300,000 to 700 Million years a.B. (aFter tHe BanG)pqcd.elte.hu/~pg/Bang/BANG-42-69.pdf · was born with the Big Bang, whether consciously or not. By unplugging the aerial from a television

300,000to700Millionyearsa.B.(aFtertHeBanG)

cHapter2andthenthereWaslight

Forthenext300,000yearsfollowingthecataclysmicperiodofinflationtherewerenomajordevelopments.thephysicalconditionsthatcontrolledtheevolutionoftheUniverseremainedmoreorlessconstant.theUniversebecamealessviolentplace.

asthetemperaturedropped,sotheprotonsandneutronsbegantoslowdown;however,radiationandmatterwerestilllinked,asweshallsee.Fromourpointofview,thebiggestdifferencebetweenthisUniverseandtheUniverseweseetodayisthatinthoseveryearlytimesitwascompletelyopaque.

electromagneticwaves,includingvisiblelight,mayalsoberegardedasastreamofphotons,whichareparticleswithzeromassthatalwaysmoveat186,000miles(300,000km)persecond.inthestrangeworldofquantummechanics(whichis,perhaps,thebesttestedtheoryofmodernscience)wenolongerhaveacleardistinctionbetween‘waves’and‘particles’,buthavetoacceptthateverythingexistsassomethingcalleda‘wave-particleduality’,intermediatebetweenthetwo.Justliketheentitieswetraditionallythinkofasparticles,suchaselectronsandprotons,lightbehavessometimesasaparticle,thephoton,andatothertimesasifitwereawave.

eachphotoncarriesawell-defined‘quantum’ofenergy,theamountofenergybeingdeterminedbythecolourofthelight,sothatitisquiteinordertosaythatelectromagneticradiationis‘astreamofphotons’.letusnowfollowthepathofoneofthesephotons,perhapsreleasedbycollisionbetweenaprotonandananti-protonintheveryearlyUniverse.insuchcrowdedconditions,nophotoncouldtravelveryfarbeforehittingandbeingabsorbedbyanelectron,whichthuswouldgainenergy.eventuallythephotonmightbere-emitted,butinalmostallcasesinadifferentdirectionfromitsoriginalheading.thisprocesswouldberepeatedagainandagain,leavingthephotonseffectivelygettingnowhereveryfast.

However,whentheUniversehadcooledtoamere30,000degrees,around300,000yearsaftertheBigBang,asuddenchangetookplace.Beforethiscriticalmoment,theelectrons–thelightest,andthereforefastestoftheconstituentparticlesofordinaryatomicmatter–hadbeenmovingmuchtoofasttobecapturedbytheheavieratomicnuclei,butatatemperatureof30,000degreestheycouldnolongeravoidcapture.thefirstneutralatomswereformed.seenonthescaleoftheatom,thecapturedelectronsorbitalongwayfromthenucleus(atomsare,afterall,mostlyemptyspace),butcomparedwiththedistancebetweenatomstheyareveryclosetotheirnuclei.alargeexpanseofspacebetweeneachnewlyformedatomthereforeopenedup,andphotonsweresuddenlyfreetotravelforgreatdistances.inotherwords,matterandradiationwereseparated,and300,000yearsaftertheBigBangtheUniversebecametransparent.

echoesoftheBigBangthecapturingoftheelectronswasamazinglysensitivetothetemperatureoftheUniverse;assoonasthisdroppedbelowthecriticalvalue,thentheprocessoccurredwithremarkablerapidity.alongwiththefactthatthetemperatureoftheUniverseisalmostexactlythesamethroughouttheentireextentofspace(thanks,remember,toinflation)thismeans

theelectromagneticspectrum

thevisible(rainbow)partofthespectrum,thelightwe

canactuallysee,isonlyaverysmallpartoftheoverall

electromagneticspectrum.overthelast70years,

astronomershavebeguntocollectinformationfrom

rightacrossthespectrum.

thefirststar

thefirststarsarebelievedtohavebeenextremely

massive.Justoneortwoperprotogalaxywouldhave

beenenergeticenoughtocauseprofoundchangesin

theirenvironments,pavingthewayfor‘normal’stars

likeoursuntoform.

visible

Infrared

45

thattheprocessoccurredalmostinstantaneouslyacrossthewholeUniverse.theresultwasthatlightcouldtraveluninterruptedacrosstheUniversesothatsome13.4billionyearslaterwecanstillseeasnapshotofthisparticularmomentintheevolutionofourUniverse.thisabilitytolookbacktooneparticularinstantoftimeisuniqueinastronomy.Usually,whenwetrytolookatthedistantpartsoftheUniverse,ourviewisobstructedbyimagesofnearbygalaxies,thatemittedtheirlightmorerecently.thismagicaleventwhentheuniversebecametransparentisobservablenow,withoutobstruction,aswhatwecallthecosmicMicrowaveBackground(orcMB).

Manyreaderswillhaveobservedthesefaintechoesofthedeathofthe‘fireball’thatwasbornwiththeBigBang,whetherconsciouslyornot.Byunpluggingtheaerialfromatelevisionorretuningitawayfromachannel,youwillseeblackandwhitestatic.onepercentofthisstaticcomesfromthecMB–justunder13.4billionyearsafterbeingemitteditisstillabletointerferewithyourviewingoftelevision.

Whenseentoday,thefrequencyofthisbackgroundradiationisconsistentwithanemitteratanaveragetemperatureofonly2.7Kaboveabsolutezero.Whysocool,ifthisradiationisreallytheechooftheBigBangitself?thereasoningisquitestraightforward;theradiationwouldhavebeenemittedwhentheUniversewasatatemperatureof3000degrees.asittravelledtowardsus,thespacethroughwhichitwasmovingwascontinuallyexpanding,stretchingthelighttolongerandlongerwavelengths,andhenceleadingtocoolerandcoolerapparenttemperatures.thisisourfirstencounterwiththephenomenonknownasredshift,whichhascometobeoffundamentalimportance.

thediscoveryofthecosmicMicrowaveBackgroundgavestrongsupporttoseveralpredictionsoftheBigBangtheory.Forinstance,itwasfoundthattheradiationemittedconformedtothatpredictedforablackbody,ahypotheticalobjectwhichabsorbsalltheradiationthatfallsonit.ifheated,itemitsradiationinaspectruminwhichtheintensityoflightatanyparticularwavelengthdependsonlyonitstemperature.inpractice,this

BigHornantenna

thetelescopewithwhichrobertpenziasandarno

Wilsonfirstdetectedthecosmicmicrowavebackground

in1964ismoreaccuratelydescribedasamicrowave

hornantenna.itisstillonshowatBelllaboratoriesin

newJersey(withoutthepigeondroppingsthatinitially

confusedtheastronomers).

themicrowavesky

thisall-skypictureoftheskyseenatmicrowave

frequenciesreveals13.4billion-year-oldtemperature

fluctuations,seenascolourdifferences,thatcorrespond

totheseedsthatgrewtobecomegalaxies,red

indicatingrelativelywarmandblue/blackrelativelycool

regions.theimagewasbasedondataobtainedbythe

WilkinsonMicrowaveanisotropyprobe(WMap)satellite.

andthenthereWaslight

300,000–700millionyearsa.B.46

tellsussomethingaboutthenatureoftheemitter–forexample,theobjectwouldhavetobeisolatedfromexternalinfluences.thehot,denseandpracticallyopaqueUniverseoftheperiodbetweentheBigBangandtransparency300,000yearslaterwouldbejustsuchanemitter.theagreementbetweentheoryandobservationisnowsoperfectthatonmostplotsofthedatathethicknessofthelineusedtoshowthepredictionislargerthantheuncertaintyinthemeasurements,asituationveryrareinscienceanduniqueinobservationalastronomy.

