June 28, 2004

Black Hole Eschatology

I've been conversing with cosmologist Milan Cirkovic about the fate of black holes. Here's his latest response to me (you can find out more about Milan at his Website):
Dear George,

Great to hear from you! The answer to your question is not known with any certainty at present -- I'll just sketch major ideas and am attaching my resource letter on physical eschatology, where (in Sec. III C) you may find pretty close to exhaustive list of references on this, very difficult issue [Milan emailed me a Resource Letter on physcial eschatology]

First of all, let us separate two issues: (1) the fate of an ideal, isolated black hole, and (2) the fate of realistic black holes in specific universe such as ours. Obviously, we would like to have a firm idea on (1) in order to try to answer (2), but this is not the case. Due to the tough problem of information loss in black holes, which was first raised by Stephen Hawking, there are basically three options for a fate of an isolated black hole (in asymptotically flat background spacetime):
i. it may evaporate completely, leaving nothing but blackbody (incoherent) radiation (this is Hawking's personal favorite, the trouble is it violates unitarity, which is the reason it is widely disliked among physicists)

ii. it may evaporate completely, leaving a mixture of blackbody radiation emitted in early phases of evaporation and COHERENT (i.e. information-rich) radiation from the later phases of evaporation (many people believe this, say t'Hooft and other bigshots)

iii. it may leave a stable remnant, amusingly called cornucopion, of several Planck masses. The remnant contains all relevant information, thus presenting an ideal memory chip ;o))))
At present, I would judge that option ii. is the most supported one, but nobody really knows.

Now, as to the fate of black holes in the real universe, we have two interesting complications here. The first, and rather obvious one, is that black holes, whatever their final fate is, are the most durable objects larger than elementary particles. That is, they are going to increase in mass, generally speaking, until at least 10^35 yrs from now. This is especially important if, as almost all particle theories suggest, proton is unstable and will eventually decay, destroying any other physical object but black holes in at least 10^34-35 yrs. Only then can they begin to really evaporate, leading to evaporation of Solar-mass black holes (or at least cornucopion formation) in further 10^65 or close yrs. The most massive black holes in galactic centers will evaporate (according to the basic picture) in about 10^98 yrs.

This basic picture is complicated and possibly disrupted by the second major issue, which is the cosmological constant (or dark energy, as it is sometimes called; people claim that "dark energy" is more general than "cosmological constant", although I do find this hair-splicing rather shallow; we should honor Einstein and retain the classical name, even if it turns out to be variable -- after all, Hubble "constant" is also changing with time!). In itself it creates an incredibly weak radiation field permeating the entire universe, which is many, many (really MANY) times order of magnitude weaker than any known source at present, and is like energy your body receives from somebody reading with 40W bulb somewhere in the Andromeda galaxy).

However, at one point in deep future time, this emission will become dominant, since all other sources will be extinct long ago. The idea -- which is still VERY controversial, see the references of Griest, Adams, Shiromizu, etc. in the attached -- is that this weak emission (of photons with wavelengths equal to the size of cosmological horizon!) limits the evaporation of black holes, since black holes will at some point absorb this energy at exactly the same rate as they lose energy by Hawking evaporation, thus making themselves eternal. Of course, this is possible only with the largest black holes, if at all. Some people find this argument false -- the question is still not settled. But again, this happens so far in the future, that it is almost irrelevant by definition: any traces of intelligent life will have to migrate to another universe (or to create one) or become extinct loooooong before it becomes an issue.

You mention the Big Rip idea. Well, this is an interesting speculation, by Bob Caldwell and some other theorists, that if the cosmological constant possesses a particular equation of state, characterizing what is called "phantom matter" -- which there is absolutely no observational, or even compelling theoretical reason to believe! -- matter could become subject to infinite repulsive forces in a finite amount of proper time and hence be obliterated. Frankly speaking, I don't know how the black holes will behave in such a scenario -- I'll do some research in literature (the entire "phantom matter" thing is just a few years old idea) and tell you. But, for the moment I won't bother too much about this, since the entire idea is just possible, but still completely unsupported speculation!

OK, I hope that this helps a bit! I'll certainly be glad to discuss these things any time! (And if you invite me to give a talk on this or some other eschatological topic, I could help even better... ;o))) )


No comments: