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…