Will Black Holes eat the Universe?

Since every galaxy seems to have a black hole won’t the black holes eventually eat every star in every galaxy?

Is the universe’s expansion enough to overcome each of the galaxy filled black hole’s pull toward each other?

According to some models, the state that maximizes entropy in the universe is a bunch of black holes and a thin cloud of evenly dispersed particles. These particles would be in steady-state since the ones that are gobbled up would be matched by those created by the black hole’s Hawking Radiation.

So, if the previously-mentioned inflation factor isn’t enough to keep the galaxies stars from the black holes, I would expect that eventually all we would have is some black holes and a thin cloud of particles. Incidentally, I wouldn’t expect the number of black holes to go significantly down, because their gravitational effects would probably not be high enough to overcome inflation.

No, no more than Jupiter will “eventually” crash into the Sun. A single star orbiting a black hole in a circular orbit is stable, just like a planet orbiting a star. The galaxy contains multiple stars, of course, and interactions between them will cause some stars to fall into the central black hole and some to be ejected from the galaxy, but there’s no reason that all of the stars will fall into the central black hole.

To the best of our understanding, yes. The expansion caused by the dark energy will cause the Universe to expand forever. This assumes, however, that the dark energy’s characteristics won’t change significantly from those we observe today, which is far from certain; but in the absence of a realistic model for the dark energy, we don’t have any reason to think this is a bad assumption.

To further expand on MikeS’s answer, the gravity of a black hole is no different from the gravity of any other object in the Universe. If I’m 150 million kilometers away from the center of a solar-mass black hole, it’s just the same as being 150 million kilometers away from the center of the Sun (where we, in fact, are). If I’m 1 million kilometers away from the center of a solar-mass black hole, it’s just the same as being 1 million kilometers away from the center of the Sun. The only difference is that you can get much closer to a black hole: If I were 5 kilometers from the center of a solar-mass black hole, I’d observe some really weird effects. If I were that close to the center of the Sun, I would observe those same effects, too, except that I can’t: If I were that close, I’d be deep inside the Sun, which of course changes things.

Like what?

For starters, my orbit wouldn’t be a nice closed loop shape around the hole, like the planets’ orbits around the Sun (approximately) are. It’d be a wild, swooping, Spirograph-like shape. Light from distant stars would be strongly blueshifted, and any transmissions I received from significantly outside the vicinity would sound like they came from Alvin and the Chipmunks. Laser beams would be significantly deflected, and measurements of angles would not match the predictions of Euclidean geometry. I’d notice significant tidal forces, trying to point me and my spaceship along a radial line and pulling me apart.

What a trip. Black holes could be the recreational drug of the future.

I’ve got a question, though - I know that the effect of gravity depends on the distance-squared, but am I correct in thinking that while this could be immeasurably small, it is never zero?

Or put another way, if the universe consisted of exactly two objects (any size, but let’s say the size of our sun) - can they never be so far away that eventually they won’t meet? Same answer if we discount the expansion of the universe? (Or have I gone too far into science-fiction will all the “ifs/ands/buts”?)


But, assuming the sun won’t explode in the meantime, won’t Jupiter eventually be pulled into the sun by gravity?
Don’t artifical satellites fall to earth, “eventually?”
This is like KidScuffy’s question. Is gravity ever equal to zero?

I can understand inflation of the universe being greater than gravity but not within the insignificant scale of a galaxy.

Yes, but not because of gravity. Artificial satellites come down because even at orbital height, there’s still some trace of atmosphere. So satellites experience some drag, and slow down, and it’s because they slow down that they don’t stay in orbit. If there were nothing but gravity involved, they’d stay up forever.

It would depend on the original vectors of the objects at an assigned time “0”. If the two objects were traveling at some radial vector with respect to the each other, then they would not collide, but would orbit their mutual center of mass, with an elliptical path determined by the masses, and the original vector of each with respect to that point.

Since no other objects are in the universe, the orbit will be exactly stable, and they will orbit forever.

Very dull universe.


As Chronos said, the eventual decay of the artificial satellite orbit is a result of the drag of the residual atmosphere at their altitude. Sychronous satellites’ orbits decay little, it at all.

The acceleration of gravity on a satellite orbiting a certain distance from the primary is balanced by the centrifugal force resulting from its velocity in orbit acting outwards on the body.

The inward force of gravity F = km[sub]s[/sub]m[sub]p[/sub]/d[sup]2[/sup]. The acceleration toward the center of the primary as a result of this force is a = F/m[sub]s[/sub] = km[sub]p[/sub]/d[sup]2[/sup]

The accelertation of the satellite resulting from its velocity in the orbit is a = v[sup]2[/sup]/d. The velocity can be replaced by the distance for one revolution around the primary divided by the time for that revolution. That gives a = (2πd)[sup]2[/sup]/t[sup]2[/sup]d = 4π[sup]2[/sup]d/t[sup]2[/sup].

If we set these two accelerations equal to each other as will be the case when the forces are balanced:

km[sub]p[/sub]/d[sup]2[/sup] = 4π[sup]2[/sup]d/t[sup]2[/sup].

this reduces to** d[sup]3[/sup]/t[sup]2[/sup] = a constant** which is Kepler’s third law for orbital motion.


I neglected to cover the third case, with two bodies alone in the universe.

