Dark Matter Theory

12 Pages·2006·0.09 MB·English

CarnegieObservatoriesAstrophysicsSeries,Vol.2:
MeasuringandModelingtheUniverse,2004
ed.W.L.Freedman(Cambridge:CambridgeUniv.Press)
Dark Matter Theory
JOSEPHSILK
DepartmentofPhysics,UniversityofOxford
Abstract
I evaluate the dark matter budget and describe baryonic dark matter candidates and their
detectability. Darkmatterissuesingalaxyformationtheoryarediscussed,andIreviewthe
prospects for detecting nonbaryonicdark matter. Indirect detection via halo annihilations
ofthefavoreddarkmattercandidate,theSUSYLSP,providesapotentialsignal. Therelic
densityofdarkmatterparticlesspecifiestheannihilationcross-sectionwithinmodeluncer-
tainties,andindirectdetectionprovidesouroptimalstrategyforconfirmingthedarkmatter
candidate. Galaxyformationsimulationssuggestthatthepredictedclumpinessofthedark
halowillfacilitateourimagingthedarkmatteringammarays,withcosmicraysignatures
providinginvaluableconfirmation. Similarly, the supermassiveblack hole in the center of
theMilkyWayshouldpresentauniquesignalamplifierbywhichwecanviewthedarkmat-
terinneutrinosaswellasingammarays. Theastrophysicaluncertaintiesaresolargethat
onehasnoalternativebuttolook.
1.1 Introduction
Therearetwotypesofdarkmatter: baryonicandnonbaryonic.Severalcandidates
forthe formerexist, butthe massfractionis unknown. Conversely,whilewe havenotyet
detectedtheleadingnonbaryonicmatter(colddarkmatter;CDM)candidate,theneutralino,
wecancalculateitsmassfractionandinteractioncross-section,withinmodeluncertainties.
Dark matter dominatesthe Universe, amountingto 90%of the matter density. Bary-
(cid:24)
onic darkmatter, as yetunambiguouslyidentified, comprisesup to a third of the baryons,
althoughtherearecompellingcandidates. Elucidatingthenatureofallofthedarkmatteris
oneof the outstandingproblemsin astrophysicsto be addressedoverthe nextdecade. As
will be discussed in this review, research at the interface of dark matter with galaxy for-
mation has been particularlyactive in recent years, but has also raised challengesthat are
leadingsometoquestiontheentiredarkmatteredifice.
Darkmatterhasavenerablehistory.Inthesolarsystem,anomaliesintheorbitofUranus
pointedtodarkmatterintheformofanewplanet,andthisledtothediscoveryoftheplanet
Neptune,followingthepredictionsofAdamsandLeverrier.Theadvanceoftheperihelionof
Mercury’sorbitalsostimulatedsearchesfordarkmatterintheformofanewplanetinterior
to the orbit of Mercury, dubbed Vulcan. This turned out to be a red herring: the orbital
anomalieseventuallyledEinsteintoproposeanewtheoryofgravitation,generalrelativity.
Similarly,thereareparallelsthatmaybedrawntoday.ThemodifiedNewtoniandynamics
1
J.Silk
(MOND) theoryseeks to modifythe law of gravity in orderto dispense with the need for
darkmatter.Thereislittleinthewayofcompellingtheoryordatatosupportsuchaposition,
but,atthesametime,MONDistenaciouslydifficulttokill(e.g.,Sanders&McGaugh2002).
Itiscertainlyworthbearinginmindthatgeneralrelativityhasbeenthoroughlytestedonly
inthe weak-fieldlimit, andonrelativelysmallscales, nowextendedup toa Mpc orso by
gravitationallensingstudies.
The modern dark matter problemwas first described in 1933 by Fritz Zwicky (Zwicky
1933),whonotedthatintheComaclusterofgalaxies,theratioofmasstolightasmeasured
by the virial theorem is about 400 M L . Since individual galaxies are found to have
(cid:12)= (cid:12)
about 10 M L , there is a serious shortfall of luminous matter. When observations of
(cid:12)= (cid:12)
clusterswereextendedtoX-rayfrequencies,asignificantcomponentofmasswasfoundthat
was notdetectedoptically. X-ray observationshave revealedthe presenceof a substantial
amountofhotintraclusterdiffusegas,contributingabout15%oftheclustermass. However,
80%oftheclustermassisnotaccountedforinanyknownform. Moreover,thisunknown
masscomponentmustbenonbaryonic,astheprimordialnucleosynthesismeasureofglobal
baryonabundance,combinedwiththemeanmatterdensity,isconsistentwiththeobserved
clusterbaryoncontentingasandstars.
On largerscales, onecan probedarkmatter via bothHubble-flowperturbationsstudied
viadeepredshiftsurveysandshearmapsfromweakgravitationallensing. Thedarkmatter
contentdominates the total matter content. The generalconsensus is that 03 with
m
(cid:10) (cid:25) : ;
about10%ofthisbeinginbaryons. Mostofthebaryonsmustalsobedark,althoughthere
arepersuasiveargumentsabouttheirnature.