atfirst,theradiationappearedtobeabsolutelyuniform;thereseemedtobenovariationslinkedwithdirection.evenaftersubtractingtheforegroundglowofmicrowavesemittedbyourowngalaxy,onepartoftheskyglowinginthecMBlookedmuchthesameasanyotherpart.ButtheUniverseweseetodayis‘lumpy’;therearehugedistances

redshift

Weneedtoconsiderlightasawave.Whenthis

ideawasfirstproposedtherewasagreatdealof

controversy–iflightisawave,whatdoesitmove

in?afterall,soundwavesdependonairtobe

transmitted,andwaterwavescanhardlyexiston

theirown.Manypeoplebelievedinafundamental

substancecalledtheether,whichwasall-pervading

andwithinwhichalllighttravelled,butduringthe

late19thandearly20thcenturiesthiswas

firmlyreplacedbytherealizationthatlightcould

beself-propagatingandwouldhavenoneedofa

surroundingmedium.

iflightisawave,therefore,ithasawavelength,

whichdeterminesbothitscolouranditsenergy.

redlight,forexample,isofalongerwavelength

andalowerenergythangreenlight,whichitself

isofalongerwavelengthandalowerenergy

thanbluelight.infraredlightisradiationwitha

theinfraredsky

thetoppanelshowsalongexposureimagetakeninthe

infraredwiththespitzerspacetelescope.atthebottom

istheresiduallightafterthesubtractionofallidentified

foregroundsources.itwasrecentlyclaimedthatthe

remainingglowcontainsultravioletlightemittedbythe

firststars,nowshiftedbycosmicexpansionintothe

infraredpartofthespectrum.iftheclaimisconfirmed,

thiswillbecomeoneoftheiconicimagesofastronomy.

47

betweentherelativelydensegalaxies,whicharethemselvesgroupedintoclusters,andtheclustersintosuperclusters.thesesuperclustersarethemselvesseparatedbyenormousvoids,nowbeginningtobeseenindetailinsurveyssuchastheanglo–australiantwodegreefield(2dFsurvey)andthesloanDeepskysurvey,whichreachoutabillionlight-yearsfromearth.WhicheverwaywepaintthepictureofourUniverseemergingfromtheseobservations,itiscertainlynotuniform,andsotherewasclearlysomethingwrong.HiddensomewhereintheseeminglyuniformearlyUniversetheremustbetheseedsofthestructureweseetoday.

thecosmicbackgroundradiationisnowthemoststudiedphenomenonofastrophysics,andithasmuchstilltotellus.itmarksourearliestviewofstructureintheUniverse.arecentmoredetailedlookatthecMBrevealedtemperaturevariationsamountingtono

wavelengthlongerthanthatoftheredlightwecan

see,andradiowavesareofstilllongerwavelengths.

attheshort-wavelengthendwecometoultraviolet

light,andthentoX-raysandgammarays.since

thecosmicMicrowaveBackgroundwasemitted,

thelightwedetecttodayhasbeentravellingtoward

usthroughanexpandingUniverse.thisexpansion

shouldnotbethoughtofasobjectsrushingaway

fromeachotherbutasanexpansionofspace

itself.asspaceexpands,itstretchesthelight

travellingthroughit,increasingitswavelength.Blue

lightbecomesgreen,thenred,theninfrared,and

wesayithasbeenredshifted.thisprocesscan

bevisualizedasaballoonbeinginflated(right).

everythingonthesurfacemovesfurtherawayfrom

everythingelse.HencethecMB,emittedinmuch

moreenergeticregionsofthespectrumisnow

detectedprimarilyaslow-energymicrowaves.

red-shiftedgalaxy

inthisdeepviewoftheUniversetakenbytheHubble

spacetelescope,thearrowpointstoaverydistant,

high-redshiftgalaxy.itappearsredwhencomparedto

therelativelynearbyforegroundgalaxiesinthisimage.


galaxies

light


morethanoneten-thousandthofadegree.theymaybesmall,butthesearetheancientseedsofthestructuresweseearoundustoday.itmaysoundstrangetomeasurevariationsindensitybymeasuringthetemperature,butthereisaverygoodreasonforit.asthecoBe(cosmicBackgroundexplorer)satelliteshowed,matteratthetimewhenthecMBradiationwasemittedwasnotabsolutelyuniform.regionswithadensityabovetheaverage,gravitationallyattractedstillmorematter.thecompressionheatedtheseregionsslightly–anditisthesevariationsthataredetectedandmeasured.

Withoutanyfluctuationsforgravitytoworkon,thetaskofproducingthenon-uniform,clumpyUniverseweseetodayfromacompletelyuniformUniverseatthetimeofthecMBwouldhavebeenimpossible.However,thedimensionsofthefluctuationsonthe

tracesoftheBigBang

capturedbythecosmicBackgroundexplorersatellite,

(coBe)thesemapsshowminutetemperature

differencesacrossthesky,reflectingdisconformities

intheearlyUniverse.attopismappedtherawdata,

themiddlemaphastheeffectoftheearth’smotion

throughspaceremoved,andatbottomweseethe

resultofcompensatingalsoforradiationfromtheMilky

Way,leavingonlythetemperaturedifferencesresulting

fromtheremainsoftheBigBang.

49

skyarealsoimportant.WhatourobservationsofthecMBproduceisessentiallyamapofthewholesky,anditiseasytoseethateachoftheblue(colder)andred(hotter)regionsappeartoberoughlythesamesize.theyareonaverageaboutoneangulardegreeacross,abouttwicetheapparentsizeofthefullMoon.Fromthissinglepieceofevidence,andsomecarefulthinking,cosmologistscanconfirmthattheUniverseisflat.thisispossiblebecauseweknowthereal,physicalsizeofthefluctuationsintheearlyUniverse;theyarepredictedbytheory.comparingthisexpectedsizetotheapparentsizetellsushowthelighthasbeenbentsinceitleftitssource,andthatdependsontheamountofmatterintheUniverse.themorematterthereis,themorethelightisbent.inaclosedUniversethelightwouldhavebeensignificantlybent,andtheneteffectwouldhavebeentomakethefluctuationsappearlargerthanexpected.inanopenUniverse–onewithoutmuchmatter–thefluctuationswouldhaveappearedmuchsmaller.infact,comparingsimulationtorealityrevealsthattheUniversehasjustthecriticalamountofmatter–andisflat.

thisdiscussionillustratesapointthatisthesourceofbothexcitementandfrustrationtocosmologists.excitement,becauseitrevealsthatthestudyofthemicrowavebackgroundcantellusnotonlyabouttheveryearlymomentatwhichitwasemittedbutalsoabouttheentirehistoryoftheUniversesincethen.thisisalsoaproblem;ifwewanttodrawfirmconclusionsabouttheearlyUniverse,wemustbecarefultodisentanglemorerecenteffects,whichcanbedifficulttodo.

thebarrieroflightWehaveseenthatbeforethecreationofthemicrowavebackgroundtheUniversewasopaque;nolightcouldtravelfarthroughit.Wecannomorelookbackintothiserathanwecanlookupfromtheearthandseetheinsideofacloud.thisanalogyisnotperfectbecauseacloudisnotinitselfluminous;abetterpictureisprovidedbythesun.thesun,viewedfromoutside,appearstohaveadefinitesurface(thephotosphere),butwhatweareseeingismerelytheboundaryatwhichthematerialbecomestransparent.insidethephotospherethegasissohot,luminousanddense,thatnophotonscanpassthroughwithoutcolliding–similartothestateofaffairsimmediatelyfollowingtheBigBang.outsidethephotospherethegasistransparentandphotonscanpassthroughfreely,similartowhathappenedintheUniverseimmediatelyaftertheeventoftransparency–themomentwhenthecMBwascreated.

