Ask Ethan #100: Why doesn’t dark matter form black holes? (Synopsis)

“All enterprises that are entered into with indiscreet zeal may be pursued with great vigor at first, but are sure to collapse in the end.” -Tacitus

If you want to form a structure like a planet, star or black hole, you need a large amount of mass together in one place. The way to bring that mass together, of course, is through the force of gravity, which attracts everything with mass towards one another in this Universe.

Image credit: ESO/VPHAS+ team, via Image credit: ESO/VPHAS+ team, via

So why, then, would the greatest source of mass in the Universe -- dark matter -- be entirely unable to form these structures at all? It has mass, it experiences gravity, and yet there's no such thing as a dark matter planet, star, or black hole.

Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC). Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).

As it turns out, gravity and mass, by themselves, aren't enough to do the job. Find out why not on this special, 100th edition of Ask Ethan!

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What I don't understand is what sort of distribution of diffuse dark matter does it take for each next further away star from the core to speed up just enough in spirals that it's as if the arms were solid. What is the formula for that?

By Chris Mannering (not verified) on 07 Aug 2015 #permalink

It is on the previous dark matter story on

Go read that.

I assume that a black hole is still able to capture dark matter, even if one won't form directly from dark matter.

Well, dark matter could still have a trajectory that ends up inside the event horizon, so by that meaning of "capture", yes.

Of course, "orbits" is also "capture", so by that, yes indeed.

It would be very hard to get dark matter to fall in to the center of the black hole because it would have to interact and radiate. Which they are famously not able to do.

Remember, though, that the event horizon is VERY VERY small. It's why black holes have accretion disks: it gives something for the normal matter to bang against and radiate their energy away otherwise they'd never enter the event horizon at all.

It can take a long time to get from the outer edge of the disk to the centre.

Ethan, excellent discussion, but your example of the path followed by a dark matter "football" is not correct. The path you show is the one that would be followed if all the mass of the earth were concentrated at the center. However, as the ball moves toward the center, the gravitational force of the spherical shell beyond the position of the ball is zero, resulting in a more complex orbit that passes through the earth and emerges at the surface somewhere in the opposite hemisphere and them falls back toward the center repeatedly.

By Philip Harrison (not verified) on 07 Aug 2015 #permalink

Ethan, as a particle physicist I need to take exception to one of your introductory comments. You wrote, "Dark matter sure does have gravity, and it sure doesn’t form black holes, dark matter stars, planets, or dark atoms."

Actually, we _don't_ know this! In fact, some would argue that this assumption, in comparison to the part of the universe we do know about, is a horrible oversimplification. Why shouldn't the various dark matter particles (why should there be only one?) have a whole suite of mutual interactions building up DM atoms, molecules, and larger structures?

For the other readers of this blog (I know that Ethan already knows all this!) the appropriate search term is "dark sector."

I can think of just two pieces of negative evidence for highly structured dark matter. First, the failure of the MACHO searches implies that there isn't a large population of DM stellar-mass objects. Second, the existence of galactic and cluster-sized DM haloes implies that DM can't have "strong" (in the colloquial sense) mutual interactions, or dissipation would have collapsed those haloes on cosmological timescales. But I'm not convinced that those arguments are really compelling enough to exclude the possibility of a dark sector.

By Michael Kelsey (not verified) on 07 Aug 2015 #permalink

And my second particle-physics comment. I'm a collaborator on the SuperCDMS experiment, one of the sources for the WIMP detection-limit plot you show at the end of your piece. That plot says *nothing* about DM self-interactions. The cross-section plotted on the Y-axis is for the (spin-independent) interaction between DM WIMPs and nulceons, which is what is needed for a direct detection.

So what we know is that DM does not interaction with normal matter very strongly. But the plot you show doesn't say anything about DM-DM interactions.

By Michael Kelsey (not verified) on 07 Aug 2015 #permalink

"You wrote, “Dark matter sure does have gravity, and it sure doesn’t form black holes, dark matter stars, planets, or dark atoms.”

Actually, we _don’t_ know this"

We do know the first one.

We have evidence in their raison d'etre to believe the others hold true.

You can work out the collapse time of a dark matter cloud of "dark hydrogen" and show that it is going to take a long time to form a gas cloud.

But as it does so, it can only radiate gravity waves. And they're not as effective as electromagnetic.

To get it to form, we'd have to know a hell of a lot more about dark matter than we do, and it would have to lead to an ability to collapse and radiate the energy away without us seeing it. And that latter bit is an EXTREMELY IMPORTANT part.

So though we don't *know* know, we know as well or better than we do that there's no life on Mars.

