The once the­o­ret­i­cal no­tion of grav­i­ta­tional waves is now the stuff of text­books. Could cos­mic strings be next? CATHAL O’CONNELL ex­plores the pos­si­bil­i­ties.

Cosmos - - Front Page - CATHAL O’CONNELL is a science writer based in Mel­bourne.

THESE HAIRLINE FRAC­TURES may still be threaded through space-time. Dubbed cos­mic strings, math­e­mat­i­cal mod­els see them as in­vis­i­ble threads of pure en­ergy, thin­ner than an atom but light-years long. The huge amount of en­ergy they con­tain also makes them ex­tremely heavy; a few cen­time­tres of cos­mic string might weigh as much as Mount Ever­est.

Pro­po­nents of cos­mic strings, like Thibault Damour, a the­o­ret­i­cal physi­cist at the In­sti­tute of Ad­vanced Sci­en­tific Stud­ies near Paris, are per­suaded by the maths that keeps pre­dict­ing their ex­is­tence. “The fact strings come up all the time makes me con­fi­dent that they ex­ist,” he says.

How­ever, as time cap­sules of the early uni­verse, cos­mic strings should re­tain fan­tas­tic en­er­gies – more than a bil­lion times greater than those re­leased by smash­ing par­ti­cles at the Large Hadron Col­lider, says Ken Olum, a the­o­ret­i­cal physi­cist at Tufts Uni­ver­sity in Bos­ton, who has con­tem­plated cos­mic strings for 20 years. “You can’t build an ac­cel­er­a­tor to test physics at that scale.”

Nei­ther can any of our astro­nom­i­cal in­stru­ments de­tect these van­ish­ingly thin, in­ter­ga­lac­tic fil­a­ments. For some physi­cists, a the­ory that can’t be tested is not worth pur­su­ing. It places cos­mic strings in the same cat­e­gory as “string the­ory”, their con­tro­ver­sial name­sake at the other ex­treme of the size scale. String the­ory in­vokes vi­brat­ing strings tinier than any sub­atomic par­ti­cle as the build­ing blocks of the uni­verse. For Matthew Bailes, an as­tro­physi­cist at Swin­burne Uni­ver­sity of Tech­nol­ogy in Mel­bourne, cos­mic strings are a “math­e­mat­i­cal cu­rios­ity” or worse, “an ex­otic fan­tasy”.

All that may be about to change. The nascent era of grav­i­ta­tional wave as­tron­omy – just two years old – may fi­nally de­liver a tool to test the ex­is­tence of cos­mic strings. We can’t see them but grav­i­ta­tional wave de­tec­tors might be able to hear the thrums and snaps cre­ated as they whip through space.

YOU MIGHT WON­DER HOW the empti­ness of space could be cracked. It helps to pic­ture the uni­verse through the eyes of a quan­tum field the­o­rist. Neo in The Ma­trix was close. He saw his world as a di­aphanous fab­ric of green­ish ones and ze­roes. Quan­tum field the­o­rists see the uni­verse as a fab­ric of all-per­vad­ing fields.

Fields fill space like a fluid, and what we call ‘par­ti­cles’ are rip­ples within the fluid. A pho­ton is a rip­ple in the elec­tro­mag­netic field (which we ex­pe­ri­ence as light), an elec­tron a rip­ple in the ‘elec­tron field’, a Higgs bo­son a rip­ple in the Higgs field, and so on. “There is noth­ing else ex­cept fields,” is the way re­tired Prince­ton physi­cist Free­man Dyson once put it.

Bri­tish field the­o­rist Tom Kib­ble, who died in June 2016, came up with the idea of cos­mic strings in 1976. He was mus­ing about the first split sec­ond af­ter the Big Bang when the uni­verse un­der­went a rapid ex­pan­sion, then cooled rapidly. This, he sug­gested, caused a phase change in the quan­tum fields, like wa­ter freez­ing to ice.

