Dogged physi­cists are me­thod­i­cally sweep­ing through all the places where the elu­sive par­ti­cles may be hid­ing. CATHAL O’CON­NELL checks their progress.

Cosmos - - Feature -

ONE SATUR­DAY I HIRED a metal de­tec­tor and drove four hours to the his­toric gold-rush town of Bright in Vic­to­ria, Australia, where my wed­ding ring lies lost, some­where on the bed of the Ovens River. I spent the evening wad­ing through the icy wa­ters in gum­boots, un­cov­er­ing such treasures as a bot­tle cap, a fisher’s lead weight and a bracelet caked in rust. I did not un­cover the ring. But that doesn’t mean the ring is not there.

Like me, physi­cists around the world are in the midst of an im­por­tant search that has so far proven fruit­less. Their quarry is noth­ing less than most of the mat­ter in the uni­verse, so-called “dark mat­ter”.

So far their most sen­si­tive de­tec­tors have found – to be pithy – nada. De­spite the lack of re­sults, sci­en­tists aren’t giv­ing up. “The fre­quency with which ar­ti­cles show up in the popular press say­ing ‘maybe dark mat­ter isn’t real’ mas­sively ex­ceeds the fre­quency with which physi­cists or astronomers find any rea­son to re­ex­am­ine that ques­tion,” says Katie Mack, a the­o­ret­i­cal as­tro­physi­cist at the Univer­sity of Mel­bourne.

In many re­spects, the quest for dark mat­ter has only just be­gun. We can ex­pect quite a few more null re­sults be­fore the real trea­sure turns up. So here is where we stand, and what we can ex­pect from the next few years.

IMAG­INE A TOD­DLER sit­ting on one end of a see­saw and launch­ing her fa­ther, at the other end, high into the air. It’s a weird and un­set­tling im­age, yet we reg­u­larly ob­serve this kind of ‘im­pos­si­ble’ be­hav­iour in the uni­verse at large. Like the lit­tle girl on the see­saw, galax­ies be­have as if they have four or five times the mass we can see.

Our first inkling of this dis­crep­ancy came in the 1930s, when the Swiss as­tronomer Franz Zwicky no­ticed odd move­ments among the Coma clus­ter of galax­ies. Zwicky’s anom­aly was largely ig­nored un­til the 1970s, when as­tro­physi­cist Vera Ru­bin, based at the Carnegie In­sti­tute in Wash­ing­ton, no­ticed that the way galax­ies spin did not tally with the laws of physics. The metic­u­lous ob­ser­va­tions by Ru­bin (who passed away in De­cem­ber 2016) con­vinced most of the astro­nom­i­cal com­mu­nity some­thing was amiss. There were two pos­si­ble an­swers to the prob­lem: ei­ther galax­ies were a lot heav­ier than they ap­peared, or our the­ory of grav­ity was ka­put when it came to galaxy-scale move­ments.

From the out­set, astronomers pre­ferred the first ex­pla­na­tion. At first they thought the miss­ing mat­ter was prob­a­bly noth­ing too weird – just reg­u­lar astro­nom­i­cal ob­jects (like plan­ets, black holes and stars) too dim for us to see. But as we sur­veyed the sky with ever big­ger telescopes, these so-called ‘mas­sive com­pact halo ob­jects’ (or MACHOS) never turned up in the num­bers needed to ex­plain all the ex­tra mass.

Other as­tro­physi­cists, such as the Morde­hai Mil­grom at Is­rael’s Weiz­mann In­sti­tute, ex­plored mod­els where grav­ity be­haved dif­fer­ently at cos­mic scales. They were not suc­cess­ful.

Slowly astronomers re­alised they had some­thing rad­i­cally dif­fer­ent on their hands – a new kind of stuff they called ‘dark mat­ter’, which must out­weigh the uni­verse’s reg­u­lar mat­ter by about five to one. “Cer­tainly, when all the ev­i­dence is taken to­gether,” Mack says, “there’s no com­pet­ing idea right now that comes any­where close to ex­plain­ing it as well.”

We know four main facts about dark mat­ter. First, it has grav­ity. Sec­ond, it doesn’t emit, ab­sorb or re­flect light. Third, it moves slowly. Fourth, it doesn’t seem to in­ter­act with any­thing, even it­self.

Like de­tec­tives in a TV mur­der mys­tery, physi­cists have com­piled a list of sus­pects. Top­ping the list are three hy­po­thet­i­cal par­ti­cles al­ready wanted on other charges: ax­ions, ster­ile neu­tri­nos and WIMPS. Be­sides nail­ing dark mat­ter, each would help ex­plain a grand mys­tery of their own.

The ax­ion is a par­ti­cle pro­posed by Roberto Pec­cei and He­len Quinn back in 1977 to ex­plain a quirk of the strong force (namely, why it can’t dis­tin­guish left from right, the way the weak force does). Thirty years on, ax­ions are still our best ex­pla­na­tion for that puzzle.

Ax­ions could have any mass, but if – and it is a big

01 Dark mat­ter can’t be de­tected but it glues galax­ies to­gether. It out­weighs or­di­nary mat­ter by five to one.

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