AlanGuth,In ationaryCosmology:IsOurUniversePartofaMultiverse,Part1,8.286OpeningLecture,September7,2016,p.1.

TheStandardBigBang

Whatitis:

Theorythattheuniverseasweknowitbegan13-14billionyears

ago.(Latestestimate:13:82�

0:05billionyears!)

Initialstatewasahot,dense,uniformsoupofparticlesthat�lled

spaceuniformly,andwasexpandingrapidly.

Whatitdescribes:

Howtheearlyuniverseexpandedandcooled

Howthelightchemicalelementsformed

Howthemattercongealedtoformstars,galaxies,andclustersof

galaxies

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{1{

Whatitdoesn'tdescribe:

Whatcausedtheexpansion?(Thebigbangtheorydescribesonly

theaftermathofthebang.)

Wheredidthemattercomefrom?(Thetheoryassumesthatall

matterexistedfromtheverybeginning.)

Inotherwords,itsaysnothingaboutwhat

banged,whyitbanged,orwhathappened

beforeitbanged!

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{2{

WhatisInflation?

In ationisa\prequel"totheconventionalbigbangpicture.

Inparticular,in ationisatheoryaboutthebangofthebigbang.

Thatis,in ationisapossibleanswertothequestionofwhat

propelledthegiganticexpansionofthebigbang.

In ationisalsoapossibleanswertothequestionofwherealmost

allofthemattercamefrom.

In ationisNOTatheoryoftheoriginoftheuniverse,butitcan

explainhowtheentireobserveduniverseemergedfromapatch

only10�

28

cmacross,withamassofonlyafewgrams.

In ationexplainstheexpansionbymeansof

GravitationalRepulsion.

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{3{

AlanGuth,In ationaryCosmology:IsOurUniversePartofaMultiverse,Part1,8.286OpeningLecture,September7,2016,p.2.

TwoMiraclesofPhysics

De�nition:A\miracleofphysics"isafeatureofthelawsofphysics

which

(a)wasnevertaughttomewhenIwasastudent;and

(b)issofar-reachinginitsconsequencesthatitcanchangeour

pictureoftheuniverse.

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{4{

MiracleofPhysics#1:

GravitationalRepulsion

Sincetheadventofgeneralrelativity,physicistshaveknownthat

gravitycanactrepulsively.

InGR,pressurescancreategravitational�elds,andnegative

pressurescreaterepulsivegravitational�elds.

Einsteinusedthispossibility,intheformofthe\cosmological

constant,"tobuildastaticmathematicalmodeloftheuniverse,

withrepulsivegravitypreventingitscollapse.

Modernparticlephysicssuggeststhatatsuperhighenergiesthere

shouldbemanystateswithnegativepressures,creatingrepulsive

gravity.

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{5{

In ationproposesthatapatchofrepulsivegravitymaterialexisted

intheearlyuniverse|

forin ationatthegranduni�edtheory

scale(�

1016

GeV),thepatchneedstobeonlyaslargeas10�

28

cm.(Sinceanysuchpatchisenlargedfantasticallybyin ation,

theinitialdensityorprobabilityofsuchpatchescanbeverylow.)

1GeV�

massenergyofaproton.

Thegravitationalrepulsioncreatedbythismaterialwasthe

drivingforcebehindthebigbang.Therepulsiondroveitinto

exponentialexpansion,doublinginsizeevery10�

37

secondorso!

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{6{

Thepatchexpandedexponentiallybyafactorofatleast1028

(�

100doublings),butitcouldhaveexpandedmuchmore.

In ationlastedmaybe10�

35

second,andattheend,theregion

destinedtobecomethepresentlyobserveduniversewasaboutthe

sizeofamarble.

Therepulsive-gravitymaterialisunstable,soitdecayedlike

aradioactivesubstance,endingin ation.Thedecayreleased

energywhichproducedordinaryparticles,formingahot,dense

\primordialsoup."Standardcosmologybegan.

Caveat:Thedecayhappensalmosteverywhere,butnotevery-

where|

wewillcomebacktothissubtlety,whichisthe

originofeternalin ation.

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{7{

AlanGuth,In ationaryCosmology:IsOurUniversePartofaMultiverse,Part1,8.286OpeningLecture,September7,2016,p.3.

Aftertherepulsive-gravitymaterialdecayed,theuniversecontin-

uedtocoastandcoolfromthenonward.

Keyfeature:Duringtheexponentialexpansion,thedensityof

matterandenergydidNOTthinout.Thedensityoftherepulsive

gravitymaterialwasnotloweredasitexpanded!

Althoughmoreandmoremass/energyappearedastherepulsive-

gravitymaterialexpanded,totalenergywasconserved!HOW????