lookingthroughcloudsontheearth,wehaveasimpleremedy–radiowaveseasilypenetratecloudsandsowecanstillgainsomeinformationfrombeyondorwithinthem.thesametrickwillnotworkwiththecMB.the300,000yearlimitappliestoallelectromagneticradiation,andseemstobeaninsurmountablebarrier.Howthencanwetalkwithconfidence,aswehavebeendoinguntilthelastfewparagraphs,aboutconditionsbeforethen?Fornow,wehavetorelyonourtheories,manyofwhichareabletomakepredictionsofhowthemicrowavebackgroundwilllook.WecanthencomparethesetheorieswiththeactualcMB,anddrawtheappropriateconclusions.

ideally,however,wewouldliketobeabletolookbackbeyondthisbarrier,andtherearenumerousproposalsastohowtoachievethis.Wemaybeabletodetecthighlyenergeticparticlesthathavesurvivedunchangedsincebeforethemicrowavebackgroundepoch.Wemaybedetectingsuchparticlesalready,intheformoftiny,almostmasslessneutrinosorotherexoticformsofmatter,butatrueneutrinotelescope,onewhichisabletodetecttheseparticlesandlocatetheirsource,hasyettobebuilt.

copernicus'sUniverse

thismapbycopernicuswasoneofthefirsttoshowthe

sunatthecentreofthesolarsystem.afundamental

assumptionunderlyingallofourattemptstounderstand

theUniverseisthattherenothingspecialaboutour

regionoftheUniverse,andthereforewecandraw

conclusionsaboutthewholefromwhatwecansee.

Weareguidedbytheso-calledcopernicanprinciple,

whichmoreformallystatesthatnotheoryshouldplace

theobserverinaspecialposition.sofarthishasbeen

vindicated–theearthisnotthecentreoftheUniverse,

andneitheristhesun.neitherofthemisatthecentre

ofourGalaxy,andourGalaxyisnottheonlyoneinthe

Universe,orevenparticularlydistinguished.



lookingbackintimecosmologistsmaynotbeabletohandlesamplesandsubmitthemtoanalysisinthelaboratory,aschemistsandphysicistscando,buttheyhaveonetremendousadvantage:theycanliterallylookbackintimeandobservetheobjectoftheirstudyexactlyasitwasmillionsofyearsago.toseefurtherandfurtherbackintime,remember,weneedonlytolookforobjectsthatarefurtherandfurtherawayfromtheearth.aswehaveseen,thisdoesnotapplytothoseeventsbeforethemomentoftransparency,whichliehiddenintheopaqueinfantUniverse,butfromnowonwearediscussingeventsthatwecouldpotentiallyobservedirectly.

thischapterbeganatthemomenttheUniversebecametransparent,amomentweseeechoedasthecosmicmicrowavebackground.recentexperiments,suchasBoomerang,MaximaandWMap(seepages52–53),haveconfirmedcoBe’sdetectionoftinyvariationsinthetemperatureofthisradiation.theseweinterpretasanindicationthattherewereindeedirregularitiesinthedensityoftheUniverseatthispointintimeofaboutonepartinten-thousand.yetthevariationsindensityweseearoundustodayaremuch,muchlargerthanthis.Weseehugegalaxysuperclusters,regionswherethousandsofgalaxiescrowdtogetherandotherareasofspacethatarealmostdevoidofmatter.

ourownMilkyWayGalaxyisjustoneofmillionsofspiralgalaxies,andyoumightimaginethatthereisnoreasontodoubtthatthegalaxies(orrather,thegroupsofgalaxies)aresimplyspreadthroughouttheUniverseatrandom.yetlarge-scalesurveysofgalaxiesrevealawealthofhoneycomb-likestructureonthelargestscale,includinga‘GreatWall’some30millionlight-yearslong.HowdidtheUniverseevolvefromitsearly,newlytransparent,almost-but-not-quiteuniformstatetoitspresentform?

Gravity,theuniversalforcetheonlyforcewewouldnormallyconsidersignificantatastronomicaldistancesisgravity,andthestrengthofthegravitationalpullofanobject–whetheritbeastar,aplanet,ahumanbeingoracloudofgas–dependsonhowmuchmatterthereiswithinit.notethat‘mass’and‘weight’arenotthesamething–massisameasureoftheamountofmaterialpresent,whereasweightdescribestheforceduetogravity.thereforeanastronautinearthorbitisweightlessbutiscertainlynotmassless.Wecoulddefinegravityasbeing‘theforcethatgivesmassweight’.Forinstance,theMoonisarelativelysmallmemberofoursun’sfamily,andhassuchaweakgravitationalpullthatithasnotevenbeenabletoholdontoanatmosphere.theearthismuchmoremassivethantheMoon,andthereforehasagreaterabilitytoattractobjectstowardit.thus,fortunatelyforus,itcanretaintheatmospherewebreathe.similarly,thedenseregionsintheearlyUniversehadagreatergravitationalpullthantheregionsthatwerelessdense,andsodrewinmaterialfromtheir

neutrinos

thesetinyparticleshavebeenstudiedby

astronomersforthelastthirtyyears.Quantities

wereproducedafewminutesaftertheBigBang;

othersareaby-productofthereactionsthatpower

stars,andareincrediblyunreactive.inthecourseof

readingthissentence,theoddsarethataneutrino

fromthesunhaspassedthroughyourbodywithout

reacting,enteredtheearthandemergedfromthe

othersideoftheplanet.inordertostudythem,

astronomersandparticlephysicistsbuilddetectors,

consistingofvasttanksofliquidwithwhichthe

occasionalneutrinomightreact.thesehavetobe

builtdeepundergroundbecauseonthesurface

therewouldbetoomuchcontaminationfrom

icecube

thelatestneutrinotelescope,icecube,currentlyunder

constructionin2006,willemploy4200detectorsin

70shaftssunkdeepintotheantarcticice.itishoped

thatthedetectorswillseeneutrinoflashesinacubic

kilometreofclearice.Hereweseeasensorbeing

loweredintoposition.

Howmanyscientistsdoesittaketochange

alightbulb?

Japan’ssuper-Kamiokandeexperimentusesmany

thousandsofphotomultipliertubessurroundinga

reservoirofultra-purewater,tocatchtheflashesoflight

fromneutrinointeractionsinthewater.in2001,mostof

thetubesexploded,necessitatinglarge-scalerepairs.

Here,scientistsinaninflatabledinghycheckthe

tubesasthetankisrefittedandrefilled.

particlessuchascosmicrays–high-energynuclei

thatslamintoourupperatmosphereatclosetothe

speedoflight,propelledacrosstheUniversebythe

mostpowerfulexplosionsknown,ofwhichmorelater.

intheircurrentform,thesedetectorsaretoosmall

tobeabletofunctionaspropertelescopes.they

cantellushowmanyneutrinosarereactingwith

thedetector,andmeasuretheirproperties,butthey

cannottellusfromwhichdirectionintheskythe

neutrinosarecoming.Forthat,wewillneedmuch

largerfacilities.oneunderconstructionisicecube,

whichwilluseacubickilometreoftheabsolutely

pure,transparenticefoundunderthesouthpoleas

avastneutrinodetector.


surroundings.this,ofcourse,furtherincreasedtheirgravitationalpull–andsoon,theprocessacceleratingallthetime.Here,ashasoftenbeenthecase,therichgotricherandthepoorgotpoorer!

insideeachofthesedenserregionstherewerefurtherlocalizedvariationsindensity,andthesamesortsofprocessesoperated–greatermass,greaterpull,morerunawaycollapses.Usingcomputers,wearenowabletoreconstructwhatwenton,andtobuildupamodelwhichgivesagoodrepresentationoftheevolutionofthelarge-scalestructurethatweseeinthepresent-dayUniverse.

