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EdLee
2006-Aug-07, 10:10 PM
Okay, I'm sorry if this is an enormously stupid question. I just don't have an answer.

So, from reading the "Black Hole FAQ" by Ted Bunn, as well as posts on this forum, I've gathered that:

1. Gravity strength is determined by an object's mass concentration.
2. Black holes are so small and massive that nothing can escape their gravity.
3. Black holes will swallow anything within a certain radius.
4. The radius is determined by the mass concentration of the black hole's singularity.

So that leads me to assume the following:

1. Objects "consumed" by a black hole get added to the singularity's mass.
2. The increased mass thereby increases the black hole's gravity.
3. The increased gravity increases the black hole's Schwarzschild radius.

So my question is:

If the radius continues to increase, would they eventually become so large as to consume the entire universe?
Stray comets/asteroids/dust keep getting sucked into any black hole, increasing the Schwarzschild radius enough to reach a planet, rinse and repeat until sooner or later they just become so large that they overlap each other, multiplying their effect, etc.?



Possible answers that I've come up with are:

1. No. Space is so damn big that even a huge asteroid wouldn't increase the Schwarzschild radius enough to reach planets. And even then, they're just too isolated. Space is huge, man.

2. No, one of my assumptions about gravity/black holes is just plain wrong.

3. Yes. Given enough time (how about eternity), the black holes would gather enough mass to swallow all matter in the universe.


Thanks for your wisdom.

Kaptain K
2006-Aug-07, 10:18 PM
Number 1 is correct.

For what it is worth, if you plug the estimated mass of the entire universe into the eqation for Schwartschild radius, you get a number on the order of the size of the universe.

EdLee
2006-Aug-07, 11:43 PM
Sorry, I don't quite understand that second part.

You're saying that, in order for a black hole to have a large enough radius to consume all of the mass in the universe, it must have the mass of the entire universe already in its singularity?

[confused about mathematics]

LucasVB
2006-Aug-08, 01:53 AM
Hmm... According to some wild estimations (Wikipedia), the total mass in the visible Universe is estimated around 3 × 10^52 kg, which gives a Schwarzschild radius of 4.45 * 10^25 m, or about 33.6% of the radius of the visible Universe (roughly 14 billion light years, or 1.32 × 10^26 m)

AGN Fuel
2006-Aug-08, 04:16 AM
Perhaps a useful way to think about this is that if our sun were to become a black hole (it won't btw), it would have a diameter of ~3km from memory. The sun contains ~99% of the total mass of the solar system, so even if the rest of the solar system were somehow captured by the black hole, it's diameter would barely increase at all.

When you consider that the next nearest star is 40,000,000,000,000 km away, you can see that the 3 km solar black hole couldn't possibly consume enough material to 'overlap' with the next nearest star.

It gets a bit more exciting down in the nucleus of the galaxy, but the same principles apply.

Kaptain K
2006-Aug-08, 04:58 AM
...about 33.6% of the radius of the visible Universe...
I said "on the order of" (factor of 10) not "on the close order of".

sol88
2006-Aug-08, 05:29 AM
I'd like to know how "they" know, regarding BH formation, Gravity strength is determined by an object's mass concentration, when readings closer to home seem to be highly variable i.e. asteroids, comets, moons etc? but they can be sure that a "black hole" of X solar masses "must" exist to account for the observations, and from cosmic distances???? :shifty:

Then, and maybe I'll get banned for this but, THIS (http://www.bautforum.com/showthread.php?t=45399) post in the ATM forum suggest a different interpretation that "fits". Recent events regarding "BH" formation appear to be turning down a much more realistic view of the cosmos, like a program on watched on “string theory” the other night, they stated all astronomical observation seem to be converging to a point, and that point is a much more elegant interpretation. (Not string theory though)


A controversial alternative to black hole theory has been bolstered by observations of an object in the distant universe, researchers say. If their interpretation is correct, it might mean black holes do not exist and are in fact bizarre and compact balls of plasma called MECOs.

I always think about answer #3 in a metaphor of putting ones cranium up ones rectum and disappearing :think:


Like I said the electric interpretation of BH, Quasars, pulsars etc etc is very elegant and much more streamlined than alternates. :whistle:

If you have the time/interest EdLee, Google it or check these links (http://www.electric-cosmos.org/links.htm) out

Sol

Ufonaut99
2006-Aug-08, 10:37 AM
There are also two other effects to consider when wondering whether BHs could eat all the mass of the universe :

1) Space isn't only huge, the accelerating expansion means it's getting huger all the time. This means that the mass will be getting more thinly spread out, so less likely to fall into a BH's maw. Of course, mass in galaxies will tend to keep close to the BHs at the centre of those galaxies.

2) BHs aren't everlasting, but do radiate away due to Hawking radiation.

Blue Fire
2006-Aug-08, 02:56 PM
2) BHs aren't everlasting, but do radiate away due to Hawking radiation.
From Wikepedia:

A black hole of one solar mass has a temperature of only 60 nanokelvins; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5 × 10²² kg (about the mass of the Moon) would be in equilibrium at 2.7 kelvins, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb, and thereby lose mass. Edited to include the quote.

So, if black holes of one solar mass or larger keep absorbing CMB, would they grow until there is no more CMB?

mugaliens
2006-Aug-08, 10:28 PM
I said "on the order of" (factor of 10) not "on the close order of".

And apparently not enough to "swallow" the universe. Is this correct?

dirty_g
2006-Aug-08, 10:33 PM
I read a book on this and it said the universe would end with one massive blackhole.

EdLee
2006-Aug-09, 03:17 AM
Perhaps a useful way to think about this is that if our sun were to become a black hole (it won't btw), it would have a diameter of ~3km from memory. The sun contains ~99% of the total mass of the solar system, so even if the rest of the solar system were somehow captured by the black hole, it's diameter would barely increase at all.

When you consider that the next nearest star is 40,000,000,000,000 km away, you can see that the 3 km solar black hole couldn't possibly consume enough material to 'overlap' with the next nearest star.

It gets a bit more exciting down in the nucleus of the galaxy, but the same principles apply.

This makes a lot of sense, thanks.

I'll have to look into the alternate theories to black holes. I'm having a difficult enough time figuring this one out though.:wall:

Van Rijn
2006-Aug-09, 08:13 AM
So, if black holes of one solar mass or larger keep absorbing CMB, would they grow until there is no more CMB?

Well, it wouldn't be microwave anymore.

The universe is expected to continue to cool. At some point the cosmic radiation background temperature will drop below a black hole's Hawking temperature and there will be a net loss of mass. Of course, that is a very low temperature, especially for supermassive black holes, so this would only happen when the universe was very, very dark.