RESEARCH BRINGING DARK MATTER OUT OF THE SHADOWS
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 per cent of the stuff in the universe.” But what is it? This question has had physicists searching for obscure new particles with the world’s largest machine, the Large Hadron Collider. Now new observations from the LIGO gravity telescope reveal a successful candidate for dark matter. Obscure particles need not apply.
LIGO stands for Laser Interferometer Gravity Observatory. It is a supersensitive instrument (actually, two instruments, 3,000 km apart) that can detect gravitational waves. These are fantastically tiny ripples in the shape of space itself. General relativity theory says two black holes will send out such ripples 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 2? 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 with masses of a few times the solar mass, which is some 300,000 times Earth’s mass. These black holes are the remains of big stars that collapse in supernovas and they move in orbits just as stars do. Astrophysics also knows of big black holes with millions or even billions of times solar mass. They sit at the centres of galaxies and their gravity sucks in vast masses of gas. But astrophysics knows no way for the universe to make black holes with intermediate masses, tens or hundreds of solar mass. Astrophysicists can see the supernovas that make small black holes. And they can detect big black holes by the rapid motions of stars near them. They have ruled out both kinds of black holes as candidates for dark matter because they know there are too few to account for the missing mass.
Thus dark matter must (they say) be something else. Physicists have dreamed up many other candidates. For example, they imagine weaklyinteracting massive particles or WIMPs. Over time the explanations have 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 tiny fraction of a second before the Big Bang. His suggestion is of special interest because new concepts for Planck-scale physics show space and matter emerging in the first 10 to 42 seconds after the beginning; these are ideal conditions 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 solar mass. In February of this year, I wrote that this event shows there are mid-sized black holes 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 signature ripples from another pair of black holes biting the dust (or strictly, chastely kissing each other’s event horizon and immediately coalescing — an invisible catastrophe of mindblowing magnitude). Observation of pair number 2 comes just 15 weeks after the first, so this is no fluke. Their masses were 14.2 solar mass and 7.5 solar mass. There must be huge numbers of intermediate (and so primor- dial) black holes out there if they can get together in twos, notwithstanding that it takes them 13 billion years to get together for 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 that it’s a real— and for now the only — candidate is convincing. Let’s recall Sherlock Holmes’ dictum to Dr. Watson: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” Now, LIGO’s repeated observation shows mid-sized primordial black holes are not improbable at all.
Colin Gillespie is a physicist and author whose most recent book is Time One: Discover How the Universe Began. He writes a weekly web blog Science Seen.