But a little bit less than we know there's a human artifact on the Moon.

Phillip, your corrections also won't account for a change in shape due to the release of gravity waves. Nor the gravity of other planets, the rest of the Milky Way, and the effect of Andromeda getting closer.

So in actual fact you are wrong too. And for many more reasons than you gave Ethan for being wrong.

Therefore you are even more incorrect.

"It is on the previous dark matter story on

Go read that"

No it isn't

By Chris Mannering (not verified) on 07 Aug 2015 #permalink

The curently leading cosmological model is lambda-CDM, where CDM means cold dark matter and cold means thermalized. To get thermalized dark matter, the dark mater must interact with itself. So the lambda-CDM model must imply some lower estimate for dark matter self-interaction cross section. Is it compatible with the claim that dark matter interacts only gravitationly?

Dark Matter also gives us a maximum estimate for dark matter self-interaction.

Oh, where is the claim it's only gravitationally? Neutrinos affect other things by gravity, but also the weak interaction. And the interactions are so small that it can pass through light years of lead before being detected by interacting.

Dark Matter interacts with normal matter almost nonexistently, POSSIBLY not at all. But not definitely not not at all.

To Wow:
Quotation from the article:
"But dark matter gets it the best in terms of freedom: as far as we can tell, it only interacts through gravity. There are no collisions at all, and so all dark matter can do is be attracted to the other sources of matter."

as far as we can tell</u

Notice that?

Is this "as far as we can tell" compatible with lambda-CDM model? This is my question.

Yes. If it weren't, then we would have evidence against CDM and we wouldn't be using it.

Do you think that the professionals who study this for a living missed out on something you found so easily and are therefore morons? That is my question.

Of course, no. But when I asked this question on other science blogs, the only answer were "Good question".

Maybe they weren't being honest with you.

Give you a pat on the head and forget it and you walk away happy and they don't have to bother with you any more.

Or the site you were visiting wasn't really very scientific and was rather pushing a "The Standard Science Is Run By A Cabal!" line.

Who knows?

Who cares? There's no value to knowing you got "good question" on some unnamed and unknown blog. What is supposed to be done with it?

@Pavel #12: You don't thermalization or self-interactions in DM to get cold dark matter.

DM can be "cold" from its creation in pair production, if the particles are massive enough. That is, you can have the DM particles produced from energy (photons) just a little bit above their mass, which would mean they appear with very low velocities.

This is different from the kind of particle production we see in colliders, where the energy is usually way above the particle masses, so they come out close to 'c'.

The high mass is why one of the leading DM candidates is the so-called "WIMP", for "weakly interacting massive particle."

You can contrast this with neutrinos. Because their mass is so close to zero (sub-eV), at their freeze-out time the temperature of the universe would have been ~MeV or higher. That would make the neutrinos highly relativistic, and with their own very small interaction cross-section, they'd remain so, and would not contribute properly to structure formation.

By Michael Kelsey (not verified) on 08 Aug 2015 #permalink

@Wow #8: In your opening, you are quite right that my comment to Ethan was unclear and misleading. We most certainly, as you say, do know that DM interacts primarily gravitationally. We also know, quite well, that DM barely, if at all, interacts with normal matter non-gravitationally (this is what my experiment, CDMS, is looking for!).

What we _don't_ know nearly as well, is whether DM has self-interactions which would form complex composite states. There are several quite good theoretical models for such a "dark sector," which should not be dismissed unnecessarily.

It was really only the latter that I was addressing, but my comment, as you noted, made it look like I was denying the primacy of the gravitational interaction. Thank you for the clarification.

By Michael Kelsey (not verified) on 08 Aug 2015 #permalink

"What we _don’t_ know nearly as well, is whether DM has self-interactions which would form complex composite states."

True, but you missed out that they have to lose energy to bind like that and that mechanism has to release energy in another form.

We don't know that many, so your state would require that we have some mechanism here for releasing that either at a rate so slow that they haven't formed any significant gathering, or in another form we haven't yet detected.

Positing a new energy type merely to get something to happen that doesn't need to (self-assembly) is unwarranted multiplication of entities.

We do also see that they haven't self-assembled as much as regular matter, otherwise their primary characteristic we invented the product for would be gone: its diffuse nature well beyond the visible galaxies that maintains the rotation speed of orbit.

I would propose finding methods of working out what dark matter is other than "Dark" and "Matter". At that point it is productive to ask "Would this stuff self assemble, and how would it do that?".

If only because it would tell us if our proposition for dark matter was right.

