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Must we have black holes?

by Nugso
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Nugso
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Mar15-14, 11:33 AM
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As the title says, do we have to have black holes for the universe to run? Every galaxy has a suppermasive black hole at its center. What if there were a star at the center of our galaxy? I guess it would have to be so big but that means the star would soon run out of energy and then explode, i.e Supernova?

And what if there were nothing at all at the center of our galaxy. No black holes, no stars, nothing?
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Simon Bridge
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Mar15-14, 11:50 AM
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Galaxies start out without dense cores, the core get denser over time since material is gravitating into them.
If they get dense enough, you get black holes.

If there were nothing in the center of the galaxy, there would probably be no galaxy.

See:
https://blogs.discovermagazine.com/b...t-black-holes/
phinds
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Mar15-14, 11:53 AM
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Quote Quote by Nugso View Post
As the title says, do we have to have black holes for the universe to run?

If you have the answer to that you should submit your name for the Nobel Prize in Physics right away.

Every galaxy has a suppermasive black hole at its center. What if there were a star at the center of our galaxy? I guess it would have to be so big but that means the star would soon run out of energy and then explode, i.e Supernova?
I'm sure that an object with the mass of whatever it is in the center of the Milky Way (and all evidence says it's a black hole) it is too massive to be a star. That amount of mass pretty much can't be anything but a black hole. It's just too much mass to do anything but collapse into one.


And what if there were nothing at all at the center of our galaxy. No black holes, no stars, nothing?
Well, there is, so I'm not sure there's any answer to that question. I think it goes with the first one.

Nugso
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Mar15-14, 12:02 PM
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Must we have black holes?

@Simon Bridge,

"If there were nothing in the center of the galaxy, there would probably be no galaxy."
Galaxies start out without dense cores, the core get denser over time since material is gravitating into them.
If they get dense enough, you get black holes.
What if it doesn't get denser over time? Then there wouldn't be a black hole? But you said there would be no galaxy. I don't get it. Could you explain?

@phinds,

Is there? Really? I google'd it but the results weren't satisfying to be honest. The astronomers says there can also be a small black holes instead of supermassive ones.

Do you know of any?
phinds
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Mar15-14, 01:42 PM
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Quote Quote by Nugso View Post
@phinds,

Is there? Really? I google'd it but the results weren't satisfying to be honest. The astronomers says there can also be a small black holes instead of supermassive ones.

Do you know of any?

I cannot imagine any astronomer worth his salt having said any time recently that there could be only a small black hole there. There has been a 10-year study that resulted in known orbits of some of the stars right at the center and the orbits cannot possibly be explained by anything other than the mass of a supermassive black hole.
Hornbein
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Mar15-14, 06:22 PM
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Quote Quote by Nugso View Post
As the title says, do we have to have black holes for the universe to run? Every galaxy has a supermasive black hole at its center. What if there were a star at the center of our galaxy?

