Black Holes and Dark Matter

[MeteorWayne]

Not really impossible to prove. A number of particles have been theorized, which would be detectable using some sophisticated data collection and analysis. Such research is underway, with some detection suggested. But it's really at the cutting edge of research right now, so we certainly don't know anything for sure yet.

 

[ramparts]

Exactly - it's absolutely not impossible to prove. There are plenty of candidates for dark matter, and particles which only weakly interact except gravitationally are plentiful in physics - have you heard of neutrinos, which quite definitely exist? Just because the dark matter particle has properties that seem strange to us doesn't make it magical. Scientists are working hard on finding the best observational evidence to constrain different theories of dark matter (in fact, I had the pleasure of watching a bit of a conference where they did just that). Unfortunately, we can't decide when a certain observation will happen, so a lot of it is waiting for the right data to come in. But I suppose in the meantime there will always be some claiming that because of that unfortunate name "dark" it must be unprovable.

 

[FlatEarth]

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.

I believe you are both right.
Overall, the gravity field of an Earth-sized planet and an equally massive black hole (theoretically possible, but to date never detected) would be the same. The result of getting too close to an Earth-sized black hole is essentially the same as getting too close to a "normal"-sized black hole. The "inversely proportional to the radius" statement is correct because the strength of gravity for a given mass increases greatly (within a certain radius) as the radius of the mass decreases.

 

[ramparts]

Yes, the gravitational fields of the two Earth-mass objects are the same, as long as you're outside both objects - so, within one Earth radius, they start looking pretty different

 

[darkmatter4brains]

Yes, the gravitational fields of the two Earth-mass objects are the same, as long as you're outside both objects - so, within one Earth radius, they start looking pretty different

Yep, I think Gauss's Law is probably the best way to picture it, if ppl are familar with it. That law can care less how the mass (or charge, since electrodynamics is the most familar place to see this law) is distributed within a volume. The field turns out to be dependent only on the amount of mass (or charge), even if the shapes/distributions are different. So, at a given radius, two masses that are similar in magnitude, but different in shape, will have the same field strength.

But, with the pea-sized Earth, you can get in a lot closer to the object, and with gravitational force being inversely proportional to the square of the radius, the force near the surface of the pea-sized Earth would be a lot stronger than at the surface of the real Earth. At least, I'm pretty sure that's how it all works out :lol:

There is also a law/eq. for white dwarfs (and I think neutron stars too), that shows how the radius of the object varies with the mass. As the mass goes up the radius gets smaller, which scientists took as a hint that white dwarfs probably had an upper mass limit. I don't know how this works out for Black holes, though. Anybody else know??

 

[ramparts]

Well, it's hard for black holes (as we know them) to have radii If they do, it's because of yet-unknown physics.

 

[darkmatter4brains]

Well, it's hard for black holes (as we know them) to have radii If they do, it's because of yet-unknown physics.

Wat about the event horizon radius though? Isn't that related to the mass somehow? Although, I guess it wouldn't be like the inverse relationship at all. The more massive the black hole, the larger the event horizon, correct?

I personally don't buy into the whole singularity thing - at least not the infinitely dense, infinitesimally small part - for no reason other than aesthetics though.

 

[darkmatter4brains]

Well, it's hard for black holes (as we know them) to have radii If they do, it's because of yet-unknown physics.

I guess this is related to me not liking the idea of a singularity too, but I can't help but wonder if there is another state of matter, that does stop the gravitational contraction of Black Hole, albeit after the point at which light can still escape.

For example, could there be a quark degeneracy pressure? They do follow the Pauli Exlcusion principle in a way - in fact, I think this was the motivation behind the quark color hypothesis? I'm totally speculating, but you can't help but wonder. If this was the case, it would avoid the singularity, and then the Black Hole itself would have a radius. I'm sure something in Quantum Chromodynamics rules this out though, or it would probably already be a theory?

 

[ramparts]

Well, it's hard for black holes (as we know them) to have radii If they do, it's because of yet-unknown physics.

