Astroboffin says 'black holes murder galaxies' About 25,000 light years from earth, nestled in the center of our galaxy, lurks a supermassive black hole. Luckily for us, our galaxy's matter-sucking hub is far less active than those at the core of many other galaxies. If it weren't, we'd all be dead. Or, more likely, our earth would never have come to be in the first place …
worst case, they get to hang around long enough that they radiate away their entire mass through Hawking radiation, but that's not exactly the fastest of processes going. In the meantime, they'll sit there getting on with, erm, not doing a great deal of anything
With a supermassive black hole at the center of each galaxy, and some galaxies that never achieve the right gravitational balance to survive this black hole…there much be the odd supermassive black hole wandering around that has succeeded in completely consuming it’s host galaxy. (I will, call them rogue hypermassive black holes as they would not merely be a supermassive around which a host galaxy orbits; they would have all the matter and thus gravity of an /entire galaxy/ packed into a neat travel-sized lump.) So out there, somewhere, are rogue hypermassive black holes with no orbiting galaxy, no accretion disk. Apart from the gravitational lensing they would provide to light, they are completely undetectable. If one of these rogue hypermassive black holes were to intersect with a galaxy, the gravitational stresses would the victim galaxy apart. Even a “close pass” with a rouge hypermassive black hole might be enough to destabilize a galaxy. Once destabilized it would begin to once more spiral into the supermassive black hole at its center. (IIRC, all galaxies essentially start off “donating” a certain amount of their mass to their dark and death-dealing friend.)
All this of course is really different then what happens when two galaxies collide. The difference of course being you see another galaxy coming.
Now you have something new to fear! Rogue hypermassive black holes! Fantastic.
I'd be worried about floating black holes, if, as you said, they were undetectable. However there isn't a "close pass" scenario. They would have been attracted to each other from the start and would begin orbiting long before it passed by in the scenario you described. Not to mention that the universe is expanding faster than an isolated black hole could catch up with it's nearest neighbour.
First, you seem to assume that all galaxies are a) moving at the same speed (they're not) and b) moving away from each other. Galactic collisions happen all the time. A short Google can show you several galactic collisions astronomers are currently tracking. (Including some interesting issues they cause when the supermassives at the center of these galaxies once more become active; not to mention the issue of the two supermassives eventually merging.)
Furthermore, we wouldn't really be able to (depending on how long ago the hypermassive consumed its host galaxy) a hypermassive black hole. With the exception of the gravitational lensing phenomenon, it would be perfect invisible. We would notice gravitational disturbances, sure. We'd have no idea what was causing those disturbances, and astronomers would spend absolutely ages arguing everything from dark mother to the aforementioned hypermassives. (For that matter, it's entirely possible that a reasonable percentage of the "dark matter" in the universe actually is a set of hypermassive black holes.)
Now, humorous "you should fear this so the government can take away more tf your rights" aside, I am perfectly aware that a hypermassive black hole is not going to come barrelling through our galaxy unannounced, particularly within our lifetimes. We would detect the gravitational disturbance of a hypermassive black hole headed our way at the latest as soon as it started pulling the edge systems off of our galaxy. This would give us, what, a quarter million years to ponder our fate?
Still, the idea of rogue hypermassive black holes slaloming around the universe periodically intersecting with (and subsequently either consuming or gravitationally disturbing to the point of partial if not total collapse) other galaxies is kind of cool. I even read somewhere (gods know if I can remember where,) that certain categories of globular cluster are probably the result of galactic collisions. (They were ejected from their host galaxy.) Others are likely the result of “near misses.’ Some astroboffin or other figured out how to tell them apart and much fun was had. In theory, since a hypermassive black hole would have the same gravity as an entire galaxy, it would be more than capable of causing the same phenomena.
While I certainly don’t think we should all run around freaking out that an unseen rogue hypermassive black hole will rampage through our galaxy in our lifetimes...knowledge that in 4.5ish billion years we are (by some calculations) scheduled to crash into the Andromeda galaxy should very well have us all freaked out.* Think of the children!
*Please disregard the part where this is far after the point where Sol wipes us all out anyways.
We should call for our politicians to immediately implement tough sanctions regime against the rogue holes. And to detain without charge anyone suspected of supporting the holes or found in possession of material potentially useful to a hole including, of course, the music and movie pirates, who must all be cut off from the internet forthwith. What?
Why the black holes will eventually be doomed - http://en.wikipedia.org/wiki/Hawking_Radiation. Of course two things make this not really matter. For a super massive black hole we are talking orders of magnitude more time needed than the current age of the universe for the black hole to evaporate and two according to the right wing nut republicans in the US if Hawking was forced to use the NIH he would have died as a child so technically the fact this theory exists should be a paradox.
As interesting as black holes evaporating over gigantic time spans I find the idea of a bubble universe forming and then overtaking our locality in the universe where the laws of physics suddenly and radically change (very likely matter will not even be possible in this new universe) to be even more mind blowing. According to most string theory and inflation theory it is not a matter of if but when (granted the time scales for such a quantum tunneling event happening are probably on par with a Boltzmann brain appearing but kiddies always remember if anything is possible given enough time it is certain to occur).
