How Did They (Black Holes) All Disappear

It was reported today that we now have x-ray evidence of the existence of a gadzillion (more or less) teeny black holes that existed at the dawn of the universe, but they are no longer around. The news article said that they consumed all the available sources of energy in their immediate environs and dissipated.

Poppycock! How is a giant mass going to dissipate when nothing can escape from it. It doesn’t need any additional source of energy. Hawkins once suggested that black holes can disappear by virtue of (no pun intended) virtual matter. Due to some quirk of quantum mechanics (which I don’t understand) virtual particles can come into existence by themselves, just so long as they go out of existence really quickly. His idea, IIRC, was that somehow virtual particles can escape from a black hole, and eventually the black hole will disappear.

Can some one explain that so I can visualize how that is possible, if that is indeed what he said?

You are on the righ track barbitu8. Hawking did indeed propose the evaporation of black holes and the method of the evaporation is (unsurprisingly) called Hawking Radiation.

Hawking Radiation:
A black hole has a boundary called an event horizon. Anything within the event horizon can never escape the black hole’s gravitation…even light. In quantum physics the creation of virtual particles is a well understood and defined property of the universe. In essence a particle and its antiparticle counterpart pop into existence and then almost immediately annihilate each other. This is called the Casimir Effect and has been well documented.

Now, when these particles pop into existence at the edge of the event horizon three things can happen.

  • Both particles are pulled into the black hole
  • Both particles escape the black hole.
  • One particle falls into the black hole and one escapes.

The third case is the interesting one. Remember these are virtual particles. When one particle escapes the black hole it becomes real. The virtual particle that fell into the black hole remains virtual but must maintain the conservation of energy by giving itself a negative mass-energy. When this particle hits the singularity it zaps an equivalent amount of mass of the black hole out of existence and so the black hole shrinks.

Primordial Black Holes:
Unfortunately the mass a black hole loses to Hawking Radiation is very small. A black hole with a few stellar masses greater than out sun would take longer than the life of the universe to evaporate this way. The energy lost via Hawking Radiation is on the order of a few millionths of a degree which is even lower than the microwave radiation background (around 3 kelvin?).

Other means of losing energy:
Someone like Chronos or DrMatrix will have to verify but I think black holes can also lose size through spinning and/or gravity. The earth, for instance, loses energy via gravity waves…much like a cork bobbing up and down in the water. The earth loses enough energy in this fashion to power an average electric toaster. Eventually the earth will spiral into the sun from this energy loss but that will happen LONG after our sun has spent itself and fizzled out. A black hole should likewise lose energy in this fashion but as has been shown it is a very small amount of energy relative to the size of the black hole.
Hope this helps…

Thanks Jeff. You clarified how a black hole can evaporate, but as you noted it would have to be a very small one to have disappeared. Hawking’s evaporation of black holes, IIRC, was in regard to very tiny ones, which he said exists all around us, and apparently still do. Can you explain that too while we’re at it?

That, however, I don’t think can explain the disappearance of the gadzillion black holes that existed at the birth of the universe, unless they were referring to very tiny ones. I doubt that in view of the x-ray evidence of their former existence along with the “fact” (?) of very tiny ones still in existence. If they are, are they evaporating and being created continually? Another question for you, Jeff, or anyone else who can answer it and wishes to do so.

I had just read a news report about the same thing and came to the board to ask exactly the same question, only to find barbitu8 beat me to it. I was even going to suggest the bit about Hawking radiation! Great minds think alike, I guess.

But from Jeff_42’s answer, it doesn’t seem that Hawking radiation explains the black hole atttrition. Could it be that many of the small, primitive black holes ate everything available and then turned cannibal, eventually forming the supermassive black holes that we see today in the center of many galaxies? Could these supermassive black holes have then acted as gravitational anchors for the large spiral galaxies that we know, love, and inhabit today?

If I understand correctly, the “tiny” (low-mass) black holes could only form under the intense pressures of the Big Bang. There is no reason to assume that they all had the same mass, or would evaporate at the same rate, though. The formation of virtual particle pairs at the event horizon is essentially a random event. A quantum black hole could exist indefinately without half of a virtual pair escaping and eroding it.

To clarify something Jeff_42 said,

‘Degrees’ is used here, as I understand it, to correspond to radiation emission equivalent to that of an object at that temperature. (black-body radiation) So, the EM radiation a multiple-solar-mass black hole gives off is extremely low wavelength and energy, like that which would be emitted from an object at a temperature of a few millionths of a Kelvin.

The complication is that the less massive the black hole, the higher the energy predicted for the emitted radiation. (The higher the equivalent temperature is for a black body radiation.) The smaller a black hole is, the faster it will lose energy. Therefore, any black holes created by the Big Bang that did not rapidly grow to considerable size will have dissipated by now.

Balance, radioactive decay of a nucleus is a random event. Yet, that’s how atomic clocks, some of the most accurate timekeeping devices we have, tell time. The key is that if you have, for example, 10^20 possible random events, (atoms in an atomic clock) then the sum of the set will be extremely predictable. Similar arguments apply to Hawking radiation and black holes.

