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A dozen black holes may lie at the centre of our galaxy

bbc.co.uk

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Re: A dozen black holes may lie at the centre of our galaxy

#71

Post by pdonis »

spchampion2 wrote: post #41

From the article: "A search for the X-ray signatures of low-mass black hole binaries in the Chandra data turned up 12 within three light-years of Sgr A*." Three light years is not that big of a distance in the grand scheme of things. For reference, the nearest star to Earth is Proxima Centauri, which is 4.2 light years away. Having 12 binaries within 3 ly sounds really dense. Having several hundred sounds crazy.

> the nearest star to Earth is Proxima Centauri, which is 4.2 light years away. Having 12 binaries within 3 ly sounds really dense. Having several hundred sounds crazy.

Not at the center of the galaxy. Stars are much closer together on average there than they are way out here in the boonies where we are.

Re: A dozen black holes may lie at the centre of our galaxy

#72

Post by BurningFrog »

coldtea wrote: post #37
jstanley wrote: post #13

Earlier quoted context omitted.

People have been saying "we might have invented everything" for hundreds of years. I like the "invention wavefront" analogy, but I unfortunately can't remember where I learnt it. Imagine the high-dimensional space of all possible things that can be invented. In the "centre" of the space is a hypersphere containing everything that has been invented to date. Every new invention is invented by "poking outwards" in one s…

> People have been saying "we might have invented everything" for hundreds of years. No, they really haven't. In fact for a good chunk of centuries (from 16th century onwards), the optimism for the eventual future developments was endless. At some point there was a few scholars saying that during the end of 19th century, but still not some widespread belief. That said, discovery probably also follows an 80/20 rule, a…

> In fact for a good chunk of centuries (from 16th century onwards), the optimism for the eventual future developments was endless

Really?

I've read some pretty authoritative writings about how humanity always thinks The End is Near, that we live in an era of decay after a lost Golden Age, that today's youth is a uniquely corrupted lost generation and so on.

Re: A dozen black holes may lie at the centre of our galaxy

#73

Post by pdonis »

everdev wrote: post #48

Is it possible that black holes could account for a good portion of "dark matter"? They literally fit the term.

> Is it possible that black holes could account for a good portion of "dark matter"?

A key issue with this hypothesis is how the holes could have formed. In order to form a black hole, a system has to gravitationally collapse. But that involves emitting a lot of energy, usually in the form of radiation, as the system becomes more and more tightly bound gravitationally.

For ordinary matter, the kind we and visible stars and galaxies are made of, that's not an issue. But there are strong limits on the fraction of the total matter in the universe that can be ordinary matter (basically because of the relative abundances of light elements, mainly deuterium, helium, and lithium, which are highly sensitive to the total number of quarks and leptons--i.e., ordinary matter--present in the early universe). The ordinary matter we can see is basically the same as the limit, which means there can't be a significant amount of ordinary matter we can't see.

But the total amount of dark matter is about an order of magnitude larger than the total amount of ordinary matter in the universe. So if a significant portion of that is black holes, the holes would have to have formed from something that is not ordinary matter. And the problem is that such a something would not be subject to any of the known interactions except gravitation, and hence would have no way to emit enough energy to gravitationally collapse to a black hole. (This is also the reason why dark matter "halos" around galaxies are much more diffuse and spread out than the visible matter--the dark matter can't clump together gravitationally the way the visible matter does.)

All of the above makes it very unlikely that black holes can account for a significant portion of dark matter.

Re: A dozen black holes may lie at the centre of our galaxy

#74

Post by pdonis »

luk32 wrote: post #60
everdev wrote: post #48

Is it possible that black holes could account for a good portion of "dark matter"? They literally fit the term.

I don't think so. I think currently dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would. Think that you have the mass spread out over a whole galaxy, vs just the middle. From infinity-like large distance it's the same, from galactic it would probably make a difference.

> dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would.

Not if the holes were spread out in a very large diffuse cloud around a galaxy. A single hole would bend light significantly, but the averaged effect of a huge diffuse cloud of holes around a galaxy would be basically zero (because there would be as many holes bending light one way as the other, so their effects overall would cancel out).

Re: A dozen black holes may lie at the centre of our galaxy

#75

Post by davesque »

Boxxed wrote: post #70
davesque wrote: post #69

I'm sure this is an oversimplified view, but doesn't it seem like, because black holes are more dense, they'd naturally tend to fall toward and gather around the center of the galaxy? Sort of like how more dense objects tend to be found at the bottom of a pond? So then perhaps it makes a lot of sense that the galactic center would contain a lot of black holes.

In a pond, you have the force of gravity (i.e., buoyant force) to do this sorting. This force doesn't exist in space, or at least not in a way that's analogous to the pond.

