Is the galaxy just an accretion disc around a supermassive black hole? All the larger galaxies so far studied seem to be orbiting supermassive black holes. Is the fate of the universe to be eaten?
Banned
Is the galaxy just an accretion disc around a supermassive black hole? All the larger galaxies so far studied seem to be orbiting supermassive black holes. Is the fate of the universe to be eaten?
Order of Kilopi
Galaxies aren't, in general, accreting on to their central black holes: most stars are in stable orbits that never come anywhere near the central black hole.Originally Posted by Project Orion
Grant Hutchison
Order of Kilopi
I assume the OP is about the very, very, very long-run. I think he is probably imagining that the black holes will slowly grow as they accrete matter. I guess the question of black hole evaporation would also come into this, so I don't really know how to answer.
By the way, why does "accrete" get tagged as a bad spelling. Accretion is OK, but not the verb?
As above, so below
Order of Kilopi
That's right. It's hard to imagine just how long of a run the OP scenario takes. According to Sean Carroll in From Eternity to Here, occasionally a couple of stars interact such that one is ejected from a galaxy. Thus the galaxy loses some energy. So perhaps billions or more times the current age of the universe, there are just a lot of black holes. Then, as you say, Jens, after another probably billion2 times the current age of the universe, Hawking radiation takes over and reduces the universe to a thin wisp of next to nothing. That's assuming there isn't a vacuum metastability event that happens first. Or a big rip.
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
But even if they do grow, the net gravity stays the same, so why would that matter?
Order of Kilopi
Wouldn't, over very large timescales, the orbits of the stars (and other stuff) decay due to gravitational radiation?
Then it would seem galaxies are bound to 'collapse' into black holes. Disregarding Hawking radiation that is.
Administrator
At some distance from the central black hole, the expansion of the universe will exceed whatever factors draw stars toward the center, preventing some/most matter from being consumed.
I haven't done ANY calculations on this, but it seems likely to be true.
Forming opinions as we speak
Order of Kilopi
Speaking of the long run, I suppose that, even disregarding disturbance factors, no orbit is actually perfectly stable, i.e. even in the stablest of orbits the initial velocity will either be slightly above or below the perfect value. The orbiting body will move ever so slowly either towards or away from the central body, as the Moon is moving away from the Earth.
Order of Kilopi
Don't think so. I'm thinking a little more or less velocity will just change the eccentricity of the elliptical orbit.Then again, even the orbital motions of the planets in the solar system are chaotic. (Laskar 2003)
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
That sort of evolution would require orbiting stars to raise tides on the black hole, which I think is impossible.
Grant Hutchison
Order of Kilopi
If nothing crossed any event horizon(ie no event horizon forms), and all you had was infalling matter, then you could raise a tide.
Order of Kilopi
To quote the Spartans: "If."
Grant Hutchison
Order of Kilopi
Similarly, if there was no universe, we wouldn't be considering the OP's question.
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
Ah, ok.
Assuming that neither of you is a NASA pansy who is being paid by the government to fool us, enslave us, and steal our money, I'll go with that.
Established Member
What about tides in the gravitational field of the black hole - is such a thing possible?
Besides, even if the blackhole can not have tides then the orbiting bodies certainly can have tides that effect their orbit.
Established Member
Or tides in the accretion disk of the black hole. But I can't imagine those being significant to most of the stars in the galaxy, even on huge timescales
Order of Kilopi
Are you sure about this? It seems to me that should be possible.
If i remember correctly, simulations of the merger of two black holes showed distortions in the EH consistent with what would be tide-raising in a less violent scenario.
Established Member
It seems to me that it shouldn't be, the horizon is a geometric phenomena with a radius that's a function of the mass within the singularity, which is point like, so they will be spherical until the merger occurs.
Order of Kilopi
The EH is an effect of the curvature of some "patch" of spacetime. When looking at one BH it will be spherical, since the source of the curvature will be point like. However when throwing other "stuff" into the mix, that might change i would think.
Established Member
Can you elucidate this "stuff"?
Order of Kilopi
I meant anything that will have a gravitational effect, though probably it would be more pronounced with high mass objects, say as in putting another black hole next to the first one.
I don't know if it would raise tides, but i have not seen anything contrary to that expectation - so i would be interested to know why one way or the other.
Order of Kilopi
I haven't done the maths but if anything 2 black holes orbiting each other would cause a dimple in their event horizons not case the EH to bubble out. Effectively what the BHs are doing is negating some of the gravitational force of the other BH.
Order of Kilopi
If it causes a dimple, might that be a mechanism for orbital decay?
It seems it would cause a dimple on the near side, but a bulge on the far side (where they work 'together').
We'll probably need someone familiar with the maths, the whole thing is probably highly non-linear around that case and analogies might fail.
Banned
The universe is expanding at an increasing rate. So how can black holes eat everything. Only the more massive stars out there will become black holes. I imagine the black holes will eat each other and dust will inherit the universe.
Established Member
Established Member
Wayne-
Is the "dimple" a tidal effect?
Invisible-
Welcome to BAUT, and the universe is expanding, but objects that are gravitationally bound, like stars in a galaxy in reach of the central clack hole, don't expand. At that range, gravity is stronger than the force that causes expansion. So, it's easy to imagine a distant future in which much of the matter in the universe is contained in massive black holes that are moving away from eachother.
Banned
Galaxies sometimes collide and merge. I know that when black holes collide they become a bigger black hole. I was just thinking how galactic star swallowers like the one at the centre of the milky way could merge in this fashion. creating stupendously massive gravity wells. I suppose there is no theoretical limit to how large a black hole might be. Do we know which came first? Galaxies or black holes?
Order of Kilopi
Yes, and a massive object in a tight orbit will raise a little bulge in the EH, too. But my point is that there's no mass being raised inside that bulge: it simply marks off the region of spacetime in which photons are doomed to hit the singularity. The EH bulge also doesn't lag because of viscous forces or the rotation of the black hole: it seems like a very different beast from a conventional tidal bulge.
(It's a bulge, not a dimple. What determines photon escape is the gravitational potential, not the net force. An object trapped between two converging black holes may experience a cancellation of forces, to some extent, but it's enmired in the potential wells of both black holes. Simulations of colliding holes show the event horizons reaching out towards each other as their gravitational potential adds in the region between the two holes.)
ETA: Some illustrations of black hole mergers.
Grant Hutchison
Order of Kilopi
I recall that the filesize of the animation Grant linked to is absurdly
large, at least considering how crude it is. I drastically reduced the
size without affecting the quality in a fit of obsessive-compulsivity:
http://www.freemars.org/jeff2/winicour_A_jsr.gif
-- Jeff, in Minneapolis
http://www.FreeMars.org/jeff/
"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"
"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves
Order of Kilopi
Thanks for the explanation.
Established Member
Let's assume that 3% of the matter in the Universe is presently in blackholes and that will be 3.000004% in another 13.7 billion years. At he end of the next 13.7 billion years the matter in black holes will be 3.000006% = decreasing gain due to the reduced density, due to expansion and less matter not trapped yet. In about one google years, the average evaporation of black holes may exceed the capture rate (if black holes evaporate). In much less than a google years lots of other stuff will likely happen, so the prediction is likely invalid.
Even if black holes have captured 99.999999% of the matter in the Universe, the last tiny bit will take almost forever to cature due to 99.999999999% of the cubic light years having no blackholes. Neil
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