AstroTalk:BehindthenewsheadlinesofNovember2015RicharddeGrijs(何锐思)(KavliInstituteforAstronomyandAstrophysics,PekingUniversity)NewdevelopmentsofferaboosttoextragalacticdistancedeterminationWhileIbrowsedthroughmycompilationofrecentdevelopmentsinastronomythispastmonth,twostoriespiquedmyinterestinparticular.ThoseofyouwhoknowmepersonallywillalsoknowthatIhaveakeeninterestinmeasuresthatallowustodeterminethesizeandshapeoftheUniverse,aswellasthephysicalprocessesunderlyingthem.Iwroteahigh-leveltextbookondistancedeterminationinastronomyafewyearsago,sowhenIcameacrosssomeexcitingnewinsightsinthiscontext,Idecidedtosharethesewithyouinthismonth’sarticle.Lessthan20yearsago,welearnedthattheexpansionrateoftheUniverseisaccelerating,propelledbyamysteriouspressurecalled“darkenergy.”ThatdiscoverywasmadepossiblebycarefulobservationsofTypeIasupernovae;extraordinarilybrightandremarkablysimilarinbrightness,theyserveas“standardcandles”—astronomicalobjectswhoseintrinsicluminositycanbecalculatedfromanotheroftheirobservableproperties—essentialforprobingtheUniverse’shistory.Infact,TypeIasupernovaearefarfromstandard.Interveningdustcanmakethemappearredder(byabsorbingandscatteringmostlylightemittedatbluewavelengths)anddimthem,andthephysicsoftheirthermonuclearexplosionsdiffers—asinglewhitedwarf(anEarth-sizedstarasmassiveasourSun)mayexplodeafterborrowingmassfromacompanionstar,ortwoorbitingwhitedwarfsmaycollideandexplode.These“normal”TypeIasupernovaecanvaryinbrightnessbyasmuchas40%.Thisrangeinbrightnessescanbereducedbywell-provenmethods,butcosmologycontinuestobedonewithcataloguesofsupernovaethatmaydifferinbrightnessbyasmuchas15%.NowmembersoftheinternationalNearbySupernovaFactory,basedattheU.S.DepartmentofEnergy’sLawrenceBerkeleyNationalLaboratoryinCalifornia,havedramaticallyreducedthescatterinsupernovabrightnesses.Usingalmost50nearbysupernovae,theyidentifiedsupernovatwins—pairswhosespectraarecloselymatched—whichreducedtheirbrightnessdispersiontoamere8%.Thedistancetothesesupernovaecanbemeasuredabouttwiceasaccuratelyasbefore.

“Insteadofconcentratingonwhat’scausingthedifferencesamongsupernovae,thesupernova-twinsapproachistolookatthespectraandseekthebestmatches,soastocomparelikewithlike,”saysGregAldering,theBerkeleyLabcosmologistwholedtheteam.“Theassumptionwetestedisthatiftwosupernovaelookthesame,theyprobablyarethesame.”

HannahFakhouriinitiatedthesupernova-twinstudyforherdoctoralthesis.Shesaysthatthetheoreticaladvantagesofatwinsmatch-uphadlongbeendiscussed

atBerkeleyLab;fortheresearchers,oneofthemaingoalswasgatheringadatasetofsufficientqualitytotesttheirhypothesis.Fakhouri’stimingwasgood;shewasabletotakeadvantageofprecisespectrophotometry—simultaneousmeasuresofspectraandbrightness—ofnumerousnearbyTypeIa’s,collectedusingtheteam’sSuperNovaIntegralFieldSpectrograph(SNIFS)ontheUniversityofHawaii’s2.2-meter(diameter)telescopeonMaunaKea,Hawaii.“Nearby”isarelativeterminthiscontext,however;someofhersupernovaearemorethanabillionlight-yearsaway.Butallyieldmorecomprehensiveanddetailedmeasurementsthanthereallydistantsupernovaethatarealsoneededforcosmology.Despitethesurprisingresults,Fakhouridescribestheinitialresearchas“alongslog,”requiringhardworkandattentiontodetail.Onechallengewasmakingfaircomparisonsoftimeseries,inwhichspectraaretakenatfrequentintervalsasasupernovareachesmaximumluminosity,thenslowlyfades;differentcolours(wavelengths)brightenandfadeatdifferentrates.Cleaningupthespectraandrankingthesupernovaefor“twinness”wasdonecompletely“blind”—theresearchershadnoinformationaboutthesupernovaeexcepttheirspectra.