In a block of ice, some re­gions can freeze with their crys­tals in dif­fer­ent ori­en­ta­tions, rather like tiles be­ing laid si­mul­ta­ne­ously at dif­fer­ent ends of a room. Where they meet, they don’t fit to­gether smoothly, re­sult­ing in a crack. Like­wise Kib­ble sur­mised that the quan­tum phase changes in the early uni­verse would have caused the fields to align in dif­fer­ent ori­en­ta­tions, again caus­ing cracks – cos­mic strings.

OUR UNI­VERSE ex­ploded into be­ing, ex­panded at a fan­tas­tic speed and cooled. Per­haps too quickly. Ac­cord­ing to some physi­cists the rapid cool­ing might have cracked the fab­ric of the uni­verse.

Some of Kib­ble’s past pre­dic­tions have paid off. He in­de­pen­dently pre­dicted the ex­is­tence of a fun­da­men­tal par­ti­cle that im­parts mass to all oth­ers, now known as the Higgs bo­son. The dis­cov­ery of that par­ti­cle in 2012 won the No­bel prize.

Cos­mic strings, how­ever, were par­tic­u­larly prob­lem­atic to put to the test. They would only ap­pear at the edges of vast re­gions about as big as the ob­serv­able uni­verse. That is why, in Kib­ble’s orig­i­nal 1976 scheme, he wrote that “look­ing for cos­mic strings di­rectly would be point­less”.

There the story of cos­mic strings might have ended, but for a re­mark­able cal­cu­la­tion by the Ukrainian physi­cist Alexan­der Vilenkin about five years later.

BY THE EARLY 1980S MOST cos­mol­o­gists ac­cepted the Big Bang the­ory – the idea the uni­verse had evolved from the ex­pan­sion of a uni­formly hot, dense state. But the idea had one big prob­lem: the lumpy dis­tri­bu­tion of gal­ax­ies. The sim­ple the­ory of gal­axy for­ma­tion holds that they formed from clouds of hy­dro­gen that con­densed un­der the pull of grav­ity. That, how­ever, should yield evenly spaced gal­ax­ies. Fur­ther­more, the ear­li­est gal­ax­ies formed too quickly to be ex­plained by this process. So how did we get a lumpy uni­verse?

Vilenkin was think­ing about this prob­lem when he picked up on an aside in Kib­ble’s 1976 pa­per: when a cos­mic string wrig­gling in the void crossed it­self, it would chop off a self-con­tained ‘loop’. These loops would be light-year-sized hula-hoops in space – and enor­mously heavy. Vilenkin ran the num­bers, and re­alised the num­ber of cos­mic loops that would have ex­isted in the early uni­verse was cu­ri­ously close to the num­ber of gal­ax­ies. Per­haps, he rea­soned, a cos­mic loop could seed a young gal­axy, much like a grain of sand seeds a pearl.

The idea caused great ex­cite­ment among physi­cists. Stephen Hawk­ing wrote pa­pers on how the loops might col­lapse to form black holes. Oth­ers got in­ter­ested in how they bend and twist in space. Some even worked out how cos­mic strings might be de­tected: if the loops were abun­dant in the early uni­verse, they would have left a pat­tern on the ra­di­a­tion left over from the Big Bang – the so-called cos­mic mi­crowave back­ground.

In Novem­ber 1989 the Cos­mic Back­ground Ex­plorer (COBE) satel­lite was launched – a US$140 mil­lion ex­per­i­ment to map the cos­mic mi­crowave back­ground. But when the data was un­veiled in 1992, the cosmos showed no hint of cos­mic strings. In­stead, it favoured the idea gal­ax­ies had seeded around tiny quan­tum fluc­tu­a­tions that had been im­printed when the uni­verse was less than the size of an atom.

“That did cause peo­ple to lose en­thu­si­asm for cos­mic strings,” ad­mits Xavier Siemens, a the­o­ret­i­cal physi­cist at the Uni­ver­sity of Mil­wau­kee, “but they were not ruled out.”