Interesting. Can you provide any more details? Obviously the force of gravity exists in space. So what's a better way to describe the difference?

Re: A dozen black holes may lie at the centre of our galaxy

#76

Post by rmcpherson »

LoSboccacc wrote: post #66
everdev wrote: post #48

Is it possible that black holes could account for a good portion of "dark matter"? They literally fit the term.

the "missing matter" is not just at the galactic core but assumed distributed more thorough the galaxy. if it just were at the center, star rotational speed would follow keplerian predictions, instead they observed the same rotational speed across whole galaxy disks. https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Baryonic matter accelerating into black holes also emits electromagnetic radiation vs. dark matter which does not interact electromagnetically by definition (and observation).

Re: A dozen black holes may lie at the centre of our galaxy

#77

Post by WJW »

azernik wrote: post #58
marcosdumay wrote: post #50

Earlier quoted context omitted.

An accretion disk is not a black hole. Current physics have no other explanation for the disks, but it does not mean that the black holes are really there. There is also some lack of precision on the LIGO experiment that does not invalidate all reasonable alternative explanations. As people already said, the evident is overwhelming on the side of black holes existing. But we don't have any definitive measurement of t…

We have direct proof of: * extremely large masses * in very small volumes * that are not, themselves, emitting anything It could be many things, but they pretty much all fit the definition of "black hole".

I think this is as definitive a proof we'll get until we get close enough to directly measure Hawking radiation. Of course, by that time we should also be close enough to "see" the black hole directly...

Re: A dozen black holes may lie at the centre of our galaxy

#78

Post by rcthompson »

davesque wrote: post #69

I'm sure this is an oversimplified view, but doesn't it seem like, because black holes are more dense, they'd naturally tend to fall toward and gather around the center of the galaxy? Sort of like how more dense objects tend to be found at the bottom of a pond? So then perhaps it makes a lot of sense that the galactic center would contain a lot of black holes.

Imagine that our sun suddenly collapsed into a 1-solar-mass black hole. What would happen to the orbits of the planets? Nothing. The black hole would still exert the same gravitational pull as the sun did. The same is true if you replace all the planets with equivalent-mass black holes.

Denser objects in an atmosphere or other fluid, where there is friction and buoyancy, will tend to sink to the bottom. However, if you're in the vacuum of space, there is no concept of "sinking". There is nothing to sink in unless you enter the atmosphere of a planet or star.

However, maybe there is enough interstellar dust near the center of the galaxy to produce a non-negligible amount of friction over millions or billions of years.

Re: A dozen black holes may lie at the centre of our galaxy

#79

Post by saiya-jin »

pdonis wrote: post #71
spchampion2 wrote: post #41

From the article: "A search for the X-ray signatures of low-mass black hole binaries in the Chandra data turned up 12 within three light-years of Sgr A*." Three light years is not that big of a distance in the grand scheme of things. For reference, the nearest star to Earth is Proxima Centauri, which is 4.2 light years away. Having 12 binaries within 3 ly sounds really dense. Having several hundred sounds crazy.

> the nearest star to Earth is Proxima Centauri, which is 4.2 light years away. Having 12 binaries within 3 ly sounds really dense. Having several hundred sounds crazy. Not at the center of the galaxy. Stars are much closer together on average there than they are way out here in the boonies where we are.

Which makes me think - if the area is relatively dense with black holes, quite often (on galactic time scale) there must be super close encounters with stars, sometimes BH swinging right through a sun (that might be a bit of a stretch)! Imagine that spectacle, devouring whole sun throwing off/swallowing it potential planets. I am sure we would be able to see it as huge burst with our current sensors.

Plus there might be huge amount of smaller and tiny BH wreaking all kinds of havoc to everything. Pretty wild place to be

Re: A dozen black holes may lie at the centre of our galaxy

#80

Post by semi-extrinsic »

wallace_f wrote: post #65
dbasedweeb wrote: post #51

Earlier quoted context omitted.

There’s a theory which accounts for that called MACHO, for MAssive Compact Halo Object. While it’s possible, over many years of observation it’s very unlikely. You can’t see a black hole directly, unless it’s “eating” of course, but you can see gravitational effects like lensing. If black holes and neutron stars made up most dark matter, we’d expect to see evidence of it, and we don’t. It is still possible, although…

My heretical theory of dark matter is that is just a byproduct of black holes gravitational effects. Probably something about so much mass punching such a deep curve in spacetime creates a widespread effect. Of course this is just a baseless theory, but it seems more reasonable to consider the possibilities of a realm which we know that we know almost nothing about, than inventing new particles or physics to fit the…

The trouble with this kind of theory is that for a force like gravity, if you consider the force outside a sphere surrounding the sun, the force is identical no matter how the mass inside is distributed, whether it's the sun or a solar mass black hole inside the sphere.
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