Published by the Association Pro ISSI

No 7, May 2001

INTERNATIONAL SPACESCIENCEINSTITUTE

In Search of the Dark Matterin the Universe

In Search of the Dark Matterin the Universe

Dark matter constitutesthe vast majority ofmatter in the universe.Recent research sheds light on what itmight be.

Dark matter constitutesthe vast majority ofmatter in the universe.Recent research sheds light on what itmight be.

Dark matter. Dark energy. Dark is beautiful!

But it is not so easy to find thedark. It requires charting the un-known as was done some six hun-dred years ago with the terraincognita. Vague contours, how-ever, are known. Dark mattermanifests itself by gravitational ef-fects on visible matter, giving ageneral direction for those whowant to move on in the dark. Butmuch more remains to be foundout.

Dark matter is a playground forcreative scientists. Imagination isrequired to define the unknownand formulate hypotheses, whichwill most likely be thrown outonce sufficient scientific progresshas been made. Karl Popper, the great Austrian philosopher,showed that rejection – not con-firmation – of a theory is the cre-ative step when it is followed bythe formulation of a more com-prehensive theory.

Dark matter is full of surprises.Dark matter is everywhere. Darkmatter may even be found in thedark underworld of Bern. Theauthor of this issue of Spatium,Professor Klaus Pretzl, head of the Laboratory for High EnergyPhysics of the University of Bern,knows where to find it. He is incharge of the ORPHEUS experi-ment, now in its final stage of im-plementation some thirty metersbelow the University of Bern.This complex set-up seeks to in-vestigate a specific class of darkmatter particles, the weakly inter-acting massive particles.

In March 2000 Professor Pretzlreported on his institute’s fascinat-ing search for dark matter to aninterested Pro-ISSI audience. Weare very grateful to Klaus Pretzlfor his kind permission to publishherewith his lecture.

Dark is beautiful.

Hansjörg SchlaepferBern, May 2001

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Impressum

S PATI UMPublished by theAssociation Pro ISSItwice a year

Association Pro ISSIHallerstrasse 6, CH-3012 BernPhone ++41 31 631 48 96Fax ++41 31 631 48 97

President Prof. Hermann Debrunner, University of BernPublisherDr. Hansjörg Schlaepfer, legenda schläpfer wort & bild,WinkelLayoutMarcel Künzi, marketing · kom-munikation, CH-8483 KollbrunnPrinting Druckerei Peter + Co dpcCH-8037 Zurich

Front CoverSpiral Galaxy NGC 1232 Copyright: European SouthernObservatory ESO, PR Photo37/e/98 (23 September 1998)

INTERNATIONAL SPACESCIENCEINSTITUTE

Editorial

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Introduction

The story of the dark matter start-ed in 1933, when the Swiss as-tronomer Fritz Zwicky publishedastonishing results from his stud-ies of galactic clusters. In his paperhe concluded that most of thematter in clusters is totally invisi-ble. Zwicky argued that gravitymust keep the galaxies in the clus-ter together, since otherwise theywould move apart from eachother due to their own motion. Bydetermining the speed of motionof many galaxies within a clusterfrom the measurement of theDoppler-shift of the spectral lineshe could infer the required gravi-tational pull and thereby the totalmass in the cluster. Much to hissurprise the required mass by farexceeded the visible mass in thecluster. His results were receivedwith great scepticism by most as-tronomers at that time. It took an-other 60 years until he was provento be right.

How much of this mysteriousdark mass is there in the universe?Where do we find it? What is its real nature? Will we be able todirectly detect it, since it does not radiate light or particles? Itdemonstrates its presence only byits gravitational pull on visiblematter. These are the questionswhich intrigue astronomers, cos-mologists, particle and nuclearphysicists alike. In this talk I willtry to give a short account of whatwe have learned about this myste-rious kind of matter which domi-nates our universe.

How was mattercreated?