Inowdescribeinmoredetailthedarkmatterbudget,describebaryonicdarkmattercan-
didatesand their detectability, and then turn to CDM issues in galaxyformationtheory. I
concludewithareviewoftheprospectsfordetectingnonbaryonicdarkmatter.
1.2 GlobalBaryonInventory
Primordialnucleosynthesisofthelightelementsdemonstratesthatthebaryonfrac-
tion is =004 0004 In effect, this is measured at z 109 when the light-element
b
(cid:10) : (cid:6) : : (cid:25) ;
abundancesfreezeout. Therearetwootherindependentmeasuresofthetotalbaryoncon-
tent of the Universe. The heights of the cosmic microwave background (CMB) acoustic
peaks, in particular the odd peaks that correspond to wave compressions and rarefactions
onthelast-scatteringsurfacescale andathalfofthisscale, arecontrolledprimarilybythe
baryondensityatz 1000 TheLy forestindirectlymeasuresthebaryondensityatz 3,
(cid:25) : (cid:11) (cid:25)
once a correction is made for the predominantionized fraction, whose density is inferred
from ionization balance by invoking the ionizing radiation field from quasars. All three
measuresofthebaryondensityconvergeon 004
b
(cid:10) (cid:25) : :
At the present epoch, the baryon inventory is rather different, however. Stars account
foramodestfractionofallthebaryonspresent,about0.0026inspheroidstarsand0.0015
in disk stars and cold gas, in units of the Einstein-de Sitter density (Fukugita, Hogan, &
Peebles1998). Thesenumbersare inapproximateagreementwith a recentdetermination,
includingallcoldinterstellarmatterandstarsinlow-massaswellasinmassivegalaxiesin
thelocalUniverse,whichyields00024 0001,correspondingto8% 4%of (Bellet
b
: (cid:6) : (cid:6) (cid:10)
al. 2003). Intraclustergasin richclusters, mappedinX-rays,accountsforanother0.0026
oftheclosuredensity,consistentwiththeconjecturethat,outsideofthegreatclusters,most
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J.Silk
ofthehotgasisabletocoolandformgalaxydisks. Intotal,onlysome20%ofthebaryons
isactuallyobservedatlowredshift.
Therearestrongtheoreticalindications,however,thatmostofthebaryons,anadditional
0.01 to 0.015 in units of the closure density, or a fraction 25% to 40% of are in the
b
(cid:10) ;
formofawarm/hotintergalacticmedium(WHIM)inlow-densityenvironmentsoutsidethe
rich clusters. A quantitative estimate of the WHIM fraction comes from large-scale nu-
mericalsimulations of the intergalacticmedium (IGM; Davé et al. 2001). The heating is
gravitational, due to accretionshocksin filamentsand sheets, and generallyoccurson the
peripheriesofgalaxies, galaxygroups,andgalaxyclusters. TheWHIM has recentlybeen
detected via excess soft X-ray emission toward clusters of galaxies (Sołtan, Freyberg, &
Hasinger2002)andalsoviaabsorptioninOVItowardquasars(Simcoe,Sargent,&Rauch
2002),althoughitisnotyetpossibletoquantifytheobservedWHIMmassfraction.
TheIGMsimulationsalsosuggestthatthereshouldbesomecoldintergalacticgas, vis-
ible as local, metal-poor,Ly forestlines, althoughquantitativepredictionsare unreliable
(cid:11)
becausemostofthecoldgaswillpresumablyhaveformedstars,oratleastfallenintogalax-
ies. However,observationsofthelocalLy -absorbingintergalacticcloudssuggestthatthey
(cid:11)
couldcontributenomorethan0.008oftheclosuredensity,or20%of ,towardthebaryon
b
(cid:10)
budget (Penton, Shull, & Stocke 2000). The net effect is that some 80% of the globally
distributed baryons are plausibly accounted for, despite the fact that no more than 20%
have hitherto been directly and quantitatively observed. The uncertainties in the gaseous
baryonfraction,especiallyforthewarmgas,aresuchthatonecaninferthatthereisnose-
riousglobaldarkbaryonproblem.However,thesituationmaybequitedifferentongalactic
scales.
1.3 GalacticBaryonInventory
Rich clusters contain a reservoir of gas that reflects the initial baryon content of
the Universe on 10 Mpc scales, and that, for the most part, consists of gas that has been
frustrated from forming disks. Processes such as galaxy collisions and tidal harassment
inhibitanaccumulationofcoolinggasingalaxyhalos. Thisfrustratesthepresumedmeans
bywhichdisksordinarilyaccretecoldgas. Itisthemaintenanceofalong-livedsupplyof
coldgasthatenablesstarformationtocontinueovera Hubbletimein low-densityregions
oftheUniverse.
