Shedding light on dark en­ergy

Two decades after its pos­tu­la­tion, dark en­ergy still pro­vokes more ques­tions than an­swers - but we could just be mak­ing some head­way in un­der­stand­ing it

All About Space - - Contents - Re­ported by Gra­ham Southorn

Has a new find­ing fi­nally solved one of the great­est mys­ter­ies in the uni­verse?

Two decades ago, as­tronomers study­ing dis­tant stars made a dra­matic dis­cov­ery. They were in­ves­ti­gat­ing the fu­ture of the uni­verse, which was known to be ex­pand­ing. Con­trary to all ex­pec­ta­tions they dis­cov­ered that the rate of ex­pan­sion was ac­tu­ally speed­ing up – the uni­verse was get­ting big­ger, and faster. To­day the fo­cus is on find­ing the cause of this un­ex­plained phe­nom­e­non, which has been given a suit­ably mys­te­ri­ous moniker: ‘dark en­ergy’.

The dis­cov­ery that the uni­verse was ex­pand­ing ever faster came as a huge sur­prise. In fact, it was so sig­nif­i­cant that it led to the award of a Physics No­bel prize in 2011 to Saul Perl­mut­ter, Brian P Sch­midt and Adam G Riess.

These were the lead­ing sci­en­tists from two com­pet­ing teams who stud­ied light from a par­tic­u­lar type of ex­plod­ing star – and lots of them. Type Ia su­per­novae flare up in a pre­dictable way, so their rel­a­tive bright­ness re­veals how far away they are. Their red­shifts – the elon­ga­tion of wave­lengths to­wards the red end of the spec­trum – showed that it was those fur­thest away that were re­ced­ing the fastest.

It was the op­po­site to what most sci­en­tists ex­pected, says Robert

Crit­ten­den, pro­fes­sor of cos­mol­ogy at the Univer­sity of Portsmouth. “We’d al­ways as­sumed that be­cause grav­ity was at­trac­tive, the rate of ex­pan­sion would be slow­ing down. Dis­tant gal­ax­ies would be grav­i­ta­tion­ally at­tracted to each other and that would be slow­ing down the ex­pan­sion of the uni­verse,” he says.

The dis­cov­ery that the uni­verse was ex­pand­ing faster than be­fore begged a ques­tion: what was caus­ing it? The sim­plest so­lu­tion, which many sci­en­tists still be­lieve will ul­ti­mately prove to be the cor­rect one, is decades old. It comes from Al­bert Ein­stein’s gen­eral the­ory of rel­a­tiv­ity, which de­scribes how grav­ity op­er­ates.

Be­fore he pub­lished it in 1917, Ein­stein wres­tled with one of its pre­dic­tions. He be­lieved that the uni­verse was static, and yet the gen­eral the­ory of rel­a­tiv­ity pre­dicted it would ei­ther ex­pand or con­tract. In or­der to keep things in bal­ance, he added a term to the equa­tions: the fa­mous cos­mo­log­i­cal con­stant.

It was ef­fec­tively a fudge, and it wouldn’t last long. By the 1930s Ed­win Hub­ble’s ob­ser­va­tions of dis­tant gal­ax­ies had shown that the uni­verse wasn’t static at all, but was ex­pand­ing, and so Ein­stein aban­doned the cos­mo­log­i­cal con­stant, re­port­edly call­ing it his “big­gest blun­der”.

In the 21st cen­tury, though, the cos­mo­log­i­cal con­stant is back as the lead­ing ex­pla­na­tion for dark en­ergy. In phys­i­cal terms it’s a num­ber that de­scribes the en­ergy den­sity of empty space. Even a per­fect vac­uum, de­void of any par­ti­cles, is not de­void of en­ergy. As the uni­verse ex­pands and more space is cre­ated, there’s more en­ergy to push things out­ward.

This isn’t hy­po­thet­i­cal. There’s a phys­i­cal mech­a­nism that ex­plains where the en­ergy comes from which can be found in an­other branch of physics: quantum me­chan­ics. It says that tem­po­rary par­ti­cles pop in and out of ex­is­tence

– a phe­nom­e­non that’s been ob­served in par­ti­cle ex­per­i­ments on Earth.

There’s just one very big prob­lem. Quantum cal­cu­la­tions have pro­duced a num­ber for the en­ergy den­sity of empty space, but it’s way, way big­ger than what’s re­quired. “The cos­mo­log­i­cal con­stant is a very sim­ple model, but when you try to re­late it to fun­da­men­tal physics the value you get is ar­guably 100 or­ders of mag­ni­tude dif­fer­ent from what you ex­pect. Apart from that it works very well in ex­plain­ing what we see, but be­cause of that prob­lem peo­ple look at other ways to solve the rid­dle,” says Crit­ten­den.

One of the other ways of ex­plain­ing dark en­ergy is an en­ergy field that isn’t con­stant. Rather, it’s dy­namic. Its value changes over space and time, driv­ing the ex­pan­sion of the uni­verse dif­fer­ently now com­pared to how it did in the past. The uni­verse may have been born a long time ago – 13.8 bil­lion years ago at the Big Bang – but dark en­ergy has only come to dom­i­nate the ex­pan­sion for the past 5 bil­lion years or so.