I recently read a book by an astronomer whose thesis was that the jets from black holes countered the force of gravity and were important in determining the rate at which stars formed. He studied ancient galaxies.
phinds
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Mar15-14, 06:39 PM
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Quote Quote by Hornbein View Post
I recently read a book by an astronomer whose thesis was that the jets from black holes countered the force of gravity and were important in determining the rate at which stars formed. He studied ancient galaxies.
I think you should put that book in the circular file.
Bandersnatch
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Mar15-14, 06:44 PM
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Rather than simply googling, go to arxiv.org and search for papers about the detection of supermassive central black holes. You'll find plenty of papers that'll let you appreciate the methods and precision of the detection.
This one as an example:
http://arxiv.org/abs/1211.0943
The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc.
which tells you that within about 20 ly radius of the centre of the galaxy in question(Circinus galaxy) there must be ten million solar masses of matter.
From stellar astrophysics we know that the largest possible stars are in the order of tens to a couple hundred of solar masses, anything more than that producing too much radiation to keep the material together. So it can't be a single star.
It could be about hundred thousands solar mass stars orbiting their common barycentre, but the gravitational models show that such configurations are unstable, leading either to ejection or collisions between the orbiting bodies. Additionally, the gaseous accretion disc that is oftern observed, mentioned in the paper cited above, would not form easily around a collection of stars.
All in all, the only stable, long-term configuration requires a single massive body, which in turn needs to be a black hole, as there can be no stars(of any kind) of that mass.
Chronos
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Mar15-14, 07:08 PM
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We know there are black hole, of various size, at the center of all our neighboring galaxies. It is considered a safe bet there is one anchoring most galaxies in the universe. Supermassive black holes are also believed to power quasars, which have been detected out to z~7. Given the universe was less than a billion years old at z=7, it raises an interesting question - how does a billion solar mass black hole form in less than a billion years? Many cosmologists believe SMBH are the seeds required to form massive galaxies. The SMBH at the center of the milky way is a mere 4 million solar masses, which is quiet modest compared to most other galaxies. The milky way is, however 13.2 billion years old, so, if this seeding idea is true, its SMBH formed when the universe was less than 600 million years old. See; http://arxiv.org/abs/0907.1608, The role of black holes in galaxy formation and evolution, and http://www.nasa.gov/mission_pages/ch.../H-11-183.html, NASA's Chandra Finds Massive Black Holes Common in Early Universe06.15.11, for discussion.
Nugso
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Mar16-14, 12:20 PM
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Thanks for the replies guys. The arxiv link Chronos provided helped me really a lot. I guess I should've arxiv'd it instead of googling all along.
abitslow
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Mar18-14, 02:56 PM
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Couple of things I didn't see in replies so far. More philosophy than science, but fwiw:
We understand 5% of the energy content we believe the Universe has, so there is more we don't know than what we do know. All the answers here will be based on that ignorance, so take them with a large grain of salt. That said, the picture is fairly consistent, gratifyingly so. Its definitely a good story, regardless of whether its the right one. Chances are it gets a lot right. So, Big Bang, hot hydrogen gas, gravitational collapse due to quantum inhomogeneities, the first stars, then the first galaxies (which came first is still somewhat controversial). Your question can be transformed to at least three better ones:
1. There is clear evidence that somethings very small and very dense exist, not only stellar sized, but super-massive sized. What is the reason we believe these things, we call "black holes" are all the same, only differing in mass, speed, location, charge, and spin (as well as the individual history). The answer is: because we have no evidence that anything can exist between neutron densities (quark/gluon plasma densities) and the singularities. Another way to put this is: because we don't know any better. It could be that states of matter/energy exist beyond what we currently understand but without the infinities inherent in black holes. My point is that what we call "black holes" may turn out to be a whole circus of different things. So, by using a single term, we put all these strange critters into the same box, but that doesn't mean doing so will turn out ultimately to be correct. Time will tell.
2. Given the average matter density of a galaxy, is formation of a central black hole the natural progression of gravitational attraction? We think the answer is yes, unless the galaxy is disturbed (by other galaxies).
3. Did the black hole come first? Almost certainly not, although this is a chicken and egg situation. The first stars are thought to have been hypergiants (many of them) with life-times of only tens of millions of years. (this is another active area of research, so tomorrow may prove me wrong (or yesterday, lol)). This means its likely that there were black holes around from the start of the galactic era. These wouldn't have started out as SMBHs, and there's no reason (afaik) that they are 'theoretically' necessary for galaxy formation, but as said, they will eventually form and almost certainly most galaxies contained BHs from day one.
This is completely outside my area of competence, but all I've read implies we expect our Universe to be more complicated than what our current understanding shows. If it turns out, for example, that there is some new force or effect which prevents the formation of singularities, then that force would have to be negligible at the energies we can observe, and only show up at much higher energy scales. You can look at the Universe as young, only 14 by old. Or you can look at it as old, 14 billion years old. Picture a forest after a forest fire. Now picture ants hatching from their eggs and coming out and trying to figure out from what they see around them, burnt trunks and smoking ashes, what the forest looked like before the big fire. Anyway, there is nothing that requires that no new forces can be added to our understanding, nor that these forces may prevent collapse to a infinitely small point (a singularity). What we can say is there is nothing in the forces we know which prevents such collapse. Required? who knows? Oh, and by the way: empty space is not nothing. Search "vacuum".
Drakkith
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Mar18-14, 06:15 PM
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Quote Quote by abitslow View Post
We understand 5% of the energy content we believe the Universe has, so there is more we don't know than what we do know. All the answers here will be based on that ignorance, so take them with a large grain of salt.
I don't necessarily agree with this. The relative ratios of matter, energy, dark matter, and dark energy have little to do with how much we know about the universe. Dark matter and dark energy could turn out to be very, very simple. We actually don't know how much we don't know.


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