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Hidden Black Holes Discovered
mknewman wrote to mention a Space.com article discussing the discovery of a large group of hidden black holes. From the article:"Black holes cannot be seen directly, because they trap light and anything else that gets too close. But astronomers infer their presence by noting the behavior of material nearby: gas is superheated and accelerated to a significant fraction of light-speed just before it is consumed. The activity releases X-rays that escape the black hole's clutches and reveal its presence. "
I think they don't really escape the black hole per se. They're just very high-frequency/high-energy radiation that leaves the super-heated gas BEFORE it 'falls over' the event horizon.
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Light and x-rays are the same things (as you imply), just at differing frequencies. Visible light can escape from outside black holes' event horizons, just as easily as x-rays.
Any electrical charge undergoing an acceleration emits radiation, if it can couple to its environment. Charges which are accelerated more emit radiation at higher frequencies, and accelerations near a black hole's event horizon are very large, so x-rays are emitted preferentially over visible light. There is also an effect of higher frequency emissions from any finitely-sized source being more "focused" than lower frequencies. This leads to more concentrated "beams" of emission from finite sources.
Finally, one of the methods of radiation from black holes is that of spontaneous particle-antiparticle production in the tremendous gravitational gradient outside a black hole. Normally, these particle-antiparticle pairs recombine quickly. However, if one travels nearer a black hole than the other (they're emitted going in opposite directions relative to their center of mass, to combine linear momentum), it can get sucked down the gravity well, and the other escapes.
Those X-rays don't "escape" the black hole because they aren't coming from inside the black hole. The idea is that as stuff falls into the black hole, it gets ripped apart at the atomic level. As it gets ripped apart, it emits x-rays. Because the matter hasn't quite reached the event horizon yet when this starts to happen, these x-rays are able to make it away from the black hole.
So in other words those x-rays aren't coming from the black hole. They're coming from just outside the black hole, the dying screams of the matter falling in. So no "escaping" is involved, not exactly.
Then there's Hawking radiation but that's different, I don't think those are X-Rays.
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I forgot to mention one of the most imporant effects: matter which is around a black hole tends to form accretion disks. It forms disks because it tends to have a net angular momentum about the black hole's center, and so spins. The fact that differing parts of the accretion disk move at different speeds means that there is a lot of rubbing going on -- not unlike with slashdot readers. This leads to heating of the accretion disk, often up to a very high temperature; the accretion disk is just like a very hot oven, which doesn't emit (much) in the visible region, but a lot in the x-ray region of the spectrum. There are also tremendous focusing effects of magnetic field lines (accelerating charges again), and so the emitted radiation tends to get focused along opposing "jets".
Nothing can escape from "inside" a black hole, from within inside the event horizon. But as matter falls into the black hole it heats up, the gravitational potential energy gets turned into heat through friction-type processes (this also happens in waterfalls, the bottom is warmer). Hot stuff glows, and the gravity near a black hole is so strong that the matter does not just get red or white hot, it gets X-ray hot. And a lot of the X-rays, since they're generated outside the event horizon but still very near the black hole, escape so we can see them.
Most stuff doesn't generate much in the way of X-rays, so it's very easy to pick out the X-rays coming from the quasar. That's not so true of visible light - no doubt visible light also escapes from right next to the black hole, but it's drowned out by the outer regions of the quasar (which are visible-light hot instead of X-ray hot) and the galaxy the quasar is in.
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Not at all. This x-ray radiation is due to the fact that the matter falling in is heated by compression. That matter adds to the mass when it finally arrives. If no matter were in the area, none of these x-rays would be produced.
Hawking radiation is completely different. What happens is that due to quantum fluctuations, random particles pop into being all the time. They pop into being in pairs, a particle and antiparticle, and normally soon annihilate each other. When they pop into existence near a black hole, sometimes the negative particle falls into the black hole, and the positive particle escapes.
The particle that goes into the black hole annihilates some of the hole, and decreases its mass. The escaping particle cannot reach its antiparticle, so it can't be annihilated. It goes out as radiation, increasing the mass of the rest of the universe.
