Astronomy Reading Set 14.docx

Astronomy Reading Set 14.docx

《Astronomy Reading Set 14.docx》由会员分享，可在线阅读，更多相关《Astronomy Reading Set 14.docx（25页珍藏版）》请在冰点文库上搜索。

AstronomyReadingSet14

Unit84DarkEnergyandtheFateofUniverse

84.1TheTypeIaSupernovaTest

84.2AnAcceleratingUniverse

84.3DarkMatter,DarkEnergy,andtheCosmicMicrowaveBackground

84.4ARunawayUniverse?

84.5OtherUniverses?

KEYPOINTS

∙FordecadesitwasthoughtthataftertheBigBangtheuniversecouldonlyslowdownbecausegravitywastheonlyforceatwork.

∙DistancesforTypeIasupernovaehaveenoughaccuracytoshowthattheuniverseisaccelerating,whichanenergyfieldcancause.

∙Thecosmologicalconstant（lumba,maybethesamethingasdarkenergy）islikeanenergyfieldassociatedwithallspace;itactstodrivetheexpansionatagrowingrate.

∙Measurementsofthefrequencyofdifferent-sizeclumpsintheCMBconfirmthepresenceofdarkenergyanddarkmatter.

∙TheCMBmeasurements（differenttemperatureatdifferentpartsofthesky,duetodifferentradiationtrends）indicatethatthereisabout2.5timesmoredarkenergythandarkmatter,andabout5timesmoredarkmatterthannormalmatter.Cosmicmicrowavebackground（CMB）,theradiationthatweseetoday.

∙Thehighlevelofdarkenergyfoundshouldcausetheuniversetobeginexpandingexponentiallyfasterinthefuture.

∙Inexponentialexpansion,distantregionsofspaceeventuallymoveawaysofastthattheypassoutsideourcosmichorizon.

∙Itisthoughtthatmatterandblackholeswilleventuallydecay,leavingnothingbutweakradiation.

∙Ifdarkenergydoesnotremainconstant,theuniversemayhavemanyalternatepossiblefates.

KEYTERMS

∙BigRip, 677

oExpansionrateisaccelerating,everythingwillbefartherapart.

oOnesuggestionisthatdarkenergyisnotaconstant,butwillundergochangesinthefuture,muchasinflationendedafteraperiodoftime.Thisopensupmanypossibilities.Darkenergymaygrowwithtime,notjustgraduallypullinggalaxiesawayfromeachotherbuteventuallytearingapartourGalaxy,thenevenourSolarSystem,andeventuallyevensmallerstructures.Inthisalternativefate,calledthe BigRip, spaceonsmallerandsmallerscalesbeginstoexpandrapidlyasduringinflation,maybeevenlaunchingnewuniverses. 

∙darkenergy, 673

oAstronomershavegiventhecosmologicalconstantthedescriptivename darkenergy becauseitisakindofcounterparttodarkmatter.

oItcausestheexpansiontospeedup.Thenatureandpropertiesofdarkenergyareunknown.（2.5,84.2）

∙heatdeath, 675

oEventually,though,theavailablefuelwillbeconsumed,convertedintoheatfromwhichnofurtherenergycanbeextracted.Thisisdescribedasthe heatdeath oftheuniverse—notbecauseitishot,butbecausewhatenergythereishasallbeenconvertedtothesamelow-gradedisorderedmotionfromwhichnofurtherpowercanusefullybeextracted.

∙multiverse, 677

otheseotheruniversesmaybeeitherexpandingatdifferentspeedsorevenhaveexpandedawhileandthencontractedbackonthemselves,orevenhavedifferentphysicallawsinoperation.Suchacollectionofindependentuniversesissometimescalledthemultiverse.

∙protondecay, 676

oOneoftheconsequencesofgrandunifiedtheories—thesametheoriesthatpredictedinflationaryexpansionintheveryearlyuniverse（Unit83.3）—isthattheypredict protondecay. 

oProtonsappeartobestableparticles,butiftheforcesareunified,thereshouldbepathwaysbywhichaprotoncanbreakdownintolighterparticles.Physicistshaveconductedmanyexperimentshuntingforthisphenomenonwithoutsuccess,butthismaybebecauseoftheextremelyimprobablenatureofsuchdecays.

oItisstillhighlyuncertain,butprotonsmaylive1040 yearsbeforedisintegrating.

Unit35OriginofSolarSystem

35.1TheAgeoftheSolarSystem

35.2BirthoftheSolarSystem

35.3FromDustGrainstoPlanetesimals

35.4FormationofthePlanets

35.5Late-StageBombardment

KEYPOINTS

∙Thedecayofradioactivematerialscauseshalfofthematerialtobreakdownineachhalf-life,leavingdecayproducts.

∙RadioactivedatingandotherevidenceallindicatethattheSolarSystemformedabout4.5billionyearsago.

∙ThesolarnebulatheoryexplainsthemainfeaturesoftheSolarSystemthroughthecollapseofarotatinginterstellarcloud.

∙CondensationofgasesinthenebulaintosolidgrainsonlyoccurredfarfromtheSun,wherethetemperaturewaslowenough.

∙OnlysilicatesandironcondensedneartheSun,butwatericecondensedbeyondthe“frostline.”

∙Grainsaccretedintoplanetesimalsthatcollidedtoformplanetswhichreflectthecondensationtemperaturedifferences.

∙Collisionsandradioactivitymeltedtheyoungplanets,allowingdensematerialslikeirontosinktothecenter（differentiation）.

