Slashdot Mirror
Black Holes Grow By Eating Quantum Foam
An anonymous reader writes "The discovery that even the most distant galaxies have supermassive black holes at their cores is a puzzle for astrophysicists. These objects must have formed relatively soon after the Big Bang. But if a galaxy is only a billion years old and contains a black hole that is a billion times more massive than the Sun, how did it get so big, so quickly? Now one cosmologist says he has the answer: black holes feed off the quantum foam that makes up the fabric of spacetime. This foam is 'nourishing' because it contains quantum black holes that can contribute to the black hole's growth. This idea leads to a prediction: that the supermassive black hole at the center of the Milky Way must also be growing in this way and at a rate that we should be able to measure. Just watch out for the burps."
In the case of a black hole? Because the radius of the event horizon - which is one of the easiest definitions of the "size" of a black hole - grows monotonically with the mass. You can see it from Newtonian physics; if you look at the distance at which a "particle" travelling at the speed of light can't escape from a body with mass M you find it grows linearly with M. It turns out, rather coincidentally, that this coincides with the event horizon of a Schwarzschild hole, which is a black hole which is perfectly spherical (ie non-rotating), uncharged black hole.
(I went looking for a reference but I gave up quickly. Basically take Newton's gravitational law, F=GMm/r^2 for a large body of mass M and an orbiting (test) body of mass m. A particle of velocity c moving in a circular orbit is experiencing a radial force of F=mv^2/r=mc^2/r. (This is the centrifugal force, and to head of pedants, in the frame of the particle it is very much experienced even if in an inertial frame it is evidently fictional.) Equating these two you quickly find GM/r=c^2, or r=GM/c^2. This is the Schwarzschild radius.)
Doesn't work - the big bang is not an expansion into pre-existing spacetime. Further, it's very hard to find a way of forcing a black hole solution of GR (Schwarzschild would be the most plausible in this context) to suddenly turn into a cosmological ("Friedmann-Lemaitre-Robertson-Walker") solution of GR. What you *can* do is embed an FLRW solution inside a Schwarzschild and get a model indistinguishable from observation, but extraordinarily contrived, and indeed pointless. (Come to that I'm not fully convinced those models genuinely work because the Schwarzschild solution is static, but if you linked that with Wetterich's recent model where the "expansion" is actually a manifestation of the increasing mass of particles you could avoid that issue, too and, indeed, avoid having to embed an FLRW inside a Schwarzschild at all.)
But if you refine what you say a bit it's not very far from an idea Penrose proposed a while back but has, unfortunately, never published in detail, although he's put out some (admittedly extremely ill-advised -- Penrose is basically a genius, but knows little of either statistics or observation) papers claiming signatures on the microwave sky. Basically Penrose points out that eventually everything will evaporate to radiation one way or another: if we follow any extension to the standard model we at least open the possibility that fundamental particles can decay, and otherwise ultimately every path every particle will take will inevitably, over an infinite period of time, take it into a black hole. If there are eventually two electrons in the universe and nothing more but radiation, they will themselves inevitably collide, after an unimaginable period, with enough energy to form a black hole (remember in this scenario the electrons are constantly buffeted by radiation of ridiculously high power... even if most of the radiation is at wavelengths of, say 10m, there will be *some* photons at a vast energy and these will interact with the electrons... eventually... and accelerate them to speeds far in excess of those reached on Earth or, indeed, in the Sun). And black holes radiate. So everything becomes radiation. But for reasons that are rather technical, it is impossible within the framework of GR to distinguish between an infinite future bathed in radiation and an infinite *past* bathed in radiation, because time and length scales become rather arbitrary. Which means that through some process Penrose has never explicated - if that's a word - the ultimate future can wrap onto the ultimate past and suddenly there's a new Big Bang.
There are also other ways of getting cyclic models, which involve a bit more new physics (new scalar fields, or branes hanging near ours, etc.) but a bit less hand-waving. Indeed, there are many ways of getting cyclic universes. But Penrose's struck me as being nearest to your suggestion.
There was some concern of the hypothetical danger creation of such black holes might pose.
More concerning to me was the uninformed speculation that lead to those concerns. As one physicist quipped here on Slashdot at the time, "You misunderstand what motivates physicists. If the LHC did get sucked up by a mini black hole we would not run from the building in fear, we would run towards it with notebooks at hand".
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.