Whereverstructuresarebeingformed,wehavetoconsidertwoopposingtendencies;theexpansionofspace,whichbeganwiththeBigBangand,locally,contractionundertheinfluenceofgravity.onceanobjectintheprocessofformationaccumulatedenoughmassitwasabletoresisttheoverallexpansion,andcollapsed.

atypicalancestorofagalaxyclusterwouldinitiallyhavebeensmall,growinginvolumewiththeexpansionoftheUniverse,allthetimeaccretingmatterfromitssurroundings.asitgraduallyranoutofmattertoaccumulate,itgrewevermoreslowlyuntilitsexpansionceased.theembryonicclusterofgalaxieshadreacheditsmaximumextentandwasthenabletocollapsetoitsfinalsize.theforceofgravityweakenswithincreaseddistance,andsoatthisstageintheevolutionoftheUniverse,collapsewasonlypossibleonsmallscales–thefirstgalaxies,stillmereagglomerationsofgas,wereforming.

GloomytimesWhatdidtheseaggregationslooklike?Wecan’tseethemaswearestilllookingatwhatMartinrees,the15thastronomerroyal,hascalledthe‘Darkages’.Duringthisperiod,whichbeganimmediatelyaftertheepochofthemicrowavebackground,therewerenotyetanystarstolighttheUniverse.

Buttherewas,ofcourse,thecomparativelyrecentechoofthemomentoftransparency.thisradiation(whichperhapsatthispointweshouldcallthecosmicelectromagneticradiationBackground,insteadofthecMB)startedlifeatabout3000degrees,aroundthetemperatureofanoxyacetylenetorch,sotherewasinfactadiffuseglow,everfading,andbecomingredder,allthroughthisperiod.infactitmaybetruetosaytheUniversewasnevercompletelydark,justgloomy!

thegravitationalcollapseofthematerialthatwouldeventuallyformgalaxiescontinuedinthefadinglightastheUniversecooled.thencameadramaticchange;thegloomwassuddenlyilluminated,whenmultitudesofstarsburstforth.theUniverseexplodedinablazeoflight.Howsuddenthiswasisstillamatterfordebate,butinanycasethetimehascometoconsidertheepochofthefirststars.

BalloonwithaviewBeforethedevelopmentofspace-basedresearch,

astronomerswereseverelyhandicapped.Ground-

basedinstrumentsweresimplyunequaltothe

taskofmeasuringvariationsinthetemperatureof

themicrowavebackground.thefirstresultswith

sufficientresolutiontoviewthevariationswere

obtainedonlyin1992bythesatellitenamedcoBe

(cosmicBackgroundexplorer).newdataarrived

in1999,notfromspacebutfromballoon-borne

equipmentcarriedinahelium-filledballoonand

takingadvantageofthedryclimateofantarctica;

someastronomersbelievethatthesouthernpolar

regionmayprovidethebestsiteforastronomical

observationonearth,andfurthertestingis

underway.thereweretwoseparateprojects,

Boomerang(theBalloonobservationsofMillimetric

VirtualUniverse

astillframefromacomputersimulationofthe

developmentoftheearlyUniverseshowsasliceone

billionlight-yearsacross.eachfilamentcontainsthe

materialthatwillclumptoformthousandsofgalaxies,

andthesimulationshowsthattheUniversebecomes

moreclumpywithtime.thissimulationincludesthe

effectofdarkmatter,whichinteractsonlyviagravity.

itdoesnot,however,takeintoaccountthepossible

effectsof‘normal’matter,whichisamuchmoredifficult

computationaltask.nevertheless,bycomparing

simulationslikethesewiththeobservedreality,

scientistshavebeenabletolearnagreatdeal

aboutourUniverse.

Boomerang

theone-million-cubic-footballoonthatcarrieda

cMBexperimentintothestratosphere.itisshown

herejustpriortolift-off,withantarctica’sMount

erebusinthebackground.

53

intheBigBangitself,toallintentsandpurposesonlythreeelementswerecreated:hydrogen,heliumandasmallerquantityoflithium;tracesofotherelementswerenegligible.alltheotherelementsknowntoustodayweresynthesizedinsidestars.ithasoftenbeensaidthat‘wearestardust’,andthisistrueenough.thematerialinoursun,andtheentiresolarsystemhasalreadybeenrecycledthroughprobablytwopreviousgenerationsofstarformation.aswewillseelater,theexplosivelifehistoryofmanystarstransformsthehydrogenandheliumintoheavierelements.thepresenceofgold,for

extragalacticradiationandGeophysics)and

Maxima(MillimeteranisotropyeXperimentiMaging

array).Boomeranghadamaintelescopewitha

primarymirror1.2mindiameter,andwascarried

byballoonuptoaheightof37km(23miles);it

coveredanareaof1800squaredegreesofthesky

andproducedaresolutionsome35timesbetter

thanthatofcoBe.theseimages,whichbrought

themicrowavebackgroundintosharpfocus,

revealedhundredsofcomplexregionsvisibleastiny

variations–eachadifferenceofjust0.0001degree.

Boomerang’simagesweresurpassedbythose

fromWMap,theWilkinsonMicrowaveanisotropy

probe.thismissionrevealedtinyvariationsin

temperature;theevidenceoftheearlieststageof

galaxyformation.



instance,isaclearindicationthatthematerialcamefromasupernovaexplosion.thefirststarstoform,ontheotherhand,wereborncontainingonlythethreelightestelements.

inordertoformastar,aparcelofgasmustcollapse,andtocollapseitmustcool.inthepresent-dayUniverse,radiationfromcarbonandoxygenatomstakesenergyfromthecollapsingclumpsofgas,butintheepochwearedescribing–withnosourceofcoolingbutmolecularhydrogen–theprocessismuchlessefficient.asaresult,onlylargeclumpscancollapseandthestarsthatformedfromthemwerealsoverylarge.thefirststarswereindeedextremelymassive–perhapsasmuchasseveralhundredtimesthemassofourownsun.Withtheirhugereservesoffuel,onemightexpecttheseleviathanstoshineforfarlongerthanthesun’slifetime,butinfacttheoppositeistrue.theearlystarslivedfastanddiedyoung,actuallysurvivingforjustafewmillionyears.Bycomparison,oursunwillhaveatotalactivelifetimeofaboutninebillionyears.

thesourceofstellarenergytounderstandwhythisis,weneedtoconsidertheconditionsdeepinthecentresofstars.onlyonestarisavailableforclosestudy–oursun.thesun,likeallnormalstars,isahugeballofincandescentgas,bigenoughtoengulfwelloveramillionglobeswiththevolumeoftheearth.itssurfaceisatatemperatureof5600°c,whilethecore,wheretheenergyisbeingproduced,reachesaround15million°c.Wecannotlookfarintothesun,butwecanexamineitsconstitution.Wecandevelopmathematicalmodelsthatseemtofittheobservations,andsohaveconfidenceinourestimateofthecoretemperature.itcontainsagreatdealofhydrogen,approximately70percentofitsmass.itisthishydrogenthatisusedas‘fuel’.andthisisthesamesituationasinthefirststars.

Wehaveseenthatahydrogenatom,thesimplestofall,hasasingleprotonasitsnucleusandoneorbitingelectron.insideastar,theheatissointensethattheelectronisstrippedawayfromitsnucleus,leavingtheatomincomplete;theatomissaidtobe‘ionized’.atthestar'score,wherethepressureaswellasthetemperatureissoextreme,thesenucleiaremovingatsuchenormousspeedthatwhentheycollide,nuclearreactionsareabletotakeplace.nucleiofhydrogenarecombiningtobuildupnucleiofthesecondlightestelement,helium.admittedlythistakesplaceinaratherroundaboutway,butintheendfourhydrogennucleicombinetomakeonenucleusofhelium.therearealsoby-products;

Galacticcensus

thetwo-DegreeField(2dF)measuredtheredshift

(equivalenttothespeedtheyaremovingaway

fromus)of240,000galaxies.theexperimenttook

measurementsintwoslicesinoppositedirections

throughtheonebillionlight-yearvicinityofthesolar

system.thesespeedstranslateintodistances,and

theresultingthree-dimensionalmaprevealshowmass

isdistributedintheUniversearoundus.itshowsonly

between20and30percentofthetotalmassneeded

fortheUniversetobe‘flat’.sinceotherexperiments

indicatethattheUniverseisflat,thereisaclearneedto

speculateontheexistenceofsomekindofdarkmatter

–matterthatmakesitspresencefeltthroughgravity,but

isnototherwisedetectable.

