vast, so a huge amount of luck is required if it is to swallow another astronomical object through a chance collision. This will sometimes happen, but before it can happen to most stars, they will end up being scattered out of their galaxies and galaxy clusters, after which cosmological expansion will keep them permanently safe from being eaten.[Adams 1997],[Baez 2004]
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]
These calculations are not controversial, and are not based on speculative physics. In fact, there are strong reasons to believe in them, because one of the world’s most talented physicists, Roger Penrose, has a pet theory called CCC[Penrose 2008] that is hard to reconcile with them, and he has spent a great deal of time and effort trying to to find a mistake or a loophole in them. He couldn’t, and was reduced to proposing various other workarounds involving speculative particle physics (such as massless charged particles), which didn’t work out.
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.) Any photons with ordinary energies, such as those originating from the present-day cosmic microwave background, will play a negligible role as their energy density becomes diluted faster than the mass-energy density of material particles. (This process has already been going on for billions of years. It’s is the reason why the cosmic microwave background, which was dominant in the early universe, is now a negligible part of the universe’s mass-energy.) However, the universe will contain photons with extremely low energies, which an observer (if one could be present and could detect them) would describe as Hawking radiation from the cosmological event horizon.[Hu 2011] These photons will have a temperature on the order of 10^-30 K, meaning a typical energy of 10^-34 eV and a wavelength of 10^28 m.
Adams and Laughlin, “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, Rev. Mod. Phys. 69 (1997) 337,http://arxiv.org/abs/astro-ph/9701131
Baez, “The End of the Universe,” 2004, http://math.ucr.edu/home/baez/end.html
Penrose, Causality, quantum theory and cosmology. In On Space and Time, ed. Shahn Majid, Cambridge University Press, Cambridge, 2008, pp. 141-195. (ISBN 978-0-521-88926-1)
Hu, “Hawking radiation from the cosmological horizon in a FRW universe,” Phys.Lett. B701 (2011) 269-274, http://arxiv.org/abs/1007.4044