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DarkenergyAlotofastronomicalobservationsindicatethattheuniverseisflat,andconsistsof4%baryonmatter,23%darkmatter,72%darkenergyandnegligibleradiationmatters.whatisdarkenergyonearth?Todescribethedarkenergy,theequationofstatehastosatisfy,itisinterestingtonotethatitisnotaneasyjobtofindasuitablecandidateforthedarkenergy.Thesimpleoneisasmallcosmologicalconstant,butitisdifficulttounderstandwhysmallerthantheexpectedvalueabout120order.Thisistheso-calledcosmologicalconstantproblem.Anotherproblemaboutthedarkenergyiscalledthecoincidenceproblemsthatwhydarkenergydensityanddarkmatterdensityareinthesameorder?Becausethetwoevolutionwiththecosmictimearesodifferent.Inrecentyearsthedarkenergyproblemhasbecomethemostactiveandimportantoneinthefieldofresearchinthemoderncosmology.DarkenergydiscoveryThemostdirectevidenceofcosmicaccelerationisfromⅠatypesupernovaobservation,thistypeofsupernovacanbeusedasastandardcandlelight.Peoplefoundindistantredshift(between0to2),Ⅰatypeofsupernova’sluminancedarkerthanoriginallyexpected,thismeansthesesupernovaearefurtherfromusthanbefore.Notasthebigbangmodelpredicted,ourtheuniverseexpansionisnotslowingdownbutspeedingup.Toexplainthecosmicaccelerationpeopleusuallyintroducedarkenergywhichhasnegativepressure.Butpeoplefirsthavereasontothoroughlyinspect.WhetherⅠatypeofsupernova'sbrightnessdimmerthanbeforereallymeansuniverse'sexpansionisaccelerating.Ifthereareothermechanismtoexplain,thedarkenergyisnotnecessary.Duringtheseyearspeopleputforwardsometheorythatdon'tneedtointroducedarkenergytoexplainthesupernova’sluminositydarkermechanism"suchasgraydust,photonaxionconversion,andtheluminosityevolution,butinrecentyearsmoreandmorehighprecisionsupernovaandotherastronomicalobservationssuchasmicrowavebackgroundfluctuations,largescalestructurehavebeengreatlycompressedtheseexplain’slivingspace.4yearsagoscientistsusedtheWilkinsonmicrowaveanisotropyprobe(WMAP)toinvestigatethemicrowaveleftbythebigbang,thisgreatlycontributedtothetestofthediscovery.Theuniversepatternsatthattimematchedtheobserveduniversebigscalestructureverywell.WMAPalsoconfirmedthatdarkenergyaccountfor74%ofthematerialenergydensityintheuniverse.Inanotherscheme,thescientistsusedifferentdistanceofexplodingstars–supernova’slightasthecosmicspeedometertotrackexpansionrateatdifferenttimeintheuniverse.Ⅰatypeofsupernovacanprovidesuchinformation,shiningjustlikethefingerprintenablethescientiststoidentifyit’sdetails.Thismodelcanalsohelpscientiststounderstandthesupernova’sintrinsicbrightness.Theresearcherscanestimatethedistanceoftheearthfromthesupernovabycontrastingthesupernova'sinherentbrightnessandbrightnessseenfromtheearth.Thenbyvirtueofthesupernova’slightcolorchangestomeasuretheuniverse'sexpansionrate,theredderthelightis,thefastersupernovaflyingaway,atthemeantimethefasterthecosmicinflationinthatdistance.Inthethreeyears,RiessandhiscolleaguesusedtheHubblespacetelescopetodiscover16supernovae,includingsixsupernovafromthefurthestdistant.Theyaddedtheseto170previouslyanalyzedsupernovae,thenlookedforthetrendofthedecelerationandacceleration.Theyfoundthatjustastheuniverse'sbirthandevolutiontheorypredicted,forthefirst7.7billionto8.7billionyears,theuniversewasexpanding,butthespeedisslow.Afterthattheuniverse'sexpansionratebegantospeedup.Ifastimegoeson,darkenergywilleventuallyproducepowerfulcracks,andtearapartthesmallatomsevenbiggalaxies.Sincearepulsiveforcethatoppositetothegravitypropertiesappear,astronomerscalleditdarkenergy,andconfirmthatintheuniverse,darkenergyismorethanthetotalqualityofknowncelestialbodiesanddarkmatter.ThedecayedcosmologicalconstantThisisacenturiedprogramputforwardtounderstandwhythecosmologicalconstantisalmostzero,whichcalledtheoldcosmologicalconstantproblems.Inthismodel,thecosmologicalconstantistimevarying,sometimescalledmodel.Commonchangedcosmologicalconstantincludingthefollowingseveralkinds:,,,,orandsoon.CoincidenceproblemInthestudyofdarkenergytheory,manydarkenergymodelaredescribedwithscalarfieldatpresent.Themostfamousscalarfieldmodelisquintessence,Itisaregularrealscalar,inthehomogeneousandisotropicuniversemodel,theLaplacedensityis(onepointdenotederivationofthecosmictime),bytheLaplacedensityitiseasytogetthepressureandenergydensityofquintessence:,Thecorrespondingstateequationparameter:Obviouslythequintessence’sstateequationparameterisbetween-1and1.Ofcourse,whenquintessencescrolldownalongthegentlepotential,thestateequationparametercanbeLessthan,thusdrivingtheuniverseacceleration,anditcanevenverycloseto1,imitateacosmologicalconstant.Takeonekindofspecialpotential,quintessencemodelcanhavescalingsolutionortrackersolution,atthistimethequintessenceenergydensityevolvewiththebackgroundmaterialenergydensity.Duetotheexistenceoftheattractor,themodelofcosmologydoesnotdependontheevolutionofinitialconditions,thuscanalleviatethecoincidence.Theobservationdatacan'texcludetheconditionthatstateequationparameterlessthan-1,Quintessencemodelcan’tmeettherequirement,sopeopleputforwardaso-calledphantom,inthismodelthestateequationparameterislessthan-1;theeasiestwaytorealizethephantomdarkenergyisascalarfieldwhichhasawrongakineticenergy.TheLaplacedensitycanbewrittenas:thepressureandenergydensity:,Thecorrespondingstateequationparameter:Obviouslythephantomstateequationparametersisalwayslessthan1,becauseitisagainstalltheenergycondition.Inthismodelsomeunwantedpropertieswillbeexist,suchasinstability,butinthephantomdarkenergymodel,Itsenergydensityevolvingwithtimenotonlynotreducebutincrease,sotheexpansionoftheuniversewillbemoreandmorequickly!FinallythefateoftheuniverseisBigRip(bigtear),sointhisdarkenergymodel,thelifeofouruniverseislimited.ThethirdkindofscalarfielddarkenergymodelisK-essence,itisakindofregularrealscalarfield,whichcontainnonlinearkineticenergyitems,theLaplacedensitycanbeany,andthefunctionofkineticenergyitems,thecorespindingpressureis,Theenergydensity,thesubscriptdenotethederivationof,taketheproperform,K-essencemodelcanalsohavescalingthesolution,also,becauseoftheexistenceoftheattractor,themodelofcosmologyevolutiondoesnotdependontheinitialcondition,inthisway,alleviatethecoincidenceproblem.Inthispaper.Wetryourbesttoshowthecurrenttheoryresearchinthefieldofdarkenergysituation,inthevastliterature,Wejustintroducedthedarkenergytheorytoresearchsomemajoraspects,since1998theuniverseac