There are many vari­a­tions of this idea, which goes by the name of quin­tes­sence, or ‘fifth force’. In physics ter­mi­nol­ogy it would be a ‘scalar field’, sim­i­lar to the Higgs field. And just as the Higgs field has an as­so­ci­ated par­ti­cle – the Higgs bo­son – so would the scalar field be re­spon­si­ble for dark en­ergy.

A par­ti­cle that could be re­spon­si­ble for dark en­ergy has never been de­tected, but strangely enough this non-de­tec­tion works in its favour as an ex­pla­na­tion. Out in the far-flung reaches of the uni­verse the field would be strong enough to fling gal­ax­ies apart, but in the pres­ence of other masses around it, like in lab­o­ra­to­ries on Earth, the force it ex­erts would be miniscule.

One such hy­po­thet­i­cal par­ti­cle is called the chameleon. It can change its mass de­pend­ing on the den­sity of its sur­round­ings, ex­plains Clare Bur­rage, as­so­ciate pro­fes­sor at the Univer­sity of Not­ting­ham. “Chameleons can self-cam­ou­flage.

They can learn about their en­vi­ron­ment and ad­just their prop­er­ties so that the mod­i­fi­ca­tion of grav­ity is hid­den from ex­per­i­ments. But they can’t hide from ev­ery­thing, so if you do a suit­ably cho­sen ex­per­i­ment you might be able to see its ef­fects. We’re us­ing a tech­nique called atom in­ter­fer­om­e­try that’s re­ally sen­si­tive to these forces.”

The chameleon force would be ex­erted solely by the outer shell of an ob­ject rather than, say, grav­ity on Earth, which is ex­erted by the en­tire planet. This idea ex­plains why the force would be so weak and why the ex­per­i­ments, which in­volve de­tect­ing the mo­tion of atoms in free fall, needs to be su­per-sen­si­tive.

“One of the in­ter­est­ing things about these the­o­ries is that you can de­sign an ex­per­i­ment on Earth that might make their ef­fects show up. It doesn’t need to be in a huge par­ti­cle col­lider. You just need to do the right kind of re­ally sen­si­tive mea­sure­ment, us­ing equip­ment that could fit on a cou­ple of ta­ble tops. It’s done on a small scale on a rea­son­ably short timescale – a very dif­fer­ent way of study­ing dark en­ergy than launch­ing a tele­scope, which can take decades,” says Bur­rage.

The Univer­sity of Not­ting­ham team has yet to pub­lish its re­sults, and a group us­ing the same tech­nique at the Univer­sity of Cal­i­for­nia, Berke­ley, has yet to spot any­thing un­usual.

While physi­cists look for tell-tale signs in the lab, many as­tronomers have fo­cused on mak­ing more pre­cise maps of how the uni­verse has evolved over time. One kind of map that’s par­tic­u­larly im­por­tant re­veals the lo­ca­tion of not just or­di­nary mat­ter, of the kind that con­sti­tutes stars and plan­ets, but dark mat­ter too.

Dark mat­ter is an­other cos­mic mys­tery all by it­self – an in­vis­i­ble form of mat­ter that, like dark en­ergy, has evaded de­tec­tion. But while the par­ti­cles mak­ing up dark mat­ter have yet to be iden­ti­fied, its ef­fects are clearly vis­i­ble. Crit­ten­den ex­plains: “We’ve known about dark mat­ter for a while longer than dark en­ergy, and we need it on a smaller scale than the uni­verse [as a whole]. Vis­i­ble mat­ter isn't nearly enough to ex­plain the dy­nam­ics of stars around gal­ax­ies and gal­ax­ies around clus­ters of gal­ax­ies – we need a sig­nif­i­cant amount of dark mat­ter as well.”

The cur­rently ac­cepted model of the uni­verse holds that it’s ge­o­met­ri­cally flat. And if this is so then dark mat­ter must ac­count for a stag­ger­ing 27 per cent of all its mass-en­ergy. Or­di­nary mat­ter would make up just 5 per cent and the re­main­ing 68 per cent would be dark en­ergy.

How­ever, dark mat­ter and dark en­ergy don’t be­have in the same way, says Crit­ten­den. “Dark mat­ter is gen­er­ally at­trac­tive – it's like or­di­nary mat­ter in how it grav­i­tates. Dark en­ergy has a very dif­fer­ent ef­fect – it doesn’t clus­ter gal­ax­ies to­gether and it's dif­fi­cult to see its ef­fect on in­di­vid­ual gal­ax­ies.”

Whereas dark mat­ter acts to pull gal­ax­ies to­gether, dark en­ergy tries to pull them apart. This

“Vis­i­ble mat­ter isn't nearly enough to ex­plain the dy­nam­ics of gal­ax­ies around clus­ters of gal­ax­ies”

Prof Robert Crit­ten­den

A Type Ia su­per­nova pic­tured by the Hub­ble Space Tele­scope in the galaxy M82

The Vic­tor M Blanco tele­scope in Chile cap­tures light for the Dark En­ergy Sur­vey

Cool­ing de­vice for the world’s largest dig­i­tal cam­era – part of the Large Synop­tic Sur­vey Tele­scope

The fi­bres of the DESI can cap­ture 5,000 gal­ax­ies si­mul­ta­ne­ously

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.