“Theunblindingprocesswassuspenseful,”Fakhourisays.“Wemighthavefoundthattwinningwascompletelyuseless.”Theresultwasarelief:thecloserthetwins’spectra,theclosertheirbrightnesses.

Theresultstronglysuggeststhatthelong-accepted15%uncertaintyinTypeIabrightnessisnotmerelystatistical;itmasksreal,butunknowndifferencesinthenatureofthesupernovaethemselves.Thetwinmethod’sdramaticreductionofbrightnessdispersionsuggeststhathiddenunknownsaboutthephysicalexplosionprocessesoftwinshavebeenseverelyreducedaswell,astrongsteptowardusingsuchsupernovaeastruestandardcandles.Theconventionalapproachhassofarbeentofitacurvethroughaseriesofdatapointsofbrightnessversustime:alightcurve.DimmerTypeIa’shavenarrowerlightcurvesandareredder;thisfactisusedto“standardize”supernovae,thatis,toadjusttheirbrightnessestoacommonsystem.Thetwinmethodbeatsthelight-curvemethodwithouteventrying.Plus,theteamfoundthatthiscanbedonewithjustonespectrum—anentirelightcurveisnotneeded.

Fakhourisays,“Supernovaeofferuniqueadvantagesforcosmology,butweneedmultipletechniques,”includingstatisticalmethodschartinghowdarkenergyhasshapedthestructureoftheUniverse.“ThegreatthingaboutNatureisthatitprovidesdifferentkindsofprobesthatcanbedecoupledfromoneanother.”

Supernovaeareasingularasset,notesAldering:“Supernovaefounddarkenergy,andtheystillprovidethestrongestconstraintsondarkenergyproperties.”

Whenbasedonareferencesampleofwell-measuredsupernovaelargeenoughforeverynewsupernovatofinditsperfecttwin,twin-supernovatechnology

couldleadtoprecisemeasuresofdarkenergy’seffectontheUniverseoverthepast10billionyears.Eachpointinspaceandtimesolabeledwillbeanaccuratemilestoneonthejourneythatledtotheuniverseweliveintoday.AtthetimewhenIcameacrossthesenewinsightsintotheuseofTypeIasupernovaeaspotential“standardizeable”candles,IwasjustabouttolectureabouttheminmygraduatecourseonthedistancescaleatPekingUniversity.Thisthusallowedmetoshowmystudentstheimmediateimpactofwhattheywerelearning–andthisalsoshowsthevalueofresearch-ledteaching!Aroundthesametime,Icameacrossanotherinterestingnewdevelopmentwithadirectbearingonfirminguptheextragalacticdistancescale,andindeedonethatwillsoonbethetopicofoneofmylectures.Thislatterapproachrelatestoanewlydevelopedtechniquetousequasars—powerfulsourcesdrivenbysupermassiveblackholesatthecentreofgalaxies—tostudytheUniverse’shistoryandcomposition.Todemonstratethenewmethod,basedonarelationbetweenaquasar’sluminosityatX-rayandultravioletwavelengths,ateamofItalianscientistsmadeextensiveuseofdatafromtheEuropeanSpaceAgency’sXMM–NewtonX-rayobservatory.TheX-rayMulti-MirrorMissionwaslaunchedinDecember1999.ThelargestscientificsatellitetohavebeenbuiltinEurope,itisalsooneofthemostsensitiveX-rayobservatorieseverflown.Morethan170wafer-thin,cylindricalmirrorsdirectincomingradiationintothreehigh-throughputX-raytelescopes.AtthecoreofmostmassivegalaxiesintheUniverseisasupermassiveblackhole—aconcentrationofmattersodensethatitattractsanythingnearby.SuchblackholeshavemassesfrommillionstobillionsoftimesthatoftheSunandaregenerallyidle,onlycapturingtheoccasionalstarorgascloudthatventurestooclosetothegalaxy’scentre.Asmallfractionofthemare,however,extremelyactive,devouringmatterataveryhighrate,causingthesurroundingmaterialtoshinebrightlyacrosstheelectromagneticspectrum,fromradiowavestoX-raysandgammarays.Insomecases,emissionfrommatterinthevicinityoftheblackholeissointensethatthecoreofthegalaxyoutshinesthestars.Theseobjectsappearaspointsourcesinthesky,likestars,andareknownasquasars,“quasi-stellarobjects.”Quasarsallowscientiststostudygravityintheverystrongfieldofthesupermassiveblackholes.Inaddition,comparingthepropertiesofquasarswiththoseofothergalaxiesthathostblackholescanrevealinterestingaspectsabouttheevolutionofgalaxiesovercosmichistory.ButoneotheraspectpiquedtheinterestoftwoscientistsfromtheArcetriAstrophysicalObservatoryinFirenze,Italy:theyrealizedthatquasarscanbeusedasprobesoftheexpansionhistoryoftheUniverse.

