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Can Primordial Black Holes Alone Account For Dark Matter? 135

thomst writes: Slashdot stories have reported extensively on the LIGO experiments' initial detection of gravity waves emanating from collisions of primordial black holes, beginning, on February 11, 2016, with the first (and most widely-reported) such detection. Other Slashdot articles have chronicled the second LIGO detection event and the third one. There's even been a Slashdot report on the Synthetic Universe supercomputer model that provided support for the conclusion that the first detection event was, indeed, of a collision between two primordial black holes, rather than the more familiar stellar remnant kind that result from more recent supernovae of large-mass stars.

What interests me is the possibility that black holes of all kinds -- and particularly primordial black holes -- are so commonplace that they may be all that's required to explain the effects of "dark matter." Dark matter, which, according to current models, makes up some 26% of the mass of our Universe, has been firmly established as real, both by calculation of the gravity necessary to hold spiral galaxies like our own together, and by direct observation of gravitational lensing effects produced by the "empty" space between recently-collided galaxies. There's no question that it exists. What is unknown, at this point, is what exactly it consists of.

The leading candidate has, for decades, been something called WIMPs (Weakly-Interacting Massive Particles), a theoretical notion that there are atomic-scale particles that interact with "normal" baryonic matter only via gravity. The problem with WIMPs is that, thus far, not a single one has been detected, despite years of searching for evidence that they exist via multiple, multi-billion-dollar detectors.

With the recent publication of a study of black hole populations in our galaxy (article paywalled, more layman-friendly press release at that indicates there may be as many as 100 million stellar-remnant-type black holes in the Milky Way alone, the question arises, "Is the number of primordial and stellar-remnant black holes in our Universe sufficient to account for the calculated mass of dark matter, without having to invoke WIMPs at all?"

I don't personally have the mathematical knowledge to even begin to answer that question, but I'm curious to find out what the professional cosmologists here think of the idea.

Can Primordial Black Holes Alone Account For Dark Matter?

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  • by Anonymous Coward


    Editors, the word you're looking for is primordial.

    • by Cryacin ( 657549 )
      Pirmodial black holes don't account for dark matter, but it does account for the Lesdyxia of the editor.
    • Re:typo in title (Score:5, Insightful)

      by ShanghaiBill ( 739463 ) on Wednesday August 09, 2017 @03:31AM (#54972305)

      The main alternative to WIMPs are MACHOs [], and black holes have long been candidates for dark matter. The problem is that they would need to have five times more mass than all the "ordinary" matter in the universe, and there is little evidence for that. For instance the amount of gravitational lensing that is observed is way less than would be expected. Dark matter appears to be more evenly distributed in galaxies and not just in the "halo". Yet we don't observe that many black holes passing through gas and dust clouds or interacting with regular stars.

      TFA says that there may be 100 million black holes in our galaxy, and that may sound like a lot, but it is actually nowhere near enough to account for all the dark matter. Even if they had 10 solar masses each (unlikely), that would still be less than 1% of the mass of the galaxy's "ordinary matter", when it should be 500%.

      • by Anonymous Coward

        If you get rid of dark energy, you eliminate the need for dark matter as well.

        Current theory about universe involves more space appearing out of thin air, causing expansion. (More vacuum!).

        Instead what should be examined is what happens to light traveling through immense amount of vacuum. We have discovered unexpected characteristics of vacuum, there may be more. It is possible that redshifting occurs through extended vacuum interaction and the output is that occasionally higher energy photons are conver

        • Re:typo in title (Score:5, Informative)

          by Sique ( 173459 ) on Wednesday August 09, 2017 @05:19AM (#54972573) Homepage
          Actually, you don't get rid of Dark Matter when you eliminate Dark Energy.

          There is evidence for more matter than visible in the galaxies, which is completely independent of Dark Energy. The most prominent evidence is the rotational characteristics of the outer parts of a galaxy. The stars there are circling the center of the galaxy much faster than expected from a Keplerian point of view. Instead of falling with r^2/3, as Kepler's Third law of motion predicts, the speed of stars remains roughly constant if you get to the outer parts of the galaxy. This means that the mass of the galaxy inside the respective orbits of the stars has to grow much faster than the mass from the additional stars within outer orbits.

          (Be careful not to confuse the speed of stars on their orbit with their angular speed! A star twice the distance from the center of a galaxy needs twice the time to complete a circle than a star closer to the center. Thus the angular speed halves, but the linear speed on the orbit keeps the same. With Kepler's Third law, we would expect the time to complete an circle for the outer star to be 2*sqrt(2) of the time the inner star needs.)