Several years passed between the prediction of micro black holes and Hawking’s prediction of radiation from black holes. There were several interesting, notable science fiction stories during that time featuring micro black holes… in a way, it’s too bad from a storytelling standpoint that all those stories don’t work with current science.

Good point, Tim. While it’s possible for any member of the set to flout the statistics, it does become highly unlikely. Out of curiosity, would you conclude that all black holes evaporate at the same rate, or that they evaporate at a predictable range of rates?

Regardless of the constancy of the evaporation rate, the black holes would still be of different masses. We could therefore reasonably expect some of them to stick around longer than others. (Unless the evaporation scales linearly with mass, which I very much doubt.)

Hawking radiation is thermal in nature. Black holes caused by stellar collapse are cooler than the background radiation. These black holes cannot shrink until the universe cools even more. But as (Tim) said, he smaller a black hole is, the hotter it is. Black holes created by the big bang are called primordial black holes these could be tiny and if they are small enough, could have evaporated via Hawking radiation already.


Your original concern is based upon misinformation:

“The news article said that they consumed all the available sources of energy in their immediate environs and dissipated.”

(Giving your recollection the benefit of the doubt…) This is bad reporting. No one said they dissipated. The most anyone is willing to commit to is to say they have become quiescent–they are practically undetectable because they have swallowed up all the nearby matter. As matter falls towards a black hole, it speeds up and bangs around and really lights up the sky. (Quasars produce 100 times the light of a galaxy but are only 1 light-month in diameter while a galaxy is 100,000 light-years in diameter. It is assumed that black holes power quasars because no one can think of anything else that size producing that much energy.) No in-falling matter, no light show, no easily detectable black hole. So where are they? Your guess is as good as anybody else’s. Floating gently through space? Gathered up into the supermassive black holes at the center of all galaxies? (Also, your reference to “teeny black holes” translates to black holes with a mass 2-5 times that of the sun. That’s what they have detected.)

Black holes are believed to shrink through radiating Hawking radiation. (If they don’t, it poses some interesting problems for the theory of black hole thermodynamics.) It is a slow process. The only black holes that would be currently evaporating out of existence would have had the mass of a small mountain at the beginning of the universe (assuming Mr. Hawking’s rate of radiation is correct) and are called primordial black holes. The last moments of a black hole involve a spectacular outpouring of gamma rays. Searches for unexplained gamma radiation have put a fairly low limit on the number of possible primordial black holes (which in turn puts some interesting constraints on the dynamics of the big bang). Making black holes of this size and mass (supposedly) can only be done during the big bang. Only black holes from large stellar masses and greater can be made in the universe today.

Astronomers have a fair sketch of how the universe evolved for the first 300,000 years based upon research in elementary particle physics and they have a fair idea what has happened from 2 billion years up to now mostly because not much has happened (cosmologically speaking). In that time gap is when stars, quasars, and galaxies formed. What happened during that time period are guesses–but slowly becoming better informed guesses that will one day be entitled to be called theories based upon better observations and computer modelling. (To start a fight among astronomers, ask: Which came first, the star or the galaxy?)

One thing you have to keep in mind with astronomy is that it has only become a rigorous field of study in the past 50-75 years. Astronomy is handicapped by being mostly science by observation and not science by experiment.

Your description was quite sufficient. Note, however, that the Cassimir Effect is just a mechanical action resulting from the existence of virtual particles. Hold two plates really close together. Only some kinds of virtual particles can be created between the plates because of the constraint on possible particle wavelengths but any virtual particle can be created on the outside of the plates, so there is a net pressure pushing the plates together. (Which strikes me as close as you are ever going to get to a source of energy for a perpetual motion machine.)

Also, if a black hole is sitting minding its own business, it will not generate any gravity waves. Bang two together, though, and you will generate waves even humans can detect in 2003 with the LIGO observatories.

I don’t think that black holes lose size through spinning or gravity.

While I appreciate the compliment, I don’t consider myself in the same league as Chronos. I took (Newtonian) physics in high school. My major in college was computer science with minor in math. No physics in college. I love physics, but what I know about physics is from reading books and articles on my own. I’m no Chronos.

As DrMatrix says a BH can’t lose mass via spinning or gravity. But if it’s orbiting another black hole or neutron star it can lose mass via gravitational radiation.

Working from memory from Kip Thorne’s Black Holes and Time Warps, some early papers on black holes showed that if a spinning black hole formed with an event horizon with a bump of some kind (say from a neutron star with a bump), the black hole would radiate gravity waves until the bump went away, and the event horizon was a smooth spheroid. But this is more of a transient effect when the black hole forms, and the black hole never comes close to going away entirely.

Zenbeam please forgive this egregious nit pick, but a gravity wave is not a gravitational wave. A water wave is a gravity wave.

Why am I driven to such extremes?