JacobBekenstein’s universe:fromBlackHolesto

modifiedtheoryofGravity

OferLahav(UCL)

Outline

• Personalmemories:fromMScatBGUin1980stoUCL’svisitin2014

• Blackholes:fromEntropytoGravitationalWaves

• ModifiedGravity: fromgalaxyrotationcurvestocosmicscales

ThankstoScottDodelson,PedroFerreira,Moti Milgrom andJonathanOppenheimforhelpfuldiscussions whilepreparingthistalk.

MSc(Ben-GurionUniversity,1985),“Coolingprocessesinformationofthefirstobjectsintheuniverse”

• Bekenstein wasverykindtosupervisemeasapart-time studentduringmyservice intheairforce.

• Heselected a‘quiet’topicwhich laterbecameveryimportant.

• Hekeptemphasizing theimportance ofTHINKING.

• Initially IcontinuedonthistopicinmyCambridgePhD,butthengot‘diverted’toworkonlargescalestructurewithGeorgeEfstathiou andDonaldLynden-Bell.

• Bekenstein’s foundationshavebeenveryinfluential formycareer.

JohnWheeleraskedhisstudentJacobBekenstein intheearly1970’s:

“Whatwouldhappenifyoupouredahotcupofteaintoablackhole?”

ThishasledtoBekenstein’s 1973paper(4842citations)

“Thereareanumberofsimilarities betweenblack-hole physicsandthermodynamics.Moststriking isthesimilarity inthebehaviorsofblack-holeareaandof entropy:bothquantities tendtoincrease irreversibly.”

BlackHoleEntropy(Bekenstein 1973)

• BH=“BlackHole”or“Bekenstein-Hawking”• ConnectingGravitytoQuantumMechanics• Area(notvolume)oftheeventhorizoniswhatmatters• Implications for“Bekenstein’s informationbound”

and“holographicPrinciple”

ThediscoveryofGravitationalWavesfromamergeroftwoblackholes

6

LIGO collaboration February 2016 (+ a second BH merger event reported in June 2016)

MeasuringtheaccelerationoftheUniverse

7

The Dark Energy problem: 20, 100 or 330 years old?

The weak field limit of GR:a = -GM/r2 + Λ/3 r

Partridge, OL & Hoffman (2014)

“I have now explained the TWO principle cases ofattraction…which is very remarkable”

Isaac Newton, Principia (1687)

e.g. the infall of the Milky Way and Andromeda is affected by Λ (increases derivded mass by 13%)

“a simple but strange universe”

But what are the Dark Matter and Dark Energy?

Paradigmshifts:anewentityoranewtheory?

OL & Michela Massimi (A&G 2014)Lucy Calder & OL (Physics World 2010)

Phenomenon New Entity New theory

Uranus’ orbit Neptune (Bessel’s specific gravity ruled out)

Mercury’s orbit (Hypothetical planet Vulcan ruled out)

General Relativity

Beta decay Neutrino (violation of angular momentum ruled out)

Galaxy flat rotation curves

Dark Matter? Modified Newtonian Dynamics?

Accelerating universe(SN Ia and other data)

Dark Energy? Modified General Relativity?

10

Galaxyrotationcurves:DarkmatterhalosorMOND?

GravitationalLensing:WeakandStrong

HST CLASH cluster MACS1206 12

TheDarkEnergySurvey

• Multi-probeapproachWidefield:ClusterCounts,WeakLensing,LargeScaleStructureTimedomain: Supernovae

• Surveystrategy300millionphotometricredshifts(grizY)over5000deg2

+2500SNIa (over27sq deg fields)

• So far about 50% of the data observed,• 69 DES papers on the arXiv

CTIO

13

WhathavewelearnedfromDESonDarkMatter?(fromgalactictoGpcscales)

• MassmapsfromWL• Theshearcorrelations asexpected inLCDM(withsigma_8=0.8)• Galaxies tracethedarkmatterfluctuations (esp clustersandvoids)

frombothDESWLandCMBWL• New‘clustered’ 17MWcompanionsqualitatively fitintothescenario of

hierarchical LSSformation• DwarfsareDMdominated (e.g.M/L=470(M/L)¤ forRet2)• ButtheNatureoftheDMstillunknown(onlyupper limitsongammaray

emission)

WL by troughs (voids)in the galaxy distribution (Gruen et al.)

New MW companions (in red) (Drlica-Wagner et al.)

RXJ224: WL DM (contours) vs stellar mass(Melchior et al, Palmese et al)

EntersTeVeS (Bekenstein 2004)

• ImpressiveRelativisticextensionofMOND,withsuccessfullensing

• Tensor,Vector,Scalar• Challenges:*TheCMBpeaks*TheBulletCluster- Moreworkisneededforfaircomparison

TestingGR:Gravitational redshiftsinClusters

Confirming Wojtak et al. (Nature 2011)See Kaiser (2012) for more wffwxtqseffects

Sadeh, Feng & OL ( PRL, arXiv:1410.5262)

16

GravitationalRedshiftinClusters

Data:WHH2011Dashed line:GR+NFWSolidline: TeVes/MONDDotted line:ahybridmodel

Actuallyatestoftheequivalence principle

The era of DESI, Euclid, LSST,…

18

AtUCL,July2014

Haaretz 5May2012

“Sometimestheoriesaresmarterthantheirinventors”(JacobBekenstein toacolleague)

Summary• Bekenstein’s work:Deepthinking,offthebeatentrack.

• BHEntropyandBound:oneofthemostimportantideasinPhysics.

• TeVeS:aremarkabletheory,facingsomeobservationalchallenges,butitmightsignalthebeginningofnewPhysics.

• Hewillbegreatlymissedintheeraofgravitationalwavesandsurveysofbillionsofgalaxies

• Bekenstein’s workwillinspiremanyfuturegenerations.