Did LIGO Detect Dark Matter?

It has often been said, including by me, that one of the most intriguing aspects of dark matter is that provides us with the best current evidence for physics beyond the Core Theory (general relativity plus the Standard Model of particle physics). The basis of that claim is that we have good evidence from at least two fronts — Big Bang nucleosynthesis, and perturbations in the cosmic microwave background — that the total density of matter in the universe is much greater than the density of “ordinary” matter like we find in the Standard Model.

There is one important loophole to this idea. The Core Theory includes not only the Standard Model, but also gravity. Gravitons themselves can’t be the dark matter — they’re massless particles, moving at the speed of light, while we know from its effects on galaxies that dark matter is “cold” (moving slowly compared to light). But there are massive, slowly-moving objects that are made of “pure gravity,” namely black holes. Could black holes be the dark matter?

It depends. The constraints from nucleosynthesis, for example, imply that the dark matter was not made of ordinary particles by the time the universe was a minute old. So you can’t have a universe with just regular matter and then form black-hole-dark-matter in the conventional ways (like collapsing stars) at late times. What you can do is imagine that the black holes were there from almost the start — that they’re primordial. Having primordial black holes isn’t the most natural thing in the world, but there are ways to make it happen, such as having very strong density perturbations at relatively small length scales (as opposed to the very weak density perturbations we see at universe-sized scales).

Recently, of course, black holes were in the news, when LIGO detected gravitational waves from the inspiral of  two black holes of approximately 30 solar masses each. This raises an interesting question, at least if you’re clever enough to put the pieces together: could the dark matter be made of primordial black holes of around 30 solar masses, and could two of them have come together to produce the LIGO signal? (So the question is not, “Are the black holes made of dark matter?”, it’s “Is the dark matter made of black holes?”)

LIGO black hole (artist's conception)

This idea has just been examined in a new paper by Bird et al.:

Did LIGO detect dark matter?

Simeon Bird, Ilias Cholis, Julian B. Muñoz, Yacine Ali-Haïmoud, Marc Kamionkowski, Ely D. Kovetz, Alvise Raccanelli, Adam G. Riess

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M≲Mbh≲100M where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they can radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will then rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2−53 Gpc−3 yr−1 rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.

Given this intriguing idea, there are a couple of things you can do. First, of course, you’d like to check that it’s not ruled out by some other data. This turns out to be a very interesting question, as there are good limits on what masses are allowed for primordial-black-hole dark matter, from things like gravitational microlensing and the fact that sufficiently massive objects would disrupt the orbits of wide binary stars. The authors claim (and quote papers to the effect) that 30 solar masses fits snugly inside the range of values that are not ruled out by the data.

The other thing you’d like to do is figure out how many mergers like the one LIGO saw should be expected under such a scenario. Remember, LIGO seemed to get lucky by seeing such a big beautiful event right out of the gate — the thought was that most detectable signals would be from relatively puny neutron-star/neutron-star mergers, not ones from such gloriously massive black holes.

The expected rate of such mergers, under the assumption that the dark matter is made of such big black holes, isn’t easy to estimate, but the authors do their best and come up with a figure of about 5 mergers per cubic gigaparsec per year. You can then ask what the rate should be if LIGO didn’t actually get lucky, but simply observed something that is happening all the time; the answer, remarkably, is between about 2 and 50 per cubic gigaparsec per year. The numbers kind of make sense!

The scenario would be quite remarkable and significant, if it turns out to be right. Good news: we’ve found that dark matter! Bad news: hopes would dim considerably for finding new particles at energies accessible to particle accelerators. The Core Theory would turn out to be even more triumphant than we had believed.

Happily, there are ways to test the idea. If events like the ones LIGO saw came from dark-matter black holes, there would be no reason for them to be closely associated with stars. They would be distributed through space like dark matter is rather than like ordinary matter is, and we wouldn’t expect to see many visible electromagnetic counterpart events (as we might if the black holes were surrounded by gas and dust).

We shall see. It’s a popular truism, especially among gravitational-wave enthusiasts, that every time we look at the universe in a new kind of way we end up seeing something we hadn’t anticipated. If the LIGO black holes are the dark matter of the universe, that would be an understatement indeed.

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62 Responses to Did LIGO Detect Dark Matter?

  1. Bob F. says:

    When we see dark matter maps like this one, from Hubble:
    http://www.nasa.gov/mission_pages/hubble/science/dark-matter-map-gallery.html

    ….does that look like the way black holes would behave? The cloud maps look more amorphous than what I’d imagine black holes would look like, with matter getting sucked in.

    [The typos above are not mine. This site doesn’t like the letter that comes between h and j in the alphabet.]

  2. Ken K says:

    Cool. But wouldn’t a “dark black hole” sweep up surrounding matter, be drawn into galaxies, and otherwise do all the things ordinary matter does that dark matter doesn’t? Or is it that the dark black holes are only a small fraction of the total dark matter and thereby go unnoticed ( until LIGO)?