Theclustergasfractionisapproximately15%. Thisplausiblyreflectstheinitialbaryon
fractioninprotodisks. Indeed,simulationsofdiskformationgenerallyrequireaninitialgas
fractionof15%to20%. Thisisnecessaryforsufficientcoolingtohaveoccurredtobeable
toformthedisks. Semi-analyticgalaxyformationpredictsthathaloscurrentlycontainlarge
amountsofgas,aboutasmuchaseventuallyformsthedisk.
Coldcloudsinthehalowouldbeeasilyobservable. Thegas,ifinthehalo,mustbeata
temperatureofseveralmilliondegrees. However,thereisequallyaproblemfordiffusegas
inthe halo: theresultingX-rayemission wouldexceedthatobservedbyupto an orderof
magnitude(Bensonetal. 2000),unlessrelativelylargesupernovafeedbackisimplemented
(Toftetal. 2002)toejectthegas. Ifthisgasweretoendupinthedisk,themassofthedisk
wouldpresumablyexceedobservedlimitsfromtherotationcurve. Onlyasmallfractionof
thediskmasscanbepresentinthehaloashotgas.
The Milky Way stellar disk mass is well constrained by a combination of gravitational
microlensing and infrared observations(Klypin, Zhao, & Somerville 2002). The mass of
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J.Silk
the disk and bulge is 6 1011M whereas the total mass, predominantly in the halo as
measuredbytherotation(cid:2)curveou(cid:12)t;tothe virialradius, is1012M Ifthe darkhaloprofile
(cid:12):
is NFW-like (Navarro, Frenk, & White 1996), there is no room for any further baryonic
componentthatmightbeinanoncompactformandnotalreadymeasuredbythepowerful
combinationofbulgemicrolensingandtheGalacticrotationcurve. Ithasbeenarguedthat
the darkhalomust actuallyhavea softer, more isothermal, corethan the power-lawNFW
profile;otherwise,thecontributionofthedarkmattertotherotationcurveintheinnergalaxy
wouldbeexcessive(Binney&Evans2001).
This leaves us in a quandary: where have the baryonsgone, which once were present?
There are two possibilities. The baryons may still be present, but hidden. For example,
they may be in dense, cold clumps in the outer halo, which are too large, by exceeding
the Einstein radius, to give a strong microlensing signal. Or they could be in the form of
compactobjects,suchasmassivecompacthaloobjects(MACHOs),sincetheobservational
limit on halo MACHOs obtained by microlensing of stars in the Large Magellanic Cloud
allows a halo fraction in MACHOs of up to 20%, in compactclump masses below a few
solar masses. The positive signal reported by one experiment indeed favors a MACHO
mass of about 05M In fact, one only needs to hide a halo mass fraction of about 6%,
: (cid:12):
and possibly as large as 10%, to arrive at an initial baryon mass fraction of 15%. An
(cid:24)
alternativepossibilityisthattheunaccountedbaryonswereejectedfromtheprotogalaxyin
anearlywind.
I nowconsiderthese variouspossibilities in turn. Cold denseclumpsof H , of Jupiter-
2
like mass and sizes of order a few astronomical units, have been invoked to account for
extreme scattering events (Walker & Wardle 1998) and for unidentified SCUBA submil-
limeter sources (Lawrence 2001). As such halo clumps orbit the Galaxy, they cross the
Galactic plane up to 100 times over the age of the disk. These traversalsresult in the gas
clumps acquiring on the order of 1 magnitude of extinction as they sweep up interstellar
dust(Kerins,Binney,&Silk2002). Inordertoavoidanexcessiverateofcollisionsandyet
keeptheclumpslargeenoughtobeJeansstableatagivenmassandtemperature,theclump
coveringfactormustbeontheorderof0.0001. Suchclumpsarepotentiallydetectablevia
gaseousmicrolensingevents(Rafikov& Draine2001),aswellasbyoccultationsofback-
groundstarsinaMACHO-typeexperimentthatmonitorsmillionsofstarsseveraltimesper
night.
Of course,suchcloudswouldcooltoafewdegreesKelvinandcollapse, unlessheated,
andtheonlyplausibleproposedheatsourceiscosmicrays.Itisnotclearifthecloudscould
maintainastableequilibrium(Gerhard&Silk1996;Wardle&Walker1999),althoughthe
reemittedradiationisconsistentwithmeasurementsofthefar-infraredbackground(Sciama
2000).Iftheclumpscollapse,theobservationalmotivationforinvokingthemisremoved.