The end result is that matter has (in effect) jumped out from the black hole into the rest of the universe.
Ewige Blumenkraft.
Wrong in part.
The X rays emitted have essentially nothing to do with the heat of matter falling in, and everything to do with acceleration of charged particles. In fact, it'd be nearly impossible to actually get any substance "xray hot" as you put it.
When you heat a substance, it radiates, of course. This occurs due to electrons changing energy levels. These energy levels are very precisely defined, and thus the emission spectrum consists of sharp lines (they are not perfectly sharp due to perturbations like spin-orbit coupling, etc.). Then, in a macroscopic situation, many of the emitted photons will scatter off of other atoms, losing some energy in the process. By this mechanism, the sharp spectral lines get very blurred, and we see an essentially continuous spectrum (as long as you restrict it to middle range frequencies) with bright lines at the spectral emission frequencies.
The reason that this process doesn't produce xrays, no matter how hot you get the substance is that the energy levels an electron could be in do not range from 0 to infinity. In fact, in the case of a hydrogen atom, suppose we take an electron in the lowest energy shell to have 0 potential energy. Well then, now we move that electron to an infinite distance from the proton. At this point, it will have lost ~13.6 electron volts of energy. Thus, the highest energy photon that a hydrogen atom can emit due to an electron changing energy levels is just 13.6 eV. This falls in the ultraviolet range. And then, by scattering off other atoms, photons only lose energy, rather than gaining it.
Now, strictly speaking, as we increase the nuclear size, the difference in the energy levels will increase, and the energy of the emitted photons will be higher. So, if we used heavy enough elements, we could conceivably get them "xray hot". But by that point, we would very likely have reached the ultra-unstable elements that have only been created for very brief periods of time in the lab before decaying. Obviously, these are not found in great quantity in nature.
So, now that we know that heat isn't the culprit, how do we get xrays from black holes?
Well, I could be mistaken, and if so, I hope someone less mistaken than me happens on this post to correct me, but I believe that it primarily occurs because first, the atoms are ripped apart by tidal forces (they are "spaghettified"), leaving the electrons and the nuclei separated. Then, obviously, these particles are accelerating, and accelerating charged particles generates electromagnetic radiation. The greater the acceleration, the higher the frequency of the radiation generated. And since the gravitational force of the black hole increases as you get closer, the acceleration will proceed at a higher and higher rate, so the frequency of emitted radiation from one individual particle should slide upwards. Of course, that doesn't take into account gravitational redshifting, so perhaps the two effects cancel each other out nicely, leaving us with xrays.
SIGSEGV caught, terminating
wait... not that kind of sig.
Could things like this be part of the explanation for that "dark matter" that scientists are always talking about? Maybe there are more and we just haven't found them.
Well, yes, but only a small part. We can put a pretty good upper bound on the amount of dark matter that can be in black holes based on gravitational lensing data. Black holes most famously absorb light that is incident inside their event horizons, but they also cause light traveling outside to curve around it. (As does all matter.) Thus, a star that is behind a black hole looks to be in a different place than it should, or even at two different places at once (more info). We can measure how much light is bent and infer how much matter is contained in high-density regions.
Obviously, gravitational lensing only happens where matter is compact. Uniformly distributed stuff won't do it. Thus, we know about how much dark matter there is, and from this, roughly how much can be in black holes. The punch line is that only a small fraction of dark matter (I don't remember the statistic off hand) can come from black holes.
The question is obviously, then, where's the rest of this matter? Some could be in other "normal" matter, like dwarf stars, but again, for various reasons that can't account for very much. Some could be in neutrinos (weakly interacting particles which are almost impossible to detect). This still leaves a whole lot of matter unaccounted for though. Maybe it's so-called WIMPs (weakly interacting massive particles) which theoretically could be very massive but interact with normal matter very little. Read more at Wikipedia.
Good question, though.
Better yet, how about its mass compared to the Moon, and how many AU is the Moon from the Earth?
