Black Holes and Dark Matter

[Randy_Shelly]

I often wonder if anyone has considered that black holes might be composes of mostly dark matter, since it is much more abundant in the universe. Since little is known about dark matter, except that it interacts gravitationally with ordinary matter, this could change the physics of black holes. For example it might have a different compressibility than ordinary matter.

 

[MeteorWayne]

First, welcome to space.com.

Second, I wish new users would spend a little time reading before posting. There are a half dozen black hole threads throughout the site; this question belongs in one of the existing ones. I'll probably merge it into one of those later today, probably the one at the bottom of this first page of the topic list.

Third, while dark matter could be a component of black holes, ordinary matter is plentiful enough to explain their existance. THey don't need to be made of dark matter.

MW

 

[Randy_Shelly]

I did read up on the other black hole subjects, but none deal specifically with the dark matter element. And I am not questioning the existence of black holes, I am just posing a question, that I think is important: how is dark matter going to fit with the current theory of black holes? I am surprised that the issue is not getting more attention.

 

[Randy_Shelly]

And by the way, MeteroWayne, there is no need to be rude.

 

[MeteorWayne]

I wasn't being rude Randy, just frustrated. When many new threads are created on the same subject, it makes the moderator's job very hard. A half dozen threads on the same subject dilute the subject. It is far more useful to keep it focused. Then everyone can comment in one discussion.

And if I was being rude, it would be far more obvious

Wayne

 

[ramparts]

Randy_Shelley, the reason is this: ordinary matter explains black holes well, dark matter doesn't So while it's a possibility, it's one with absolutely zero evidence - and thus zero reason to believe it.

A good deal of evidence (both observational and theoretical) suggests that black holes are the remnants of very massive stars, so the "stuff" going into making a black hole needs to originate from the stuff that makes stars (ordinary matter). We don't know what dark matter is, but we have some ideas, and none of them can originate from ordinary matter. They're different types of things. So that's one big problem - if black holes form from stellar material, how does that become dark matter?

I can understand the mental connection - after all, they're both dark, right? But to the best of our knowledge, they're dark for entirely different reasons - black holes have very strong gravitational fields, and dark matter doesn't interact except through gravity so doesn't emit much light. So we're talking about two very different kinds of darkness.

 

[Randy_Shelly]

Understood that black holes formed from star remnants would be mostly regular matter. I'm thinking more in terms of those found in the center of galaxies that might have formed as the galaxy was forming. If dark matter reacts gravitationally with itself and regular matter, one would expect that it would clump together as regular matter would (dark matter stars?).

I realize that it lacks any evidence, but exploring these hypothetical scenarios, could help to understand the nature of dark matter.

 

[FlatEarth]

I often wonder if anyone has considered that black holes might be composes of mostly dark matter, since it is much more abundant in the universe. Since little is known about dark matter, except that it interacts gravitationally with ordinary matter, this could change the physics of black holes. For example it might have a different compressibility than ordinary matter.

I think you make a valid argument, because there is no way to know what black holes are made of since they do not emit light that allows us to measure their composition. (Of course we know they are not made of regular elements because of the extreme gravity, but you know what I mean.) So in theory a black hole could be "normal" matter or dark matter, and we wouldn't be able to tell.

The problem I have with the idea of dark matter anywhere in the universe is that science does not predict it to exist, except that observations indicate there is a lot of missing matter in the universe. This seems like a big jump in logical reasoning. It's like saying I witnessed some lights in the sky I couldn't explain and therefore they must be alien spacecraft. Naturally, the alien spacecraft explanation is the most unlikely answer. Dark matter seems more unlikely than alien spacecraft! At least they would be made of normal matter!

This leads me to the question "Why dark matter?". Why can't all the missing matter be trapped in invisible, inactive black holes, and not be dark matter after all? It seems that this is "low hanging fruit", and there must be a reason scientists came up with the dark matter concept, but I have yet to come across it in the context of my question.

Flat

 

[SpeedFreek]

This leads me to the question "Why dark matter?". Why can't all the missing matter be trapped in invisible, inactive black holes, and not be dark matter after all? It seems that this is "low hanging fruit", and there must be a reason scientists came up with the dark matter concept, but I have yet to come across it in the context of my question.

Flat

The original observations that prompted idea that there was missing matter came from looking at the speeds at which stars orbit their galaxies at various distances from the centres of those galaxies.

