The Uni­verse Is Ring­ing

Air & Space Smithsonian - - Front Page - BY MATTHEW FRAN­CIS

A new as­tron­omy of un­der­ground de­tec­tors and miles-long waves.

Grav­i­ta­tional waves were pre­dicted in Al­bert Ein­stein’s 1916 the­ory of gen­eral rel­a­tiv­ity. Ein­stein pos­tu­lated that the grav­ity of mas­sive ob­jects would bend or warp space-time and that their move­ments would send rip­ples through it, just as a ship mov­ing through wa­ter cre­ates a wake. Later ob­ser­va­tions sup­ported his con­cep­tion.

The im­print of this type of ra­di­a­tion on the old­est light in the uni­verse—the cos­mic mi­crowave back­ground (CMB)—IS one pre­dic­tion of in­fla­tion the­ory, which was first pro­posed in 1979. That the­ory states that the uni­verse, orig­i­nally chaotic quan­tum noise made of un­sta­ble par­ti­cles and space-time tur­bu­lence, ex­panded at an unimag­in­able rate, cre­at­ing these grav­i­ta­tional waves, smooth­ing out the chaos, and leav­ing the or­derly cos­mos we see to­day.

“Grav­i­ta­tional waves al­low us to see all the way back to the start to the uni­verse,” says Katherine Doo­ley, a post­doc­toral re­searcher at the Cal­i­for­nia In­sti­tute of Tech­nol­ogy in Pasadena. “The early uni­verse was too dense such that stan­dard elec­tro­mag­netic waves”—light—“would get scat­tered off of all the ma­te­rial, and could not travel to us to­day.” Ob­serv­ing these grav­i­ta­tional waves might con­firm what we know about gen­eral rel­a­tiv­ity, or they might give us new in­sight into the na­ture of the uni­verse, like whether the Big Bang was the be­gin­ning of all time, or if another uni­verse pre­ceded ours. The story of the uni­verse’s ori­gin is best told through this

THINK OF IT AS A LOW HUM, a rum­ble too deep to no­tice with­out spe­cial equip­ment. It per­me­ates ev­ery­thing—from the emp­ti­est spot in space to the dens­est cores of plan­ets. Un­like sound, which re­quires air or some other ma­te­rial to carry it, this hum trav­els on the struc­ture of space-time it­self. It is the trem­ble caused by grav­i­ta­tional ra­di­a­tion, left over from the first mo­ments af­ter the Big Bang.

pri­mor­dial rum­ble…if we can fig­ure out how to de­tect it. A few grav­i­ta­tional wave ob­ser­va­to­ries have been built—none has yet de­tected a wave—and more are planned over the next few decades. It’s an ex­cit­ing time for astronomers, who may soon have real ev­i­dence on which to ground this new branch of one of the old­est sci­en­tific dis­ci­plines.

PRAC­TI­CALLY EV­ERY AC­TION makes grav­i­ta­tional waves— you can cre­ate them by wav­ing your arms—but it takes se­ri­ous as­tro­nom­i­cal do­ings to gen­er­ate any­thing pow­er­ful enough to be de­tected. Earth or­bit­ing the sun pro­duces them, but they are low en­ergy (which is good for the long-term sta­bil­ity of our so­lar sys­tem); two pul­sars, the ul­tra-com­pact rem­nants of mas­sive stars, locked in bi­nary or­bit pro­duce far more sub­stan­tial waves. As those bod­ies sweep around each other, they com­press and ex­pand the struc­ture of space-time it­self, cre­at­ing a dis­tur­bance that trav­els out at the speed of light.

Grav­i­ta­tional waves from bi­na­ries like this are reg­u­lar, like a pure note from a sin­gle string of an in­stru­ment. In prin­ci­ple we could trace such a sig­nal back to its source, though, as with sound, tri­an­gu­la­tion is less pre­cise than for light. Pri­mor­dial ra­di­a­tion, on the other hand, comes from ev­ery place at once, since it was pro­duced ev­ery­where, when the uni­verse was much smaller, and trav­eled in all di­rec­tions from where it was cre­ated. The ul­ti­mate sources were tiny fluc­tu­a­tions in the quan­tum chaos that was the cos­mos right af­ter the Big Bang; the grav­i­ta­tional rip­ples cre­ated by the fluc­tu­a­tions stretched out when the uni­verse ex­panded rapidly into large, so­lar sys­tem-span­ning waves.

