Did di­nosaurs fall foul of dark mat­ter?

Cosmos - - Spectrum - AN­DREW MASTER­SON is is an au­thor and jour­nal­ist based in Mel­bourne, Aus­tralia.

“I mean, it re­ally is just en­ergy,” says Lisa Ran­dall. “So to ac­tu­ally say it’s some­thing is wrong.”

At first blush, this seems like a strangely blasé way for a the­o­ret­i­cal physi­cist to re­fer to dark en­ergy, the prop­erty that com­prises just a shade less than 70% of the ob­serv­able uni­verse and about which we know (to use the tech­ni­cal term) three-fifths of stuff-all.

Then again, Ran­dall, the Frank B Baird Jr Pro­fes­sor of Sci­ence at Har­vard, is among the hand­ful of peo­ple in the world for whom dark en­ergy is less an ex­otic mys­tery and more just an­other worka­day prob­lem. For her, dark en­ergy, and all the ques­tions sur­round­ing its den­sity and dy­namism, can just take a ticket and wait its turn. Right now she’s con­cen­trat­ing on dark mat­ter.

At least, that’s the case in the par­al­lel non-aca­demic ca­reer she en­joys – that of a best-sell­ing pop-sci­ence au­thor. Her lat­est book, which rev­els in the ex­cel­lent hot-but­ton ti­tle Dark Mat­ter and the Di­nosaurs, was re­leased in late 2015 and promptly went gang­busters.

The book glosses the gen­er­ally ac­cepted idea that the Cre­ta­ceous-palaeo­gene mass ex­tinc­tion 66 mil­lion years ago – the event that wiped out the di­nosaurs and pretty much ev­ery­thing else – was prompted by an ex­trater­res­trial ob­ject whack­ing into Earth. Ran­dall ex­tends the con­cept, sug­gest­ing the ob­ject was a me­te­oroid dis­lodged from the dis­tant Oort Cloud af­ter a col­li­sion with a thin flat disc of dark mat­ter.

The disc, she sug­gests, is a per­ma­nent fea­ture in the Milky Way, the ro­ta­tion of which brings our own so­lar sys­tem into its prox­im­ity roughly ev­ery 30 mil­lion years. The sug­ges­tion po­ten­tially ex­plains ob­ser­va­tions that in­di­cate that while lit­tle as­ter­oids and comets tend to bang into plan­ets on a sta­tis­ti­cally reg­u­lar ba­sis, re­ally big bug­gers from the Oort only hit, yes, ev­ery 30-odd mil­lion years.

The idea, she adds, re­quires only a pro­por­tion­ately tiny amount of the dark mat­ter as­sumed to be in the lo­cal area to form it­self into a disc. The propo­si­tion im­plies that dark mat­ter, which ac­counts for roughly 27% of the stuff in the uni­verse, is sub­ject at least some­times to the same forces that af­fect vis­i­ble (or “bary­onic”) mat­ter. The Milky Way it­self, of course, is disc-shaped.

Which is all well and good, ex­cept we don’t ac­tu­ally know what dark mat­ter is ei­ther.

English physi­cist, tele­vi­sion pre­sen­ter and au­thor Brian Cox, in­ter­viewed re­cently in Cos­mos, sug­gested it “must” be some kind of par­ti­cle. Ran­dall isn’t pre­pared to go that far.

“It’s more likely than not that it’s a par­ti­cle,” she says, “but you know, I’m a model builder and I’m a the­o­rist, and what we do is we make sug­ges­tions and then we see how to test them. So he [Cox] can imag­ine what­ever he wants. The fact is un­til we ac­tu­ally find it or mea­sure what it is, we don’t know for sure.”

She notes that re­search into dark mat­ter in­volves dis­cov­er­ing the types of con­straints that can be ap­plied – in essence, iden­ti­fy­ing what it can’t be, and thus ever lim­it­ing the pos­si­bil­i­ties of what it can. In cer­tain mod­els, she notes, the in­flu­ence of a black hole of a par­tic­u­lar size can’t yet be dis­counted.

If push comes to shove, though, she’ll put her money on par­ti­cles.

“I would sus­pect that it’s even more than one kind of par­ti­cle,” she says. “We as­sume it is just one kind of par­ti­cle, even though our own mat­ter is many dif­fer­ent types of par­ti­cle. So it’s very likely, but I’m not go­ing to tell you it must be true un­til I know it must be true.”

“I think what dark en­ergy is telling me is that we feel like maybe we’re miss­ing some re­ally big idea.”