55

aswellasthelightwereceivefromthestars,therearestrangeparticlescalledneutrinos,ofwhichmoreanon.intheprocessofhelium-building,alittlemassislostandalotofenergyisreleased.itisthisreleasedenergythatkeepsthestarsshining,andthelossinmassinoursunisequaltofourmilliontonneseverysecond.thesunismuchlessmassivenowthanitwaswhenyoustartedtoreadthisparagraph.thesupplyofhydrogenfuelcannotlastindefinitely,butthereisnoimmediatecauseforalarm.thesunwasbornaroundfivebillionyearsago,andasyetisnomorethanmiddle-agedbystellarstandards.Whenalltheavailablehydrogenhasbeenusedup,thesunwillnotsimplyfadeaway;butthatisanotherstorytobetoldinanotherchapter.

so,inthesunatleast,itisthelossofmassintheconversionoffourhydrogennucleiintoalightersingleheliumnucleusthatprovidestheenergythatpowersthestar.themostfamousequationintheworld,e=mc2,tellsusthatmass(m)isequivalenttoenergy(e).theconvertingfactor(c2),equaltothespeedoflightsquared(multipliedbyitself),islarge,sothatatinyamountofmasslossproducesavastamountofenergy.thesunlosessomefourmilliontonnesofmattereverysecond!

insidethesun

Fromthecoretothephotosphereisadistanceofnearly

435,000miles(700,000km),approximatelythesame

asatriptotheMoonandback.



Howdoesthisdisappearanceofmasscomeabout?eachofthefourhydrogennucleiisasingleproton–hydrogenisthesimplestofallatomsandconsistsmerelyofanelectronorbitingaproton–whereastheheliumnucleusismadeupoftwoneutronsandtwoprotons.However,aneutronisslightlyheavierthanaproton,sothatifwejustaddupthemassesoftheparticlesontheirownthenitseemstheheliumnucleusmustbeheavierthanfourhydrogenatoms;massseemstohavebeengained.yetaheliumnucleusreallydoesweighlessthanfourprotons,despitebeingcomposedofheavierparticles.Weareforcedonceagaintorememberthatweareintherealmwherequantumphysicsanditsassociatedeffectsdominate,andthesolutionlieshere.itistruethatifwecanmeasurethemassofaprotononitsownitisslightlylessthanthatofaneutron,butthesubatomicparticlesarenotfree.inaheliumnucleustheyareboundtogetherbythestrongnuclearforceandarelessfreetomove.thecreationofthesebondsbetweensubatomicparticlesreleasesenergyandwemeasureadropinthemass.

Whydoesthenucleusweproducehavetwoprotonsandtwoneutrons?lifeforastrophysicistsattemptingtostudythesereactionswouldbemuchsimplerifitwerepossibletoformastablebondbetweentwoprotonsalone.this‘lighthelium’couldthenbeproducedbythedirect,head-oncollisionoftwoprotons,whichwouldreleaseelectromagneticradiation.However,theforcesactingbetweentwoprotonsarenotquitestrongenoughtoholdthemtogetherwhen,astheybothcarrythesamepositivecharge,theelectromagneticforceisattemptingtopullthemapart.insteadofthissimplepictureofcombiningprotons,theinsideofthesun,andindeedofallstars,ishometoasubtleandasurprisinglyslowprocess.

aswecannotsimplyaddtogethertwoprotonswemustfindawaytobypassthisstate,whichblockstheformationofmorecomplicatednuclei.inthisdiscussionweonlyneedtoconsidernuclei,notwholeatoms,becauseatthetemperaturesthatprevailatthecentreofastartheelectrons,whichnormallyorbitthenuclei,makingupanatom,havefartoomuchenergytobecaptured.theonlyforcethatcanhelpistheweaknuclearforce,whichspontaneouslycausesprotonstodecayintoneutrons,releasingapositronandaneutrino.thenewly-createdneutroniscapturedbyapassingproton,creatingadeuteriumnucleus.Deuteriumisessentiallyheavyhydrogen,withaneutronaddedtotheusualproton.theweakforcelivesuptoitsname,andthisisthestepthattakeslongest–aprotonmightspendanaverageoffivebillionyearsatthecentreofthesunbeforeitisabletoformadeuteriumnucleus–butfromhereoninthingsspeedup.

inanaverageofasecondorso,thedeuteriumnucleuswillsnapupanotherprotonformingastablenucleuswithtwoprotonsandaneutron–helium-3,alightformofhelium.inanaverageof500,000yearsthisnucleuswillcollidewithanother,formingthemorefamiliarheliumnucleuswithtwoprotonsandtwoneutronsandreleasingtwoprotons,whichstartthecycleagain.thisstepisdelayedbythedifficultyofforcingtwolargepositivelychargednucleitogether.thestrongforce,whichoperatesoverextremelyshortdistances,pullsthenucleitogether,buttheyarerepelledbytheelectromagneticforce,whichkeepspositivelychargedparticlesapart.eventuallythenucleiwillpasscloseenoughforthestrongforcetoact,andweareleftwithenergyintheformofradiation,apositron,whichcombineswithitsantiparticleandreleasesmoreenergy,andaneutrino.

neutrinosaretinyparticlesthatmoveathighspeedandrarelyinteractwithotherparticles,andsotheyshootoutfromthecentreofthesunrelativelyunimpededbythemassofgassurroundingthecore.someofthemreachearth,wherevastdetectorshave

nuclearfusion

Hydrogenatomsfusetoformheliumintheheartof

thesun,generatingtheenergythatgivesuslight,

heatandlife.thisprocessisknownastheproton–

protoncycle.

beenconstructedtofindthem.Formanyyears,thiscausedaproblemasthereweresimplytoofewneutrinosbeingdetected–itwasthoughtonemustbeproducedforeachsequenceofcollisionsthatproducedaheliumnucleus.However,itturnsoutthattheneutrinoshavearemarkableabilitytochange‘flavour’,ortype,enroute.particlephysicistsknowthattherearethreekindsofneutrino,anditturnsouttheyhavetheabilitytoswitchbetweenkindsovertime.theoriginalexperimentsweresensitiveonlytooneparticularkindofneutrino,andthereforemissedalltheothers.theseexperimentsconfirmedthatourpictureofwhatisgoingoninthecentreofthesun,atfarhighertemperaturesthananyexperimentonearthcouldhopetoreach,isbasicallycorrect.they

theactivesun

thisimageshowsahugehandle-shapedprominence

–acloudofrelativelydenseplasmasuspendedinthe

sun'scoronaatatemperatureof60,000degrees.the

hotterareasarewhite,thecoolerdark.

spectrasirisaacnewtonwasthefirsttopassarayofsunlightthroughaglassprism,andtorealizethatthelightwasamedleyofwavelengthsfromred(long)throughtoviolet(short).Hepassedthesunlightthroughaholeandaprism,anddrewitoutintoacolouredsequence–thefirstspectrumintentionallyproduced.newtonnevertookthisexperimentmuchfurther(possiblybecausethelensesavailablewereofpoorqualityglass,butalso,nodoubt,becausehehadotherthingsonhismind),andthenextrealdevelopmentwasduetotheenglishscientistW.H.Wollaston,in1801.Wollastonusedaslitratherthanaholeinhisscreen,andthespectrumofthesunshowedasacolouredbandcrossedbydarklines.Wollastonbelievedthelinessimplymarkedtheboundariesbetweenthedifferentcolours–andtherebymissedthechanceofmakingagreatdiscovery.themanwhodidso,morethantenyearslater,wastheGermanoptician,JosephvonFraunhofer.

likeWollaston,Fraunhoferproducedasolarspectrum.Hemappedthedarklinesandfoundthattheydidnotvaryeitherinpositionorinintensity;forexamplethereweretwoveryprominentdarklinesintheyellowpartoftheband.Whatcausedthelines?theanswerwasgivenin1858byGustavKirchhoffandrobertBunsen,whomaybesaidtohavelaidthefoundationsofmodernspectroscopy.

