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Chapter 5 Cosmology002（第1页）

Chapter5ology002

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，wemaythereforehavemisidehem。Futureastronomicalobservatiooihispossibilityandtofurtherexplorethepropertiesofdarkmatteranddarkenergy。

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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。