Wat about the event horizon radius though? Isn't that related to the mass somehow? Although, I guess it wouldn't be like the inverse relationship at all. The more massive the black hole, the larger the event horizon, correct?

That's true, but that's not really a radius in the same way, just a distance past which light can't escape (and spacetime behaves differently). That's directly proportional to the mass - in particular, it's equal to 2GM/c^4.

I personally don't buy into the whole singularity thing - at least not the infinitely dense, infinitesimally small part - for no reason other than aesthetics though.

A lot of folks do believe that quantum considerations will prevent the formation of a singularity, but we need to wait on a quantum theory of gravity to say more.

Normally I'm all into aesthetics as a guiding principle in physics, but there are some big caveats associated with that. That's a topic for another thread though

 

[darkmatter4brains]

That's directly proportional to the mass - in particular, it's equal to 2GM/c^4.

Is that the correct eq. for an event horizon radius? It seems to come out in the wrong units (s^2/m) and gives an event horizon radius of ~3*10^-13 meters for an 8 solar mass black hole? It looks like it would work if it was a c^2 in the denominator? Units come out right, and you get an event horizon radius of ~24,000m?

anyhow, thanks, that's basically what I was looking for. I guess if we ever figure out exactly what is at the center of a black hole, there might be an equation for that radius as well. Assuming it's not a singularity!

 

[csmyth3025]

We're all aware of the Newtonian concept that the force of gravity (like that of electromagnetic radiation) is inversely proportional to the square of the distance between two objects. We deduce this from measurements in our immediate neighborhood (cosmically speaking). Is it possible that the proportionality factor by which gravity reduces differs ever so slightly (say, by r^1.999 instead of r^2) over vast regions from the force/distance relationship we've been able to discern in our local region of space?

Chris

 

[ramparts]

That's directly proportional to the mass - in particular, it's equal to 2GM/c^4.

Is that the correct eq. for an event horizon radius? It seems to come out in the wrong units (s^2/m) and gives an event horizon radius of ~3*10^-13 meters for an 8 solar mass black hole? It looks like it would work if it was a c^2 in the denominator? Units come out right, and you get an event horizon radius of ~24,000m?

anyhow, thanks, that's basically what I was looking for. I guess if we ever figure out exactly what is at the center of a black hole, there might be an equation for that radius as well. Assuming it's not a singularity!

Yes, you're right, it's c^2. I'm used to working in so-called "natural units" where c is equal to 1, so sometimes I get the factors confused It's easy to work out the right one with some dimensional analysis though, as you did.

 

[ramparts]

We're all aware of the Newtonian concept that the force of gravity (like that of electromagnetic radiation) is inversely proportional to the square of the distance between two objects. We deduce this from measurements in our immediate neighborhood (cosmically speaking). Is it possible that the proportionality factor by which gravity reduces differs ever so slightly (say, by r^1.999 instead of r^2) over vast regions from the force/distance relationship we've been able to discern in our local region of space?

Chris

Yeah, theories like that have been put forth, but haven't gained much traction. For one, experiments put the term very very very close to exactly 1/r^2. Also, I think some dynamics would be changed.... for one thing, I believe in a lot of theories like that, there can't be stable elliptical orbits (like there are in the solar system).

Of course, over very large scales (or around very massive bodies), general relativity comes into play and 1/r^2 is no longer quite accurate.

 

[darkmatter4brains]

Yes, you're right, it's c^2. I'm used to working in so-called "natural units" where c is equal to 1, so sometimes I get the factors confused It's easy to work out the right one with some dimensional analysis though, as you did.

ok, thanks! yep, I remember natural units, they were a pain in the ....

 

[pjay]

can this sphere of unseen matter actually be a multiude of black holes distributed evenly throughout the galaxy. Has this been ruled out?

Good question, but yes, this can be ruled out. There is a lot more to dark matter than just the rotational speed of galaxies. One important source of information is the cosmic microwave background. This is the radiation remnant from the big bang that can still be observed today. The CMB is extremely evenly spread accross the whole sky, but there are some extremely small variations in temperature which are the seeds for the earliest concentrations of matter in the uniform radiation background. Since the CMB is about the oldest form of electromagnetic radiation in the universe, these variations date back to a time before the universe became transparent for radiation. Something must have interacted with matter that did NOT interact with radiation because otherwise the immense radiation pressure would have spread matter more evenly. This something cannot be normal baryionic matter, it cannot be blackhole matter, since baryionic matter would have interacted with radiation. That's why the concept of dark matter is pretty much unavoidable.

