Winnipeg Free Press - Section D - - SCITECH - COLIN GILLE­SPIE

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.

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