# Do black holes interact with dark matter/energy?

1. Mar 6, 2007

### thechosenone

the title pretty much says it all. i am just wondering if dark matter (which doesn't interact with light i believe) gets eaten by black holes, or is unaffected by them.

2. Mar 6, 2007

### mathman

Dark matter is known by gravitational interaction. Presumably it would interact with black holes like ordinary matter.

Dark energy is different - it acts like negative gravity - I won't try to guess what happens with black holes.

3. Mar 6, 2007

### thechosenone

hold up, what do you mean by dark energy acts like negative gravity? like dark energy is repelled by large masses?

4. Mar 6, 2007

### marcus

you will probably get confused if you begin thinking about DE and "negative gravity" because that means trying to understand things in the wrong order.

if you understand the basics first then the negative effect will ultimately be easy to understand

the first thing is that there are lots of different versions of DE that people have thought up and the simplest one (which most cosmologists assume is right) is the DE corresponding to Einstein's cosmological constant Lambda

so that is the one to focus on, and it is pretty simple.

Lambda corresponds to a CONSTANT ENERGY DENSITY unchanging throughout all space and time of about 0.6 joules per cubic kilometer.

That is all, IT DOESNT FLOW around under influence of gravity. It cant heap up into blobs or get sucked into things or swirl around. It is just this perfectly even energy density of 0.6 joule per cubic km everywhere and at all times.

So first off you can see that DE doesnt feed black holes to any significant extent

a kilogram of matter is worth 9 x 1016 joules.

To encompass that much energy in the form of DE you'd have to surround a volume on the order of 1017 cubic kilometers

Dark energy is just to sparse and dilute to be of interest. The reason is that it doesnt flow so it can't collect in concentrated blobs. It is tied to the space it belongs to.
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I agree with the other poster, mathman, that DM, if it exists, probably feeds BH, because DM flows and coagulates in blobs under the action of gravity. So it can get sucked into holes.

but I think you can safely forget about DE feeding holes.

5. Mar 6, 2007

### marcus

no.

DE cannot flow. It is not attracted or repelled by things.

to understand the "negative gravity" effect you have to understand ordinary gravity is caused by both energy density and PRESSURE

mentally convert all mass into energy and think of the sun as a concentration of energy

this concentration of energy attracts the earth and we orbit around it.
but the positive PRESSURE inside the core of the sun also contributes to the attraction.

positive pressure is something that if you have it in a cylinder you have to DO work to compress, but you can GET work if you let expand.

negative pressure is the opposite kind of thing, if you have it in an ideal abstract cylinder and you want to pull the piston out it takes work.

We have no everyday examples of something with negative pressure.
Dark Energy (so far an imaginary hypothetical thing) has negative pressure.

If you could put enough negative pressure in the core of the sun to cancel the positive pressure and the energy density effect----then it wouldnt attract any more.

That is why the expansion of the universe is supposed to not be slowing down, because of negative pressure of the DE.

Simply as energy, DE has the usual attractive gravity effect that other energy does! The kicker is the pressure.

So you read cosmology papers these days and you will see they are always talking about "w". This is the ratio of DE pressure to energy density.

p = w rho. In the standard Lambda CDM model, where DE corresponds to Lambda, it is assumed that W EQUALS MINUS ONE. w = -1

in other words p = - rho
PRESSURE EQUALS MINUS ONE TIME ENERGY DENSITY.

If this ratio, this "w", also called "the DE equation of state" is not exactly minus one then you don't have an Einsteinian cosmological constant, you have some other weird mess. the words "quintessence" and "big rip" come to mind. But over the past few years they have been measuring "w" and narrowing it down closer and closer to -1, so not to worry.

Either DE doesnt exist at all, and there is some other clever explanation for the data. Or else DE exists and very probably then w = -1 and it is just the Lambda that Einstein originally put in the equation. And it has this peculiar negative pressure, which is hard to swallow but heck it could be worse.

6. Oct 18, 2007

### keinve

if it exists, black holes affect it.

7. Oct 19, 2007

### Chronos

I view it highly likely that galactic core SMB's originated almost entirely from dark matter.

8. Oct 20, 2007

### Wallace

I'm not so sure about that. The thing about DM is that it (if it exists) is thought to be essentially collisionless, so that not only would you not notice if DM passed through you (which it presumably does all the time) but that two lumps of DM would notice passing through each other. This is in contrast to normal matter which obviously collides with itself at the molecular level.

In the context of the formation of structure this is very important. Due to the collision between gas particles, a collapsing gas cloud is able to turn the kinetic energy of the particles in the cloud into heat via collisions, which is then able to be radiated away (i.e. the collisions excite electrons to higher energy levels which relax to the ground state emitting photons).

This removal of kinetic energy allows gas clouds to 'condense' into compact dense objects much more easily. This is why the typical model of a galaxy is a large smooth close to spherical halo of DM with a highly concentrated central region containing most of the gas and stars. Even though the ratio of DM to 'normal' matter in the universe is though to be ~ 10:1, there is much more 'normal' matter than DM in the bulk of an actual galaxy. DM and the whole rotation curve issue kicks in as you get towards the edge of galaxies.

With all this is mind it would seem that extremely dense collapsed objects, black holes, are therefore likely to have accreted far more normal matter than DM.

I've not read any detailed technical discussion of this issue though, so I could be very wrong but based on my understanding of the formation of structure ( be that as it may) I would suspect that normal rather than dark matter is the delicacy of choice for most black holes.

9. Oct 20, 2007

### Chronos

The gravitational wells necessary to coalesce galaxies in the early universe are not sufficiently explained without a substantial dark matter component, IMO. I do, however, defer to your judgement. My understanding of the early universe is profoundly incomplete. My thinking is DM was so prevalent in the early universe that it was hugely important in structure formation. Perhaps the early universe was almost entirely composed of DM that decayed into 'normal' matter.

Last edited: Oct 20, 2007
10. Oct 20, 2007

### Wallace

According to current understanding the ratio of dark to ordinary matter is the same for the entire history of the universe, back until the very first moments where we really don't have a clue anyway. Of course we really have no clue what DM is anyway so that could be wrong.

As you say, the gravitational wells that formed the first structures (as well as later structures) are caused predominantly by DM. The point is that the ordinary matter tends to condense to the very centre of these DM created potential wells and hence in the regions where the black holes themselves would form, there is more regular than dark matter. But it's the DM that drives the creation of the potential well in the first place.

11. Oct 21, 2007

### Garth

It would be wrong, for example, if DE is nothing more than the cosmological constant ($\omega$ = -1).

In that case DE would grow with the volume of the universe whereas the total mass of ordinary matter/energy in a representative volume would remain nearly constant.

Garth

12. Oct 21, 2007

### Wallace

You're confusing your darks Garth! We were discussing dark vs baryonic matter not dark energy. Dark and baryonic matter densities both drop with the inverse of the volume of the universe, hence their ratio's remain the same.

13. Oct 21, 2007

### Garth

Point taken! The general thread was discussing DM and DE, and as you said, I got confused as to which one we were talking about here.

Garth

Last edited: Oct 21, 2007