In the pipe or­gan that is the grav­i­ta­tional-wave uni­verse, in­fla­tion would be the long­est, largest pipes, pro­duc­ing sounds so low-pitched they are felt rather than heard. Bi­nary pul­sars would lie to­ward the mid­dle reg­is­ter, and vi­o­lent catas­tro­phes

like su­per­novas or cos­mic col­li­sions would be the short, pic­colo pipes. “Hear­ing” each type of wave re­quires equip­ment tuned to the ap­pro­pri­ate reg­is­ter.

The prin­ci­ple of de­tec­tion is sim­ple: As grav­i­ta­tional waves pass by, they mas­sage mat­ter, squeez­ing and stretch­ing it along the waves’ crests and troughs. The ef­fect of the wave is recorded as its “strain” on the de­tec­tor, though that strain is tiny by ev­ery­day stan­dards. So far, no­body has de­tected grav­i­ta­tional waves di­rectly, though in­di­rect de­tec­tions abound. The most fa­mous is the Hulse-tay­lor bi­nary pul­sar, named for the two re­searchers who dis­cov­ered it 40 years ago. Rus­sell Hulse and Joseph Tay­lor earned a No­bel Prize in physics for their ob­ser­va­tion that the two pul­sars were or­bit­ing closer and closer to­gether, and the en­ergy leav­ing the sys­tem as the or­bit de­cayed was the same as what the gen­eral the­ory of rel­a­tiv­ity pre­dicted would be lost due to grav­i­ta­tional waves. Since then, other astronomers iden­ti­fied even stronger grav­i­ta­tional wave sources, in­clud­ing a pair of white dwarfs—the last life stage of stars less mas­sive than the sun—which take just 12 min­utes to or­bit each other.

But this in­di­rect de­tec­tion isn’t sat­is­fy­ing: Astronomers want to de­tect the waves them­selves. “As with all new win­dows you open on the uni­verse, there’s go­ing to be things we’re go­ing to find [with grav­i­ta­tional waves] and we have no idea what the hell they are,” says Matt Be­nac­quista of the Univer­sity of Texas at Brownsville. His­tor­i­cally, ev­ery new type of as­tro­nom­i­cal ob­ser­va­tion, from ra­dio waves to gamma rays, has led to un­ex­pected dis­cov­er­ies, and grav­i­ta­tional waves are likely to be no dif­fer­ent. “That in many ways is the most ex­cit­ing part about do­ing this,” says Be­nac­quista.

Yet for a num­ber of rea­sons, the prob­lem of di­rect de­tec­tion is vex­ing. Like sound, grav­i­ta­tional waves are com­pa­ra­ble in size to what­ever pro­duces them. Large sys­tems, like big black holes or­bit­ing each other at the cen­ters of gal­ax­ies, will make very­long-wave­length, low-fre­quency waves, which re­quire suit­ably huge de­tec­tors. Even rel­a­tively small sources, such as a pair of pul­sars very close to col­li­sion, re­quire de­tec­tors mea­sur­ing more than a mile across. If they ex­ist, pri­mor­dial grav­i­ta­tional waves from in­fla­tion would ex­ist at a wide range of wave­lengths, but only ex­tremely long ones—those with a wave­length com­pa­ra­ble in size to Earth’s or­bit around the sun—would pro­vide a large enough sig­nal for astronomers to de­tect.

Tech­ni­cally we are bathed in the “sound” of grav­i­ta­tional ra­di­a­tion all the time, but the sound is faint and usu­ally too low-pitched. Grav­ity is by far the weak­est of the four fun­da­men­tal forces of na­ture, and its in­flu­ence grows smaller with dis­tance, so when a grav­i­ta­tional wave—even a rel­a­tively pow­er­ful one—passes by, very lit­tle en­ergy gets trans­ferred. To make mat­ters more dif­fi­cult, since the ef­fect on mat­ter is to push it around, de­tec­tors on the ground must deal with in­ter­fer­ence by any­thing that could make them vi­brate, from earth­quakes to big trucks rum­bling by.

So ob­ser­va­to­ries must be large, sen­si­tive to faint sig­nals, and iso­lated as com­pletely as pos­si­ble from any stray vi­bra­tions. That’s a tall or­der, solved best by build­ing mul­ti­ple ob­ser­va­to­ries or launch­ing de­tec­tors into space. Sci­en­tists, be­ing re­source­ful crea­tures, are do­ing both.