The very first instants

Our knowledge about the begin-ning of the universe is rather ob-scure, since it was born out of astate which is not describable byany physical law we know of. Wesimply call its indescribable mo-ments of birth the Big Bang. Afterthe Big Bang the universe rapidlyexpanded starting from an incred-ibly small region with dimensionsof the order of 10–33 cm and an un-thinkably high energy density of1094 g/cm3. Grand Unified Theo-ries (GUT), which aim to describethe physical laws at this young ageof the universe, tell us that physicswas much simpler at that time,since there was only one force rul-ing everything. Today, however,we distinguish four fundamentalforces: the force of gravity, whichattracts us to the earth and theplanets to the sun, the electromag-netic force, which keeps the nega-tively charged electrons in theouter shell of an atom attracted tothe positively charged nucleus inthe center, the weak force, whichis responsible for the radioactivitywe observe when unstable nucleidecay, and the strong force, whichholds the protons and neutronstogether in the nucleus. All theseforces were unified in a singleforce at this early time. Physicistscall this a state of symmetry, sincethe forces all have the samestrength and are therefore indis-

tinguishable from each other.However, while the universe wasexpanding rapidly this symmetrywas quickly broken and gravity,the strong and the electro-weakforces appear today as separateforces with vastly differentstrength and reach.

The enormous isotropy and ho-mogeneity of the universe, whichwe encounter when looking at thedistribution of galaxies through-out space, and which we obtainfrom measurements of the cosmicmicrowave background radiation(CMB), is very puzzling. This isbecause there are regions in theexpanding universe which havenever been in causal contact witheach other, i.e. light never had suf-ficient time since the Big Bang totravel far enough to transmit in-formation from one to the otherof these regions. To solve this puz-zle the astrophysicists A. Guth andA. Linde introduced a scenario ac-cording to which the universe ex-perienced an exponentially rapidexpansion during a very short pe-riod of time between 10–36 to 10–34

seconds after the Big Bang. Dur-ing this period the universe wasrapidly growing in size by about a factor of 3 x 1043. Within thisscenario of inflation it becomespossible to causally connect re-gions of the universe which seemto be otherwise disconnected fromeach other. Nevertheless, after thatshort period of inflation the uni-verse continued its expansionwith retarded speed. Inflation isvery much favoured by most cos-mologists and strongly supportedby the recent observations of thecosmic microwave background

In Search of the Dark Matter in the Universe *)

Klaus Pretzl, Laboratory for High Energy Physics, University of Bern

*) Pro ISSI lecture, Bern, March 15th 2000

radiation by the Boomerang andMaxima experiments (see alsobelow).

According to this scenario the in-flationary phase of the universe

was abruptly ended by the cre-ation of matter and radiationabout 10–34 seconds after the BigBang. At that time the universecontained all the basic buildingblocks of matter, the quarks, the

gluons and the electrons, and theirantiparticles. Nevertheless, mostof the energy of the universeresided in radiation, mainly pho-tons and neutrinos, etc . . . How-ever, as the universe cooled by ex-pansion, radiation lost its energyfaster than matter and when theuniverse became about ten thou-sand years old the energy balanceshifted in favour of matter.

The quark gluon phase of matterended about 10–6 seconds after theBig Bang, when the universecooled to a temperature of 2 x 1012

Kelvin. At that temperature aphase transition from a quarkgluon plasma to a nucleonic phaseof matter took place (Figure 1),where the protons and neutronswere formed. In this process threequarks of different flavour (so-called up-quarks and down-quarks) combine together to forma proton (two up-quarks and onedown-quark) or a neutron (twodown-quarks and one up-quark)and similarly antiprotons (two an-tiup-quarks and one antidown-quark) and antineutrons (twoantidown-quarks and one antiup-quark). The gluons were giventheir name because they providethe glue for holding the quarks to-gether in the nucleons. After thisphase transition one would expectto end up with the same numberof nucleons and antinucleons,which annihilate each other aftercreation, leaving us not a chanceto exist. Fortunately this was notthe case. The reason that we livein a world of matter with no anti-matter is due to a very subtle ef-fect, which treats matter and anti-matter in a different way during

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Figure 1Temperatures of about 2 x 1012 were reached in this collision experiment atCERN. A lead target was bombarded with highly relativistic lead nuclei. At that tem-perature, the quark gluon plasma occurs. The picture shows the outburst of manyparticles which looks typical when transitions from the quark gluon to the nucleonicphase takes place. (NA49 experiment at CERN).

the phase of creation. This effect,which is known as CP-violation(Charge conjugation and Parityviolation), was first discovered inan accelerator experiment by V.Fitch and J. Cronin in 1964, forwhich they got the Nobel prize in1980, and was used by A. Sakha-rov to explain the matter antimat-ter asymmetry in the universe.