“ThehistoryofcosmicexpansionholdsawealthofinformationabouttheUniverse,includingitsageandtherelativeabundanceofitscomponents,andtopinitdownweneedtoobserveastronomicalsourcesatawiderangeofdistancesfromus,”explainsGuidoRisaliti,oneofthescientistswholedthestudy.“ButdeterminingdistancesintheUniverseisnotatalltrivialand

canbebestperformedonlywithafewclassesofsources.Inthisstudy,weshowhowitcanbedonewithquasars.”

Themainobstacletomeasuringdistancestoastronomicalobjectsliesinourignoranceoftheirtruebrightness,whichmakesitvirtuallyimpossibletoassesswhetherasourceisintrinsicallybrightorwhetheritjustappearssobecauseitisveryclosetous.ForrelativelynearbystarsinourownMilkyWaygalaxy,astronomerscangetaveryprecisehandleondistancesusingparallaxes,thetinyapparentshiftofastar’spositionintheskywhenviewedfromdifferentlocationsintheEarth’sorbit.However,thegreaterthedistancethesmallertheparallax,whichrestrictsthereachofthismethodtoourlocalcosmicneighbourhood.Fartheraway,astronomershavetorelyonstandardorstandardizeablecandles,liketheTypeIasupernovaeusedbytheBerkeleylabteamwediscussedatthebeginningofthisarticle.

“TypeIasupernovaeareapowerfultoolforcosmology,buttheycannotbeobservedatverylargedistancesfromus,sotheyaremostlyusedtoprobetherelativelyrecentUniverse,”saysElisabetaLusso.

FewTypeIasupernovaehavebeenobservedinearliercosmicphases,whenouralmost14billion-year-oldUniversewasyoungerthan5billionyears.

“ThisiswhywesuggesttocomplementTypeIasupernovaewithquasars,whichcanbeobservedinlargequantitiesouttomuchgreaterdistances,probingcosmichistoryuptotheepochwhentheUniversewasonlyonebillionyearsofage,”sheadds.

Todeterminehowfarquasarsarefromus,RisalitiandLussousedaninterestingpropertyofthesesources:alinkbetweentheamountoflighttheyemitatultravioletandX-raywavelengths,whichhasbeenknownsincethelate1970s.Bothtypesofemissionderivefromtheblackhole’sactivity,althoughtheyarecausedbydifferentprocesses.Astheaccretedmaterialflowstowardstheblackholethroughan“accretiondisk,”itisheatedbyfrictionandshinesbrightlyatvisibleandultravioletwavelengths.Then,partofthelightemittedbythediskinteractswithnearbyelectrons,receivinganextraenergyboostandturningintoX-rays.Thekeypointunderlyingtheapplicationofthisrelationtocosmologyisthatthelinkbetweentheluminositiesatthetwodifferentwavelengthsisnotlinear.Thismeansthattheratiobetweenaquasar’smeasuredX-rayandultravioletemissionisnotfixed,butvariesdependingontheultravioletluminosityitself.Sobymeasuringaquasar’sX-rayandultravioletemissionthescientistscanestimatetheluminosityatultravioletwavelengths;inturn,thiscanbeusedtogaugethequasar’sdistance.Whilethephysicalmechanismunderlyingthisrelationisunclear,RisalitiandLussocouldstilluseittotreatquasarsasstandardcandlesandemploythemasdistanceindicatorsforcosmologicalstudies.Todoso,theycompiledasampleof1138quasarswithbothultravioletandX-raymeasurements.

“First,weverifiedthattherelationbetweenultravioletandX-rayluminosityholdsforquasarsobservedatanycosmicepoch:thisisanessentialconditionifwewanttotreatthemascosmologicalprobes,”explainsRisaliti.

