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The Dark Matter Mystery Is Solved! LIGO’s observations show mid-sized primordial black holes are not improbable.

For years Dark Matter has confronted physics with a massive mystery. A recent article describes it as ‘the invisible substance that astronomers believe accounts for about 80% of the stuff in the universe.’ But what is it? This question has physicists searching for obscure new particles with the world’s largest machine, the Large Hadron Collider. Now observations from the LIGO gravity telescope reveal the successful candidate for Dark Matter. (Obscure particles need not apply.)

LIGO stands for Laser Interferometer Gravity Observatory. It is a super-sensitive instrument (actually, two instruments, 3,000 km apart) that can detect gravitational waves that make fantastically tiny changes in the shape of space itself. General relativity theory says two black holes will cause such changes when they collide. In February of this year, LIGO researchers reported the first observation of such an event.

Now, hot off the press, the news that both LIGO instruments detected another pair of black holes merging might seem a tad old hat. Who cares about number two? But this second observation has its own unique news value and it is a big deal. Here’s why.

Standard astrophysics provides for ‘small’ black holes of a few M (M is the mass of our Sun, some 300,000 times Earth’s mass). They are remnants left when stars collapse in supernovas; they move in orbits just as stars do. Astrophysics also knows of big black holes with millions or even billions of times the solar mass that sit at the centers of galaxies with their gravity sucking in vast masses of gas. But it provides no way for the universe to make black holes with intermediate masses, tens or hundreds of M. Astrophysicists see the supernovas that make small black holes. And they detect big black holes by the rapid motions of stars near them. They rule out both kinds of black holes as candidates for Dark Matter because they know there are too few to account for the missing mass.

Sherlock Holmes by Paget - Image for The Dark Matter Mystery Is Solved!

Sherlock Holmes by Paget

Thus Dark Matter must (they say) be something else. Physicists have dreamed up many other candidates. For example, they imagine weakly-interacting massive particles or WIMPs. Month by month the explanations become weirder. There is no evidence for any of them. Experiments designed to find them have all failed.

Five years ago British astronomer Mike Hawkins said Dark Matter could be primordial black holes (created in the first fraction of a second before the Big Bang). Two years ago I shared his view. My reason was that new concepts for Planck-scale physics show the emergence of space and matter in the first 10-42 second after the beginning as ideal for creating vast numbers of primordial black holes with a range of masses.

Late last year came the initial observation by LIGO. Before they merged, these first-ever directly-detected black holes weighed in at about 29 and 35 M. In February of this year I wrote that this event shows there are mid-sized black holes, a class that standard astrophysics says does not exist and ‘intermediate black holes like these may be the universe’s missing Dark Matter.’ Last month, astrophysicist Simeon Bird and seven colleagues at Johns Hopkins University said this too. But this notion rested on a single observation. Could it have been a fluke?

Now the LIGO team reports another pair of black holes bit the dust (or strictly, chastely kissed each other’s event horizon and immediately coalesced—an invisible catastrophe of mind-blowing magnitude). Observation of pair number two came just 15 weeks after the first, clearly no fluke. Their masses were 14.2 M and 7.5 M. There must be huge numbers of intermediate (and so primordial?) black holes out there if they can get together in twos, notwithstanding that it takes them thirteen billion years to get to their first—and final—kiss.

Does this prove Dark Matter is primordial black holes? Not quite. An upgraded LIGO and other gravitational-wave observatories will soon give us more data from which we can work better numbers. But tentatively I’m convinced. Remember Sherlock Holmes’ dictum to Dr. Watson that ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth’. And now LIGO’s twin observations show mid-sized primordial black holes are not improbable at all.

Sources:

Mike Hawkins (2011), “The case for primordial black holes as dark matter”, Monthly Notices of the Royal Astronomical Society, June, p. 415; http://arxiv.org/abs/1106.3875

Double your fun:

Rod Stewart & Tina Turner (1966), “It Takes Two”, https://www.youtube.com/watch?v=1mzDQbmtPZ0

Image credit: Sidney Paget

4 Comments

  1. Ivan Faught June 20, 2016 at 10:41 am #

    It’s not solved at all. The writer’s headline is misleading. You cannot claim a Sherlock Holmes quote as scientific explanation. Probability alone definitely doesn’t make anything true.

    • Colin Gillespie June 20, 2016 at 11:49 am #

      Thanks for your comment, Ivan. Your critical caution is to be commended. And of course the headline is not an explanation. For that, you can read the article. But science does not make anything true. It does not even assign a probability. It just tests for failure. But please take a closer look. The claim is that the mystery is solved. What was the mystery? As the post makes clear, it is that there is evidence for the matter but no viable candidate remained for what it could be. There is now a viable candidate. No mystery.

      • Ivan Faught June 20, 2016 at 12:28 pm #

        Colin Gillespie I read your article that is why I made my comment. I realized the headline is not an explanation, but the first part is stated as fact. The second part shows the mid-sized primordial black holes as probable. That gives the impression that you use your heading to sensationalize your article. Old journalistic trick to draw readers at first glance.

        The rest of your article is good. You explain in great detail why you came to your conclusion that Dark Matter are not primordial black holes quite yet. But according to me by throwing in the Sherlock Holmes quote made everything seems so improbable by itself :-)

        And I agree that there is a viable candidate for the claim, as there are many more viable claims, but that there is sudden evidence for the matter is still far off. I personally think Dark Matter is a bit more complicated, and I am hoping the James Webb Telescope will bring some of the answers. Although it is still a long wait. Lets hope the LHC can bring us light on that quicker.

        I still enjoy reading your articles though.

        • Colin Gillespie June 20, 2016 at 2:32 pm #

          Fair comment, Ivan. I think our difference of view if any may come down to this. I agree the headline is sensational. I don’t mind it giving that impression if it is accurate. I think the ‘sensation’ is warranted by the new information; it’s not a trick. And you seem to think this too.

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