After further expansion the uni-verse cooled to a temperature of109 Kelvin, when protons andneutrons started to hang on toeach other to form the light ele-ments like helium, deuterium,lithium and beryllium. This phaseof nucleosynthesis began a fewseconds after the Big Bang. Theheavier elements were only

formed many million years later,mainly during star formation andsupernova explosions. After theirformation the light nuclei hadhundred thousand years of time tocatch electrons to build atoms.

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Figure 2The evolution of the universe. Modern physics and experimental observations document the history of the universe from anincremental fraction of time after the Big Bang 15 billions of years ago up to its present state. Dark matter is seen today as havingplayed a key role in the formation of stars and galaxies. (© CERN Publications, July 1991)

radiationparticles

heavy particlescarrying theweak force

quarkanti-quarkelectron

positron (anti-electron)

protonneutron

mesonhydrogendeuterium

heliumlithium

1032 degrees

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The cosmic backgroundradiation

About three hundred thousandyears after the Big Bang radiationhad not enough energy left tointeract with matter and the uni-verse became transparent for elec-tromagnetic radiation. This radia-tion from the early universe wasfirst discovered by R. Wilson andA. Penzias in 1965. They receivedthe Nobel prize for this finding in1978. Their discovery was madeby chance, since they were on amission from Bell Laboratories totest new microwave receivers torelay telephone calls to earth-or-biting communication satellites.No matter in what direction theypointed their antenna, they alwaysmeasured the same noise. At firstthis was rather disappointing tothem. But they happened to learnof the work of the astronomers R.Dicke and P. Peebles and they re-alised that the noise they weremeasuring was finally not thenoise of the receiver, but ratherthe cooled down cosmic mi-crowave background radiation(CMB) from the Big Bang. Fromthe frequency spectrum andPlanck’s law of black body radia-tion the temperature of CMB wasderived to be 2.7 Kelvin. Regard-less which direction the cosmic ra-diation was received from, thetemperature came out to be thesame everywhere, demonstratingthe enormous homogeneity of the universe. The most accurateCMB measurements come fromthe Cosmic Background Explorersatellite (COBE), which was sentinto orbit in 1989. They found

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Figure 3Full sky map of the cosmic background radiation as seen by the COBE mission.After subtraction of the dipol anisotropy (top) and our own galaxy’s emission (center)temperature variation of 0.01% unveil matter density fluctuations in the very earlyuniverse (bottom). (© Physics Today)

temperature variations only at alevel of one part in a hundredthousand.

Where does dark matter come from?

After this short description of thehistory of the universe and thecreation of matter we ask our-selves: What about the darkmatter? When and how was itcreated? A partial answer to thisquestion is given to us by theCOBE cosmic microwave back-ground radiation measurements.They show islands of lower andhigher temperatures appearing onthe map of the universe which aredue to density fluctuations of mat-ter (Figure 3). They were alreadypresent at the time radiation de-coupled from matter, three hun-dred thousand years after the BigBang, long before matter wasclumping to form galaxies andclusters of galaxies. We have rea-sons to believe that these densityfluctuations are due to the darkmatter, which was probably creat-ed from quantum fluctuationsduring the inflationary phase ofthe universe. These tiny fluctua-tions expanded first through in-flation and then retarded theirexpansion due to gravitationalbinding forces. They then formedthe gravitational potential wellsinto which ordinary matter fell toform billion years later galaxiesand stars. All galaxies and clustersof galaxies seem to be embeddedinto halos of dark matter.

How muchmatter is in theuniverse?

At first this question seems to behighly academic. It is not. The fateof our universe depends on itsmass and its expansion velocity.

The destiny of the universe

In the 1920s the famous as-tronomer Edwin Hubble demon-strated that all galaxies are movingaway from us and from eachother. His discovery was the foun-dation stone of modern cosmolo-gy, which claims that the universeoriginated about 15 billion years

ago in an unthinkably small vol-ume with an unthinkably high en-ergy density, the so-called BigBang, and is expanding ever since.However, this expansion is coun-teracted by the gravitational pullof the matter in the universe. De-pending on how much matterthere is, the expansion will contin-ue forever or come to a halt,which subsequently could lead toa collapse of the universe endingin a Big Crunch, the opposite ofthe Big Bang. The matter densityneeded to bring the expansion ofthe universe to a halt is called thecritical mass density, which todaywould be roughly the equivalentof 10 hydrogen atoms per cubicmeter. This seems incrediblysmall, like a vacuum, when com-pared to the density of our earthand planets, but seen on a cosmicscale it represents a lot of matter.

