thE、origin oF thE、univErsE
1 AstronomErs BEliEvE thAt thE univErsE BEgAn with A lArgE, DEnsE mAss oF gAs Consisting mAinly oF hyDrogEn, thE simplEst oF All thE nAturAlly oCCurring ChEmiCAl ElEmEnts. thE mAss oF hyDrogEn wAs vEry hot AnD CAusED intEnsE light AnD muCh ExpAnDing motion.As thE univErsE ExpAnDED, its light BECAmE DimmEr, yEt EvEn now somE oF thE primEvAl light mAy BE prEsEnt.
2 thE originAl univErsE unDErwEnt A physiCAl trAnsition thAt grADuAlly DiFFErEntiAtED it into gAlAxiEs, stArs, AnD plAnEts.As thE originAl mAss oF gAs ExpAnDED AnD CoolED, lArgE ClouDs sEpArAtED thEmsElvEs From thE pArEnt mAss. grAvity plAyED An importAnt rolE in this mEChAnism. mAttEr is suBjECt to grAvity, yEt mAttEr is Also thE CAusE oF grAvity sinCE it is mAttEr’s mAss thAt DEtErminEs thE strEngth oF thE grAvitAtionAl ForCE.
3 sCiEntists BEliEvE thAt thE originAl mAss oF gAs in thE univErsE wAs not ComplEtEly uniForm, AnD thErE wErE somE rEgions thAt wErE slightly DEnsEr AnD CApABlE oF gEnErAting strongEr grAvitAtionAl FiElDs thAn othErs. sinCE grAvity tEnDs to pull mAttEr togEthEr, thE DEnsEr rEgions tEnDED to BEComE EvEn morE CompACt. thus, smAll vAriAtions in thE originAl mAss EvolvED into DEnsEr ClouDs thAt grADuAlly sEpArAtED From thE ExpAnDing pArEnt mAss. From thEsE ClouDs, thE gAlAxiEs wErE FormED、
4 At thE EnD oF thE First phAsE oF thE univErsE, A grEAt numBEr oF hugE ClouDs hAD BEComE sEpArAtE EntitiEs thAt CoulD stArt thEir own inDEpEnDEnt Evolution. thEsE turBulEnt ClouDs-- AnCiEnt gAlAxiEs-ContAinED vAriAtions thAt grEw in importAnCE ovEr timE. thE ClouDs DiviDED into smAllEr AnD smAllEr "ClouDlEts" thAt grAvity CAusED to ContrACt. thE inCrEAsE in prEssurE From this ContrACtion CAusED thE tEmpErAturE to risE until thE ClouDlEts BEgAn to glow As inDiviDuAl, luminous stArs.
5 AstronomErs BEliEvE thAt thE EArliEst gAlAxiEs wErE smAll whEn thEy FormED most oF thEir stArs, But ACCumulAtED most oF thEir mAss lAtEr through Collisions. lArgE gAlAxiEs FormED in stAgEs As smAllEr gAlAxiEs wErE AttrACtED to onE AnothEr By grAvity.As thE smAllEr gAlAxiEs wErE pullED togEthEr ovEr timE, thEy mErgED into lArgEr AnD lArgEr struCturEs, EvEntuAlly Forming mAssivE gAlAxiEs.As mAny As hAlF oF All gAlAxiEs ArE thought to hAvE BEEn involvED in somE sort oF Collision.
thE worD CompACt in pArAgrAph 3 is ClosEst in mEAning toA．DistinCt
B．DEnsE
C．DisConnECtED
D．DistAnt