Justasatelescopecollectslight,soaspectroscopesplitsuplightintoitsfullspectrum,verymuchlikearainbow.examinethespectrumofaluminoussolidorliquid,andyouwillseeacontinuousbandofrainbowcolours.Butaspectrumofagasunderlowpressurewillbequitedifferent;insteadofarainbowtherewillbeisolatedbrightlines–anemissionspectrum(seeright).KirchhoffandBunsensawthateachlinewasthetrademarkofoneparticularelementorgroupofelementsandcannotbeduplicated.thussodiumyieldstwobrightyellowlinesaswellasahostofothers.someelementshavecomplicatedspectra.iron,forinstance,hasthousandsoflines.Butthegreatinsightwastorealizethatthedarklinestheysawcrossingthecontinuousspectrumofthesuncorrespondedexactlytothebrightemissionlinesemittedbyglowinggasesinthelaboratory.Wenowknowthateachspectrallineisgeneratedbyaparticulartransitioninthestateofanelectronintheshellofagasatom.ifthegasishot,weseeanemissionline,astheelectrondropsdownanenergylevel,emittingenergy,andif

newton'ssketch

areproductionofanoriginalsketchbysirisaac

newton,showingthelayoutofhisfamousexperiment

tosplitwhitelightintoitscomponentcolours.


absorptionspectrumthispictureillustratestheappearanceofabsorption

lines.theintenselyhotsurface(photosphere)ofthe

sunemitswhitelight,whichpassesthroughtheslightly

coolerouterregions(thechromosphere,featuring

somesolarprominencesisseenhere).thislight,being

splitupintoitsconstituentcolours,orfrequencies,by

theprism,isrevealedtobemadeupofacontinuous

humped-back‘blackbody’spectrum,typicalofan

incandescentobject,crossedbythedark‘Fraunhofer’

lines,evidencethatthegasesinthesun’scoolerlayers

haveremovedtheseparticularfrequenciesfromthe

picture.

thegasiscool,viewedagainstabrightcontinuousbackgroundlikethesun,weseeadarkabsorptionline,sincetheelectronsaremovingupastepinenergylevel,andabsorbingenergyatthissamefrequency.thatdistinctivepairofdarklinesintheyellowpartofthesun’sspectrumisaclearsignatureofthepresenceofrelativelycoolsodiumgas.FromastudyoftheseFraunhoferlinesithasbeenpossibletoestablishtheabundanceofallgaseouselementsinthesun’sinneratmosphere,aregionoftenreferredtoasthe‘reversinglayer’.

thedarklines,nowcalledFraunhoferlines,cangiveinformationaboutmotionand,indirectly,distance.listentoanambulancesoundingitssiren.Whenthecarisapproaching,moresoundwavespersecondreachtheearthanwouldbethecaseifthecarwasstationary;thewavelengthiseffectivelyshortened,andthenoteofthehornishigh-pitched.Whenthecarhaspassedby,andhasstartedtorecede,fewersoundwavespersecondreachyou,thewavelengthislengthenedandthenotedrops.thisistheDopplereffect,namedaftertheaustrianwhofirstexplainedit.exactlythesamethinghappenswithlight.Foranapproachingsource,theshortenedwavelengthmakesthelightmoreblue;witharecedingsourcethelightisreddened.thecolourchangeistooslighttobenoticed,buttheeffectshowsupintheFraunhoferlines.ifallthelinesareshiftedtowardsthered,orlongerwavelengths,thesourceisreceding.thegreatertheredshift,thegreaterthevelocityofrecession.

nowletusreturntothesolarspectrum.thesun’sbrightsurface,orphotosphere,givesacontinuousspectrum.aboveitisalayerofgasatmuchlowerpressure(thechromosphere)andthismightbeexpectedtoyieldanemissionspectrum.infactitdoesso,butseenagainsttherainbowbackgroundthelinesare‘reversed’,andlookdarkratherthanbright.thepositionsandintensitiesarenotaffected;thetwodarklinesintheyellowpartofsunlightcorrespondtotheemissionlinesofsodium,andsowecanprovethatthereissodiuminthesun.

Historicspectra

thisfrontispiecefromnormanlockyer’s1874textbook

elementarylessonsinastronomy(neverbettered!)

neatlyillustratesthecorrespondencebetweenemission

andabsorptionspectra.thetwodistinctiveyellow

sodiumlinesarehereseenontheirowninemission

(spectrum5)andinabsorptionagainstacontinuous

spectrum(6).theyarealsovisiblebelowasFraunhofer

linesinthespectraofsirius(7),oursun(8),and

Betelgeux(9).theotherlinesinthesespectraindicate

thepresenceofmanyotherelements.

andthenthereWaslight 59

1

2

3

4

5

6

7

8

9

10

11

spectrum no.

alsoprovidedthefirstfirmevidencethatneutrinoshadafinite(althoughsmall)mass,foriftheywere,ashadbeenbelieved,completelymasslesstheycouldnotswitchfromonekindofparticletoanother.

thelifeofthefirststarsasthefirststarstoappearintheUniverse–thosewhoselightendedtheDarkages–weremassive,eachperhapsmatchingtheweightofasmanyas150suns,theincreasedgravitationalpressuresthatcamewiththeirimmensesizeheatedtheircorestoveryhightemperatures.thenuclearreactionsthatpowerstarsmusthaveproceededfaster,andthematerialwasuseduprapidly.thefirststarsranoutoffuelinaperiodperhapsasshortasamillionyears.

Beforethebirthofthefirststars,theUniversewasaseaofatoms,mainlyhydrogen.thegiantstarsignited,andtheirradiationspreadoutwards,knockingelectronsoutofatoms–ionizingthem.Gradually,eachnewstarwassurroundedwithabubbleofionizedgas.themorepowerfulstarswouldhaveproducedlargerbubbles.thestar’senergycouldonlyinfluencethegasouttoacertaindistance,butthesestarswerelargeenoughandenergeticenoughtocreatehugebubbles,tensofthousandsoflight-yearsacross.

Whathappenednext?occasionally,thebubblesaroundtwodifferentstarsmet.assoonastheydidso,allthematterwithinthemwasexposedtothecombinedlightofthetwo

centralMilkyWay

thisinfraredimagefromnasa'sspitzerspace

telescopeshowshundredsofthousandsofstarsatthe

coreofourGalaxy.invisiblelightthesestarscannotbe

seenatallbecausedustlyingbetweenearthandthe

galacticcentreblockstheview.

stars.poweredbytwicetheenergy,thebubbleexpandedmuchfasterandfurther.thismeantthattherewasagreaterlikelihoodthattheexpandedbubblewouldcollidewithanotherneighbour,andthewholeprocessaccelerated.overarelativelyshortperiod,aUniversedominatedbyneutralhydrogenevolvedintooneinwhichmorethan99percentofthematerialwasionized.