Mean­while, Kib­ble’s strings were pop­ping up in other fields of physics. In 1996, two pa­pers in the same is­sue of Na­ture de­scribed ex­per­i­ments where liq­uid he­lium – a model for the early uni­verse – had been rapidly cooled. String-like de­fects ap­peared. Other string-ish flaws were found dur­ing phase changes in liq­uid crys­tals and su­per­con­duc­tors, ex­otic ma­te­ri­als whose prop­er­ties also fit Kib­ble’s equa­tions. “In fact, one might say de­fects and or­der­ing pro­cesses of the type Kib­ble dis­cov­ered have been found and stud­ied al­most ev­ery­where ex­cept in the uni­verse,” writes physi­cist Neil Turok, of Canada’s Perime­ter In­sti­tute, in his 2013 book Sym­me­try and Fun­da­men­tal Physics.

The cos­mic string idea also cropped up in the physics of the very small. In 2003 one sys­tem­atic re­view pub­lished in Phys­i­cal Re­view D con­cluded that al­most all the­o­ries of su­per­sym­me­try – the idea that all fun­da­men­tal par­ti­cles have as-yet-un­seen part­ners – pre­dict cos­mic strings of one form or an­other. Mean­while Olum and oth­ers have run com­puter sim­u­la­tions show­ing that, if this pre­dic­tion holds true, there should be at least a bil­lion cos­mic string loops sprin­kled through the ob­serv­able uni­verse.

What was miss­ing was the real-life ob­ser­va­tion. But how do you de­tect some­thing thin­ner than an atom, as long as a gal­axy, and in­vis­i­ble to boot?

EN­TER GRAV­I­TA­TIONAL WAVES. In Septem­ber 2015 the Laser In­ter­fer­om­e­ter Grav­i­ta­tional-wave Ob­ser­va­tory (LIGO) de­tected grav­i­ta­tional waves re­ver­ber­at­ing from col­lid­ing black holes. (See Cosmos 68, p34) That added a new di­men­sion to astronomers’ abil­ity to scan the uni­verse. “Af­ter LIGO’S dis­cov­ery,” Damour says, “I im­me­di­ately thought, ‘Aha! Now it would be good if cos­mic strings were de­tected.’”

Cos­mic strings can’t be seen but they might be heard. Grav­i­ta­tional waves are rip­ples in space­time gen­er­ated by mas­sive ob­jects mov­ing ex­tremely fast – like a pair of in­spi­ralling black holes or neu­tron stars. (see p109) Or a writhing cos­mic string.

“What hap­pens is like a whip,” ex­plains Damour, who worked out the idea with Vilenkin in 2000. The crack of a bull­whip is ac­tu­ally a sonic boom caused when part of its tail moves faster than the speed of sound. Like­wise, as a cos­mic string loop wig­gles and bounces, some parts would be whipped up to the speed of light – and emit a burst of grav­i­ta­tional waves. The two physi­cists cal­cu­lated such a burst might be de­tectable by LIGO.

From 2005 to 2010, LIGO lis­tened but heard no whip crack. Since Septem­ber 2015, ad­vanced LIGO, an up­graded ver­sion which is four times more sen­si­tive, has con­tin­ued the vigil.

One dif­fi­culty in de­tect­ing the crack is that it would only be emit­ted in a par­tic­u­lar di­rec­tion, like the beam of a flash­light. LIGO would have to be right in the path of the beam.

That is why our best hope of de­tect­ing cos­mic strings is prob­a­bly not from their whipcracks but from their ro­ta­tions. As a loop of cos­mic string spins like a hula-hoop, it would emit grav­i­ta­tional waves – one wave for each turn of the hoop. Since the hoops could have a cir­cum­fer­ence of light-years, it could take decades to fin­ish a sin­gle spin.