∙Theforerunnersofthegasgiantsgrewlargeenoughtogravitationallycapturegasfromthesolarnebula.

∙Atmospheresoftheterrestrialplanetswereprobablyaddedlaterbyoutgassingandcollisionswithvolatile-richcomets.

KEYTERMS

∙accretion, 254

oThetinyparticlesthatcondensedfromthenebulastucktogetherintobiggerpiecesinaprocesscalled accretion. 

oTheadditionofmattertoabody.Examplesaregasfallingontoastarandasteroidscollidingandstickingtogether.（35.3）

∙condensation, 254

oConversionoffreegasatomsormoleculesintoaliquidorsolid.Asnowflakeformsinouratmospherewhenwatervaporcondensesintoice.（35.3）

oCondensation occurswhenagascoolsanditsmoleculessticktogethertoformliquidorsolidparticles.Forcondensationtohappen,thegasmustcoolbelowacriticaltemperature,thevalueofwhichdependsonthesubstancecondensingandthesurroundingpressure. 

∙differentiation, 257

oHeavierelementswillbeinthecenter

omanyelementsinthesolarnebulawereradioactiveandwouldreleaseheatastheydecayed.Theheatmeltedtheyoungplanetsandallowedmatterwithhighdensity（suchasiron）tosinktotheircores,whilematterwithlowerdensity（suchassilicaterock）“floated”totheirsurfacesinaprocesscalled differentiation

oTheseparationofpreviouslymixedmaterialsinsideaplanetorotherobject.Anexampleofdifferentiationistheseparationthatoccurswhenadensematerial,suchasiron,settlestotheplanet'score,leavinglightermaterialonthesurface.（35.4）

∙frostline, 254

oabout4AUfromtheSun

othedistancefromtheSuninthesolarnebulabeyondwhichitwascoldenoughforwatertocondenseandthereforebecomeamajorcomponentofplanetesimalsformingthere.（35.3,44.4）

∙half-life, 251

ohalfoftheelementwilldecayinthishalf-lifetime.

oIfEhasahalf-lifeof20years,thenamong100Eatoms,50ofthemwilldecayin20years.Inanother20years,amongtheremaining50atoms,25ofthemwilldecay.

∙interstellarcloud, 252

oacloudofgasanddustinbetweenthestars.Suchcloudsmaybemanylight-yearsindiameter.（35.2,60.2）

oanenormousrotatingaggregateofgasanddust 

∙interstellargrain, 253

oSomeelements,suchasiron,silicon,andoxygen,formsmallsolid“dust”particlescalled interstellargrains.

omicroscopicsoliddustparticlesininterstellarspace.Thesegrainsabsorbstarlight,makingdistantstarsappeardimmerthantheytrulyare.（35.2）

∙isotope, 250

oanatomthathasthesamenumberofprotonsbutadifferentnumberofneutronsinitsnucleus.（35.1,52.2）

∙meteorite, 252

othesolidremainsofameteorthatfallstotheEarth.（35.1,50.0）

∙planetesimal, 255

osmall,planetlikebodies.

ooneofthenumeroussmall,solidbodiesthat,whengatheredtogetherbygravity,formaplanet.（35.3）

∙radioactivedecay, 250

oThedecayofanelement

othespontaneousbreakdownofanatomicnucleusintosmallerfragments,ofteninvolvingtheemissionofothersubatomicparticlesaswell.（4.3,35.1,37.2）

∙solarnebula, 253

oTheinterstellarcloudthatformedthesolarsystem

otherotatingdiskofgasanddustfromwhichtheSunandplanetsformed.（35.2）

∙solarnebulatheory, 252

oTheSolarSystemoriginatedfromarotating,disk-shapedcloud（nebula isLatinfor“cloud”）ofgasanddust,withtheouterpartofthediskbecomingtheplanetsandthecenterbecomingtheSun.ThistheoryoffersanaturalexplanationfortheflattenedshapeofthesystemandthecommondirectionofmotionoftheplanetsaroundtheSun.

oTheSunandplanetsformedallatapproximatelythesametimefromarotatingcloudofgasanddust.（35.2）

∙Kuiperbelt

oObjectoutsideNeptune

oSimilartoasteroids,butdifferentcomposition.Outside:

moreiceandwater.MostcometsareKuiperbeltobjects.

oDifferentfromoldclouds,asitisadiskinsteadofacloud.

Unit36OtherPlanetarySystems

36.1YoungPlanetarySystems

36.2DetectingExoplanetsbyDopplerShifts

36.3MigratingPlanets

36.4NewExoplanetDetectionMethods

Keypoints:

∙Astronomershavedetectedmaterialaroundsomeyoungstarsconsistentwiththeearlystagesofsolarnebulatheory.

oDiskaroundthestarasevidenceofplanetformation

∙ProperMotionMethod:

itusesthesameeffectstudiedwiththeDopplershiftmethod,butitobservesthesidewaysshiftinthesky.Thesidewaysshiftislargerthefartherawayaplanetisfromthestar,somore-distantplanetsproducelargershifts.Becauseorbitsthisfarouttakealongtimetocomplete,though,themethodwillrequiremanyyearsofhigh-precisionpositionmeasurementstoyieldresults. 

∙

∙DopplerShiftMethod:

BymeasuringtheDopplershiftofstars,itispossibletodetecta“wobble”duetothegravitationalpulloforbitingexoplanets.Hundredsofexoplanetshavebeendetected,andseveralexoplanetshavebeendetectedaroundsomestars.ThewobblecreatesachangingDopplershift（Unit25）inthestar'slight（Figure36.3