Blackholes–aonewaytripthereisanotherpossiblecandidateforthecauseofthisfirstionization.(ratherillogically,thisperiodisknownas‘reionization’.)almosteverygalaxy,includingours,hasamassiveblackholeatitscentre.ablackholeistheproductofthecollapseofamassivestar.ithasagravitationalpullsopowerfulthatnotevenlightcanescapefromit;itsescapevelocityistoolarge.theconceptofescapevelocityisstraightforwardenough;itisthevelocityanobjectmustattaintoescapefromthegravitationalfieldofamoremassivebody.eventuallytheescapevelocityofacollapsingstarrisesto186,000miles(300,000km)persecond,thevelocityoflight.lightcannolongerbreakfree,andsincelightisthefastestthingintheUniverse,theoldstarhassurroundeditselfwithaforbiddenzonefromwhichnothingcanescape.obviouslywecannotseeit,becauseitemitsnoradiationatall,butwecanlocateitbecauseofitsgravitationaleffectuponobjectsthatwecandetect–forexamplewhentheblackholeisonecomponentofabinary-starsystem.


theresultisthattheblackholeiscutofffromitssurroundings,andsincenoradiationcanescapewehavenowayofprobingtheinterior.Wecanonlyspeculateaboutconditionsinside.Fallingintosuchabodywouldcertainlybeaonewaytripandisemphaticallynottoberecommended;scientistshavecoinedtheword‘spaghettification’todescribethisprocess–warningenoughforanyonetemptedtotryavisit.

ablackholeisusuallyproducedbythecollapseofastar,eightormoretimesthemassofoursun,butthismaynotbetruefortheverylargeblackholesinthecentresofgalaxies,whichcontaintheequivalentofmillionsofsolarmasses.itmaywellbethattheseverymassiveblackholesformedatanextremelyearlystageoftheUniverse.ifthisisso,thenthefirstlightmayhavecomenotfromstars,butfrommatterheatingupasitasitfellintotheseblackholes,andthiswouldhavebeensufficienttocausewidespreadionization.inthiscase,theblackholesresponsiblearethenstillwithus,embeddedinthecentresoftoday’sgalaxies.itisnotyetclearwhichofthetwopossiblemechanismsofreionizationisactuallyresponsible.Weneedtolearnagreatdealmoreaboutthiscuriousepochbeforethisargumentcanbesettled.

supernovaeWhichevertheoryiscorrect,atsomepointthesefirst,curiouslylargestarsexisted,andtheirinfluenceontheirsurroundingsdidnotendatthetimeofreionization.Wehave

escapevelocity

throwanobjectupward,anditwillrisetoacertain

height,stop,andthenfallbacktotheground.throwit

faster,anditwillrisehigher.throwitupataspeedof

7miles(11km)persecond(admittedly,rathera

difficultthingtodo)anditwillneverfallback;the

earth’sgravitationalpullwillnotbestrongenough,

andtheobjectwillescapeintospace,whichiswhy

thisvalueisknownastheearth’sescapevelocity.the

escapevelocityofthesun,anormalstar,is386miles

(618km)persecond,whiletheescapevelocityofthe

Moon,whichhasonlyaneightiethofthemassofthe

earth,isamere1.4miles(2.4km)persecond.this

isnothighenoughtoholddownanatmosphere;any

airontheMoonhaslongsinceescapedintospace.

(actually,theMoondoeshaveanextremelythin

atmosphere;itiscontinuallyreplenishedwithdustfrom

thesurfaceandcontinuallylost.)toescapefromthe

earththeapolloastronautsrequiredamassivesaturnV

rocket,whereastoescapetheMoontheyonlyrequired

thesmallenginesonthelunarmodule,asseenhere.

63

alreadyseenthattheyledverybrieflives;moreover,theirdeathswereviolent.Unliketherelativelyquietfuturethatawaitsoursun,suchamassivestarmaybedestinedtosufferacataclysmicexplosion.

theouterlayersofastararesupportedbytheenergyproducedinthenuclearreactionstakingplaceinitscore.Whenthefuelforthisprocessisexhausted,theseouterlayerscollapse,increasingthepressureandthetemperatureofthecore.thesechangesallowheliumnuclei,theproductoftheprevioussetofreactions,tocollideandreactwitheachothertobuildupheavierelements.Meanwhile,hydrogenaroundthecorewillstillbebeingburnt;theresultisratherlikeanonionwithmanylayers,assuccessivelyheavierelementsareproducedinthecore.eventuallyironisproduced,andherethecyclestops.thenucleiofironarethemoststableofall,andthereforewhentheycollideenergyislostratherthanproduced.onceamassivestarformsacoreofiron,nothingcanpreventtheouterlayersfromcrashinginwards.adensecorequicklyforms,andashockwaverushesthroughthestar,propellingtherestofthematerialoutwardinavastexplosionofheatandlight–whichweseeasasupernova.

supernovaoutburstsarecertainlyveryviolent.evenmoreextremearehypernovae,whicharecreatedinverymuchthesamewaybutinvolveexceptionallymassivestars.yetwehavenotyetwitnessedtheultimate:themostcatastrophicphenomenaweknowarecalledgamma-raybursts.

supernovarings

astronomersarestillwaitingforthefirstobservable

supernovainourGalaxysincetheinventionofthe

telescope.insupernova1987awehadthenext

bestthing–asupernovaintheneighbouringlarge

Magellaniccloud.sevenyearsaftertheevent,the

Hubblespacetelescopeimagedthreeextraordinary

ringsaroundthesiteoftheexplosion.


attheheightofthecoldWar,militarysatelliteswere

launchedtolookforsuddenburstsofgammarays,

whichareoneofthesignaturesofnucleartesting.

theamericansatellitessentupforthispurpose

diddetectbursts,althoughtheywerenotinthe

leastwhathadbeenexpected.theburstslasted

foranythinguptoafewminutesandsometimesno

morethanafewseconds.

allthatcouldbefoundoutaboutthemwasthat

theyseemedtobedistributedevenlyaroundthe

Gamma-rayburstsGammaraysarethemostenergeticformofelectromagneticradiation,andhaveveryshortwavelengths–shortereventhanX-rays,atwavelengthsbelow0.01ofananometre(ananometreisonebillionthofametre).althoughthereisamoreorlessuniformbackgroundglowingammaraysthatisconstantalloverthesky,afewdiscretesourcesarefound.thesesuddenburstsofgammarays,lastinguptoafewminutes,areextremelypowerfulandcanbeseenrightacrossthevisibleUniverse.theinitialburstofgammaraysisfollowedbyan‘afterglow’inotherregionsofthespectrum,andtheidentificationofthisfading‘smokinggun’wasthekeythatallowedastronomerstodeterminethedistancetothemorerecentbursts;wenowknowthatthegamma-rayburstsareindeedveryremote.

supernova1987a

left,beforetheexplosiononFebruary23,1987,and

right,10daysafter.asinglesupernovacanoutshine

anentiregalaxy.

thegammaraystory

skyratherthanbeingsituatedatonegeographical

location,whichthankfullyruledoutanucleartest

astheorigin.Formanyyears,itprovedimpossible

todecidewhethertheburstswereweak,and

thereforenearby,orextremelypowerfuland

thereforeextremelydistant.itisnowbelievedthat

theseburstsemanatefromsourcesaroundabillion

light-yearsfromus,andareincrediblypowerful

–probablythebiggest‘bangs’sincetheBigBang

itself.

thepoweremittedinasingleburstisalmostunimaginable–duringitsentirelifetimethesunwillnotemitasmuchenergyasasingleburstwillmanageinafewminutes.

itseemsthat,althoughdifferentburstsmaybeduetodifferentcauses,manygamma-rayburstsareproducedbythedeathsofexceptionallymassivestars.rememberthatoncesuchastarhasrunoutoffueltopowernuclearreactions,theradiationemittedfromitscoreisswitchedoff,andgravityfinallywinsthebattle.theouterlayersofthestarrushinward,andthecentralregionscollapsecompletelytoformblackholes.theouterlayers,meanwhile,reboundandarethrownoutwardattremendousspeed.theenergyissogreatthattheatomicnuclei,assembledduringthestar’slifetime,arerippedapartandbrieflyeverythingrevertstohydrogen.However,theenergyavailableinthismassiveexplosion

crabnebula(M1)

thisisthefamousremnantofasupernovathat

explodedinaD1054,observedbychinese

astronomers.Withinthisnebulalurksaspinning

neutronstar,whichisallthatremainsofthestar’score.


canthendrivefurthernuclearreactions,whichfusethehydrogenatomsintoheavierelements,including,significantly,thosemoremassivethaniron.

