Chris Lintott

$VWURQRPHUV DUH WU\LQJ WR UHVROYH WKH FRQ LFW be­tween lo­cal and pan-Univer­sal ob­ser­va­tions

Sky at Night Magazine - - NEWS -

Sky at Night pre­sen­ter

What do you do when your ob­ser­va­tions don’t agree? Chris delves into one of the en­dur­ing chal­lenges of par­ti­cle physics.

Mod­ern cos­mol­o­gists have a dif­fi­cult time of it. Thanks to 30 years or more of hard ob­ser­va­tional work, their free­dom to cre­ate new Uni­verses, each one more com­plex and in­ter­est­ing than the last, is se­verely cur­tailed. Any model they build has to match ob­ser­va­tions on a wide range of scales, pro­duc­ing sen­si­ble gal­ax­ies and ex­plain­ing the whole Uni­verse, from the near-be­gin­ning of its story to the present day.

It’s a com­plex jig­saw, so it’s lit­tle won­der that the com­mu­nity jumps on any sign that the pic­ture doesn’t quite fit to­gether. There don’t seem to be any ob­vi­ously miss­ing pieces just yet, but there are a few places where things aren’t as sim­ple as they could be. For ex­am­ple, those who study gal­ax­ies and those who use data from ESA’s Planck satel­lite to study the cos­mic mi­crowave back­ground dis­agree on the value of Hub­ble’s con­stant, which mea­sures the rate at which the Uni­verse is ex­pand­ing.

An­other source of ‘ten­sion’ (the word physi­cists use for dif­fer­ences that aren’t stark, but which won’t go away) is ex­am­ined in a new pa­per by Ian McCarthy of Liver­pool John Moores Univer­sity and col­leagues. They com­pare ob­ser­va­tions of the cos­mic mi­crowave back­ground with those of the cos­mic web of gal­ax­ies in the lo­cal Uni­verse. Each is sen­si­tive to both the to­tal amount of mat­ter in the Uni­verse, a quan­tity known as ‘Omega_M’ and how clumpy the ar­range­ment of that mat­ter is, cap­tured in a mea­sure­ment known as ‘sig­ma_8’.

There are lots of dif­fer­ent ob­ser­va­tions that can pro­duce mea­sure­ments of th­ese num­bers, which are re­lated to each other. A Uni­verse with more mat­ter is one in which grav­ity plays a more im­por­tant role, and there­fore one which will be lumpier. Mea­sure­ments taken by look­ing at lo­cal gal­ax­ies seem to favour lower val­ues of both than mea­sure­ments that look at the cos­mic mi­crowave back­ground, whereas if all is well they should agree.

The au­thors are the first to point out that es­ti­mat­ing the po­ten­tial mea­sures in each of the mea­sure­ments is dif­fi­cult, and it’s cer­tainly pos­si­ble that the ‘ten­sion’ could be re­solved by cor­rect­ing ei­ther set slightly. The ap­proach here is dif­fer­ent,

though; us­ing the won­der­fully named BA­HAMAS sim­u­la­tions, they in­ves­ti­gate how to change the Uni­verse to bet­ter agree with the ob­ser­va­tions.

The re­sults are sur­pris­ing. The team found that fix­ing the de­tails of the recipe that con­trols the sim­u­la­tion doesn’t help, but al­low­ing neu­tri­nos to have a more sub­stan­tial mass than nor­mally as­sumed re­ally does. The spread out mass of neu­tri­nos in sim­u­la­tions of the Uni­verse nor­mally helps to smooth

out clumpy struc­tures. But adding mass makes them more sig­nif­i­cant. We know that neu­tri­nos can’t be com­pletely mass­less, but the sim­u­la­tions sug­gest they’re about three times heav­ier than oth­er­wise as­sumed.

This is an ex­cit­ing re­sult. The kinds of masses needed are al­lowed by our cur­rent un­der­stand­ing of par­ti­cle physics, and it may be that the sub­tle dif­fer­ences in cos­mo­log­i­cal ob­ser­va­tions are telling us some­thing pro­found about th­ese tiny par­ti­cles. Some­times, if you want to un­der­stand the very small, you have to think big.

CHRIS LINTOTT was read­ing… The BA­HAMAS project: the CMB – large-scale struc­ture ten­sion and the roles of mas­sive neu­tri­nos and galaxy for­ma­tion by Ian G McCarthy et al Read it on­line at https://arxiv.org/abs/1712.02411

“There don’t seem to be any ob­vi­ously miss­ing pieces yet, but there are a few places where things aren’t as sim­ple as they could be”

The cos­mic mi­crowave back­ground is an im­print of ra­di­a­tion; the ‘af­ter­glow’ of the Big Bang

CHRIS LINTOTT is an as­tro­physi­cist and co-pre­sen­ter of The Skyat Night on BBC TV. He is also the di­rec­tor of the Zooni­verse project.

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