# Black Holes vs. Dark Matter

1. Dec 17, 2009

### mintparasol

Hi all,
Apologies if my understanding is a bit simplistic but even if my question turns out to be idiotic, I'm sure I'll learn from your replies. Thanks in advance!

So the reason dark matter was postulated in the first place is because the observed gravitational effects in the known universe far exceed the observable matter in the universe, right?
But we're pretty sure there's a supermassive black hole at the centre of our galaxy and of most other galaxies and that our galaxy probably contains a large amount of smaller black holes, right?
So we've never directly observed a black hole. Is it not then possible that this 'extra' matter that we can't see exists in the form of black holes and not dark matter?

2. Dec 17, 2009

### nicksauce

The short answer is that it is very unlikely that black holes constitute a significant portion of dark matter. Observations of Baryon Acoustic Oscillations and the observed chemical abundances show that there needs to be a large amount of non-baryonic matter in the universe.

3. Dec 17, 2009

### DaveC426913

No, it is because the observed gravitational effects within the discs of galaxies is not a simple curve as it should be. There must be some extra mass that is unaccounted for distributed throughout the galaxy disc.

But that doesn't refute your idea, so:

The thing about blackholes is that they are very visible. They are surrounded by an accretion disc of infalling matter that radiates characteristically in the X-ray band. We don't see this, thus no unaccounted-for BHs.

4. Dec 17, 2009

### mintparasol

Hmm, I wasn't saying they were not accounted for, just that maybe there's more mass within them than they're given credit for....
And, no, actually, there could be countless black holes unaccounted for. Can you show me a picture of an accretion disc?

5. Dec 17, 2009

### mintparasol

But how can we talk about chemical abundances in relation to black holes? Surely matter as we know it ceases to exist within the event horizon of a black hole?

6. Dec 17, 2009

### DaveC426913

Not so much pictures of accretion dics, but observational evidence of black holes:
http://apod.nasa.gov/apod/ap951230.html
http://apod.nasa.gov/apod/ap970516.html
http://apod.nasa.gov/apod/ap050128.html
My point was not simply that accretion discs alone are the signature of BHs, but that BHs - unlike what most laypeople seem to assume - are not undetectable. They are quite detectable.

7. Dec 17, 2009

### nicksauce

The abundances of various light elements depend strongly on $\Omega_B$. See the first figure here http://www.talkorigins.org/faqs/astronomy/bigbang.html#lightelements. If dark matter actually was baryonic (i.e., black holes) and $\Omega_B$ was 10 or so times bigger, then we would expect to see about 10 times more Li-7 than we do, and around 10 times less Deuterium than we do (as you can read off of that graph). Unless by some unknown process Li-7 preferentially falls into black holes compared to other elements, and by some other unknown process deuterium is created in large amounts, the situation is impossible.

Last edited: Dec 17, 2009
8. Dec 18, 2009

### Garth

You may find the following eprint paper interesting Nucleosynthesis in a Simmering Universe , which was published in a peer reviewed journal as NUCLEOSYNTHESIS IN A UNIVERSE WITH A LINEARLY EVOLVING SCALE FACTOR.

The Coasting Cosmology model, (R(t) = t) does not have a horizon or flatness problem, (so the model does not require Inflation), in addition the baryon density is much higher than in the $\Lambda$CDM model, and so could accommodate all Dark Matter.

This DM might be in the form of quiescent Black Holes, IMBHs perhaps as the remnant of an epoch of Pop III stars, or including much smaller and up to intermediate mass Primordial Black Holes.

In order to obtain a linear expansion factor the DE would have to have an equation of state $\omega = -\frac{1}{3}$, dominated by a form of DE called 'K Matter' by Kolb in A coasting cosmology , as would be delivered by cosmic string networks.

Garth

Last edited: Dec 18, 2009
9. Dec 18, 2009

### mdnazmulh

No, it’s not possible because scientists confirmed the existence and positional mapping of dark matter by a method called gravitational lensing. They send light rays in the space to locate dark matter. When light rays pass through a glass, they get bent. Similarly dark matter bends the light rays. Even though we can’t see dark matter, the bending and distortion of light rays tell us there is ‘some thing’ which is causing it. This ‘some thing’ is dark matter. A black hole can’t be dark matter for if we send light rays to a black hole, those light rays can never escape its gravitation. Like black holes, dark matter is not an eater of light. It only bends light.

10. Dec 18, 2009

### Garth

mdnazmulh BHs do bend light rays and would cause gravitational lensing.

See Stephen Brooks Website for simulated illustrations.

Garth

Last edited: Dec 18, 2009
11. Dec 18, 2009

### twofish-quant

Last edited by a moderator: Dec 18, 2009
12. Dec 18, 2009

### Garth

Some might think the standard $\Lambda$CDM model looks a bit odd with its requirement of Inflation, DM and DE, all of which are undiscovered (so far) in laboratory experiments!