The MACHOoptionmostlikelycentersonhalowhite dwarfs. Therehavebeenclaims
andcounterclaimsofdetectionsofold,high-velocitywhitedwarfs. Thefewdetectedhave
generallybeenattributedtothethickdisk. Ifevenafewpercentofthehalomasswasinthe
formofwhitedwarfs,amostunusualstellarinitialmassfunction(IMF)wouldberequired
fortheprecursorstars, peakedinthe intermediate-massrange. With a solar neighborhood
IMF, one could hide only of order 0.1% of the stellar mass in the Milky Way disk in our
halo. Iftheprecursorstarshadprimordialabundances( 10- 4 solar),onecouldplausibly
<
suppress helium flashes and associated C or N dredge-(cid:24)up, thereby avoiding the obvious
concernaboutstellarejectaoverpollutingtheinterstellarmediumandprotogalaxybyafactor
4
J.Silk
of up to 1000 in the elements primarily producedby intermediate-massstars. One would
stillperhapshavetotolerateagreatlyenhancedrateofTypeIasupernovae(SNIa).However,
thelikelihoodthatmostoftheSNIaejectaescapefromthegalaxyviawinds,combinedwith
theunknownscenarioforSNIaformation,whichitselfinvariablyinvolvesoldwhitedwarfs
inbinarysystems,renderssuchestimateshighlyuncertain.
1.4 Outflows
AnalternativeviewisthatgalaxiessuchasourMilkyWayhaveundergonemassive
outflowsinthepast. Thereisstrongevidencethatmanygalaxiespresentlyundergoingstar-
burstsaredrivingsuperwinds,withoutflowsontheorderofthecurrentstarformationrate.
The high-redshiftevidenceis especially compelling. At z 3 the Lyman-breakgalaxies,
(cid:25) ;
manyofwhichhavehighstarformationrates,occasionallyrevealinversePCygniprofiles
and,moregenerally,broadlinewidths, with inferredoutflowvelocitiesoftheorderofthe
escape velocity from the galaxy (Pettini et al. 2002; Shapley et al. 2003). Studies of the
Ly forestinthevicinityoftheLyman-breakgalaxies,viaabsorptioninthespectraofback-
(cid:11)
groundquasars,revealMpc-sizedholes. TheseareseenbothintheHIforestandinCIV
absorption, centered on the Lyman-break galaxies (Adelberger et al. 2003). A vigorous
windcouldexcavatesuchholes,althoughonecannotexcludeaphotoionizationsource.
Itmaybethatspiralshaveexpelledamassingascomparabletothatremaininginstars,
whereas, on the basis of gravitational lensing of background quasars, the time-delay ev-
idence suggests that many massive ellipticals have not lost a significant fraction of their
initialbaryoncontent.Indeed,ellipticalsneedtohaveconservedtheprimordialbaryonfrac-
tion,andpossiblyevenaccretedmorecoldbaryons,inordertoretainconsistencybothwith
theHubbleconstantandaNFW-likeprofile(Kochanek2003).
This would also be in accordance with the following conjecture, namely that the dark
halosinmassiveellipticalshavepristinedarkmatterprofiles,asgeneratedbymajormergers
andpredictedbyN-bodysimulations,whereasthedarkmatterconcentrationsinthehalosof
manyspiralsmayhavebeenmodifiedbytheastrophysicsofdiskformation.Suchmodifica-
tionscouldplausiblyoccurasaconsequenceofthedynamicalheatingandassociatedearly
outflows,linkedtotheformationofmassive,transientprotogalacticgaseousbars(Weinberg
& Katz 2002). An alternativepossibility appealsto the formationof supermassivecentral
black holes and associated early AGN activity that would have resulted in massive winds
(Binney,Gerhard,&Silk2001).
Thedesiredmodificationsinhalosofspirals,bothmassiveanddwarf-like,consistofthe
following. Theamountofsubstructuremustbesuppressed,bothtobeconsistentwiththat
vieweddirectlybycountingvisibledwarfs,andinferredindirectlybythenear-conservation
ofthe initialangularmomentumacquiredviatidaltorqueswithneighboringprotogalaxies
thatis necessary to accountfor the observedsizes of galactic disks. The concentrationof
darkmatterwithinacoupleofdiskscalelengthsmayneedtobereducedfromthe50%orso
bymasspredictedbythesimulationstothe10%orlessinferredfrommicrolensingforthe
MilkyWayGalaxyandfromdynamicalfrictionofbarsactingonthedarkhalosforspirals
withrapidlyrotatingcentralbars(Debattista&Sellwood2000).
Actually,thelatterargumentiscontroversial,asitissubjecttoignoringdestructionand
reformationofbarsthatcouldoccurseveraltimesoverthediskage,ifgasaccretionoccurs
overasimilartimescaleandataboutthesamerateasthediskstarformationrate(Bournaud
&Combes2002).Thismightallowbarstoberelativelyshort-lived,decayingviadynamical
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J.Silk
friction against the dark matter if the halo profile is NFW-like, or decaying on a similar
timescaleduetotheeffectsofdestabilizationviacentralgasaccretionandbulgeformation.
Provided the disk that reformed via cold gas accretion was sufficiently cold, it would be
unstabletobarformation.Presumablythestellaragesofbarswouldcontaincluesaboutthe
starformationhistorythatcouldperhapsbeunravelledbydetailedspectrophotometry.