As several commentors are alluding to, a mass of gravitationally bound particles has to be able to lose the kientic energy of its random motions in order to contract in size. Normal matter accomplishes this primarily via thermal radiation. DM particles do not have an effective means for doing so, so their distribution remains diffuse at least on time scales comparable to the lifetime of the universe.

Now DM is not prohibited from falling into BHs, presumably unlucky DM particles do pass through event horizons and contribute in some very small way to the masses of BHs.

By Omega Centauri (not verified) on 09 Aug 2015 #permalink

August 8, 2015

Wow replies "Yes it is."

But you said you never go to So how you do know it is there?

You're a very naughty boy mini-me

By Chris Mannering (not verified) on 10 Aug 2015 #permalink

"But you said you never go to So how you do know it is there? "

Because it also appears on this site as a sort of "Abstract".

You know, just like this thread ATL has a subject and then links to the discussion.

Or had you not read the site you were commenting on before commenting on what you thought it might be saying?

I'm not sure what I was doing.

Could I possible trouble you for a link to that article. I'm not sure what is wrong with me but I can't find it.

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

Not to worry I think I found it

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

Next time look before asking. That would have saved, what, six posts and three days.

"There are several quite good theoretical models for such a “dark sector,” which should not be dismissed unnecessarily."

Looked again at this thread and I didn't really note this part.

No matter how unnecessary I think it is to have dark matter create composite products, there's absolutely, 100% no problem with working on a "what if..." on dark matter like this.


Because we have nearly no damn idea what the bloody stuff IS. Any idea that can turn up results that could be found or fail to be found will let us know at least SOMETHING more than we did.

A bit like the FTL neutrinos.

Even the fact that it appears to be a problem with a cable being loose helps work out what to check for when doing such time critical and statistically difficult investigating. And hopefully we will do better next time.

There are limitations on what alternatives aren't already falsified, which many other hypotheses still being punted have failed, but within that framework, not a problem.

I suppose the summation of it is that if you'd said "Well, we don't know for SURE that they don't create composites, there are several theories that are being looked at where they COULD do that" would have prompted me to interest rather than caution based counterargument.

Of course, some interest would have been blunted by the many cases of quackery being honestly held as truth on the internet, so links would have been approached with extreme caution....

Wow you totally love it and that's reason enough to go the slowboat way. I was reading your behaviour. You are continually in arguments with people, normally involving your behaviour in which you presume yourself a status of seniority and authority. You're a weird guy.

Anyway I don't care about that. I still can't find it. Was it the 7 things you need to iknow about Dark Matter? That was the last one before the current.

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

No, Christ, I don't love being surrounded by the proof of morons living around me.

It's depressing.

You, meanwhile appear to love making shit up.

That too I find depressing.

So kindly fuck off you miserable little turd.

Your knowledge is just get off on telling other people how to suck eggs. But you're absolutely average.
What the fuck are doing here all the time you little cunt? I mean are you Ethan's enemy or you are jealous of something? Because you chronically undermine this blog.

you little cunt

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

Yet here you are, and the only thing you've done is argue with others.

My knowledge beat yours, punk. "Delusions of inadequacy" would be true for you, but not in a good way. Inadequate would still be a step up.

So you're a slimball projection queen. Think all you got to do is mirror the complaint about back to the complainant? You worm, what a nasty dishonest way to live. You don't even do it well. You are all mouth. You do all this putting down and swearing online because you haven't got the bottle to do it face to face. You worm, you projection queen, you wanker.

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

Your knowledge beats mine? No it doesn't....I've seen what you've got. You just a sad fuck that hangs around someone elses blog day and night.

By Chris Mannering (not verified) on 11 Aug 2015 #permalink

@Wow (several): Glad you were able to work out what I was trying to say. You've got it quite right: the so-called "dark sector" models are akin to the various models of supersymmetry: we don't know what the stuff _really_ is, so we (the theorists, not me!) try to explore the model space comprehensively. That gives us both "predictions" of things to possibly search, as well as limits on what we could possibly discover.

By Michael Kelsey (not verified) on 11 Aug 2015 #permalink

Michael, yeah, it sounded like you were talking about dark matter analogues of stars and so forth, which is what I think Ethan was talking about with the "we know it doesn't form...".

Though thinking about it, it would be odd if it weren't for the big rip in that we'd be able to have dark stars and planets. With enough time, even losing energy by gravity waves would *eventually* allow a dark matter cloud to collapse into a gravitationally cohesive body.

Which would be weird.

Chris, you've done nothing but bitch and whine on here. Then bitch and whine because I wouldn't treat you like the special little flower you believe yourself to be.

If you didn't find what you were looking for them you must not have understood your question.

So knob off.