In her pop-sci­ence writ­ing, Ran­dall has been rightly praised for her abil­ity to ren­der the com­plex­i­ties of cos­mol­ogy ac­ces­si­ble to non-spe­cial­ist read­ers – such as the be­wil­der­ing idea of mul­ti­di­men­sional ob­jects dubbed “branes”.

Her aca­demic work, in con­trast, delves deeply into the mul­ti­ple po­ten­tials of su­per­sym­me­try, a field that posits warped branes and cre­ates warped brains with equal vigour.

Her best-known con­tri­bu­tion to the field – and one ex­plored in a pre­vi­ous book, Warped Pas­sages (2005) – be­gan in 1999 when, in con­junc­tion with Univer­sity of Mary­land the­o­ret­i­cal par­ti­cle physi­cist Ra­man Sun­drum, she for­mu­lated what be­came known as the Ran­dall-sun­drum Model.

The model sug­gests that our world is lo­cated in a higher di­men­sional uni­verse that com­prises a five-di­men­sional neg­a­tively curved man­i­fold, in which all el­e­men­tary par­ti­cles (ex­cept the gravi­ton) are clus­tered lo­cally in a four­di­men­sional brane.

There are two ba­sic ver­sions of the model, known as RS1 and RS2, which dif­fer pri­mar­ily in the dis­tance as­sumed to ex­ist be­tween two branes – one has them rel­a­tively close to each other, the other in­fin­itely far apart. Ran­dall-sun­drum is taken very se­ri­ously in the field, with re­sults from CERN’S Large Hadron Col­lider in Switzer­land from as re­cently as Au­gust this year be­ing used to con­tin­u­ally re­fine its mea­sure­ments and pre­dic­tions.

Given, thus, the depth, com­plex­ity and longevity of her re­search into what ex­actly makes the uni­verse the uni­verse, it’s hardly sur­pris­ing that she has lit­tle time for por­tray­als of dark en­ergy as enig­matic.

“Quite se­ri­ously, dark en­ergy is just that: it’s just en­ergy

“What dark en­ergy is telling me is that maybe we’re miss­ing some re­ally big idea. Be­cause we would naively ex­pect the amount of en­ergy to be greater.”

per­me­ated through the uni­verse,” she says. “It’s not a mat­ter of say­ing what it is – it’s re­ally about say­ing some­thing dif­fer­ent: why it car­ries the amount of en­ergy it does.

“It’s en­ergy. It’s not car­ried by par­ti­cles; it’s not car­ried by mat­ter. We would like to know why there is the amount of it that there is. And that’s been mea­sured. The sur­pris­ing thing is that there isn’t an enor­mously big­ger amount of en­ergy. That’s re­ally what’s sur­pris­ing.”

In essence, then, the key ques­tion for Ran­dall is not what dark en­ergy is, but what it isn’t. Dark en­ergy has been in­voked since the 1990s to ex­plain ob­ser­va­tions that sug­gest that not only is the uni­verse ex­pand­ing, but that it is do­ing so at an ac­cel­er­at­ing rate.

The prob­lem is it’s not all that good at do­ing so. Whether dark en­ergy is a con­stant or a dy­namic force, ho­moge­nous or var­ied, and why it seems much less dense than cal­cu­la­tions pre­dict, are all open ques­tions.

To Ran­dall, this makes one thing very clear: there’s a lot more sci­ence still to be done.

“I think what dark en­ergy is telling me is that we feel like maybe we’re miss­ing some re­ally big idea,” she says.

“Be­cause we would naively ex­pect, not just through the stan­dard model [of par­ti­cle physics] but also through quan­tum field the­ory – that com­bines to­gether quan­tum me­chan­ics and spe­cial rel­a­tiv­ity – the amount of en­ergy to be enor­mously greater.

“The fact that it isn’t tells us there must be some­thing else go­ing on. And that’s the chal­lenge: to un­der­stand what that thing is. Is there some un­der­ly­ing prin­ci­ple – that this is the only place where we could end up with the kind of things we see? We re­ally don’t know the an­swer here.”

IMAGES 01 Ull­stein Bild / Getty Images 02 Roger Har­ris / Getty Images

in Dark Mat­ter and the Di­nosaurs, Lisa Ran­dall sug­gests the mass- ex­tinc­tion event at the end of the Cre­ta­ceous-palaeo­gene Pe­riod was caused by a me­teroid dis­lodged from the dis­tant Oort Cloud af­ter a col­li­sion with a disc of dark mat­ter. 02

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