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Einstein proved right: LIGO, U of T astrophysicists detect gravitational waves

February 12, 2016

“We see today that black holes exist in the uni­verse and they do col­lide!” Asso­ciate Pro­fes­sor Har­ald Pfeif­fer says (image cour­tesy NASA)

For the first time, sci­en­tists have observed grav­i­ta­tion­al waves – rip­ples in the fab­ric of space­time from a cat­a­clysmic event in the dis­tant uni­verse, like­ly the col­li­sion of black holes.

The dis­cov­ery, made by the team at the Laser Inter­fer­om­e­ter Grav­i­ta­tion­al Obser­va­to­ry (LIGO), promis­es to open a new win­dow into the cos­mos.

A team of astro­physi­cists at the Uni­ver­si­ty of Toron­to played an instru­men­tal role in pro­vid­ing some of the cal­cu­la­tions that enabled a suc­cess­ful search for the waves. They’re part of the LIGO Sci­en­tif­ic Col­lab­o­ra­tion, a group of more than 1,000 sci­en­tists from uni­ver­si­ties in the Unit­ed States and 14 oth­er coun­tries.

“It is absolute­ly stun­ning to see two ground-break­ing dis­cov­er­ies at once,” said Asso­ciate Pro­fes­sor Har­ald Pfeif­fer, U of T’s lead on the col­lab­o­ra­tion. “Not only were grav­i­ta­tion­al waves mea­sured for the very first time pass­ing through Earth, but these waves were caused by astro­nom­i­cal objects that have nev­er been observed before.

“We see today that black holes exist in the uni­verse and they do col­lide!” said Pfeif­fer.

A sci­en­tist at the Cana­di­an Insti­tute for The­o­ret­i­cal Astro­physics (CITA) at the Uni­ver­si­ty of Toron­to, Pfeif­fer is also a fel­low at the Cana­di­an Insti­tute for Advanced Research (CIFAR) and hold­er of the Cana­da Research Chair for Numer­i­cal Rel­a­tiv­i­ty and Grav­i­ta­tion­al Wave Astro­physics.

The grav­i­ta­tion­al waves were detect­ed on Sep­tem­ber 14, 2015 by the twin Laser Inter­fer­om­e­ter Grav­i­ta­tion­al-wave Obser­va­to­ry (LIGO) detec­tors, locat­ed in Liv­ingston, Louisiana and Han­ford, Wash­ing­ton, USA. The break­through has been accept­ed for pub­li­ca­tion in Phys­i­cal Review Let­ters.

Grav­i­ta­tion­al waves car­ry infor­ma­tion about their dra­mat­ic ori­gins and about the nature of grav­i­ty that can­not oth­er­wise be obtained.

The detect­ed waves were pro­duced dur­ing the merg­er of two black holes that occurred more than a bil­lion years ago, cre­at­ing a sin­gle, mas­sive spin­ning black hole. This col­li­sion of two black holes had been pre­dict­ed but nev­er observed.

Based on the observed sig­nals, LIGO sci­en­tists esti­mate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 bil­lion years ago. By look­ing at the time of arrival of the sig­nals – the detec­tor in Louisiana record­ed the event sev­en mil­lisec­onds before the detec­tor in Wash­ing­ton – sci­en­tists can say that the source was locat­ed in the South­ern Hemi­sphere.

In addi­tion to Pfeif­fer, U of T team mem­bers include post­doc­tor­al fel­low Prayush Kumar and PhD can­di­date Heather Fong, both at CITA. For­mer CITA Senior Research Asso­ciate Kipp Can­non had led the group since its incep­tion in 2010, before depart­ing just weeks ago for Tokyo Uni­ver­si­ty.

Can­non spear­head­ed the devel­op­ment and oper­a­tion of one of the search strate­gies that iden­ti­fied the grav­i­ta­tion­al wave in the detec­tor data.

“This detec­tion is beyond our wildest dreams,” Can­non said. “This sig­nal is so loud that we can see clear­ly the bend­ing of space­time by col­lid­ing black holes imprint­ed in the wave­form. It’s hard to exag­ger­ate how much these few sec­onds of data have advanced our under­stand­ing of nature.”

Pfeiffer’s con­tri­bu­tions to the LIGO project are informed by his work as the head of the Numer­i­cal Rel­a­tiv­i­ty group at CITA. The group is among the world’s lead­ers in sim­u­lat­ing col­li­sions of black holes on high-per­for­mance super­com­put­ers. Their cal­cu­la­tions pro­duced the grav­i­ta­tion­al wave­forms – the shapes of the sig­nals – that LIGO search­es for.

The results rely in part on the cal­cu­la­tions per­formed on Cana­di­an super­com­put­ers, includ­ing U of T’s SciNet. The SciNet sys­tems have been instru­men­tal dur­ing the past years for cal­cu­la­tions of bina­ry black hole merg­ers. These cal­cu­la­tions enabled sci­en­tists to cross-check and ver­i­fy some of LIGO’s results report­ed today.

“I start­ed grad­u­ate school at U of T because I want­ed to study grav­i­ta­tion­al waves, and CITA is cur­rent­ly the only research facil­i­ty in Cana­da involved in this excit­ing field,” said Fong, who, togeth­er with Kumar, con­tributed to and val­i­dat­ed the wave­forms.

Accord­ing to gen­er­al rel­a­tiv­i­ty, a pair of black holes orbit­ing around each oth­er lose ener­gy through the emis­sion of grav­i­ta­tion­al waves, caus­ing them to grad­u­al­ly approach each oth­er over bil­lions of years, and then much more quick­ly in the final min­utes.

Dur­ing the final frac­tion of a sec­ond, the two black holes col­lide into each oth­er at near­ly one-half the speed of light and form a sin­gle more mas­sive black hole, con­vert­ing a por­tion of the com­bined black holes’ mass to ener­gy, accord­ing to Einstein’s for­mu­la E=mc². This ener­gy is emit­ted as a final strong burst of grav­i­ta­tion­al waves. It is these grav­i­ta­tion­al waves that LIGO has observed.

“This first obser­va­tion of grav­i­ta­tion­al waves by LIGO is as excit­ing as the first look through tele­scopes was to the ear­li­est astronomers,” Kumar said.

Sup­port for the CITA mem­bers’ con­tri­bu­tions to the research comes from CIFAR, the Cana­da Research Chairs Pro­gram, the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da, the Province of Ontario and the Uni­ver­si­ty of Toron­to.