Thedarkhalosofdwarfspiralsgenericallyseemtobemostlycuspless,insomecasesin
possiblecontradictionwiththeextentofacentralcuspthatispredictedbyCDMsimulations.
The dark matter profilescan be fit either by CDM profiles with anomalouslylow concen-
trationsor by dark matter profileswith nearly isothermalcores (de Blok & Bosma 2002).
Again,thiscouldbeaconsequenceofvigorousearlywinds,whichwouldbeconsistentwith
thelowobservedmetallicitiesofdwarfs.
Finally, a substantialsubsetof massiveellipticalshassoftcores. Thesegalaxiesareas-
sociatedwithslowrotationandboxyprofiles. Numericalsimulationssuggestanoriginvia
majormergers(Burkert&Naab2004). Onecouldplausiblyimaginethatsuchmergersre-
sultedinsubstantialcentralgasflowsandgeneratedluminousstarburststhatdrovemassive
winds. Supermassiveblackholeformationbymergersofblackholesof comparablemass
wouldalsoheatandscouroutthedarkmattercusp,producingasoftcoreorevenacentral
minimuminthestellarluminositydensity(Laueretal. 2002).
Itisindeedpossiblethatgalaxieshaveundergonea widevarietyofformationhistories.
Some of these, such as massive gas accretion onto disky ellipticals and major mergers to
form boxy ellipticals, may have had a strong but possibly indirect influence on the dark
matterprofiles,includingcusp,substructureandconcentration.
One could possibly distinguish between these alternative hypotheses by the prediction
that a massive early starburst would result in boosting the rate of Type II supernova pro-
ductionrelativeto the longertime scale forSNIa explosions,andhencelead to an [ Fe]
(cid:11)=
enhancement in the stellar core. Boxy ellipticals should therefore display systematically
higher[ Fe] thandiskyellipticals, whichoftenhavecentralcusps, orpower-lawprofiles
(cid:11)=
in surface brightness, and have also presumably undergone more quiescent accretion and
extendedperiodsofstarformationthatledtodiskformation.
Anotherlensingresultfromtheimageratiosofquadruplelensimagesofquasarscanonly
be explained if the massive elliptical lenses have substructure on subgalactic mass scales
(Dalal&Kochanek2002).Asimilarconclusionisinferredfromthebendingofradiojetson
milli-arcsecondscales(Metcalf2002).Somemassiveellipticalshavesubstructureconsistent
withthefullpredictedpowerofCDM,thatistosayoforderafewpercentofthehalodark
matter in inhomogeneities, with much of this in million-solar mass clumps. Some of the
massive ellipticals, those with disky isophotes, as well as low-mass ellipticals and bulges,
oftenhavecentralpower-lawstellarcusps. Itwouldofcoursebeinterestingtoknowifthese
propertieswere indeed correlated, and characteristic of the disky ellipticals and spheroids
thatarethoughtnottohaveundergonemajormergers.
Thereisonecrucialissuetobeaddressedifmassivewindswithoutflowratesontheorder
ofthestarformationratearetobeconsideredseriouslyinthecontextofgalaxyformation.
Simulationsfailtoproducesuchwindsinmassivegalaxies.Onefindsthatthewindsinteract
withambientgas,undergostrongradiativecooling,andarequenched.Ihavesuggestedthat
afundamentalproblemwithallexistingwindsimulationsistheomissionofcrucialsubgrid
physics(Silk 2003). In particular,Rayleigh-Taylorinstabilities, as the wind interactswith
thesurroundingmatterandtheshockedshellofdense,cooledgasdecelerates,increasethe
6
J.Silk
interstellarmediumporosity. This enablesthe galactic outflowsto proceedwith enhanced
efficiency. Simultaneously,Kelvin-Helmholtzinstabilitiesentraincoldinterstellargasinto
the outflows. Ablationand evaporationof the cold cloudsaddsto the mass loadingof the
wind. Theenhancedefficiencymeansthatevenmassivegalaxiescanhavewinds. Thestar
formationratepeaksintheprotogalacticphase.Thisiswhentheinterstellarcoldgasdensity
isgreatest,andhencecoolingismostimportant.Ifawindisindeeddriven,thewindoutflow
isoforderthestarformationrate.
However,asuccessfulwindrequiresthattherebesufficientenergyintheoutflowtodrive
the gas out of the galactic potential well. Normal supernovae of Type II, associated with
thestarburstthatcharacterizestheelevatedstarformationrate,maynotsufficetogenerate
enoughenergyifthegalaxyescapevelocityexceeds 100kms- 1. However,Ihaveargued
(cid:24)
thatevenmassivegalaxiesshouldhaveundergoneamassiveoutflowphase. Theresolution
mightliewithhypernovae,withcharacteristickineticenergies 1053 erg. Ithasbeensug-
(cid:24)
gestedthathypernovaeareassociatedwiththedeathsofstarsinexcessof30M thehigh
(cid:12);
energiesbeing generatedvia release of bindingenergyfrom infall onto the formingblack
hole. Therearecluesfromtheobservedmetallicitiesinatleastonestarburstgalaxyandin
metal-poorhalostarsthathypernovaemaymakeanimportantcontributiontotheobserved
chemical abundances. Indeed, hypernovae may even be the dominant contributors to the
abundancepatternsinextremelymetal-poorhalostars(Umeda&Nomoto2003). Ifindeed
all stars above 30M explode as hypernovae in metal-poor environments, the specific
(cid:24) (cid:12)
energyinputintotheinterstellarmediumwouldbeenhancedrelativetothatofordinarysu-
pernovaebyanorderofmagnitude.Thiswouldsufficetodrivewindsfromthemostmassive
protogalaxies.