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NGC 3198

Figure 4The observation of constant orbital velocities of stars around the galactic center(here the spiral galaxy NGC 3198) as a function of the radial distance provides con-vincing evidence for the presence of an extended halo of dark matter surroundingthe galaxy. The expected curve from Kepler’s law if there would be no dark matter isalso shown.

How can we find out how muchmatter there is? When estimatingthe visible matter in the universe,astronomers look in a very wideand very deep region in space andcount the number of galaxies.Typical galaxies containing hun-dreds of billions of luminous starshave a brightness which is propor-tional to their mass. Thus, by sim-ply counting galaxies over a largevolume in space and by assumingthat galaxies are evenly distrib-uted over the entire universe, onecan estimate the total mass theycontribute in form of visible massto the universe. However, it turnsout to be only 1% of the criticalmass of the universe. Therefore, ifthe visible matter in form of stars

and galaxies were the only matterin the universe, the universewould expand forever. We neg-lected here the amount of matterin form of planets, since they con-tribute not more than a few per-cent of the mass of a star. How-ever, it came as a surprise whenVera Rubin and her team foundout in the 1970s that the visiblestars are not the only objects mak-ing up the mass of the galaxies.They measured the orbital speedsof stars around the center of spiralgalaxies and found that they movewith a constant velocity inde-pendent of their radial distancefrom the center (Figure 4). This isin apparent disagreement withKepler's law, which says that thevelocity should decrease as thedistance of the star from the galac-tic center increases, provided thatall mass is concentrated at the cen-ter of the galaxy, which seems to

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Figure 5A galaxy as seen schematically from a distant point in the galactic plane. Darkmatter forms a large halo extending far outside the outer edges of the galaxy.

Figure 6Experimental analysis of the rotational velocities in the Andromeda GalaxyM31 from optical observations (V. Rubin et al.) and radio observations at 21cm wave-length (B. Roberts et al.).

Luminous Galaxy

150,000 Light-years

Envelope of Dark Matter

Sun

be the case if only the luminousmatter is considered. If howeverKepler’s law, which describes theorbital motion of the planets inour solar system very correctly, isvalid everywhere in the universe,then the rotational velocities ofthe stars can only be explained ifthe mass of the galaxy is increasingwith the radial distance from itscenter. Numerical calculationsshow that there must be at least anorder of magnitude more matterin the galaxies than is visible.From their measurements, whichthey repeated on hundreds of dif-ferent galaxies, they conjecturedthat each galaxy must be embed-ded in an enormous halo of darkmatter, which reaches out evenbeyond the visible diameter of thegalaxy (Figure 5). Spiral galaxies aresurrounded also by clouds of neu-tral hydrogen, which themselvesdo not contribute considerably tothe mass of the galaxy, but whichserve as tracers of the orbital mo-tion beyond the optical limits ofthe galaxies. The hydrogen atomsin the clouds are emitting a char-acteristic radiation with a wave-length of 21 cm, which is due to a hyperfine interaction betweenthe electron and the proton in the hydrogen atom and which can be detected. These measurementsshow that the dark matter halo ex-tends far beyond the optical limitsof the galaxies (Figure 6). But, howfar does it really reach out? Veryrecently gravitational lensing ob-servations seem to indicate thatthe dark matter halo of galaxiesmay have dimensions larger thanten times the optical diameter. It is quite possible that the darkhalos have dimensions which are

already typical for distances be-tween neighbouring galaxies with-in galactic clusters.

Determining the mass in the universe

The effect of gravitational lensingis a consequence of Einstein's gen-eral relativity. It was first observedin 1919, when an apparent angularshift of the planet Mercury closeto the solar limb was measuredduring a total solar eclipse. Thiswas the first, important proof forthe validity of Einstein’s theory,according to which light comingfrom a distant star is bent whengrazing a massive object due tothe space curvature caused by thegravity of the object (Figure 7). Itwas Fritz Zwicky in 1937 who re-alized that the effect of gravita-tional lensing would provide the

means for the most direct deter-mination of the mass of very largegalactic clusters, including darkmatter. But it took more than 50 years until his suggestion wasfinally realized and his early deter-mination of the mass of theCOMA cluster, in 1933, was con-firmed. With the Hubble tele-scope in space and the Very LargeTelescopes (VLT) at the SouthernObservatory in Chile astronomersnow have very powerful tools,which allow them not only to ex-plore the visible, but also the darkside of the universe with gravita-tional lensing.