If the matter that we could see was all there was, then it was concentrated in the centre of those galaxies. Just as the Sun is where most of the matter in our Solar System resides, the further you look from the centre, the slower the orbital velocity of the objects orbiting that centre. In our Solar System, Mercury travels the fastest and Neptune the slowest.

But when we look at galaxies, we find that the outer stars orbit at similar speeds to those near the centre. This was unexpected. Even if there is a supermassive black hole at the centre of the galaxy, the stars should orbit at slower speeds, the further away they are from the centre.

The solution was unseen mass, spread relatively evenly throughout the galaxy and beyond, so that the orbital speeds remain constant at increasing distance from the centre - as if the galaxy is "lodged" within a sphere of unseen matter, a sphere much larger than the galaxy itself.

The concept seems to have been confirmed by observations of gravitational lensing. We see distant galaxies whose image is lensed by closer galaxies, forming arcs of distorted and magnified images of the distant galaxies around the centre of mass of the closer galaxy. We can measure those arcs and work out the mass required to form them - the calculations confirm that the lensing galaxy must have a lot more mass, spread over a much larger area than the visual galaxy itself, in order to create those gravitational lenses.

(In the photo above, the centre of mass is near the centre of the photo. The magnified arc of a galaxy 3 times more distant is shown on the right. If you look at the shape of the arc, you can see that it is not centred on the galaxy just to the left of it, but is centred nearer the galaxy near the centre of the photo!)

 

[MeteorWayne]

As usual, a very erudite explanation SF!

 

[origin]

Excellent post speedfreak.

 

[darkmatter4brains]

Although Black Holes do form from regular matter, i.e. from the death of massive stars, it's conceivable they contain dark matter, especially large ones at the centers of galaxies. However, the whole statement, that black holes contain "matter" of any form has to be taken with a grain of salt. Matter, as we know it, probably ceases to exist once it enters a black hole. The "matter" in neutron stars is already EXTREMELY exotic ... it's hard to even imagine what is in black holes.

Dark matter has been postulated to be anything from MACHOS ( massive compact halo objects ) to WIMPS (weakly interacting massive particles) to the neutrino mass, to brown dwarfs, to floating Extra Terrestial crap, etc, etc. In the end, I bet it's probably a mix of a few things.

Since black holes pretty much "suck" up anything with mass (or w/o mass!) that comes near them, it's conceivable that one that has been in existence for a while has devoured quite a bit of "dark matter" in one form or another, at one time or another. But, once again, it was probably "annihilated" as it crossed the even horizon. So, although interesting, I'm not sure dark matter and black holes together form anything super important theoretically.

Given how a black hole forms from a massive stellar object, one that formed SOLELY from dark matter, I think, would come from an extremely exotic object, of which we have never seen. Now, if we find something like that, that will be BIG news.

Interesting, if slightly silly, side note. They always say how a neutrino, because it interacts so weakly with "matter", can pass through a light year of lead before interacting with it. Can it pass through 10 km of black hole?? :shock: :? What happens to neutrinos (and photons) that get "sucked" up by a black hole?

 

[Astro_Robert]

I too have long had a major conceptual problem with Dark Matter (and even Dark Energy, although that is a separate issue).

I understand that there must be more matter or gravity causing stuff around than what emits detectable radiation to our telescopes. But why does it have to be this mythical stuff that hangs around and conveniently does not form compact stellar or planetary masses of its own?

I am not going to go into Modified Newtonian Gravity or any such thing as I am a layperson. But when astronomers say that there must be this halo or cloud of dark matter around a galaxy or cluster to explain its gravitational effects I think its like a large molecular cloud of regular matter. So if it has gravitational influence, then why doesn't it behave like normal matter (at least with itself) and form dark matter onjects.

I mean the implication from the popular description of Dark Matter is that a cloud of it could 'float' though me right now, without being gravitationally drawn into the Earth. This seems to be preposterous, and therefore I disbelieve Dark Matter. If said Dark Matter does get drawn into the gravity well of Normal Matter objects, then there should be a detectible change in their net gravitation. If the Dark Matter is neutrinos, then how does it hang out in a Halo, instead of flying off to eh ends of the universe.

Thanks

 

[FlatEarth]

The original observations that prompted idea that there was missing matter came from looking at the speeds at which stars orbit their galaxies at various distances from the centres of those galaxies.

If the matter that we could see was all there was, then it was concentrated in the centre of those galaxies. Just as the Sun is where most of the matter in our Solar System resides, the further you look from the centre, the slower the orbital velocity of the objects orbiting that centre. In our Solar System, Mercury travels the fastest and Neptune the slowest.