Ithasalsobeensuggestedthathypernovaearepreferentiallyproducedviastellarmergers
(PortegiesZwartetal. 2002),whichareprimecandidatesforproducingthemassive,rapid
rotatorspreferredinhypernovamodels(Nakamuraetal. 2001). Suchmergersarelikelyto
bemostimportantinhigh-densitycoresassociatedwithstarbursts,andmightbeespecially
frequentduringtheprotogalacticstarburstphaseassociatedwithspheroidformation.
1.5 ProspectsforNonbaryonicDarkMatterDetection
CDM,despitethecurrentissuesthatarebeingraisedconcerningsubstructureand
darkmatterconcentrationonsmallscales,hasbeenremarkablysuccessfulinaccountingfor
large-scalestructureandinleadingtothepredictionoftheamplitudeoftheCMB temper-
ature fluctuations. However, detection, director indirect, has beenelusive, at least for the
dominantnonbaryonicdarkmattercomponent.
Massive neutrinosare the only formof nonbaryonicdark matter known to exist. How-
ever, they are subdominant. The relic neutrino number density is 3n where the relic
photon density n is 024(kT hc)3 and T is the measured CMB tem11pe(cid:13)r;ature of 2725
(cid:13) : =(cid:22) : (cid:6)
0002K A lower boundto the mass comesfromatmospheric oscillations, and is
e
m: 0:1eV Upperlimits come fromparticle physicsand from(cid:23)(cid:22)a!stro(cid:23)nomy. Now, h2=
>
m(cid:23) (cid:24)9:15eV: Theneutrinolessdoublebetadecaylimitontheelectronneutrinomas(cid:10)s,(cid:23)com-
(cid:6) (cid:23)= : :
binedwith recentreactorconstraints(usingKamLANDdata) onoscillations, setan upper
bound h2 007 (Minakata & Sugiyama 2002). A similar, but stronger bound comes
<
fromla(cid:10)rg(cid:23)e-sc(cid:24)ale:structure,andinparticularthelinearpowerspectrumderivedfromthe2dF
galaxyredshiftsurvey, such that the sum of the neutrinomasses must be less than 2.5 eV
7
J.Silk
(Elgarøyetal. 2002;Hannestad2002). Neutrinoscanbecomparabletothebaryondensity
incontributingtothemattercontentoftheUniverse.
Neutrinos decouple relativistically, since decouplingat kT 1MeV 2m c2 is con-
dec e
(cid:25) (cid:25)
trolled by neutron freeze-out. Generic massive weakly interacting particle candidatesthat
oncewere in thermalequilibriumdecouplenon-relativisticallyatkT m c2 20 and are
(cid:31) (cid:25) (cid:31) = ;
suppressedinnumberdensitybythecorrespondingBoltzmannfactorrelativetothephoton
numberdensity. ThereistheoreticalprejudicethatfavorsthelightestSUSYparticle,which
is stable if there is a commonly adopted symmetry among the SUSY family of particle
partnersto the knownbosonsand fermions. Constraintsfromthe LargeElectronPositron
Collider set a lower bound on the particle mass of about 50 GeV. A generic upper bound
comesfromtheory: theSUSY scale, andhencetheWIMPmass, cannotbemuchabove1
TeV if SUSY is to be relevant for accounting for the electroweak scale. Since the cross-
sectionforannihilationdecreasesintheunitaritylimitwithincreasingmassabove100GeV,
one cannot go to too high a mass, typically less than a few TeV, without overclosing the
UniversewithWIMPs. RealisticmodelswiththeobservedCDMdensity(Melchiorri&Silk
2002) =012 004h- 2 andincludingallcoannihilationchannels(Edsjöet al. 2003)
cdm
(cid:10) : (cid:6) :
suggestanupperboundof1500GeV.
One may hope to detect CDM in the form of the LSP via its annihilations in the halo.
Theannihilationproductsincludehigh-energypositrons,antiprotons,gammarays,andneu-
trinos. If the halo is uniform, the predicted fluxes fall a factor of 100 or more below ob-
servationallimitsasset, forexample,by thediffusehighgalacticlatitude EGRETgamma
rayflux. However,secondaryproductionofgammaraysvia p- pand p- interactionsof
(cid:11)
cosmicraysresultsina spectralenergydistributionthatissofterthantheobserveddiffuse
gammarayflux,suggestingthatamoreexoticexplanation,suchasthatfromannihilations,
maypossiblybemerited.