  3. Sean Carroll says:

    The gravity around black holes is only really strong once you get right up next to them. At any fixed distance, the gravitational pull of a 30-solar-mass black hole is exactly the same as that of a star of 30 solar masses. Namely, pretty insignificant on galactic scales. Black holes wouldn’t sweep up an appreciable amount of matter.

  4. Sean Carroll says:

    For a while there a plugin was deleting all appearances of the letter “i” in comments; should be fixed now.

  5. Ray Gedaly says:

    Hi Sean. I count five phenomena acting against gravity at various scale sizes to prevent ordinary matter from more quickly collapsing to form black holes: (1) the (Big Bang) primordial expansion of the universe, (2) the additional expansion of space from (the repulsive nature of) dark energy, (3) angular momentum of rotating and orbiting bodies, (4) the electromagnetic force on the atomic scale (and the outward pressure of thermal radiation at larger scales), and (5) the strong nuclear force on the nuclear scale. But dark matter doesn’t feel the electromagnetic or strong forces, so wouldn’t it be a preferential source material for black holes forming in the later universe?

  6. David Graff says:

    Hi Sean,

    There are limits on primordial black holes over a huge range of masses covering 14 orders of magnitude from dynamical, gravitational microlensing and wide binary arguments. See http://arxiv.org/abs/astro-ph/0307437

    In short, if the Milky Way halo were full of black holes of mass 10^-7 — 10^1 msun, it would have been seen in gravitational microlensing experiments. If the black holes had mass > 10^7 sun, they would have heated the disk too much. That is, they would have perturbed the orbits of disk stars to the point that they would move out of the plane of the galaxy, making the disk thicker than it is and with a higher velocity dispersion. If they were in the range 10^2 — 10^7 msun, they would knock apart wide binary stars as they passed between them.

    There may be a very narrow window of possible masses, roughly 10 – 100 msun, which does include the LIGO masses.

  7. MPS says:

    It’s been a long time but i thought the CMB said no to this.

  8. John Barrett says:

    I don’t think it would be possible for just 2 black holes to account for all the dark matter in the universe, which would be an overstatement. It could perhaps account for the dark matter in one region of space. I was under the impression that the suppermassive black hole in the center of the Milky Way was accurately predicted to be the number of solar masses it was found to have. I assume these black holes are not in the Milky Way, so it wouldn’t account for the orbits of the stars in our own galaxy. Then they still say that dark matter would have to be able to hold our galaxy together so tightly… If dark matter is just black holes, it would seem like there would have to be more black holes than we think, and there would have to be another black hole in our galaxy. Then I guess we just “discovered” that there is a black hole in the center of almost every galaxy, which has been considered to be a strong theoretical possibility for a very long time. Although, it makes me wonder if predictions of dark matter really considered black holes in the center of galaxies at all.

  9. Mahdi says:

    But if PBB constitute majority of DM, how should we explain observation of merging galaxies and the lag caused by their collision ? how about DAMA’s signal annual modulation ? How about our dream of DM being BSM particle? they r all gone for the sake of tiny BB. not a fair trade!

  10. Finn Dublin says:

    John, your comment reminds me of some AI projects I had that attempted to grasp the meaning of unstructured text. You failed to generalize the idea. These 2 black holes are only the tip of the iceberg. If the idea is true, then there will be billions, zillions of these primordial black holes around the universe, making up the missing matter, but we only see a few of them at any year with LIGO when 2 of them collide.

  11. Bee says:

    I also thought the CMB said no to this. See fig 9, left http://arxiv.org/abs/0709.0524

  12. “Bad news: hopes would dim considerably for finding new particles at energies accessible to particle accelerators. “

    Why is that bad news?

  13. As far as I know, LIGO can’t distinguish between dark-matter black holes and other types of black holes. Indeed, according to the no-hair theorem, nothing can.

    Primordial black holes have not been discussed as dark-matter candidates very much since the microlensing results ruled out a large part of the parameter space. So, has the community overlooked part of parameter space which has been missed, or did someone just want to get “LIGO” and “dark matter” into the same paper?
    There is also the danger that someone ruled out this mass range years ago, but people have forgotten the paper. Massive black holes will distort the structure of radio-source jets in VLBI. Does the absence of observations here rule out this mass range?

  14. “I count five phenomena acting against gravity at various scale sizes to prevent ordinary matter from more quickly collapsing to form black holes: (1) the (Big Bang) primordial expansion of the universe, “

    Totally irrelevant on the scales in question. Even Woody Allen is now convinced that Brooklyn is not expanding.

    “(2) the additional expansion of space from (the repulsive nature of) dark energy, “

    Ditto.

    “(3) angular momentum of rotating and orbiting bodies, “

    It can’t prevent collapse, only increase the threshold.

