When astronomers saw a star ex­plode they knew – thanks to Ein­stein – that they could watch it again a year later. Some­times that dis­tor­tion can be so ex­treme and com­plex that we see mul­ti­ple images of the same galaxy, as if look­ing through warped, un­even

Cosmos - - Contents - KATIE MACK is a the­o­ret­i­cal as­tro­physi­cist who fo­cuses on find­ing new ways to learn about the early Uni­verse and fun­da­men­tal physics.

— Su­per­nova déjà vu

IN LATE 2015, the Hub­ble Space Tele­scope turned to­ward a dis­tant galaxy to watch the ex­plo­sive demise of a doomed star. Su­per­nova Refs­dal was a re­play, hav­ing al­ready been seen and mea­sured a year be­fore. But be­tween here and there was a re­gion of space so crowded and man­gled with gal­ax­ies, the ex­plod­ing star’s light rays flowed through it like a river over rapids, twist­ing and bend­ing along mul­ti­ple paths. Hub­ble was watch­ing an­other an­gle on the same ex­plo­sion, from light rays that took the scenic route.

That light can split and bend is a fa­mil­iar con­cept. Ev­ery time we look through a lens, see a re­flec­tion off glass, or watch the dance of sun­light un­der­wa­ter, we are watch­ing light be­ing bent or split by the mat­ter through which it flows.

In space, as long as it doesn’t run into any­thing, such as a galaxy, there’s no mat­ter for light to flow through: it should travel in straight lines, unim­peded. But thanks to Ein­stein we know it doesn’t. He hy­poth­e­sised that when there’s mat­ter nearby, empty space it­self can bend, stretch and com­press, car­ry­ing light beams along with it. In fact, this phe­nom­e­non of grav­i­ta­tional lens­ing (see page 106) was one of the first pieces of ev­i­dence to sup­port Ein­stein’s gen­eral rel­a­tiv­ity the­ory.

He re­cast grav­ity, not as a force but as a con­se­quence of the dis­tor­tion of space by mas­sive ob­jects such as stars or gal­ax­ies. Imag­ine plac­ing a bowl­ing ball on the mid­dle of a tram­po­line, and then rolling a ten­nis ball past it. The ten­nis ball won’t travel in a straight line, but will in­stead cir­cle around or fall into the dent in the cen­tre. This new pic­ture of grav­ity ex­plained why a planet or­bits a star. It also pre­dicted that light can’t pass by a mas­sive ob­ject in a straight line. The path of light through curved space would bend, too.

The first time grav­i­ta­tional lens­ing was ob­served, it rocked the sci­en­tific world. Ein­stein’s the­ory pre­dicted that stars in the same part of the sky as the sun would ap­pear to be shifted from their true po­si­tions, as the light pass­ing by the sun would be curved around by its dis­tor­tion of space. Dur­ing the next to­tal so­lar eclipse, when the moon blocked out the sun’s light, astronomers were able to see the back­ground stars and mea­sure the dif­fer­ence, ex­actly as pre­dicted, be­tween the stars’ charted po­si­tions and where they ap­peared. A fa­mous New York Times head­line pro­claimed “Lights All Askew in the Heav­ens.” Ein­stein be­came an overnight sensation and our un­der­stand­ing of the na­ture of space and time changed for­ever.

These days, astronomers can use grav­i­ta­tional lens­ing to mag­nify dis­tant gal­ax­ies, help­ing us see and study the far reaches of the Uni­verse. In some cases, it can help us mea­sure the shape of space it­self on the largest scales. It can also map out in­vis­i­ble dark mat­ter: any­thing that has mass dis­torts space and bends the light be­hind it, giv­ing away its pres­ence.

Some­times that dis­tor­tion can be so ex­treme and com­plex that we see mul­ti­ple images of the same galaxy, as if look­ing through warped, un­even glass. Light orig­i­nally trav­el­ling off to one side might be pulled through a strongly curved part of space to reach us from an­other an­gle. Be­cause a curved path is longer than a di­rect one, there can be a de­lay be­tween two images of the same dis­tant light source.

When astronomers dis­cov­ered Su­per­nova Refs­dal, they knew to be ready for a re­play, be­cause they saw an­other im­age of the host galaxy, but in that one the su­per­nova had yet to go off. It in­volved a for­tu­itous align­ment: the very dis­tant host galaxy, where the su­per­nova oc­curred, and a gi­ant clus­ter of gal­ax­ies in be­tween. The com­bined grav­ity of an en­tire clus­ter turned space into a dis­tort­ing lens be­tween the su­per­nova’s host and us, mak­ing the host ap­pear to be in sev­eral places be­hind it.

Af­ter the com­bined grav­ity of the clus­ter split the im­age, a sin­gle galaxy acted as an­other lens, split­ting one of the re­sult­ing mi­rages four more times. When the orig­i­nal su­per­nova was seen in 2014, it was seen there, in qua­dru­pli­cate, fram­ing the in­ter­loper galaxy like a cross. In an­other part of the clus­ter, be­cause the light had taken a longer route, the doomed star was still in­tact. Astronomers cal­cu­lated the light trav­el­ling on that path would take about a year more. And then they saw the ex­plo­sion, again, right on time.

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