ASTR 1120ASTR 1120General Astronomy:General Astronomy:Stars & GalaxiesStars & Galaxies!NNOUNCEMENTS!NNOUNCEMENTS• Midterm #3 this Thursday 11/17; covering lect. 15-21 (and corresponding reading in Ch. 19-22) • last part of class today will be review• HW #8 due today, 5pm• HW #9 due Tue, 12/01, by 5pm ALTERNATE FINAL on Monday, Dec.7th, 5:30pm-ALTERNATE FINAL on Monday, Dec.7th, 5:30pm-7:00pm,7:00pm, in in Muenzinger Muenzinger E131E131Numerical simulations of structure formationNumerical simulations of structure formationREVIEWLessons from ImaginaryLessons from ImaginaryUniversesUniverses••Cold (Slow) dark matterCold (Slow) dark matterworks better than works better than hot (fast)hot (fast)dark matterdark matter••Neutrinos are too fastNeutrinos are too fast––structure would bestructure would besmeared outsmeared out••What is slow and darkWhat is slow and darkenough? We donenough? We don’’t knowt knowyet!yet!––Particle experiments underParticle experiments underwayway……....Dark Matter and the Fate of theDark Matter and the Fate of theUniverseUniverse••Expansion begins with the Big Bang (weExpansion begins with the Big Bang (we’’ll talk aboutll talk aboutthis after the break)this after the break)••At that point, everything in the universe is flungAt that point, everything in the universe is flungapart atapart at outrageous speeds!outrageous speeds!••Several different models for Several different models for PastPast and and FutureFuturedepending upon the amount of depending upon the amount of dark matterdark matterVery important diagramVery important diagram““Average distanceAverage distancebetweenbetweengalaxiesgalaxies”” measure ofmeasure of““expansion factorexpansion factor of Universeof Universe””Hubble constantHubble constant NOWNOWsets how fastsets how fastuniverse isuniverse isexpandingexpanding NOWNOWBig Bang = when distance zeroBig Bang = when distance zeroTIMETIMESIZESIZENOWNOWThe expansion rate of the universe isThe expansion rate of the universe isnot necessarily constant for all timenot necessarily constant for all time••GRAVITYGRAVITY should SLOW should SLOWexpansion rate expansion rate !!decelerationdeceleration••Different models forDifferent models fordifferent amounts of darkdifferent amounts of darkmattermatter––LetLet’’s ignore s ignore acceleratingaccelerating for fornownowSince gravity is what pulls everythingSince gravity is what pulls everythingback in,back in, there must be athere must be a magic numbermagic number••Just the right amount of mass (in our currentJust the right amount of mass (in our currentuniverse) to slow down expansion but notuniverse) to slow down expansion but notenough toenough to cause cause recollapserecollapse••We call this exact amount of matter, theWe call this exact amount of matter, theCRITICAL DENSITYCRITICAL DENSITY~10~10-29-29 grams/cm grams/cm33 = a few atoms in a closet = a few atoms in a closetCriticalCritical Universe Universe••Density ofDensity ofmatter =matter =““criticalcriticaldensitydensity””••Will expandWill expandforeverforever, but, butmore andmore andmore slowlymore slowlywith timewith timeFlatFlat UniverseUniverseRecollapsing Recollapsing UniverseUniverse••Dark matterDark matterdensity isdensity isgreatergreater than than““critical densitycritical density””••Expansion willExpansion willstop in thestop in thefuture, willfuture, willcollapse backcollapse backinin––Big CrunchBig Crunch––Oscillations?Oscillations?Closed UniverseClosed UniverseCoasting UniverseCoasting Universe••The universeThe universehas alwayshas alwaysexpanded at theexpanded at thesame ratesame rate(no deceleration(no decelerationdue to gravity!)due to gravity!)••The age of theThe age of theUniverse = 1/HUniverse = 1/HooOpen UniverseOpen UniverseThree models for fates of universeThree models for fates of universeCLOSEDCLOSEDOPENOPENFLATFLATWhich model predicts theWhich model predicts the youngestyoungestageage for the universe today? for the universe today?A.A. RecollapsingRecollapsing (closed)(closed)B.B. CriticalCritical (flat) (flat)C.C. CoastingCoasting (open) (open)Clicker QuestionClicker Question••A.A. RecollapsingRecollapsing••Age of universe isAge of universe ishow far to lefthow far to leftcurves hitcurves hithorizontal axishorizontal axis(distance between(distance betweengalaxies = 0)galaxies = 0)What determines the future averageWhat determines the future averagedistance between galaxiesdistance between galaxies??A. The rate of expansion, with a slower expansionrate meaning a greater average distanceBB.. The rate of expansion, with a faster expansionrate meaning a greater average distanceC.C. Only on the density of matter in the universe.Clicker QuestionClicker QuestionWhat determines the future averageWhat determines the future averagedistance between galaxiesdistance between galaxies??A. The rate of expansion, with a slower expansionrate meaning a greater average distanceBB.. The rate of expansion, with a faster expansionrate meaning a greater average distanceC.C. Only on the density of matter in the universe.Clicker QuestionClicker QuestionWhich model has the slowest future rate ofWhich model has the slowest future rate ofexpansion?expansion?A.A. RecollapsingRecollapsingBB.. CriticalCriticalC. CoastingC. CoastingClicker QuestionClicker QuestionWhich model has the slowest future rate ofWhich model has the slowest future rate ofexpansion?expansion?A.A. RecollapsingRecollapsingBB.. CriticalCriticalC. CoastingC. CoastingClicker QuestionClicker Question••Recollapse Recollapse to big crunchto big crunch::––Crushing heatCrushing heat––Destruction of all matterDestruction of all matter––Rebirth?Rebirth?••Eternal expansionEternal expansion::––Cold, galaxies dimmingCold, galaxies dimming––Star formation slowingStar formation slowing––Everything winds up as a brown dwarf, blackEverything winds up as a brown dwarf, blackdwarf, neutron star or black holedwarf, neutron star or black holeWhat is the fate of the Universe?What is the fate of the Universe?Is there enough dark matterIs there enough dark matterto to recollapse recollapse the universe?the universe?Baryonic matter:Baryonic matter: only a few % of critical density only a few % of critical densityDark matter:Dark matter: only about 25% of what is needed only