But when we look at galaxies, we find that the outer stars orbit at similar speeds to those near the centre. This was unexpected. Even if there is a supermassive black hole at the centre of the galaxy, the stars should orbit at slower speeds, the further away they are from the centre.

The solution was unseen mass, spread relatively evenly throughout the galaxy and beyond, so that the orbital speeds remain constant at increasing distance from the centre - as if the galaxy is "lodged" within a sphere of unseen matter, a sphere much larger than the galaxy itself.

The concept seems to have been confirmed by observations of gravitational lensing. We see distant galaxies whose image is lensed by closer galaxies, forming arcs of distorted and magnified images of the distant galaxies around the centre of mass of the closer galaxy. We can measure those arcs and work out the mass required to form them - the calculations confirm that the lensing galaxy must have a lot more mass, spread over a much larger area than the visual galaxy itself, in order to create those gravitational lenses.

Hello, SpeedFreek. Thanks for your response, and I'm glad you are talking to me again.

I understand the reason for needing a sphere of mass to explain the movement of stars in a galaxy, but the question I have is can this sphere of unseen matter actually be a multiude of black holes distributed evenly throughout the galaxy. Has this been ruled out?

 

[MeteorWayne]

Again, you seem to be under the misconception that black holes have some magical powers. Black holes (in the gravitaional scheme of things) have no more mass than what they were made of, which is the baryonic mass of the galaxy. Or the mass that is concentrated into their event horizon. Small black holes have stellar masses, so it's the same as if it was a star or a stellar system. There are not enough black holes to account for the mass described by the "dark matter" placeholder. While the stuff beneath the event horizon is invisible, the energy in the accretion disks and polar jets would reveal their existance. There is jut no evidence whatsoever for millions or billions of rogue black holes out there.

 

[FlatEarth]

Interesting, if slightly silly, side note. They always say how a neutrino, because it interacts so weakly with "matter", can pass through a light year of lead before interacting with it. Can it pass through 10 km of black hole?? What happens to neutrinos (and photons) that get "sucked" up by a black hole?

My guess is that a neutrino would be trapped in the black hole because space-time is wrapped around itself and nothing escapes, except for Hawking radiation.

 

[FlatEarth]

Again, you seem to be under the misconception that black holes have some magical powers. Black holes (in the gravitaional scheme of things) have no more mass than what they were made of, which is the baryonic mass of the galaxy. Or the mass that is concentrated into their event horizon. Small black holes have stellar masses, so it's the same as if it was a star or a stellar system. There are not enough black holes to account for the mass described by the "dark matter" placeholder. While the stuff beneath the event horizon is invisible, the energy in the accretion disks and polar jets would reveal their existance. There is jut no evidence whatsoever for millions or billions of rogue black holes out there.

Who is implying that black holes have magical powers? (Black holes: no, me: yes.) They obviously are only as massive as the mass they have "swallowed", but the fact remains that the only way they are detected is by their interaction with matter. I believe I'm correct when saying we would need to be really lucky to find an inactive one all by itself. What if most of the matter at the Big Bang ended up in black holes that were evenly distributed in the universe, and are now inactive. I wouldn't be so quick to dismiss this possibility.

 

[darkmatter4brains]

Interesting, if slightly silly, side note. They always say how a neutrino, because it interacts so weakly with "matter", can pass through a light year of lead before interacting with it. Can it pass through 10 km of black hole?? What happens to neutrinos (and photons) that get "sucked" up by a black hole?

My guess is that a neutrino would be trapped in the black hole because space-time is wrapped around itself and nothing escapes, except for Hawking radiation.

I think you're right about that. And same with the photon. But the photon is interesting, because it has to always travel at C according to special relativity. Hard to picture that happening as it gets crammed down into a singularity! Of course, relativity breaks down at that point too.

Also, accreting matter ultimately makes a black hole more massive. But, what addition does a photon make to the black hole?

 

[darkmatter4brains]

Again, you seem to be under the misconception that black holes have some magical powers. Black holes (in the gravitaional scheme of things) have no more mass than what they were made of, which is the baryonic mass of the galaxy. .

Who is implying that black holes have magical powers? (Black holes: no, me: yes.) They obviously are only as massive as the mass they have "swallowed"

They might be only as massive as the objects the swallow but, if I'm not mistaken, they have much more gravitational pull, which is proportional to the amount of mass AND inversely proportional to the radius.

Hence, the saying, "if you shrunk the size of the Earth down to a pea, it would turn into a Black Hole."