If the two bodies are moving at a speed greater than the force of their combined gravity will overcome (Mutual Escape Velocity) and are not on a collision vector, they will move away from each other in a hyperbolic path about the center of their masses. (An open orbit, if you please.) They will, after their closest approach, they will decelerate, but move farther apart forever.

Briefly more interesting, but eventually still fairly boring


But he’s right, though, isn’t he?

Given that gravity decreases at the square of the distance from an object but is never zero it would be proper to say that ALL objects in the universe are exerting pull on all OTHER objects regardless of distance. Is that right?

I understand that it will be infinitesimal for most objects but the pull is there and never ends, right?

I dunno. If gravity is carried by gravitons, which are the excitation of the gravity field, I would suspect that quantum gravity would take hold and there might be a lower bound past which no gravity would be expected (unless you happened to run into the random graviton.)

It’s just like the situation for photons. At any distance from a massive object there will be an expected (in the statistics sense) rate at which you’ll see gravitons. Far enough away, that number will get arbitrarily small, but it won’t ever go away.

Well, if we’re talking about the eventual entropic state of the universe we should probably be careful about saying “forever.” General relativity predicts gravitational radiation from bodies orbiting each other, which will remove energy from the system and eventually cause decay of Keplerian orbits even in the absence of any nongravitational interactions. For systems like the Sun-Jupiter system, of course, the timescales are all on the order of 10[sup]big[/sup] number of years, but still…

A question about the nature of said black holes.

If they expand to consume the collapsed stars that fall into their gravitational pull, will they eventually expand to the point where they lose the intensity of their pull? As the non-physics guy, I am struggling to understand how a black hole is changed in terms of gravitational/other related forces over it’s lifetime.

I’m envisioning a laser beam opposed to a flashlight. Both throw, but one is much more focused than the other. Or, a Shop-Vac with a 2 inch hose mouth opposed to a Shop-Vac with a 1/2 inch hose mouth. ( perhaps this is the better analogy? ) The motor making the machine suck sucks at the same pressure, but the pressure appears greater in a narrower area, yes? The air is sucked in let’s say at 100 lbs/per square inch, and with a 2 inch mouth, it has a pull of X. With a 1/2 inch hose mouth, it has a pull of… 4X ? ( I am trying. I suck at math. :smiley: ).

So, a black hole. Does it’s gravitational pull alter the more stars it “eats” or does it remain a constant source of heavy pull? Where do the stars go as they are sucked into the black hole?


Yes, the gravity increases.

A mass has a particular gravity because of its magnitude (size). That attraction acts on the particles of the mass itself, as well as every other mass in the universe (Well, maybe). That gravity is expressed as an acceleration, at any particular distance from the center.

You might recall that Earth has a gravity of one G. This is not a coincidence. The rate at which an object falls on Earth (32 ft/sec/sec) is an acceleration, and is called one G. Now as a result of that, moving away from earth without constant energy input requires that you move fast enough to keep moving even though you loose 32 feet/second of speed every second. (At first, later you loose less) That means on earth you have to go 25,000 miles an hour in order to “escape” from Earth.

Every body in the universe has an escape velocity for every distance from that body’s center. When you are very far away, that velocity is fairly low, unless the object is very large. The closer you are, the higher that velocity is.

Now the black hole part. If the escape velocity from an object at some point is greater than the speed of light, nothing can move outside of that radius, since nothing can even reach the speed of light. Since the attraction of a particular mass varies with the square of distance, even a moderately large mass could have that high an attraction,* if you were close enough to the center of mass.* But, if the object itself is so dense that all of its own mass is inside that radius (The Swartzchild Radius) the object is a black hole.

If more mass enters the black hole, that mass too becomes part of the black hole. It is now a larger black hole, and the Swartzchild radius becomes larger. It grows. In fact, on a normal scale of objects, that is the only thing that can happen. Stuff falls in toward the black hole, and either orbits, and gets heated up by various effects, or it hits, and becomes part of the black hole. The critical size is proportional to the total mass. The possibility of small black holes with masses smaller than say, the Moon, can be described, but none have been observed. There is some conjecture that they might, in fact be rather common. Such a black hole could be only millimeters in radius, or even less. Their stability is a subject of some debate. Galaxy sized black holes are thought to exist, as well. These are thought to be stable of cosmic time scales.

So, to answer your question, the stars that fall into the black hole don’t go anywhere. They are still there, and in fact their magnetic and electrical properties are there as well. The total gravitational attraction of the old black hole and the new stars is added together. It all gets added to the mass of the object that we call a hole, even though it is not really a hole at all. Nothing comes back out, except Hawking Radiation, and an occasional Tee shirt.


This is a little deep for me. I understand that the prediction is that gravity radiates but where to? Heat passes from hot to cold and I suppose that gravity would pass from more massive to a less massive body. So eventually will all bodies wind up as having the same mass?

Thank you so much, Tris. That made perfect sense to me. :slight_smile:

If there may be black holes a millimeter, or a few millimeters across and there may or may not be many of them, is it thought that they are stationary in relation to bodies near them of much greater masses or are they constantly on the move? Are they constantly growing by dint of the effect you described, wherein the start as 1 MM across, and then continuously grow as they absorb matter around them.

And, if there were in fact a black hole 1MM across, what matter would it absorb? Could a black hole continue to exist and grow if it absorbs pollen and flys as efficiently as it’s big sister who eats dead stars for breakfast? What matter of… .er…matter must any black hole attract and absorb to keep on keeping on?

And, tell me more about these t shirts.