Then,thescientistsdetermineddistancestothequasarsintheirsampleandusedthesetostudyhowtheexpansionoftheUniversechangedinthespanofcosmichistorycoveredbythesesources.

“QuasarsarealessprecisetooltomeasuredistancesthanTypeIasupernovae,buttheyyieldcomplementaryinformationaboutthedistantUniversethatisinaccessibletosupernovaobservations,”saysLusso.

ThepowerofthisnewapproachisbestunleashedthroughthecombinationofquasarsandTypeIasupernovae,spanningover13billionyearsofcosmicevolutiontoinvestigatehowtheUniversechangedacrossmostofitshistory.Infact,combiningdatafromcurrentsurveysofbothtypesofsourcesyieldsconstraintsontherelativeabundanceofdarkmatteranddarkenergythataretighterandmoreprecisethanthoseobtainedfromsupernovaealone.ThemethoddevelopedbyRisalitiandLussoappearsespeciallypromisinginlightoffuturesurveys,sincealargerquasarsamplemeanssmallererrorsonthecosmologicalparameters.Itappears,therefore,thatwearenowfinallygettingclosetobeingabletomeasurehigh-accuracydistancestoobjectsthatexistedwhentheUniversewaslessthanitscurrentage!Veryexcitingdevelopmentsindeed…

Figure1:Configurationofprogenitorbinarysysteminthecaseofared-giantstar(left)andawhitedwarf(right,buttoosmalltobeseen).Gasisflowingfromtheredgiant.Thewhitedwarfaccretesapartofthegasthroughtheaccretiondisk(bluewhitedisk

aroundthewhitedwarf).Oncethewhitedwarfhasattainedacertainminimummass(approximately1.4timesthemassofourSun),itwillexplodeasaTypeIasupernova.

Figure2:TypeIasupernovaeresultfromtheexplosionsofwhitedwarfstars.Thesesupernovaevarywidelyinpeakbrightness,howlongtheystaybright,andhowtheyfadeaway,asthebottomgraphshows.Theoreticalmodels(dashedblacklines)seektoaccountforthedifferences,forexamplewhyfaintsupernovaefadequicklyandbrightsupernovaefadeslowly.AnewanalysisbytheNearbySupernovaFactoryindicatesthatwhenpeakbrightnessesareaccountedfor,asshowninthetopgraph,thelate-timebehaviouroffaintandbrightsupernovaeprovidessolidevidencethatthewhitedwarfsthatcausedtheexplosionshaddifferentmasses,eventhoughtheresultingblastsareall“standardcandles.”

Figure3:Supernova2011fewasdiscoveredbythePalomarTransientObservatoryjusthoursafteritexplodedintheBigDipper.StudiesbytheNearbySupernovaFactoryofitscoloursandspectrumastheyevolvedovertimehaveproducedabenchmarkatlasofdatabywhichtomeasureallfutureTypeIasupernovae.(Credit:B.J.Fulton,LasCumbresObservatoryGlobalTelescopeNetwork)

Figure4:Thisartisticviewshowsanaccretingsupermassiveblackholeatthecoreofagalaxy.Activelyaccretingsupermassiveblackholesdevourmatteratveryhighrate,causingthesurroundingmaterialtoshinebrightlyacrosstheelectromagneticspectrum,fromradiowavestoX-raysandgammarays.Insomecases,emissionfrommatterinthevicinityoftheblackholeissointensethatthecoreofthegalaxyoutshinesthetotal

luminosityofitsstars:asaresult,theseobjectsappearaspointsourcesinthesky,likestars,andareknownasquasars.Astheaccretedmaterialflowstowardstheblackholethroughan“accretiondisk,”itisheatedbyfrictionandshinesbrightlyatvisibleandultravioletwavelengths,shownhereinredandyellow,respectively.Partofthelightemittedbythediskinteractswithhighlyenergeticelectronsinacoronanearthedisk(showninblue),receivinganextraenergyboostandturningintoX-rays.(Credit:ESA,C.Carreau)

Figure5:Thisgraphshowshowmeasurementsofdistantastronomicalobjectscanbeusedtostudytheexpansionhistoryoftheuniverse.(Credit:Risaliti&Lusso,AstrophysicalJournal,Volume815,2015)