An observer sees a distorted mul-tiple image of a light source in thefar background, when the deflect-ing massive object in the fore-ground is close to the line of sight.The light source appears to be aring, the so-called Einstein-Ring,

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Figure 7Space is curved by gravity. The light rays from a distant star are bent by the grav-ity field of the sun. The distant star therefore appears at a different position.(Spektrum der Wissenschaft)

Earth Sun

star (true

position)

star (apparent position)

when the object is exactly in theline of sight (Figure 8). If one knowsthe distance of the light sourceand the object to the observer oneis able to infer the mass of the ob-ject from the lensing image. Withthis method it was possible to de-termine the mass of galactic clus-ters, which turned out to be muchlarger than the luminous matter. Itseems that the gravitational pull ofhuge amounts of dark matter is pre-venting individual galaxies frommoving away from each other andis keeping them bound togetherin large clusters, like for examplethe famous Coma cluster.By addingthe total matter (dark and lumi-nous matter) in galaxies and clus-ters of galaxies one ends up with atotal mass which corresponds toabout 30% of the critical mass ofthe universe. With only 1% lumi-nous mass this would mean thatthere is 30 times more dark massin the universe. In addition, theuniverse would be growing forev-er, since its total mass is subcriticalto bring the expansion to a halt.But as we will see, this seems notto be the full story.

The discovery of dark energy

The big surprise came in 1998 froma supernovae type1a survey per-formed by the Super CosmologyProject (SCP) and the High z-Su-pernova Search (HZS) groups. Su-pernovae type1a are a hundredthousand times brighter than or-dinary stars. They are still visible atvery great distances for which theirlight needed several million yearsto travel until it reached us. In prin-ciple we experience now super-

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Figure 8Gravitational lensing occurs when the gravity field of a massive celestial objectbends the path of light emitted by a distant source. Einstein predicted the deflectionof starlight by the sun (top) and the ring that would appear if the star and the celestialbody were aligned perfectly (center). Lens systems found to date result from thealignment of extra-galactic quasars and galaxies (bottom). (© Scientific American,July 1988)

novae explosions which happenedseveral million years ago. Since inevery supernova type1a explosionthere is always the same totalamount of energy released, theyall have the same brightness andtherefore they qualify as standardcandles in the cosmos. Their dis-tance from us can then be inferredfrom the measurement of their ap-parent brightness. By probing spaceand its expansion with supernovaedistance measurements astrophysi-cists learned that the universe hasnot been decelerating, as assumedso far, but has rather been expand-ing with acceleration (Figure 9).More measurements are still need-ed to corroborate these astonish-ing findings of the supernovaesurvey. But it already presents asurprising new feature of our uni-verse, which revolutionizes ourprevious views and leaves us witha new puzzle. In order to speed upthe expansion of the universe anegative pressure is needed, which

may be provided by some uniden-tified form of dark energy.

This ubiquitous dark energyamounts to 70% of the criticalmass of the universe and has thestrange feature that its gravitation-al force does not attract, on thecontrary it repels. This is hard toimagine since our everyday expe-rience and Newton’s law of gravi-ty tell us that matter is gravitation-ally attractive. In Einstein’s law ofgravity, however, the strength ofgravity depends not only on massand other forms of energy, butalso on pressure. From the Ein-stein equation, which describesthe state of the universe, it followsthat gravitation is repulsive if thepressure is sufficiently negativeand it is attractive if the pressure ispositive. In order to provideenough negative pressure tocounterbalance the attractiveforce of gravity, Einstein originallyintroduced the so-called cosmo-

logical constant to keep the uni-verse in a steady state. At thatearly time all observations seemedto favour a steady state universewith no evolution and no knowl-edge about its beginning and itsend. When Einstein learned aboutthe Hubble expansion of the uni-verse in 1920 he discarded thecosmological constant by admit-ting that it was his biggest blunder.For a long time cosmologists as-sumed the cosmological constantto be negligibly small and set itsvalue to zero, since it did not seemto be of any importance in de-scribing the evolution of the uni-verse. This has changed very re-cently, since we know about theaccelerated expansion of the uni-verse. However, there remainburning questions like why is thecosmological constant so constantover the lifetime of the universeand did not change similar to thematter density, and what fixes itsvalue. Besides the cosmologicalconstant, other forms of dark en-ergy are also discussed by cosmol-ogists, like for example vacuumenergy, which consists of quantumfluctuations providing negativepressure, or quintessence, an ener-gy source which, unlike vacuumenergy and the cosmological con-stant, can vary in space and time.