KATHERINE DOO­LEY GOT HOOKED on grav­i­ta­tional ra­di­a­tion re­search dur­ing a sum­mer un­der­grad­u­ate fel­low­ship at Cal­tech, which, with the Mas­sachusetts In­sti­tute of Tech­nol­ogy, op­er­ates the two Laser In­ter­fer­om­e­ter Grav­i­ta­tional-wave Ob­ser­va­to­ries (LIGO) in the United States: one in Rich­mond, Wash­ing­ton, and the other in Liv­ingston, Louisiana. For her doc­toral dis­ser­ta­tion, she spent four years de­sign­ing the ap­pa­ra­tus in Liv­ingston to be more sen­si­tive through the use of more laser power. She moved to Han­nover, Ger­many, to do her post­doc re­search at GEO600, a grav­i­ta­tional wave ob­ser­va­tory that be­gan op­er­a­tion in 2002. She’s now back work­ing with LIGO, just in time for the in­au­gu­ra­tion of the up­grades she helped in­sti­tute.

With her ex­pe­ri­ence in de­tec­tor design, Doo­ley un­der­stands the chal­lenge of grav­i­ta­tional wave ob­ser­va­tion bet­ter than most peo­ple. Ground-based ob­ser­va­to­ries like LIGO and GEO600 are sim­i­lar: pow­er­ful laser beams travel down two “arms,” at the ends of which the light strikes a mir­ror, which re­flects it back to its source. When a grav­i­ta­tional wave passes by, the mir­ror should move, chang­ing the dis­tance the light trav­els ever so slightly. By run­ning its two ob­ser­va­to­ries si­mul­ta­ne­ously, LIGO can bet­ter elim­i­nate lo­cal dis­tur­bances—when a grav­i­ta­tional wave passes through Earth, both ob­ser­va­to­ries should feel it. The size of grav­i­ta­tional wave sources ne­ces­si­tates long arms: At the LIGO fa­cil­i­ties the arms are four kilo­me­ters (two and a half miles) long; GEO600’S are 600 me­ters (hence the name).

Be­nac­quista is the kind of grav­i­ta­tional wave as­tro­physi­cist who prefers to take notes with a foun­tain pen. Like Doo­ley, he has worked with two ob­ser­va­tory projects, one of which was LIGO, from 2006 to 2013, al­beit from the the­o­ret­i­cal side: He char­ac­ter­izes the sources of grav­i­ta­tional waves that de­tec­tors might see. In 1995, a sum­mer re­search pro­gram at NASA’S Jet Propul­sion Lab­o­ra­tory con­nected him with an ex­cit­ing project just get­ting started, the Laser In­ter­fer­om­e­ter Space An­tenna (LISA). As the name sug­gests, LISA will be a space-based ob­ser­va­tory de­signed to or­bit the sun, made of three small space­craft in a V-for­ma­tion, each sep­a­rated by a mil­lion kilo­me­ters. The ba­sic op­er­a­tion is sim­i­lar to LIGO: By mea­sur­ing the dis­tance be­tween each space­craft us­ing laser light, re­searchers can de­tect a grav­i­ta­tional wave as it com­presses or ex­pands the space-time be­tween the space­craft.

Con­ceived as a joint project be­tween NASA and the Euro­pean Space Agency, LISA was orig­i­nally pro­jected to launch be­tween 2012 and 2016. How­ever, NASA with­drew par­tic­i­pa­tion in 2011, leav­ing the very ex­pen­sive project en­tirely up to ESA. By cut­ting back on the am­bi­tion a bit, the project sur­vived as Euro­pean LISA, or ELISA, but now the launch date is 2034, which is far enough in the fu­ture to make any fore­casts doubt­ful.

“I’m still kind of pes­simistic about LISA,” Be­nac­quista says. He’s hope­ful that when LIGO de­tects the sig­nal from col­lid­ing neu­tron stars in the next five years or so, the LISA launch might get pushed up by a few years, but that still doesn’t place it in the next decade. “Hope­fully I’ll still be alive!” he laughs rue­fully.