High-resolutionnumericalsimulationsofrealizationsofgalaxyhalospredictstrongclumpi-
ness on small scales. This would help boost the expected annihilation signal. A reported
high-energypositronfluxshowsaspectralexcessrelativetopredictedcosmicraysecondary
positronsnear50GeV.Withaboostfactorof100to1000,thiscouldbeduetoWIMPanni-
hilationsinthehalo,thefittothedatacertainlybeingimprovedbyacarefullytunedWIMP
mass(Baltzetal. 2002).
Oneconsequenceisthepredictionofaprimaryhigh-energy pfeaturethatmightbepo-
(cid:22)
tentially observable with the forthcomingPAMELA and AMS experiments. The possible
spectralsignaturesofthediffusegammarayandneutrinofluxesfromhaloannihilationsare
moreelusive. Withregardtothegammarays, oneexpectsa 0 decaysignaturebothfrom
(cid:25)
cosmicrayinteractionswith interstellargasandfromtheannihilations,althoughthe latter
wouldproduceaharderspectrumformassiveWIMPs. However,onemayhopetoidentify
individualhaloclumpsaspointlikegammaraysourcesatthesensitivityandangularresolu-
tionoftheGLASTsatellite,tobelaunchedin2007. Gammaraylinesareanotherpotential
signal,thatwouldbeauniquetracerofannihilations.
One concern is that the clumpiness of the dark halo may be greatly overestimated by
the numerical simulations. This is because significant astrophysics is omitted from halo
modeling. Clumps self-destructvia dynamicalinteractionswith each other and especially
withtheGalactictidalfield. Nevertheless,eventheneutrinofluxfromannihilationsmaybe
detectablewithamoreextremestrategythattargetstheGalacticCenter.
Observationsstronglysuggestthattheformationofthespheroidstarsiscloselycoupled
8
J.Silk
totheformationofthecentralsupermassiveblackhole. OneobservesthatM 10- 3M
sph
(cid:15)(cid:25)
overawidedynamicrange,withonlymodestdispersion.InadiskgalaxysuchastheMilky
Way,itislikelythatthesupermassiveblackholeattheGalacticCenterof 26 106M
(cid:24) : (cid:2) (cid:12)
(Ghez2004)formedbyacombinationofgasaccretionontoaseedblackholeandmerging
ofsmallerblackholes,ratherthanbymajorblackholemergingevents. Cosmology,andin
particularstudiesofprimordialstarformation,suggeststhattheseedblackholemassesare
likelytobearound100M InferencesabouttheprimordialIMFleadtothelikelypresence
(cid:12):
in the protogalactic halo of large numbers of 100M black holes (Islam, Taylor, & Silk
(cid:12)
2002;Volonteri,Haardt,&Madau2003). Gasaccretioninthegas-richprotogalaxyisthen
thefavoredformationmodelforthecentralsupermassiveblackhole.
Another optionwould be for the black hole and accompanyingspheroidto form by hi-
erarchicalclusteringandmergingofroughlyequal-massblackholesthatareembeddedin
mini-spheroidsofstars. ThisseemsimplausiblefortheMilkyWayblackholeandspheroid
because the merging time scales would seem to be very long, and also because of the re-
markableobservedchemodynamicalcontinuitybetweenthindisk,thickdisk,andspheroid
stars. Thisissuggestiveofarelativelynonviolenthistory.Thelastsignificantmergerwould
havetohaveoccurredatleast12Gyragoandprecededdiskformation.Thestellaragesand
chemicalpropertiesofthediskandbulgesuggestacoevalformation.Inordertoaccountfor
barandbulgeformation,secularevolutionratherthanmergingisusuallypreferredinMilky
Way-typegalaxies,andmoregenerallyinlate-typedisks.
A rather differenthistory may be applicable to massive spheroidformation. Of course,
ellipticalgalaxiesmaywellbeolder,onaverage,andformindenserenvironments. Major
mergersthattriggeredbursts of star formationmostlikely played an importantrole in the
formationofmassiveellipticals,asinferredforexamplefromthehighcentralstellarsurface
densitiesandmuchcircumstantialevidencefromultraluminousinfraredgalaxies.Ourhalo,
however,appearstohaveretainedthekinematicsubstructurecharacteristicofminormergers
(Gilmore,Wyse,&Norris2002).
TheMilkyWayisanespeciallyinterestingcase. Thereisacentralbar,andtherearealso
indicationsfromtherotationcurveandthegravitationalmicrolensingopticaldepththatno
morethan10%ofthemasswithintwodiskscale lengthscanbenonbaryonicdarkmatter.
Theformationofthesupermassiveblackholeaddsafurthercomplicationthatmaymodify
thedarkmatterprofileclosetothecenter.Infact,wecantakeadvantageofthis,iftheCDM
consistsofthelighteststableSUSYparticle.
