Lasttime:generalrelativity,blackholes,&gravitationalwaves
• Massesbendspacetime;otherobjectsmovealongtrajectorycorrespondingtoshortestdistanceincurvedspace.
• Gravitybendslightaswell• Blackholesarecollapsedstarsfromwhichlightcannolongerescape
• Acceleratingmassesemitveryweakgravitationalwaves• Thesewavesweredetectedfor1sttimelastyearbyLIGOproject(Caltech,MIT,Florida)
Eddington1919expt.confirmsbendingoflight
Einstein theory of general relativity says any mass curves spacetime around it:
Light travels on a geodesic – the shortest path between 2 pts. on the curved surface
Cosmologyin20thcentury
Measuringlargedistances
• HenriettaLeavitt:In1908studiedCepheidvariables–starsthatoscillateinbrightness.Shefoundintrinsicrelationbetweenbrightnessandperiod�comparestarswithsameperiod.Iftheyaredimmer,thenfurtherawaybecausebrightness�1/r2
CompositionofStars• CeceliaPayne-Gaposchkin:Ph.Dthesis(1925)showedthatspectrallinesobservedinstarsshowedoverwhelminglyevidenceforhydrogenandhelium,unlikeEarth
• HansBethe1939:showsenergyofsuncomesfromnuclearfusionofhydrogen
E=mc2 Mass of fusion products less than inputs
Dopplershift/Redshift
• Dopplershift(usuallyforsound)occurswhenemmitterismovingwithrespecttotheobserver:changeinapparentfrequency
https://www.youtube.com/watch?v=h4OnBYrbCjY
• RedshiftisDopplershiftforlight
Hubble’sDiscovery• EdwinHubble’sobservationsofremotegalaxies,andtheredshiftoftheirspectrallines(1924).
• Hubblenoticedthatthefurtherawaythegalaxy,thegreatertheredshiftofitsspectrallines.
• ThislinearrelationshipiscalledHubble’sLaw.
http://rst.gsfc.nasa.gov/Sect20/A9.html
Hubble’sLaw
• v=H0d• v=recessionalvelocityofthegalaxy• H0=Hubbleconstant*• d=distanceofgalaxytoearth• Galaxiesaregettingfartherapartastimeprogresses,thereforetheuniverseisexpanding.
Note1/H0isatime–meaning?
Hubble’slaw:data
Hubble’sConstant
• ExpansionratemeasuredusingType1ASupernovae.
• TheageoftheuniversecanbederivedfromHubble’sconstantbyworkingbackwardsd�0
• T0=1/H0
– CurrentmeasurementsH0=73km/s*Mpc,then T0=13.4billionyearsold(ageofuniverse)
Einsteindoesn’tlikeit.
• AllthesolutionstoE’sequationswhichgiveHubble’slawhavesingularities
• Einsteinthoughtthatsingularitiessuchastheseindicatedthattherewereimportantphysicaleffectsnotaccountedforinhisequations.Healsothoughtthattherightanswerwouldinvolvestaticbehavior:large-scalestructureshouldnotchangewithtime.
• Healsosawhowhecould“fix”thefieldequationtoeliminatesingularanddynamicsolutions:introduceanadditionalconstantterm,whichbecameknownasthecosmologicalconstant,torepresentthemissing,unknown,physicaleffects.
Einsteingivesup.• TheUniverseisobservedtobeexpanding;itisnotstatic.– ThustherealUniversemaybedescribedbyoneofthedynamicsolutionstotheoriginalEinsteinfieldequation.
– ThusthereappearedtobenopointinEinstein’scosmologicalconstant,soheletitdrop,callingit“mygreatestblunder.”
– Thereafterhebegantryingtoshowthatthesingularitiesinthedynamicsolutionssimplywouldn’tberealized.Hiseffortresultedinthesteady-statemodeloftheUniverse,whichwe’lldescribelater.
• Cosmologicalconstant,willcomeback,though.
HowtheUniverseExpands• Thespacebetweengalaxiesexpands,notthegalaxiesthemselves
• Example:raisinsinaloafofbread.– Asthedoughrises,theoverallloafofbreadexpands;thespacebetweenraisinsincreasesbuttheraisinsthemselvesdonotexpand.
CenterofUniverse?