A supermassive black hole will eventually lose all its mass due to Hawking radiation. However "eventually" is a very, VERY long time. The black hole will not shrink until losses from Hawking radiation exceed gains from miscellaneous matter (interstellar dust , cosmic rays etc) and incoming energy (light from other galaxies and assorted stars, the cosmic microwave background radiation). Even with no incoming energy and matter, evaporation would take so long that the numbers would be meaningless to us. We're talking at least high double digits of powers of ten - I don't have the math to figure it out.
As long as the universe is expanding and the mass per cubic meter is failling we'll hit that point eventually. Don't let it worry your grandchildren though.
As interesting as looking at known physcial processes when contemplated over uncontemplatable elapsed times - http://en.wikipedia.org/wiki/1_E19_s_and_more . What I love is the nature of true randomness and entropy. Most people would look at a number repeated a million times and tell you that there is no way it is a random sequence when in fact not only could it occur with only a million numbers (ungodly unlikely but possible) but is certain to occur if the sequence is large enough. Again two books I love that cover the universe and randomness is Susskind's (cofounder of string theory and the fact that our universe is a giant hologram and life takes place in two not three dimensions) The Cosmic Landscape and The Drunkards Walk.
Meant to say single digit decimal number instead of any given number. Not stoned so don't want to think if you could ever even select two true random numbers that are the same if the domain is infinite. Possible but sequence would also have to be nearly infinite (though possible in small sequence but oh oh so unlikely), ouch my brain hurts.
Anyone else find it a bit odd the way they're vilifying the black hole as some galactic nuisance? ... like a snail in the garden eating all the radishes? Come on, it's *ALL* just a bunch of stuff out there. There's nothing precious about a galaxy. The universe just *IS*. There should be no reference to right, wrong, good or bad outside of our little human ideologies.
Hawking radiation increases in magnitude as the notional temperature of a black hole increases. Sadly, this temperature decreases with increasing mass.
According to http://en.wikipedia.org/wiki/Hawking_radiation, a one-solar-mass black hole has a notional temperature of 60 nanokelvin (moderately cold), so it acquires mass faster by absorbing cosmic background radiation than it loses it by Hawking radiation. The break-even point is one lunar mass, around 1e22 kilograms. Less massive than that, and the black hole is hotter, and evaporation is faster than the absorption of cosmic background. More massive, and it is slower...
There are formulas for calculating this stuff. The last 85 tonnes will evaporate in about 5ms...
I may be wrong (I often am), but...
I thought the accretion disc was only present while the black hole was 'feeding'. And black holes have periods of feeding and not feeding depending on how much material is close enough to be devoured. When not feeding, there is no accretion disc and therefore it will cool, and/or when less is being devoured then the accretion disc will be less/cooler and star formation can occur. So the galaxy is not doomed at all.
But then I also though every galaxy had a black hole at it's core, so what do I know!
I don't get why after the black hole warms its galaxy above star-formation temperature but then switches off (when it pretty ma!uch stops growing and eating gas), the gas doesn't cool down and start making stars after all.
Or does the black hole in fact eat all of the mass in the galaxy, gas and all? Being in orbit around a black hole at long range, as the gas is, isn't dangerous... unless matter falls into the black hole from outside and increases its mass. Then, without changing course, you're in a less safe orbit - and so on...
As always - why are we talking about it on this site? Why not (Very) Bad (News) Astronomy?
Firstly, this research seems to ignore the fact that the galaxy size and black hole mass are closely correlated. They have found that each galaxy has a supermassive black hole of about one thousandth the mass of its parent.
Secondly, even if the galaxy was too hot for star formation for some period of time, all that gaseous hydrogen does not just disappear, in fact it stays there, in warm lumps presumably. Once the galaxy approaches "death" as dramatically reported here, it will again be cold enough for star formation.
A low-quality research finding of dubious accuracy.
The conclusion says that because stars do not renew themselves, the black hole itself brings its own doom.
NOT true.
Star renewal is completely irrelevant to the fate of the black hole.
Whether the particles and star dust are accreting into stars or not, the total amount of matter available to the black hole does not change.
There is neither more nor less matter to swallow for the black hole whether it prevents stars from forming or not (ok, there a tiny bit less as some dust will stray from the galaxy while stars would be less likely to, but that's negligible).
The black hole will still die some day because of Hawking radiation, but neither slower nor faster than if it swallowed a galaxy of aggregated dust (stars) instead of the same galaxy made of unaggregated dust.
This explains the Fermi paradox nicely. Advanced civilizations that could not progress enough to exit their galaxies and avoid the gamma ray roasting are long gone as their old galaxies' supermassive black holes got to behemoth oven size. And the odds of a civilization sufficiently advanced as to exit a galaxy are how much? The Drake equation omits this parsing effect.
Nature has a way of recyling itself, from rotting plants to exploding suns.
So it stands to reason 'black holes' do too. Now the timeframe in which this happens may be truly galactic, but methinks it unlikely something would suck up a galaxy worth of mass and then just lie there untill the end of time.
What goes up must come down.
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