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Chapter5ology002
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SoourtoverallpictureoftheUniverseisasfolloertoftheeheUheformofnormalmatter;about25pertisthoughttobeintheformofthegravitationallyattractivedarkmatter;andtheremaiisthoughttobeintheformofthegravitationallyrepulsivedarkehesepriveortakeafeoihere，seemsuittoexplainallastronomicalobservationsthathavebeee.Thetotalofallthreeofthesetypesy，addedtogether，alsoseemstobejusttherightamounttomakespaceflat（ratherthanpositivelyativelycurved，likeasphereorasaddle，asillustratediheflatUniverse，filledwithmyanddarkmatter，isusuallyreferredtoastheodeloftheUniverse.AmongastroisnowthesusviewthatthisisthemodeloftheUbestfitstheirdata.
Theodel，andalloftheobservationsthathaveledtoit，isundoubtedlyagreatatof21st-turyphysics.However，itislyhestory.NardtoofthehistoryoftheUhregardstothewayweusts，ravitatiohinit.Tobeblunt，theodelhasanumbers.First，itappearstohavestartedofinaparticularlyspefiguration.Forspacetobesoclosetoflat，ahebadradiatioributionofgalaxieslooksoeveheearlyUohavebeeremelyclosetoperfeiy.SeeoftheripplesweseeintheCMBappearterthaacouldhavetravelledsiheBigBang.Nothingshouldtravelfasterthanlight，iheory，sothisisgenuinelypuzzling.Third，wehavedarkmatterreallyis.Weonlyknowthatitshouldgravitate，andthatitshouldhlight.Itdoesn’tshowupiaiclephysics，laceforeveryotherkidithasbeenseeninanyparticlephysicsexperiment.Fourth，theexistencey，anditsrepulsivegravitationalfield，seemstorequireenormousfiuniohavetheefectthatweseetoday.Abitmoreofitandgalaxieswouldneverhaveformed.Abitlessahaveeverall.
Thesefourproblemsarethefoutionforphysicists.ThefirsttwhttobesolvedbyaperiodofveryrapidexpansionintheearlyUniverse，i.I’lldesiiishopedthatthethirdwillbesolvedbyexteaiclephysidthereareanumberofproposalsonhowthiscouldbedoime，itisthoughtthatthepropertiesofdarkmatterpartibeilybyusingtheLargeHadronCollider（LHeva.WhetherornotnaturewillhavebeeoallowthemtofallinteylevelsthattheLHainstobeseeoftheseproblemsisprobablythemostmystifyingofall.ThelengthsthatsomephysicistshavegooexplairulyextraaihisfurtherinChapter6.
Ofcourse，thereremainafewphysiciststhatarescepticalthatdarkenergyanddarkmatterexistatall.TheymaintaiouaailhowgravityworksonthescaleoftheUniversebeforewebesurethattheyarereallythere.Afterall，it’shtheirgravitatiohatweknowaboutthesesubstaall.Ifwe’vemisuoodgravity，wemaythereforehavemisideheFutureastronomicalobservatiooihispossibilityandtofurtherexplorethepropertiesofdarkmatteranddarkenergy.
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Itoftenturnsouttobefollytotrytopredictthefutureiseemsreasohe21stturywillseesignifiology.alotaboutthewaytheUniverseisexpandistrutheUniversehaveformed，butouroarfedbyobservationsthatecoupleofdecades.Muotivationforthisworkatterahesearchforthesedarkmaterialswillshedfurtherlightongravity.
&withtheostobservationsofthisradiationhavefoeasuriureiionsonthesky，andtryiheripplesintheearlyUhavelookedlike.ThegedgeinthisworkhassofarbeenthePlahismissionwassosuccessful，however，thatitisalmostimpossibletodoahfuturespace-basedmissions.Whate，however，isbuildbiggertelesthesurfaceoftheEarth.ThisistlybeiheAtacamaDesertitheSouthPole.Thesearetwoofthelowest-humiditylotheplahethindryairmakesthemidealplacesfielescopestheCMBthresolution，andwillsupplyawealthofinformationaboutthestructuresthatexistintheUniverse.
&emperature，therearesthatbeobservedia.Astronomerseasureitspolarizatioioicwaves，asillustratedihepolarizationofthebadradiatioionalinformationabouteheearlyUniverse，andbylookingforparticularpatternsastronomersferwhatthegravitationalfieldlookedlikeveryearlyonintheUory.
&hisinformationduplicateswhatbededuperature，butsomeofitisentirelynew.Inparticular，bylookingforacharacteristiccurlpatterninthepolarization，itispossibletodeducewhetherthereweregravitatioravellingaroundtheearlyUniverse.YouwillreChapter4，thatsiderableeforthasbeeodirectlydetectthegravitatioravelthroughtheEarth.ThepolarizationoftheCMBprovidesthesilarexperimentsiirelydiferente.
11.Anillustrationof（a）pht;and（b）u.Theorientationofthewavesintheuisrandom，whilethewavesinthepolarizedcasearealigned.Arrowsdeioninwhichthelightistravelling.
InMarch2014，stistswontheBICEP2experimehPoleaheyhadusedthismethodtravitatioheearlyUhetime，itappearsthattheextgehisaremature.Whilethestistshadseeterninthepolarizatiohiswasgeedbysourcesclosertousandnravitatioall.Thisdoesgravitatiooutthere，intheearlyUureexperimentswillmeasurethepolarizationofthebadradiationtoevenhigheracdinmorefrequends.IfgravitationalwaveswerepresentintheearlyUoanysiderablelevel，thenit’slikelywe’llknowaboutthemidecadeorso.ThesuccessortoBICEP2hasalreadybeenbuilt，andwillgiveitsfirststificresultssoon.
Alsoofgreatpromisearetheionofgalaxysurveymissions.Wediscussedthe2dFandSDSSsurveysearlier，whichwereveryambitiousattemptstorecordthepositionsofthegalaxiesintheUniverseweseearoundus.Futuresurveyswillbemuch，muchbigger.ThreeofthegrahesewillbetheLargeSynopticSurveyTelescope（LSST），whichisalreadyuruChile;theSquareKilometreArray（SKA），whichwillbeginstru2018;andtheEuropeanSpacy’sEuclidsatellite，whichisexpectedtolaund2020.Thesemissionswillmeasurebillionsofastronomicalsources，whichwillbeusedtoapsoftheUniverseoedscales.
&heSKA，aellitewillturnologyintoaprestheyareiion，muchmoreaboutthehedarkmatterahroughtheefectstheyhaveoureintheUhewayinwhichlighttravelstousfromthosestrudfromthewaythattheyevolvei，itisexpectedthatthisinformationwillbesoprecisethat，forthefirsttime，wewillbeabletostartperftestsofgravitythatwillrivalthoseerformintheSolarSystemandinbinarypulsarsystems.Thiswillopenupaongravity，andwillallowustotestitinnewwaysaancescales.
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