Whenthestarinvolvedisaslargeasthosebelievedtomakeupthefirstgeneration,thisoutpouringofenergywillbegreatenoughtopoweragamma-rayburst.inthenearbyUniverse,wherethelargeststarsareonly20to30timesthesizeofthesun,weseetheirdeathsasrelativelymodestsupernovae.thelightfromasinglesupernova,however,isstillenoughtooutshinetheentiregalaxyinwhichitlies,andsohypernovaeshouldbevisiblefromrightacrosstheobservableUniverse.

Followingthisviolentdeath,ashockwaverippledoutfromtheexplosionatclosetothespeedoflight.asimilarprocesscanbeseeninHubblespacetelescopeimagesofnearbysupernovae.aswellasheatingthesurroundinggas,thespreadingshockwavefromadyingfirst-generationstarcausedsurroundinggascloudstocollapseinturn,triggeringtheformationofthenextgenerationofstars.asthesenewstarswereforming,theyaccumulatedelementsproducedinthefirstgenerationofstars,whichhadnotbeenavailableintheearlierperiod.theseatoms,particularlycarbonandoxygen,efficientlyradiatedawayenergyfromthecollapsingcloud.thisallowedittocoolandfragment,producingsmallerclumps,andeventually,smallerstars.consequently,thesesecondgenerationstarswereverylikethosethatweseetoday.thesmallestofthem(andhencethosewiththelongestlifetimes)mayevenstillbeshiningtoday,andwemaywellhavedetectedtheminourownGalaxy.

theexactmassofthesestarshadaprofoundeffectontheirfate.Forexample,starswithmassesgreaterthanabout300solarmasseswouldcollapsedirectlytoformmassiveblackholeswithnomaterialexpelledandnospreadingshockwaves.astarwithamassinanarrowbandaround160solarmassesproduceswhatisknownasapair-instabilitysupernova.theseexplosionshappentoproducevastnumbersofpositrons,theantiparticleoftheelectron.Whenparticleandantiparticlemeet,theyannihilate,producingenergy,andinthesesupernovaethisenergyisgreatenoughtopreventthecorecollapsing.noblackholeorneutronstarisformed,andallthematerialisthrownoutward,becomingavailablefortheformationofthesecondgenerationofstars.WebelievelargenumbersofstarsofthissizeformedearlyintheUniverse’shistory,andthismechanismisjustwhatthedoctorordered.

supernovaremnant

thisHubblephotographoftheremainsofsupernova

lMcn49showsbeautifulsilk-likesheetsofdebristhat

willeventuallyberecycledtoformnewstars.

Gamma-raybursts

awhole-skymapofthe2704gamma-raybursts

thatwererecordedoveraperiodofnineyearsby

thecomptonGamma-rayobservatory.theplaneof

ourGalaxyrunshorizontallyalongthecentreofthis

representation,from+180to–180.

+90

–180+180

–90


relativity–anobserver’sguidethephysicsofblackholesisnaturallywritteninthelanguageoftheGeneraltheoryofrelativity,anditisworthtakingthetimetotrytolearnsomeofthislanguage.accordingtoeinstein,iftwodifferentobservers,eachwiththeirseparateframeofreference,areaccelerating(ordecelerating)relativetoeachother,theirtimescaleswillnotagree.inotherwords,whileimayobservetensecondselapsing,you,whoareacceleratingawayfromme,mayobserveonlysix.

thetemptationisfirstofalltoaskwhoisright,andthentolookforsomesubterfugethatmayhavealteredtheclocks.yetrelativitytellsusfirmlythatbotharerightandthereisnotrick–differentobserversreallydoexperiencetimeflowingatdifferentrates.somerulesofcommonsensearepreserved;twoobserverswillalwaysagreeontheorderofevents,forexample.soalthoughonemaybelieveaprecededBbyaminute,andanotherthataandBweresimultaneous,itisimpossibleforanyobservertoseeBprecedinga.Hencecauseandeffectarepreserved,butmanyothercommon-senseideasthatseemsecondnaturetousmustbeabandoned.

Whyaresuchseemingparadoxesnotpartofoureverydayexperience?Wenevernoticeclocksrunningatdifferentrates,afterall.theansweristhat,fortunately,wedon’tliveanywherenearablackhole.Withoutextremeaccelerations,orhugevelocitiesneartothespeedoflight,orverylargeconcentrationsofmass,theeffectsaresosmallthatnewton’slawsofmotionstillworkverywell.einsteindidnotprovenewtonwaswrong,heextendednewton’sideastobeaccurateinthesemoreextremecases.

somuchfortheeffectoftheblackholeonthepassageoftime,butrelativityalsotellsushowitsimmensemassaffectsthespacearoundit.oneofthereasonsrelativity

Kepler'ssupernovaremnant

thiscombinedimagefromX-ray,infraredand

visiblelightobservationsshowshowKepler's

supernova,whichexplodedinourGalaxyover400

yearsago,appearstoday.amongthefeaturesareafast

movingshellfromtheexplodedstar,surroundedbyan

expandingshockwavesweepingupgasanddust.

69

isdifficulttounderstandisthatitsmathematicsisframedinafour-dimensionalform–thethreefamiliardimensionsofspaceplusoneoftime.spaceandtimenolongerexistindependently–Minkowski,whoprovidedmuchofthemathematicalstructureofrelativity,wentsofarastowritethat‘spacebyitself,andtimebyitself,havevanishedintothemerestshadows,andakindofblendofthetwoexistsinitsownright.’

canyouimaginewhatafour-dimensionalspherelookslike?neithercanwe,butwecangetsomeideaofitspropertiesbyconsideringjusttwodimensions,picturingspacetimeasaflatsheetofbedlinen,beingheldtautatitsfourcorners.now,putatennisballorsomeotherweightinthecentre,andthesheetwillbedistorted,justasthetheorytellsusmassiveobjectsdistortspaceandtime.alightraytravellinginthisdistortedspacetimewillhaveitspathdistortedandbent.aroundamassiveblackholethiseffectmaybelargeenoughtoallowasuitablyplacedobservertoseethefrontandbackofthesurroundingdisksimultaneously.

Wormholes–factorfiction?Wecandonomorethanspeculateaboutconditionsinablackhole.Doesthelucklessstarcrushitselfcompletelyoutofexistence?somehaveproposedtheideathatblackholesdistortspaceandtimetosuchanextentthattheycouldformgatewaysbetweendifferentlocationsandtimeintheUniverse,orevenbetweendifferentuniverses.thisconcept,knownasawormhole,currentlybelongstotherealmofsciencefiction,whereitoffersausefulplotdevicethatallowscharacterstodothingsbeyondtheconstraintsofmodernphysics.However,itmustbesaidthattheseideasformapartoftheseriousacademicstudyoftheseexoticobjects.

perhapsthepresentsituationisbestsummarizedbysayingthatnothinginanytestedtheoryrulestheideaofwormholesout,butthereisnopositiveevidenceintheirfavour,either.inanycase,itseemsthatinsideablackhole,alltheordinaryrulesofsciencebreakdown,alongwithanychanceofapplyingwhatwethinkofasourintuitionbasedoncommonsense.

imaginingspacetime

amassiveobjectdistortsspaceandtime.arayof

lightisbent,sowhatweseeappearstocomefrom

somewhereotherthanwhereitactuallyoriginated.the

dottedlineshowsthepathundisturbedlightwouldtake.

Withamassiveobjectintheway,thelightfollowsthe

pathshownbytheunbrokenredline.


Documents

300,000 to 700 Million years a.B. (aFter tHe BanG)pqcd.elte.hu/~pg/Bang/BANG-42-69.pdf · was born with the Big Bang, whether consciously or not. By unplugging the aerial from a television