 

[csmyth3025]

by pjay » Thu Sep 17, 2009 4:11 pm

That's why the concept of dark matter is pretty much unavoidable.

It's probably best to keep an open mind about this question. Remember - a hundred years ago the concept of luminiferous aether was thought to be pretty much unavoidable by quite a few respectable scientists.

 

[pjay]

Sure, let's be careful. We don't know what dark matter is, although there are some good candidate theories. Anyway it's not black holes, THAT concept is, "falsified". Anyway any theory that tries to explain the missing gravity phenomenon and rotational speed complexities also needs to account for the anisotropies of the CMB. Any idea how to do that?

 

[ramparts]

Sure, let's be careful. We don't know what dark matter is, although there are some good candidate theories. Anyway it's not black holes, THAT concept is, "falsified". Anyway any theory that tries to explain the missing gravity phenomenon and rotational speed complexities also needs to account for the anisotropies of the CMB. Any idea how to do that?

Eh, it's not impossible. I'm reading a paper on TeVeS right now, incidentally, which mentions that things the vector field in that theory does can reproduce the observed power spectrum without cold dark matter. But don't ask me how, because I don't know much about TeVeS, lol, this is the first paper I've really read on it.

So it's not impossible. Just highly improbable :lol:

 

[csmyth3025]

Sure, let's be careful. We don't know what dark matter is, although there are some good candidate theories. Anyway it's not black holes, THAT concept is, "falsified". Anyway any theory that tries to explain the missing gravity phenomenon and rotational speed complexities also needs to account for the anisotropies of the CMB. Any idea how to do that?

I'm completely clueless about how to account for "the missing gravity phenomenon and rotational speed complexities".

I take it that your reference that "it's not black holes, THAT concept is falsified" is in the context of explaining the aforementioned missing gravity phenomenon. The concept of black holes, per se, has most definitely not been falsified.

Chris

 

[ramparts]

Indeed - black holes have most certainly not been falsified on their own, just black holes as the "missing matter"

 

[FlatEarth]

Indeed - black holes have most certainly not been falsified on their own, just black holes as the "missing matter"

Much of the missing matter is most definitely hidden in black holes or is dark in the sense that it has eluded detection, but is still baryonic matter. I questioned earlier if there was conclusive evidence to rule out that all or most dark matter was in the form of black holes, and the consensus opinion is that there must be additional non-baryonic matter to explain observations. I do accept that. Certainly much of dark matter is in the form of MACHOs.

 

[csmyth3025]

Indeed - black holes have most certainly not been falsified on their own, just black holes as the "missing matter"

Much of the missing matter is most definitely hidden in black holes or is dark in the sense that it has eluded detection, but is still baryonic matter. I questioned earlier if there was conclusive evidence to rule out that all or most dark matter was in the form of black holes, and the consensus opinion is that there must be additional non-baryonic matter to explain observations. I do accept that. Certainly much of dark matter is in the form of MACHOs.

Are you just voicing your opinion, or are there observations (or at least a logical argument) supporting your statement that: "...Much of the missing matter is most definitely hidden in black holes or is dark in the sense that it has eluded detection, but is still baryonic matter..."?

Chris

 

[MeteorWayne]

Indeed, the search for MACHOS's has turned up nowhere near the required mass to explain the irrefutable observations.

 

[FlatEarth]

Indeed, the search for MACHOS's has turned up nowhere near the required mass to explain the irrefutable observations.

I am not suggesting dark matter is comprised of MACHOs exclusively, but that it is partially made up of black holes, brown dwarfs, ect. Certainly dark matter is not completely made of WIMPs as is being suggested here.

 

[MeteorWayne]

Who suggested it is made up exclusively of WIMPS?