In contrast to dark matter, which isgravitationally attractive, dark en-ergy cannot clump. Therefore it isthe dark matter which is responsi-ble for the structure formation inthe universe. Although the truenature of the dark energy and thedark matter is not known, the lat-ter can eventually be directly de-tected, while the former cannot.

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Figure 9Recent supernovae distance measurements show that the expansion of the uni-verse is accelerating rather than decelerating as assumed before. This observationsuggests the presence of dark energy. (© Scientific American, January 1999)

What is thenature of thedark matter?

Baryonic dark matter

Before speculating with exoticmatter, the obvious thing is tolook for non-luminous or veryfaint ordinary matter in form ofplanetary objects like jupiters orbrown dwarfs for example. Ifthese objects represent the darkmatter, our galactic halo must beabundantly populated by them.Since they may not be visible evenif searched for with the best tele-scopes, B. Paczynski suggested to look for them by observingmillions of individual stars in theLarge and the Small Magellan-ic Cloud to see whether theirbrightness changes with time dueto gravitational lensing when amassive dark object is movingthrough their line of sight (Figure10). Several research groups, so-called MACHO, EROS andOGLE followed Paczynski's idea– Paczynski himself was memberof the OGLE group – to look forthese so-called Massive Astro-physical Compact Halo Objects(MACHOs) using gravitationallensing. They found some of thesedark objects with masses smallerthan the solar mass, but by far notenough to explain the dark matterin the halo of our galaxy. Otherobjects like black holes or neutronstars could also have been detect-ed by this method, but we knew

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Figure 10Massive dark objects (Massive Astrophysical Compact Halo Objects MACHOs)moving through the line of sight between the observer and a distant star in the LargeMagellanic Cloud cause the apparent luminosity to change. (© Bild der Wissenschaft,2/1997)

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already that there are not many ofthem in the galactic halo.

Do we know how much ordinarymatter exists in the universe?Under ordinary matter or so-called baryonic matter (barysmeaning strong or heavy in an-cient Greek) we understand mat-ter in form of chemical elementsconsisting of protons, neutronsand electrons. About 3 minutesafter the Big Bang the light ele-ments, like hydrogen, deuteriumand helium, were produced vianucleosynthesis. From the meas-urement of their present abun-dances we can estimate the totalamount of the baryonic matterdensity in the universe. Thisamounts to not more than 6% ofthe critical mass density of theuniverse. It shows that most of thebaryonic matter is invisible andmost of the dark matter must beof non-baryonic nature.

Non-baryonic dark matter

The most obvious candidates fornon-baryonic matter would bethe neutrinos, if they had a mass.Neutrinos come in three flavours.If the heaviest neutrino had a massof approximately 10–9 times themass of a hydrogen atom, it wouldqualify to explain the dark matter.This looks like an incredibly smallmass, but the neutrinos belong tothe most abundant particles in theuniverse and outnumber thebaryons by a factor of 1010. For along time it was assumed thatneutrinos have no mass. The stan-dard model of particle physics in-cludes this assumption. All experi-

mental attempts to determine themass of the neutrinos ended inproviding only upper limits.However, in 1998 an under-ground detector with the nameSUPER-Kamiokande in Japan ob-served anomalies in the atmos-pheric neutrino flux which ishighly suggestive that neutrinoshave indeed a mass. These obser-vations will have to be corroborat-ed by planned accelerator experi-ments, like K2K in Japan, MINOSin the USA and OPERA in Eu-rope. The OPERA experimentwill be constructed in the under-ground Gran Sasso laboratory,which is located about 100 kmnorth east of Rome. For this ex-periment, a neutrino beam will besent from CERN to the GranSasso laboratory. If neutrinos havea mass they would change theirflavour during their journey overthe 735 km distance from CERNto the Gran Sasso. They wouldstart as muon-neutrinos at CERNand would arrive as tau-neutrinosat the Gran Sasso. This change offlavour can be detected by theOPERA experiment, in which ourgroup in Bern is also participating.The future will show whetherneutrinos will be able to con-tribute significantly to the missingmass in the universe. Massiveneutrinos may also provide the so-lution to the puzzle of the missingneutrinos from our sun.