On track, how­ever, is the LISA demon­stra­tion mis­sion slated to launch later this year. Called LISA Pathfinder, it will test the

in­stru­men­ta­tion and phys­i­cal con­cepts the ob­ser­va­tory will use: lasers and masses, which are, like the mir­rors on LIGO, de­signed to move in­de­pen­dently of the space­craft. Ad­di­tion­ally, the GRACE Fol­low-on (Grav­i­ta­tional Re­cov­ery And Cli­mate Ex­per­i­ment) mis­sion, tar­geted to launch in 2017, will ob­serve tiny fluc­tu­a­tions in Earth’s grav­i­ta­tional field by mea­sur­ing the dis­tance be­tween two in­de­pen­dently fly­ing space­craft, just as LISA will do. The mis­sion is a fol­low-up to the previous GRACE and GRAIL (Grav­ity Re­cov­ery And In­te­rior Lab­o­ra­tory) probes, which per­formed the same duty for the moon in 2012. To grav­i­ta­tional wave re­searchers, the suc­cesses of these mis­sions, cou­pled with the de­lays on LISA, are a si­mul­ta­ne­ous joy and frus­tra­tion.

“BICEP2 IS AN EX­PER­I­MENT THAT AIMS to do just one thing and do it well,” says Walt Og­burn, a cos­mol­o­gist at Stan­ford Univer­sity who spent the sum­mer of 2009-2010 at the South Pole in­stalling the tele­scope. That one thing BICEP2 was de­signed to do: mea­sure the po­lar­iza­tion of the cos­mic mi­crowave back­ground. The CMB comes from a time when the uni­verse cooled enough to be­come trans­par­ent, about 380,000 years af­ter the Big Bang. Sim­i­lar to the way po­lar­iz­ing sun­glasses re­duce glare by fil­ter­ing light, var­i­ous cosmological ob­jects—big gal­ax­ies, cos­mic dust mol­e­cules, and grav­i­ta­tional waves—fil­ter the cos­mic back­ground ra­di­a­tion in in­ter­est­ing ways.

“Since these fluc­tu­a­tions [waves] are in space-time, they stretch or com­press space—and par­ti­cles—as they pass,” says Renée Hložek, a post­doc at Prince­ton Univer­sity in­volved with the Ata­cama Cos­mol­ogy Tele­scope po­lar­iza­tion project, or ACTPOL, in Chile, another ex­per­i­ment to mea­sure the po­lar­iza­tion of the CMB. The par­ti­cles Hložek refers to in­clude pho­tons, the par­ti­cles of light. Be­cause grav­i­ta­tional waves squeeze space-time in one di­rec­tion and stretch it in the other (some­thing known as ten­sor modes), Hložek says, “the pat­tern of po­lar­iza­tion in­duced from these grav­i­ta­tional waves is very spe­cific.” Ex­per­i­ments like BICEP2 and ACTPOL are try­ing to con­firm in­fla­tion the­ory by dis­cov­er­ing the pri­mor­dial grav­i­ta­tional waves such rapid ex­pan­sion would have cre­ated; they are ob­serv­ing the light—the Cmb—the waves should have po­lar­ized.

But while these ob­ser­va­to­ries are very good at mea­sur­ing po­lar­iza­tion, they can’t tell us ex­actly what is caus­ing it. In March 2014, re­searchers with BICEP2 an­nounced they de­tected the po­lar­iza­tion—ev­i­dence of grav­i­ta­tional waves—and thus con­firmed in­fla­tion the­ory. Stan­ford Univer­sity even re­leased a mov­ing video show­ing pro­fes­sor and BICEP2 re­searcher ChaoLin Kuo bring­ing news of the ex­per­i­ment’s re­sults to An­drei Linde, one of the most in­flu­en­tial au­thors of in­fla­tion the­ory. Chao-lin sur­prised Linde at his home with cham­pagne to toast the “smok­ing gun” that proved Linde’s life’s work to be true. The ex­cite­ment turned out to be pre­ma­ture. More or­di­nary phe­nom­ena—such as dust par­ti­cles in the Milky Way— can po­lar­ize light in a sim­i­lar way, and af­ter fol­low-up study by the Euro­pean Space Agency’s Planck space­craft, the BICEP2 team re­vised its ini­tial find­ings to say that it’s pos­si­ble the en­tire sig­nal was caused by cos­mic dust.

