Eu­clid: map­ping the dark uni­verse

The space­craft’s in­stru­ments will ob­serve bil­lions of faint gal­ax­ies to re­veal the sig­na­ture of dark en­ergy

All About Space - - Dark Energy -

“If we can be ex­act with Eu­clid, we’ll be able to tell if dark en­ergy is con­stant or if it’s dy­namic”

Paniez Paykari

a 3,200-megapixel cam­era – the world’s largest. Cur­rently un­der con­struc­tion in Chile, its 8.4-me­tre (27.5-foot) di­am­e­ter pri­mary mir­ror will cap­ture an im­age of the whole sky ev­ery three nights over a ten-year pe­riod.

And then there’s the Euro­pean Space Agency’s Eu­clid tele­scope, which is ex­pected to be launched into space in 2021. It will cover 15,000 square de­grees of sky – that’s ten-times the area of the Kilo-De­gree Sur­vey – ob­serv­ing a bil­lion gal­ax­ies and peer­ing 10 bil­lion years into the past. The im­ages it sends back to Earth will be spec­tac­u­lar – sim­i­lar in res­o­lu­tion to those taken by the Hub­ble Space Tele­scope.

In prepa­ra­tion for the launch, Paniez Paykari is mod­el­ling Eu­clid to en­sure the in­stru­ment per­forms as in­tended. A re­search as­so­ciate at the Mullard Space Science Lab­o­ra­tory, she ex­plains that while all mea­sure­ments come with sta­tis­ti­cal un­cer­tainty, Eu­clid will give cos­mol­o­gists more pre­cise data to work with. “In the past 20 years ev­i­dence from all probes has pointed to the ex­is­tence of dark en­ergy – that it’s prob­a­bly a cos­mo­log­i­cal con­stant but noth­ing more. If we can be ex­act with Eu­clid, we’ll be able to tell if dark en­ergy is con­stant or if it’s dy­namic.”

What if the trea­sure trove of data from new ob­ser­va­tions, ex­pected in the mid-2020s, sounds the death knell for the cos­mo­log­i­cal con­stant? One pos­si­bil­ity could be that, de­spite pass­ing ev­ery test thrown at it for decades, Ein­stein’s gen­eral the­ory of rel­a­tiv­ity is wrong. Or at least that grav­ity may, in some cir­cum­stances, work dif­fer­ently than the the­ory de­scribes.

“It might be grav­ity is not at­trac­tive on all scales. If you put two things close to each other they will at­tract be­cause of grav­ity, but it may be that if you put them far enough apart they will re­pel. It could be that grav­ity is re­pul­sive or some­how needs chang­ing on cos­mic scales,” says Kitch­ing.

Many mod­i­fied-grav­ity the­o­ries have been cooked up by the­o­ret­i­cal physi­cists, and only more data will de­ter­mine whether or not they’re cor­rect. By a quirk of fate, many of these mod­els were ruled out last year thanks to a grav­i­ta­tional wave sig­nal named GW170817.

“It came out of the blue,” says Kitch­ing. “A merger of two neu­tron stars pro­duced a grav­i­ta­tional wave sig­nal and a flash of light at the same time. These ar­rived [at Earth] at the same time too, which means that light and grav­i­ta­tional waves travel at the same speed.”

At a stroke, all the mod­i­fied-grav­ity the­o­ries that also pre­dicted dif­fer­ent speeds for light and grav­i­ta­tional waves bit the dust. But there are plenty left, and it’s likely to be quite some years be­fore the dark en­ergy rid­dle is solved once and for all.

So­lar ar­rayThe so­lar ar­ray con­sists of three pan­els which to­gether sup­ply up to 2,430 watts of elec­tric­ity de­pend­ing on the space­craft’s ori­en­ta­tion to the Sun. Vis­i­ble im­ager (in­side)VIS will take high-qual­ity im­ages equiv­a­lent in res­o­lu­tion to those cap­tured by the Hub­ble Space Tele­scope. It’s de­signed to mea­sure the shapes of gal­ax­ies. ThrustersThe thrusters are pow­ered by cold ni­tro­gen gas so as not to dis­turb any mea­sure­ments. This is sup­plied by four high­pres­sure tanks, hold­ing a seven-year sup­ply. Dichroic plate (in­side)The dichroic plate splits in­com­ing light, send­ing vis­i­ble light to the VIS in­stru­ment and near-in­frared light to NISP, al­low­ing ob­ser­va­tions of both si­mul­ta­ne­ously. Sun­shieldThe sun­shield pro­tects the pay­load mod­ule, which con­tains the in­stru­ments, from the Sun, and sup­ports the so­lar pan­els on the other side. Tele­scopeThe tele­scope has three mir­rors. Its pri­mary mir­ror is 1.2 me­tres (4 foot) in di­am­e­ter and made of sil­i­con car­bide with a sil­ver coat­ing.Near-In­frared Spec­trom­e­ter and Pho­tome­ter (in­side)The NISP in­stru­ment’s pho­to­met­ric mea­sure­ments will be in the near-in­frared to ob­tain the red­shifts of mil­lions of gal­ax­ies. Star track­ersThree star track­ers mea­sure the tele­scope’s at­ti­tude (the di­rec­tion it’s point­ing) by com­par­ing what it sees to a built-in star cat­a­logue.

The Abell 370 clus­ter con­sists of hun­dreds of gal­ax­ies held to­gether by grav­ity

Sun­set over the Large Synop­tic Sur­vey Tele­scope, un­der con­struc­tion in Cerro Pachón, Chile

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