    “(4) the electromagnetic force on the atomic scale (and the outward pressure of thermal radiation at larger scales), and”

    Once a certain density is reached, this pressure is overwhelmed by gravitation.

    “(5) the strong nuclear force on the nuclear scale.”

    Ditto.

  15. marten says:

    As I understand it the quantities of mass and energy involved did not give an unexpected result, so why would any dark matter be involved?

  16. Josh says:

    If dark matter is the bulk of the matter in the universe and also attracts to regular matter via gravity, shouldn’t we intuitively expect (by mere statistical probability), that most black holes would be made up considerably of dark matter?

  17. “If dark matter is the bulk of the matter in the universe and also attracts to regular matter via gravity, shouldn’t we intuitively expect (by mere statistical probability), that most black holes would be made up considerably of dark matter?”

    No. Black holes don’t go around sucking up stuff. In most cases, all that is in them is what they formed from. Since baryons can lose energy via radiation, they can form objects such as stars which can later become black holes. Dark matter probably can’t. (We don’t know for sure, of course.)

  18. “As I understand it the quantities of mass and energy involved did not give an unexpected result, so why would any dark matter be involved?”

    You are a victim of the hype. Due to the no-hair theorem, nothing we can observe about a black hole can tell us what went into it. So, at best, any hint of black holes which formed from dark matter would be indirect.

  19. marten says:

    Philip,

    ?

    I am only expressing my doubt that LIGO’s signal would have anything to do with dark matter. To give an example of observations with unexpected results that are explained by the gravitational inflence of dark matter: higher than expected orbital speeds around spiral nebulae.

  20. Joel says:

    Hm. So most of the mass that came out of the Big Bang was in black holes, of a wide variety of sizes. The biggest ones attracted lots of other mass around them and eventually developed into galaxies. Smaller ones drifted off on their own and are still out there: dark matter.

    To this layman it all sounds totally plausible. So plausible, in fact, that I’ve got to think there’s something missing. Why didn’t anyone think of this before?

  21. John Barrett says:

    Finn, black holes inside of our own galaxy shouldn’t be very difficult to find. There have already been a lot of theories out there that predicted a suppermassive black hole in the center of the galaxy before it was ever found. These predictions were made, most of the time, to account for the amount of dark matter in our own galaxy. So, where is the recognition that someone should have received for predicting the correct mass of the suppermassive black hole in our galaxy? If that was the only thing that was throwing General Relativity off, then those predictions should have been correct. But, the only thing that seems to have happened is that they spotted one, and the old theories that predicted it went, apparently, mostly ignored… It makes me wonder if they were wrong, and something else is needed to account for the amount of dark matter in our own galaxy or if no one really bothered to check. I know it is a theoretical possibility that is so old that I don’t even remember their names.

  22. DLB says:

    Nobody seem to wonder why these black holes aren’t in the same disc as ordinary stars in galaxies. What keeps them from following the flow of matter on average? How can they form a sphere (roughly)?

  23. antonioncarlosbmotta says:

    The the idea of that black holes contain dark matter and theirs produced major part of the strongest gravitational fields.then could perceive that quantum vacuum generate fantástico quantity esbofetear energy that produced distortions in the spacetime changinthe metrics of the b fábrics of spacetime that modifying the topological geometrics structures.then great part of the radiation emmite by the blackholes.that hide in its interior the dark matter, for that reason then gravity create leasing that are leis produced by the dark matter.then irradiation of energynretired of the quantum vacuum paírs of particles and antiparticles that disappear in photons with differents energies, that does appear the black photons with dístints polarizations breaking symmetries as cp and pt.the idea of gravitational waves generated by the collapse of dark matter would produced the increase mente of entropy generating giant ripples in the spacetimes with differents frequencies of the common matter

  24. ArXivGuy says:

    excluded. See Carr+, arXiv:0912.5297

  25. Mike P says:

    People! It’s not that these black holes were made from dark matter. It’s that if there’s a lot of unseen black holes like these out there, in fact, so many that their total mass is 5 times that of all the ordinary stuff we know about, then dark matter *is* these black holes. But then how were they made, out of what? Well, they had to be made before nucleosynthesis, within 3min after the big bang. That’s because known matter is the right amount to get the H/He ratio right. Too much or too little ordinary matter at that time, and that’s not our universe. So dark matter had to be dark by then. That’s why these black holes have to be “primordial”. Meaning, formed by the big bang or shortly after. It also solves another problem: the LIGO black holes are too big to be made by collapsing a star. Several stellar black holes would need to merge to make each of the LIGO black holes. Lots of primordial black holes would explain why LIGO saw a merger so soon. Why don’t we see lots of lensing within our galaxy from all these black holes? Because mostly, they aren’t in galaxies. Some are, sure. But these things are primordial. They’ve always been everywhere. Most of them still are. (Hypothetically!)