 

[MeteorWayne]

They might be only as massive as the objects the swallow but, if I'm not mistaken, they have much more gravitational pull, which is proportional to the amount of mass AND inversely proportional to the radius.

Hence, the saying, "if you shrunk the size of the Earth down to a pea, it would turn into a Black Hole."

The point being, even if you shrunk it down to that size, it would have no more gravitational effect than if it was spread across a hundred or a thousand AU.

In no way would that explain the evidence for "Dark Matter" whatever it is.

 

[darkmatter4brains]

In no way would that explain the evidence for "Dark Matter" whatever it is.

Agreed - I definitely wasn't trying to say that, or defend that.

 

[ramparts]

I too have long had a major conceptual problem with Dark Matter (and even Dark Energy, although that is a separate issue).

I understand that there must be more matter or gravity causing stuff around than what emits detectable radiation to our telescopes. But why does it have to be this mythical stuff that hangs around and conveniently does not form compact stellar or planetary masses of its own?

I am not going to go into Modified Newtonian Gravity or any such thing as I am a layperson. But when astronomers say that there must be this halo or cloud of dark matter around a galaxy or cluster to explain its gravitational effects I think its like a large molecular cloud of regular matter. So if it has gravitational influence, then why doesn't it behave like normal matter (at least with itself) and form dark matter onjects.

I mean the implication from the popular description of Dark Matter is that a cloud of it could 'float' though me right now, without being gravitationally drawn into the Earth. This seems to be preposterous, and therefore I disbelieve Dark Matter. If said Dark Matter does get drawn into the gravity well of Normal Matter objects, then there should be a detectible change in their net gravitation. If the Dark Matter is neutrinos, then how does it hang out in a Halo, instead of flying off to eh ends of the universe.

Thanks

I think this post has gotten lost in the commotion (sorry Robert!). The reason dark matter doesn't form objects like regular matter is that it actually doesn't act like regular matter - it interacts gravitationally, but doesn't have any other interactions (at least not noticeably). If it interacted through another force, say, the electromagnetic force, we'd see signatures of that - in the case of E&M, it would emit light Most of the interactions of regular matter actually have to do with gas dynamics - stars wouldn't be able to form, for example, if there were no friction. Dark matter doesn't really experience these things, though, because they're not gravitational effects.

 

[SpeedFreek]

Hello, SpeedFreek. Thanks for your response, and I'm glad you are talking to me again.

Heheh, I didn't stop talking to you, I thought that you had stopped listening!

 

[FlatEarth]

In no way would that explain the evidence for "Dark Matter" whatever it is.

Agreed - I definitely wasn't trying to say that, or defend that.

Et tu, Brute.

 

[FlatEarth]

I too have long had a major conceptual problem with Dark Matter (and even Dark Energy, although that is a separate issue).

I understand that there must be more matter or gravity causing stuff around than what emits detectable radiation to our telescopes. But why does it have to be this mythical stuff that hangs around and conveniently does not form compact stellar or planetary masses of its own?

I am not going to go into Modified Newtonian Gravity or any such thing as I am a layperson. But when astronomers say that there must be this halo or cloud of dark matter around a galaxy or cluster to explain its gravitational effects I think its like a large molecular cloud of regular matter. So if it has gravitational influence, then why doesn't it behave like normal matter (at least with itself) and form dark matter onjects.

I mean the implication from the popular description of Dark Matter is that a cloud of it could 'float' though me right now, without being gravitationally drawn into the Earth. This seems to be preposterous, and therefore I disbelieve Dark Matter. If said Dark Matter does get drawn into the gravity well of Normal Matter objects, then there should be a detectible change in their net gravitation. If the Dark Matter is neutrinos, then how does it hang out in a Halo, instead of flying off to eh ends of the universe.

Thanks

I think this post has gotten lost in the commotion (sorry Robert!). The reason dark matter doesn't form objects like regular matter is that it actually doesn't act like regular matter - it interacts gravitationally, but doesn't have any other interactions (at least not noticeably). If it interacted through another force, say, the electromagnetic force, we'd see signatures of that - in the case of E&M, it would emit light Most of the interactions of regular matter actually have to do with gas dynamics - stars wouldn't be able to form, for example, if there were no friction. Dark matter doesn't really experience these things, though, because they're not gravitational effects.

You must admit that for dark matter to exist, it must have a combination of properties that make it...um...magical (thank you, MeteorWayne ). I'm surprised that it has become the most accepted theory to explain what is happening in the cosmos, because it is such an unlikely scenario, and impossible to prove!