          • You seem to know what you're talking about. Kepler's "laws" describe two bodies in orbit around each other, with one much more massive than the other, and at a substantial distance. Obviously (well, I _guess_ it's obvious), the rotation of gaseous bodies (like the Sun) is different; an hydrogen atom in the photosphere is not in _orbit_, so its behavior is not described by Kepler's (or Newton's) laws.

            While it's a long ways to the next star in the arm of a galaxy, it's still not quite the same as if our Sun

        • Normally in science the simpler the answer that fits the given data is considered the most likely. Given Dark Matter and Dark Energy are more or less scientific terms for paranormal activity ( just so people just don't go thinking it is cause by ghosts ) but right now they are a name of something, we are not quite sure how to measure and observe, it is just a variable if filled into the equation seems to fit the data that we observe in the universe.
          Now as we learn more and collect more data and get better

          • There's nothing paranormal about Dark Matter or Dark Energy. Dark Matter is matter that has gravity but doesn't interact electromagnetically. We actually have an example of such matter in the neutrino, although what we're observing isn't what neutrinos would do.

            Like many things in science, they're a name for something we haven't been able to observe more directly. Are you familiar with how we found Uranus and Neptune? Astronomers observed anomalies in the orbit of Saturn, and figured that these could

        • If you get rid of dark energy, you eliminate the need for dark matter as well.

          That's completely wrong. Dark energy is needed to explain the acceleration in the expansion of the universe. Matter - either dark or ordinary - is gravitationally attractive and can never cause the expansion to accelerate. Dark Matter is needed to explain the "clumpiness" of the Cosmic Microwave Background, the rotation curves of galaxies and gravitational lensing observations e.g. bullet cluster. Black Holes have been considered a dark matter candidate before (MACHOs) and have been ruled out.

          I think th

          • by Mkkby ( 4973999 )
            The data that needs to be explained is galaxies are repelling each other, and the red shifts accelerate the further out you look. Black holes cannot explain this because black holes are an attractive force. Both astrophysics and particle physics are in chaos now. Somehow stars and galaxies are created by coalition of matter, yet on the larger scale they are being pushed apart. This is so bizarre it may never be explained. At the same time, the energy level of the newly discovered higgs boson has trashe
            • Both astrophysics and particle physics are in chaos now.

              No, we are not. There are two unexplained phenomena at galactic and intergalactic scales but either could easily be explained by adding a new field to the physics we already have. This is no more 'chaotic' than it was before we found the Higgs boson or the top quark.

              At the same time, the energy level of the newly discovered higgs boson has trashed decades of string theory.

              That is simply not true. The Higgs mass is at the upper end of the allowed range for SUSY but it is still in the allowed range to avoid vacuum instability. I believe that there were some papers that suggested that the mass was closer to this l

      • But, if there were black holes scattered between galaxies, that would be a different story.

    • Well, isn't a black hole just that: black matter?
  • by wherrera ( 235520 ) on Wednesday August 09, 2017 @03:16AM (#54972243) Journal

    See this link: Content of the Universe 2016 []

    So, the problem is that there is so much of it, you would think we'd see it perturbing stallar orbits more, it it were concentated in many, many discrete points of star gravitational influence. There would be a lot more stars orbiting pulsar type objects, perhaps?

    A real cosmologist would know the odds of the galaxy looking the way it does if all the extra mass were in scattered black holes of a certain size. Probably low.

  • by dyfet ( 154716 ) on Wednesday August 09, 2017 @03:23AM (#54972271) Homepage [] [] If dark matter were simply some existing form of baryonic matter, even if trapped in black holes, then a phenoma like this where dark matter halos separate from collided galaxies and behave under different rules to continue on their existing path should not be possible at all, because it, like all the other ordinary matter involved, it should have followed the same paths gravitationally bound.

    • by Anonymous Coward

      That's not quite right.

      There are three types of stuff involved in the Bullet Cluster collision: stars, gas, and dark matter. The stars are the smallest fraction of the mass, the gas is a larger chunk of it, and the dark matter makes up most of it. We can see the stars and the gas directly (from visible and X-ray observations), and we can figure out where the dark matter is from gravitational lensing (which shows where the mass is).

      In the collision, the stars in each cluster passed right through each other

    • by hAckz0r ( 989977 )
      Abell 520 is yet another instance, where the bizarre physics of the so called Dark Matter from two colliding galaxies turns to stationary filaments of gravitational lensing rather than clustering along with the normal baryonic matter that kept passing through. Doing this would require not only new physics but also lots of energy to stop dead in its tracks and make a left hand turn for no reason what so ever. The Elephant in the room is tap-dancing and nobody is even paying attention!
  • by CptJeanLuc ( 1889586 ) on Wednesday August 09, 2017 @03:38AM (#54972329)

    Quote: "Dark matter [...] has been firmly established as real [...] There's no question that it exists." There is still plenty of controversy related to the idea of dark matter, and there is no such thing in physics as proving something exists - you can only prove something to be false. I'm not saying dark matter does not exist, only that statements like the above are too assertive.