• ThereisNOCENTERtotheuniverse– Expansionlooksthesameregardlessofwhereyouareintheuniverse.
– Everypointappearstobethecenteroftheexpansion,thereforenopointisthecenter.
– Theuniverseisinfinite.
EvidenceforExpansion
• Thelightfromremotegalaxiesandotherobjectsisredshifted.
• Thisredshiftiscalledcosmologicalredshiftbecauseitiscausedbytheexpansionoftheuniverse,notbytheactualmovementoftheobject(Dopplerredshift).
OriginsoftheBigBangTheory• GeorgesLemaître(1927)consideredideaofexpandinguniverse,realizingthattheuniversewassmalleryesterdaythantoday,andsoonuntila“daythatwouldnothavehadayesterday”:themomentofcreation.
• Thisideawasn’twidelyacceptedatfirst:FredHoyledismissed“thishotBigBang”,notingthattherewasn’tanyrecordorremnants.Hearguedfora“steadystate”universe.
OriginsoftheBigBangTheory• GeorgeGamow(1948)suggestedthatiftheuniversewascreatedwitha“hotBigBang”,then:– Variouselements,suchasHandHe,wouldbeproducedforafewminutesimmediatelyaftertheBigBangduetotheextremelyhightemperaturesanddensityoftheuniverseatthistime.
– Thehighdensitywouldcauserapidexpansion.– Astheuniverseexpanded,HandHewouldcoolandcondenseintostarsandgalaxies.
– Today,duetocontinuedcooling,radiationleftoverfromtheepochofrecombination,whenneutralatomsformed(~380,000yearsafterBigBang)shouldbeabout3K(hesaid10K!).
EvidencefortheBigBangTheory
• Gamow’stheorywasrevistedinthe1960’sbyBobDickeandJimPeeblesofPrincetonUniversity.– Believedthatthiscooledradiationwouldberedshiftedtothemicrowaveregionoftheelectromagneticspectrum.
– Dickemadeareceivertodetectthisradiation,butwasunsuccessful.
EvidencefortheBigBangTheory• Theradiation,sofarundetectedbythePrincetonteam,wasposingaproblemforNJBellTelephoneLabs,whereArnoPenziasandRobertWilsonweredevelopinganewmicrowave-satellitetechnologyforphonecalls.– Puzzledbysteadyhissthattheyreceivednomatterwhereintheskytheypointedtheirantenna.
– ThisfaintbackgroundnoisetheyweretryingtogetridofwasexactlywhatthePrincetonteamwastryingtodetect:evidenceoftheBigBang.
http://nobelprize.org/educational/physics/star_stories/overview/index.html
Cosmicmicrowavebackgroundradiation
• DetectionofthisCosmicMicrowaveBackgroundradiation,wonPenziasandWilsontheNobelPrizeforPhysicsin1978.
• CMBradiationcanbedetectedbyyourTVaswell-1%ofstaticseenonachannelthatyourTVdoesn’treceiveisfromthebirthoftheuniverse.
CMBRadiation• COBE(CosmicBackgroundExplorer)1989:detectoroutsidetheatmosphere:– MeasuredtheblackbodyspectrumofCMBradiationtobeatT=2.725K-consistentwiththeory.
– CMBradiationalmostentirelyisotropic
• WMAP(WilkinsonMicrowaveAnisotropyProbe)(2002)improvedpictureofCMBRadiation.
HomogeneoussolutionstoEinsteinequations:thestructureoftheexpandingUniverse
All three expanding solutions predict that the matter in the universe was concentrated at earlier times, and that the expan-sion started as an explosion of this concentration.
Typical distance between galaxies
Time
Unbound (open)
Bound (closed)
Marginal (total kinetic energy = binding pot. energy)
Big Bang Big Squeeze
You are here
Thelatest:darkmatteranddarkenergy
• https://www.youtube.com/watch?v=QAa2O_8wBUQ
Questionsunansweredorunasked
• Whatwastherebeforethebigbang?• Whatcausedthebigbang?• Istheuniverseopenorclosedormarginal?• Whatisthemeaningoflife?• Whatistheairspeedofanunladenswallow?
“ThemoreIlearn,themoreIrealizehowmuchIdon'tknow.”---A.Einstein
And there are many more but…