There may be a lot of problems with the coasting cosmology model, however you have to remember it has a lot more baryonic mass, and if the DM is dark baryonic material, say in the form of BHs, then a cloud of BHs would form galactic haloes and attract the visible matter as in the standard model. ( Non-interacting, etc.)

Garth

Last edited: Dec 18, 2009
13. Dec 20, 2009

### mintparasol

Won't light be lensed by a black hole in much the same way? Light passing near the event horizon but not beyond it should lens in a very similar way, no?

14. Dec 20, 2009

### Orion1

Universe black hole density...

The Universe black_hole density should be considered 'baryonic' because of theoretical Hawking radiation.

Universe black_hole density with respect to Milky Way galaxy:
$$\Omega_{bh} = \left( \frac{M_{bh}}{M_{MW}} \right) \Omega_b = 3.38 \cdot 10^{-7}$$

$$\boxed{\Omega_{bh} = 3.38 \cdot 10^{-7}}$$

$$M_{bh}$$ - Milky Way galactic nucleus mass
$$M_{MW}$$ - Milky Way galaxy mass
$$\Omega_b$$ - Universe baryon density

Reference:
http://en.wikipedia.org/wiki/Black_hole#Galactic_nuclei"
http://en.wikipedia.org/wiki/Milky_Way" [Broken]
http://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters"

Last edited by a moderator: May 4, 2017
15. Dec 20, 2009

### DaveC426913

That is correct. The only thing required for lensing is mass.

16. Dec 20, 2009

### twofish-quant

Yes, but the oddness has been quantified, and after taking some sledgehammers to the theory, you end up with something that reproduces observations. I rather seriously doubt that if you take the coasting model as is, that you are going to be able to reproduce the galactic power spectrum or the Hubble measures. You may be able to get somewhere by taking a sledgehammer to the theory, but if you end up with more unknown entities it doesn't help.

Which is the the problem. If you have baryonic matter it interacts more strongly with visible matter so you end up with a power spectrum that is much less "fluffy" than with cold dark matter. Also a lot of the power spectrum calculations model things as sound waves through dark matter. If you have dark matter in black holes, things will look different....

Also there is the matter of converting the baryonic matter to black holes. If you do with the standard supernova formation, then you already have to have the galaxies form because you end up with black holes, and you should also be able to see massive amounts of star formation. If you do it through some non-standard black hole mechanism, it would help a lot of people mentioned what that mechanism might be.

One thing about these sort of objections is that they could be overcome. If you do a simple coasting universe model, and it happens to replicate one aspect of the standard model, that's a paper. The trouble is that if you do a coasting universe model, and then work through the numbers and it doesn't work out, that's not publishable.

17. Dec 20, 2009

### Garth

Yes twofish-quant, I agree there are a lot of problems with the freely coasting model although there are a few people (mainly in India) working on it who think they may be overcome such as: A Concordant “Freely Coasting” Cosmology.

It has a number of positive features too, such as: no need for Inflation, a ready explanation why the age of the universe happens to be exactly Hubble time and a solution to a possible age problem in the early universe.

The important thing (IMHO) for cosmology is to have alternative models against which the standard model may be compared and tested.

Garth

Last edited: Dec 21, 2009
18. Dec 21, 2009

### mintparasol

So why dark matter then if we're saying that lensing leads us to postulate dark matter in the first place? Black holes are the most massive objects in the known universe. Why postulate dark matter if we already have the culprit nabbed?

19. Dec 21, 2009

### twofish-quant

They aren't. If you take a 2 solar mass star and then turn it into a black hole. It's still 2 solar masses. If the black hole was formed by collapse of massive amounts of normal matter than you have a problem since it seems that dark matter isn't "normal matter" and you have to assume a massive amount of star formation in the early universe. If black holes aren't "normal matter" then you have an even bigger mystery of explaining how those black holes formed.

The other problem is that the black holes are in the wrong place. It appears that most galaxies have a massive supergiant black hole in the middle of them, but the galaxy rotation curves say that there is missing mass in the edges. Since we haven't seem supermassive black holes anywhere other than at the center of galaxies, we have a mystery.

20. Dec 21, 2009

### Garth

The mass in black holes formed from collapsed stars originally had to be baryonic (hydrogen/helium etc.) in nature.

The baryonic density limit of about 4% total (closure) density in standard BBN is not enough to create sufficient BH's in the standard model. The DM in that model (another 23% closure density) therefore has to consist of more exotic, non-baryonic massive particles, which are as yet undiscovered.

Given the undiscovered nature of 96% of the uiverse in that standard model, I like to keep an open mind on other possibilites, such as is provided by the lineraly expanding - coasting cosmology model.

Garth

Last edited: Dec 21, 2009
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