A suc­ces­sor ex­per­i­ment, BICEP3, in­stalled in Antarc­tica

early this year, along with ACTPOL and other stud­ies, will help by adding more data, but grav­i­ta­tional wave sig­nals might still hide from such de­tec­tion. That brings us back to the ques­tion of di­rect de­tec­tion. As Walt Og­burn points out, the Big Bang Ob­server would be able to set­tle the is­sue of in­fla­tion for good. A pos­si­ble fol­low-up to LISA, the project as ini­tially pro­posed would con­sist of 12 satel­lites in three group­ings that would or­bit around the sun. The vast scale would pro­vide the abil­ity to mea­sure grav­i­ta­tional ra­di­a­tion with wave­lengths com­pa­ra­ble to the size of the so­lar sys­tem—what we would ex­pect from in­fla­tion.

Not only could the Ob­server con­firm re­sults from po­lar­iza­tion ob­ser­va­to­ries, it also could dis­cover things about the fun­da­men­tal struc­ture of the cos­mos. As Og­burn points out, these early waves “also rep­re­sent new physics at en­er­gies a tril­lion times higher than what we can reach at the [Large Hadron Col­lider].” They could even help set­tle one of the loom­ing co­nun­drums in mod­ern physics: un­der­stand­ing the quan­tum me­chan­i­cal prop­er­ties of grav­ity.

Even though LIGO and GEO600 are re­mark­ably sen­si­tive in­stru­ments, they are sim­ply too small to ob­serve pri­mor­dial grav­i­ta­tional waves. Their mis­sion is else­where: de­tect­ing waves caused by col­lid­ing black holes, neu­tron stars, and other rel­a­tively com­pact ob­jects that pack a lot of en­ergy. And as large as LISA will be, its mil­lion-kilo­me­ter arms will still be too short for the largest grav­i­ta­tional waves. The Big Bang Ob­server is cur­rently the best hope we have, and it is far in the fu­ture.

Like other grav­i­ta­tional wave re­searchers, Katherine Doo­ley and Matt Be­nac­quista are philo­soph­i­cal about the lack of di­rect de­tec­tions so far. Grav­i­ta­tional wave re­search is dif­fi­cult, and the waves that would be eas­i­est to de­tect be­cause they’re the most com­mon—those from bi­nary pul­sars and black holes— could be de­tected only by big­ger de­tec­tors than we can build on Earth’s sur­face.

For that rea­son, no­body in the field was re­ally sur­prised when the first it­er­a­tions of LIGO didn’t see any­thing. Each phase of LIGO was al­ways in­tended to be a learn­ing process, much as engi­neers build and test many rocket pro­to­types be­fore set­tling on a design to launch valu­able pay­loads aboard. The deep­est con­cern now is “noise hunt­ing,” says Doo­ley, identi- fy­ing all the en­vi­ron­men­tal and tech­ni­cal dis­tur­bances that could get in the way of be­ing able to see a clear sig­nal from a grav­i­ta­tional wave when one comes along.

Ad­vanced LIGO, the ver­sion with Doo­ley’s up­grades that be­gan op­er­a­tion last Fe­bru­ary, has ten times the sen­si­tiv­ity of the orig­i­nal ex­per­i­ment. In prac­ti­cal terms, that means it can “hear” ten times as far, which rep­re­sents a vol­ume of the uni­verse that is a thou­sand times greater. In that pocket of space, says Doo­ley, sig­nals from col­lid­ing neu­tron stars “could be as in­fre­quent as once a year or once ev­ery other year, or even as fre­quent as al­most ev­ery day.” If Ad­vanced LIGO de­tected one grav­i­ta­tional wave sig­nal per month, that would be enough to keep re­searchers busy and happy for some time.

When astronomers fi­nally de­tect grav­i­ta­tional waves, what doors to our un­der­stand­ing of the uni­verse will open? Be­nac­quista doesn’t mind not know­ing in ad­vance: “That’s one of the things I re­ally like about as­tro­physics. It’s like a game where you’ve been told the con­clu­sion to a story, and now you have to in­vent the story that got you to that point.” With ev­ery new field in as­tron­omy, sci­en­tists dis­cov­ered some­thing un­ex­pected: ra­dio as­tron­omy led to pul­sars, X-ray astronomers found black holes, mi­crowave an­ten­nas de­tected the cos­mic mi­crowave back­ground. If this first gen­er­a­tion of grav­i­ta­tional re­searchers at last hears the rum­ble of the first mo­ments of the uni­verse, they may find them­selves, thrillingly, at the be­gin­ning again.

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