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{8{

MiracleofPhysics#2:

EnergyisConserved,ButNotAlwaysPositive

Theenergyofagravitational�eldisnegative(bothinNewtonian

gravityandingeneralrelativity).

Thenegativeenergyofgravitycancelledthepositiveenergyof

matter,sothetotalenergywasconstantandpossiblyzero.

Thetotalenergyoftheuniversetodayisconsistentwithzero.

Schematically,

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{9{

EvidenceforInflation

1)Largescaleuniformity.

Thecosmicbackgroundradiationis

uniformintemperaturetoonepartin100,000.Itwasreleased

whentheuniversewasabout400,000yearsold.Instandardcos-

mologywithoutin ation,amechanismtoestablishthisuniformity

wouldneedtotransmitenergyandinformationatabout100times

thespeedoflight.

InflationarySolution:

Inin ationarymodels,theuniverse

beginssosmallthatuniformityiseasilyestablished|

justlike

theairinthelecturehallspreadingto�llituniformly.Then

in ationstretchestheregiontobelargeenoughtoincludethe

visibleuniverse.

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{10{

2)\Flatnessproblem:"

WhywastheearlyuniversesoFLAT?

Whatismeantby\ at"?

Flatdoesnotmean2-dimensional.

FlatmeansEuclidean,asopposedtothe

non-Euclideancurvedspacesthatarealso

allowedbyEinstein'sgeneralrelativity.

3-dimensionalcurvedspacesarehardto

visualize,buttheyareanalogoustothe

2-dimensionalcurvedsurfacesshownonthe

right.AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{11{

AlanGuth,In ationaryCosmology:IsOurUniversePartofaMultiverse,Part1,8.286OpeningLecture,September7,2016,p.4.

Accordingtogeneralrelativity,the atness

oftheuniverseisrelatedtoitsmassdensity:

(Omega)=

actualmassdensity

criticalmassdensity;

wherethe\criticaldensity"dependsonthe

expansionrate.=1is at,greater

than1isclosed,lessthan1isopen.

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{12{

Auniverseatthecriticaldensityislikeapencilbalancingonits

tip:Ifintheearlyuniversewasslightlybelow1,itwouldrapidly

falltozero|

andnogalaxieswouldform.

Ifwasslightlygreaterthan1,itwouldrapidlyrisetoin�nity,

theuniversewouldrecollapse,andnogalaxieswouldform.

Tobeasclosetocriticaldensityaswemeasuretoday,atone

secondafterthebigbang,musthavebeenequaltooneto15

decimalplaces!

{13{

InflationarySolution:

Sincein ationmakesgravitybecome

repulsive,theevolutionofchanges,too.isdriventowards

one,extremelyrapidly.Itcouldbeginatalmostanyvalue.

Sincethemechanismbywhichin ationexplainsthe atnessof

theearlyuniversealmostalwaysovershoots,itpredictsthateven

todaytheuniverseshouldhaveacriticaldensity.

Until1998,observationpointedto�

0:2{0.3.

LatestobservationbyPlancksatellite(combinedwithother

astronomicalobservations):

=1:0008�

0:0040(95%con�dence)

Newingredient:DarkEnergy.In1998itwasdiscoveredthatthe

expansionoftheuniversehasbeenacceleratingforaboutthelast

5billionyears.The\DarkEnergy"istheenergycausingthisto

happen.

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{14{

3)Smallscalenonuniformity:

Canbemeasuredinthecosmic

backgroundradiation.Theintensityisalmostuniformacrossthe

sky,buttherearesmallripples.Althoughtheseripplesareonly

atthelevelof1partin100,000,thesenonuniformitiesarenow

detectable!Wheredotheycomefrom?

InflationarySolution:

In ationattributestheseripplesto

quantum

uctuations.

In ationmakesgenericpredictionsfor

thespectrumoftheseripples(i.e.,howtheintensityvarieswith

wavelength).Thedatameasuredsofaragreebeautifullywith

in ation.

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{15{

AlanGuth,In ationaryCosmology:IsOurUniversePartofaMultiverse,Part1,8.286OpeningLecture,September7,2016,p.5.

RipplesintheCosmicMicrowaveBackground{

16{

CMB:

Comparison

ofTheory

and

Experiment

GraphbyMaxTegmark,

forA.Guth&D.Kaiser,

Science307,884

(Feb11,2005),updated

toincludeWMAP

7-yeardata.

AlanGuth

MassachusettsInstituteofTechnology

8.286OpeningLecture,September7,2016

{17{

CMB:

Comparison

ofTheory

and

Experiment

GraphbyMaxTegmark,

forA.Guth&D.Kaiser,

Science307,884

(Feb11,2005),updated

toincludeWMAP

7-yeardata.

AlanGuth

Massachusetts

InstituteofTechnology

8.286OpeningLecture,September7,2016

{18{

SpectrumofCMBRipples

{19{