  • by NoNonAlphaCharsHere ( 2201864 ) on Wednesday August 09, 2017 @03:41AM (#54972341)
    26% of the mass of the universe is made up of your simplifying assumptions: space is flat and uniform everywhere and everywhen, gravity is constant everywhere and everywhen, the speed of light is constant everywhere and everywhen, the Higgs field isn't really the luminiferous aether with a fancy new name, etc. ...

    So so so much of the Standard Model (and astrophysics in general) starts out like "Given a spherical cow of uniform density at STP...".

    We can basically derive ALL of chemistry from first principles involving (protons, neutrons, electrons) (and their charges), electron shell configurations, etc. Does the Standard Model provide an explanation for the mass of the electron, or any of the other 92 empirically derived "constants" that make up the current orthodoxy? Does calling the gap between reality and our understanding of it really benefit from calling it "Dark Matter", or "Dark Energy", or should we just call it "phlogiston"?

    I'm not trolling, I'm serious. The Standard Model has lots of (statistical) predictive power, but absolutely no explanitory power -- back to the chemistry example, it's as though we have atomic weights and molar values, but no notion of electron shells -- we can predict, but we can't explain, at least not in a meaningful way -- yet.
    • Re: (Score:3, Interesting)

      by Anonymous Coward

      Well, not that many simplifications. Actually, many people try out many different things to see if your experimental observations are affected by the underlying theory you are basing your assumptions.

      I want to point out some contradiction in your message:
      - On one hand, you criticize the standard model for its high number of parameters (19, I believe that is the number, actually). And it is a fair criticism, you would like your theory to predict nature with the least number of empirical parameters. If every

    • by Anonymous Coward

      IMHO (based on playing with the dipole model).

      Time is flat and constant and causlity happens in forward time.
      Gravity and mass are the same, the mass of something is the potential of it to attract via the dipole binding force (DBF).
      There is no momentum as such, only a reduction in the dbf cause by velocity effect on the component of the twist subtended onto the other particle. (do it numerically you'll see what I mean, if you move a particle faster away from a clump of dipole stuff the binding force reduces

      • by hord ( 5016115 )

        This is how I think of things. Basically just regions where energy is denser and lighter and flows in between them. The EM and Weak fields are just particular configurations or patterns of this and the entire thing is perfectly deterministic. We are living in a wrinkled energy background that constantly vibrates.

      • Time is flat and constant and causality happens in forward time.

        In which case you're throwing out Special Relativity, and need to explain what it explains. It's one of the most tested theories in modern physics, and has been shown to offer very exact explanations of a variety of things.

        For example, we know that unstable subatomic particles, when at near-light speeds, decay more slowly than if they were still.

    • No model can explain the "why" of fundamental particles or constants. They just "are". Science is not about Truth, it's about usefulness.
    • by Ramze ( 640788 ) on Wednesday August 09, 2017 @07:08AM (#54972869)

      That's the thing with (supposedly) fundamental particles -- you can't explain them in terms of something else... because then they wouldn't be fundamental. If you're talking about why they have certain properties -- like why there are 3 generations of matter (separated only by mass) and why they have the masses that we measure (as opposed to some other mass), maybe one day when we find a way to merge gravity with the standard model and/or figure out why the Higgs mechanism gives different masses to different particles, we'll find out.

      But, if you mean you want to have explanations for things like "charge," "spin," "color charge," and why only certain ones exist -- we may never know. If they're fundamental properties, there may not be any real explanation other than "they just are." That's the universe we appear to live in.

      String theory and some other interesting quantum theories are trying to explain deeper meanings and use expected symmetries to figure out missing particles and new physics... and they helped to tease out the Higgs Boson and its field to explain why all fundamental particles don't move at the speed of light. There may be more than one Higgs field & that may explain more if we find it. If there are hidden, curled up dimensions, we may be able to explain all the properties of particles in terms of vibrating strings or membranes in higher dimensions, but until string theorists can decide on what the shape of those curled up dimensions might be for our universe, they can't help much with predictions, much less explanations. Trouble is, there are a heck of a lot of possibilities for those curled up dimensions, and there aren't a lot of ways to discern which ones match our known universe yet. Sure, they can whittle them down to a subset that matches known properties of the universe, but that leaves a massive subset to eliminate false positives from.