In other words, this cos­mic hula hoop would gen­er­ate grav­i­ta­tional waves at an ex­tremely low fre­quency – way too low for LIGO to de­tect. You need an en­tirely dif­fer­ent kind of grav­i­ta­tional wave de­tec­tor; luck­ily we have one wait­ing in the wings.

A PULSAR TIM­ING ARRAY is a grav­i­ta­tional wave de­tec­tor the size of the gal­axy. Pul­sars are spin­ning neu­tron stars (col­lapsed cores of ex­ploded stars) emit­ting in­tense beams of light that ap­pear to blink on and off with a pre­ci­sion ri­valling atomic clocks. The North Amer­i­can Nanohertz Ob­ser­va­tory for Grav­i­ta­tional Waves (NANOGRAV) has been ob­ses­sively tim­ing a few dozen pul­sars for a decade.

Any de­vi­a­tion from the norm could in­di­cate a pass­ing grav­i­ta­tional wave has stretched or squeezed the space­time be­tween us and the pulsar – caus­ing a slight lag, or ad­vance, in the tim­ing.

“We’re about to open a new win­dow on grav­i­ta­tional waves at low fre­quen­cies,” says Siemens, who is also di­rec­tor of NANOGRAV. To keep tabs on pul­sars across the whole sky, NANOGRAV is linked with two other pulsar tim­ing ar­rays, one us­ing ra­dio tele­scopes across Europe, and the other based at the Parkes Ob­ser­va­tory, in New South Wales.

So far the searches have drawn a blank, as Siemens and Olum an­nounced last Septem­ber.

“In physics, when you don’t find some­thing it’s not a fail­ure,” Olum says. “It’s a suc­cess of a dif­fer­ent kind, be­cause it tells us some­thing new about the uni­verse.” The no-show of cos­mic strings at cer­tain en­er­gies can al­ready be used to rule out some the­o­ries of su­per­sym­me­try.

The next level up in the search for cos­mic strings, and per­haps our only hope of a de­fin­i­tive an­swer, will come with the Laser In­ter­fer­om­e­ter Space An­tenna (LISA), a space-based grav­i­ta­tional wave de­tec­tor due to launch in 2034, which will lis­ten to the fre­quency band be­tween the high-pitched chirps caught by LIGO and the sub-bass mur­murs to which pulsar tim­ing ar­rays are at­tuned.

Even if the ev­i­dence con­tin­ues to come up neg­a­tive, some physi­cists are un­likely to let go of cos­mic strings. Siemens says the strings might have been formed with too low an en­ergy to give off any sig­nals “de­tectable in the near fu­ture”. An­other pos­si­bil­ity is that an­cient cos­mic strings ra­di­ated away their en­ergy and faded to noth­ing­ness too quickly af­ter the Big Bang to have left a last­ing im­pres­sion.

For now, cos­mic strings sit on the shelf along­side other beau­ti­ful ideas that could com­plete our un­der­stand­ing of the uni­verse, but lack em­pir­i­cal sup­port. “This is the beauty and the dan­ger of physics,” Damour says. “Some­times things ex­ist that we can never see.”

IM­AGES 01 Tatyun / Getty Im­ages 02 David Cham­pion / NASA

“In physics, when you don’t find some­thing it’s not a fail­ure,” Olum says. “It’s a suc­cess of a dif­fer­ent kind, be­cause it tells us some­thing new about the uni­verse.”

THE GRAV­I­TA­TIONAL SPEC­TRUM Grav­i­ta­tional wave de­tec­tors are our best hope for lis­ten­ing in on the cracks and hums of cos­mic strings. They are all tuned to dif­fer­ent fre­quen­cies. LIGO and VIRGO might hear high-pitched whipcracks, pulsar tim­ing ar­rays...


STRINGS AND LOOPS Many the­o­ries about the birth of the uni­verse sug­gest it is threaded through with cos­mic strings: cracks in space-time cre­ated dur­ing rapid cool­ing af­ter the big bang. This model shows the strings in or­ange and many smaller loops in...

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