Thepresenceofthesupermassiveblackholeallowsapotentiallyimportantprobeofthe
dark matter density concentrationin the inner galaxy. As the black hole forms within the
preexistingdarkmatterpotentialwell,adarkmattercuspdevelopswithinthezoneofgrav-
itational influence of the black hole, or about 0.1 pc. The cusp slope is r- 9 4 for a soft
=
galacticcore,andissteeperforaninitiallypower-lawprofileinthegalaxyinnerhalo. Dark
matterannihilationsarecorrespondinglyboosted,withinaradiusbetween0.1pcandabout
100Schwarzschildradii,withinwhichthedarkmatterannihilatesinlessthanadynamical
crossingtime.Oneconsequenceisthatapointsourceofhigh-energyneutrinosshouldbede-
tectabletowardtheGalacticCenter(Gondolo&Silk1999;Ullio, Zhao,&Kamionkowski
2001). However, this is not the end of the story, for there are also radio and gamma ray
signatures.
Infact,theadvection-dominatedaccretionmodelforSgrA ,fortheflowintheaccretion
(cid:3)
diskaroundthecentralblackholeintheMilkyWay,explainsalloftheobservationsexceptin
9
J.Silk
thelow-frequencyradioandgammarayregimes(Quataert&Narayan1999).Anadditional
nonthermalsourceofenergeticparticlesisneeded.Annihilationsboostedbytheblackhole–
enhancedcuspprovideanattractivepossibleexplanation.
The annihilations into e+e- pairs generate synchrotron emission. Remarkably, the pre-
dicted spectral shape, dominated by synchrotron self-absorption, matches that observed.
MoreovertheunidentifiedEGRETgammaraysourceattheGalacticCentercanalsobefit
spectrally,andthemagneticfieldstrengthcanthenbeinferredbycomparingtheradioand
gammarayfluxes(Bertone,Sigl,&Silk2002). Theinferredfieldstrengthdependsonthe
initial profile and the WIMP cross-section at a specified dark matter density, both leading
toseveralordersofmagnitudeuncertaintyintherequiredmagneticfieldstrength,whichin
factspanstheequipartitionestimate. Inpractice,theprofileuncertaintiesdominate.
The observed radio and gamma ray luminosities can then be explainedas being a con-
sequenceofthe energyinputfromannihilationsifthe initialcentralcoredensityprofileis
reasonably soft, as compared to the NFW profile. Detection of a neutrino source by the
Northernhemisphereunderwaterneutrinotelescopes ANTARES or NESTOR couldeven-
tuallyprovidethesmokinggunthatconfirmsannihilationsasasignificantenergysourcein
theinner0.1pcoftheMilkyWay.
1.6 Conclusions
Darkmatterhasbeenremarkablyelusive.Despite70yearsoverwhichtheproblem
ofthedarkmatterhasbeenrecognized,therehasbeennoconfirmationofitsnature. How-
ever, observations are proceeding at a great pace, both from the observational astronomy
side,especially,butnotexclusively,viagravitationallensing,andontheexperimentalside
by the enormousprogressbeingmade in directand indirectdetection experiments. There
isincreasingrecognitionthattwo of the greatestunresolvedissues in physicalcosmology,
the nature of the dark matter and the formation of the galaxies, are intimately connected.
Supermassiveblack holesand active galactic nucleiare importantingredientsthat need to
beincorporatedintoourmodelingbeforewecanunderstandhowprotogalaxiesevolve.
Imagingdarkmatterwouldbeawonderfulachievementtocrownourstudiesoftheseas-
trophysicalpuzzles. Ifourprejudicesaboutdarkmatterarecorrect,annihilationsprovidea
potentialsignal. Directdetectioncannotcompete: attheLargeHadronCollideranySUSY
evidence will have no direct relevance for dark matter, although, of course, the comple-
mentarity is an essential strategy. Even direct detection experimentsin deep underground
laboratoriesaresearchingforaSUSY-inspiredscatteringsignalthatmaybefarbelow10- 10
picobarns,andsowillbebeyondanypossiblereachintheforeseeablefuture,wherethegoal
forton-scaledetectorsisoforder10- 8picobarns.
The relic density of dark matter particlesspecifies the annihilationcross-section within
modeluncertainties,andindirectdetectionprovidesouroptimalstrategyforconfirmingthat
theSUSYLSPisthedarkmattercandidate. Itmaywellbethattheclumpinessofthedark
halo will facilitate our obtainingthese images in the gammaray domain. The cosmic ray
signatureswill provideinvaluableconfirmation,aswill detectionofhigh-energyneutrinos
fromtheSun thatare generatedby annihilationsofWIMPstrappedin thesolar core. The
supermassive black hole in the center of the Milky Way may present a unique signal am-
plifier bywhich we can view the dark matter, in neutrinosas well asin gammarays. The
uncertaintiesinthehalofine-structureandinhowthedarkmatteraggregatedandthecentral
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