      I'd say string theory is your best bet for explaining why things are as they are one day... but it may be that some things just are, and that's as fundamental as they get -- at least as far as we can tell from experimental data from within the universe. Anything deeper is speculation or philosophy -- unless it can fit the math perfectly and explain things other models can't. For instance, we've never directly observed quarks, but we've been able to indirectly observe them and figure out their properties from subatomic collisions. At one time, people debated if they really existed or if they just helped the math work... but physicists generally agree they exist today. Maybe we'll find something more fundamental in time that will explain more. My bet is on strings, but... who knows?

    • 26% of the mass of the universe is made up of your simplifying assumptions: space is flat and uniform everywhere and everywhen, gravity is constant everywhere and everywhen, the speed of light is constant everywhere and everywhen, the Higgs field isn't really the luminiferous aether with a fancy new name, etc.

      Addressing the first part of your post only, this is one reason I like the Timescape Cosmology [] model, that basically posits that some dark matter effects and all dark energy effects are just arising from GR. Most cosmology simplifies the universe to a homogeneous soup, which conveniently ignores small-scale GR effects that could be very important.

    • Or maybe, just maybe, if you're not an astronomer or a physicist then there are other people who know more about this than you.

      Perhaps the idea that it's all a simplifying assumption is your simplifying assumption.

    • There was a recent interesting paper, "Concordance cosmology without dark energy" [], which explained how dark energy was actually not required to explain the structure of the universe, if one just used a more accurate numerical model to simulate how the universe evolved. They even resolved a long-standing issue in cosmology whereby different ways of estimating the Hubble constant from observations gave different results. I'm looking forward to seeing how this theory develops, and how their findings are receiv
  • "What interests me..."

    Thanks for letting us know in an entirely opinionated piece you lack the science background, but this fascinates you.
    My dog feels the same way about asphalt and rabbits.


  • WIMPs are unknown particles with unknown properties and the speculative detectors we hoped might detect them haven't yet ... ...this does not mean they don't exist, it just means the things we don't know the properties of we still don't know the properties of ...

    • Is there a good idea to think they're WI and not just MP? There's the practical matter that we can go looking for WIMPs, but I don't know of any reason why nonstandard matter would be affected by the weak nuclear force.

  • by jfdavis668 ( 1414919 ) on Wednesday August 09, 2017 @08:34AM (#54973193)
    Astronomers have observed the gravitational lens effect of dark matter. Dark matter normally surrounds normal matter, but is sometimes found separated. It appears that during galactic collisions, the dark matter can be separated from the normal matter, gas and dust of a galaxy. To do that, dark matter would have to interact with itself in a manner that does not involve gravity. A bunch of black holes would not interact in this way, so it is unlikely that dark matter consists solely of black holes.
    • Why would it have to interact with itself through non-gravitational means? It doesn't clump like regular matter does, so it would seem that it doesn't have anything corresponding to electromagnetism.

      • If it separates during collisions, it is because dark matter is stopping dark matter. If it was caused by gravity, the same thing would happen to normal matter. No, they don't think it uses electro-magnetism, or it wouldn't be dark. It may interact via the weak nuclear force like neutrinos do.
  • I've never understood the emotional response to the theory of dark matter being some sort of mass we can't see. It seems like the theory personally affronts people for some reason.

    The idea that a form of matter doesn't interact with three of the four fundamental forces doesn't seem all that crazy to me. I mean, the photon and W and Z bosons don't interact with the strong nuclear force, do they? (I'm actually asking - I'm no particle physicist.) And the neutrino interacts with the weak force and gravity, b
    • Some people just love to get in on the action, and if they can't do it by understanding the science, they'll do it by blindly dismissing the science. It's an easy route to giving yourself the impression that you're "in the know," putting yourself in the contrarian position (which is not always an incorrect thing to do if you've got the understanding back it up) and making yourself feel part of a more exclusive group. Eventually this all feeds back and you get people who think that being in the minority itse

      • Thing is, dark matter is easy to understand. WIMPs are at least much like large slow neutrinos, and the idea of "something like regular matter but - " shouldn't be hard to grasp.

  • Well.... they'd need to be pretty small, and also to not evaporate via Hawking radiation. Tiny black holes evaporating would release light at a known frequency that hasn't been detected (or hadn't been a decade ago, when I was paying attention). And the black holes would need to be primordial, because otherwise they'd affect the proportion of Lithium in the interstellar dust. So they can't have been engaging in nuclear reactions while that was being formed (*quite* early in the process). And they'd need

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