Think of it this way:
Most black holes are for obvious reasons of stellar mass, i.e, less than 20 times the mass of our sun. 20 AU doesn't even get you out of this star system--Pluto is 30 AU or so out. So the contribution of those black holes is going to be completely swamped by the sun.
The supermassive black hole at the center of the Milky Way is thought to be in the neighborhood of a 10^6 solar masses; the galactic center lies about 2x10^9 AU in the general direction of Sagittarius, so any contribution from it will also be swamped by the sun.
Nothing outside our solar system is likely to have any measureable gravitational effect on anything inside it other than the entire system orbiting the galactic center.
http://www.universetoday.com/am/publish/spitzer_f
http://www.nasa.gov/home/hqnews/2005/aug/HQ_05211
http://www.esa.int/esaCP/SEMPHV1P4HD_index_0.html
http://www.msnbc.msn.com/id/8812911/
More information of hidden black holes and their discovery.
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Example: It is possible, at room temperature and pressure, to have pure water at 105 degrees celcius and NOT have it boil. It is very unstable and will generally boil vigorously the moment you get any kind of circulation within the water.
Second, Quasars (Quasi-Stellar Objects) are, as yet, undefined. Nobody knows what drives them, so to call them super-active Black Holes is blatantly absurd. They are also frequently at the very edge of the visible Universe, making it very unlikely anything large enough to collapse into a super-massive Black Hole could have existed - let alone existed long enough to actually undergo gravitational collapse.
Besides which, such objects are not near. This is important. Black Holes evaporate, but they don't evaporate THAT quickly. A Black Hole the size of a typical Quasar would need to be absolutely gigantic and would not have evaporated in this time even if no other matter had fallen in.
Indeed, there are NO quasars closer than 5 billion light-years away - a distance referred to as the "red-shift cutoff". If Quasars were galaxy seeds, you would expect them to fade into the age of galaxies, not dramatically and suddenly cut off entirely.
The idea that Quasars then formed into galaxies is improbable - the diameter of a Black Hole is a direct function of the mass of the Black Hole (which includes the mass and effective mass of everything it consumes). It is unlikely that there are any galaxies large enough to have a Black Hole of the kind of mass implied by the output of a typical Quasar.
If a Quasar were powered by a Black Hole, it would be typically 100,000 times more massive than the Black Hole at the Black Hole at the center of our own galaxy. Given that the presence of a galaxy implies that the Black Hole is still being fed matter and energy, it would be quite impossible for a Black Hole to evaporate to 0.00001% of its original size in the time available.
Remember, Earth is 4 billion years old, the Universe is only 15 billion years old. And of those 15 billion years, the Black Holes would only start to really evaporate relatively late on as the density of matter and energy declined. Actually, you don't even get all 15 billion years of that. Quasars peaked at about 12 billion years ago and as already noted, vanished entirely at 5 billion years ago. This gives you a paltry 7 billion years to shrink to the required size.
Now we get into a real mess. The Milky Way galaxy is ALSO estimated at 12 billion years old, based on the ages of known structures. There are no structures around Quasars. They'd be blown to bits. For the Milky Way to have formed around a "dead" Quasar, the Quasar must have formed considerably earlier. There are a LOT of galaxies out there as old as, or older than, the Milky Way. If all of them formed around Quasars, there would have needed to have been more of these really early starters than existed at the height of the reign of Quasars.
There is another problem. The Milky Way belongs to a local cluster of galaxies. If they ALL had formed around dead Quasars, the Quasars would have fallen into each other from their gravitational pull LONG before there was any possibility of a galaxy forming.
Nor are Black Holes strictly "hidden". They always emit Hawking Radiation, although there are no good detectors for this at present. That is hardly the fault of the Black Holes, though - if they're not seen, it's because the observers aren't looking.
As for the number of Quasars - there are only 39 known Type II Quasars
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Dark matter is far, far too large an error (around 90% of the universe's mass is "missing") for it to be accounted for by these few black holes.
Many scientists believe that there is no missing matter, and that the theory which predicts it is simply wrong.
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