A cocktail of non-baryonicdark matter

Computer models allow us tostudy the development of smalland large scale structures under

the hypothesis of various non-baryonic dark matter candidates.Two main categories are distin-guished, namely the so-called hotand cold dark matter. Neutrinoswould qualify under the categoryhot dark matter, since their veloc-ities were very large when theydecoupled from matter, a few mil-liseconds after the Big Bang. Be-cause of their speed they were notable to clump on small, typicalgalactic scales, but their gravita-tional force would still allow forclustering on very large, typicalsupercluster scales. Thus in a hotdark matter dominated universeonly the formation of large scalesuperclusters would be favoured.In such a model superclusterswould fragment into smaller clus-ters at a later time. Hence galaxyformation would be a relativelyrecent phenomenon, which how-ever is in contrast to observation.Cold dark matter candidates, onthe other hand, would have smallvelocities at early phases andtherefore would be able to aggre-gate into bound systems at allscales. A cold dark matter domi-nated universe would thereforeallow for an early formation ofgalaxies in good agreement withobservations, but it would over-populate the universe with smallscale structures, which does not fitour observations. Questions like,how much hot and how muchcold or only cold dark matter, are still not answered. Some com-puter models yield results whichcome closest to observations whenusing a cocktail of 30% hot and70% cold dark matter.

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Neutralinos

Exotic particles like neutralinosare among the most favoured colddark matter candidates. Neutrali-nos are stable elementary particleswhich are predicted to exist bySuper Symmetry (SUSY), a theo-ry which is an extension of theStandard Model of elementaryparticles. Thus if they exist, theywould solve two problems at thesame time: namely the dark mat-ter as well as SUSY, which is aprerequisite for the unification ofall forces in nature, the so calledgrand unification theory (GUT).Experiments at the Large HadronCollider (LHC) at CERN, whichis under construction and will beoperational in 2005, will alsosearch for these particles.

Weakly interacting particles

If the dark matter consisted of neu-tralinos, which would have beenproduced together with other par-ticles in the early universe andwhich would have escaped recog-nition because they only weaklyinteract with ordinary matter, spe-cial devices would have to be builtfor their detection. These detec-tors would have to be able tomeasure very tiny energies whichthese particles transfer in elasticscattering processes with the de-tector material. Because of thevery weak coupling to ordinarymatter these particles are alsocalled WIMPs, for Weakly Inter-acting Massive Particles. Theywould abundantly populate thehalo of our galaxy and would havea local density in our solar system

which would be equivalent to onehydrogen atom in 3 cm3. Sincethey would be bound to ourgalaxy allowing for an average ve-locity of 270 km /s, their flux(density times velocity) would bevery large. However, because theyonly weakly interact with matter,the predicted rates are typicallyless than one event per day perkilogram detector material.

WIMPs can be detected by meas-uring the nuclear recoil energy inthe rare events when one of theseparticles interacts with a nucleusof the detector material. It is likemeasuring the speed of a billiardball sitting on a pool table after ithas been hit by another ball. Be-cause of the background comingfrom the cosmic rays and the ra-dioactivity of the material sur-

rounding the detector, whichyield similar signals in the detec-tor as the WIMPs, the experimentmust be carried out deep under-ground, where cosmic rays cannotpenetrate, and must be shieldedlocally against the rest radioactivi-ty of materials and the radioactivi-ty in the rock.

The race for WIMPs

Several groups in the USA, in Eu-rope and in Japan are searchingfor WIMPs employing differenttechniques. However, in order tobe sensitive to very small nuclearrecoil energies, innovative newcryogenic detectors have been de-veloped. At temperatures near theabsolute zero point, at –273 °C,where the heat capacity is very

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Figure 11The ORPHEUS detector contains billions of small tin granules with a diameter of 30 micrometers. They are cooled down to –273 °C, where they are in a super-conducting state. An interacting WIMP may generate enough heat in a granule to cause a phase transition from the superconducting to the normal conducting state.This phase transition of the granule can be measured with a sensitive read-outsystem.

15SPAT IUM 7

small, a very tiny recoil energy canbe transformed into a measurablesignal. One of these innovativedetection systems has been devel-oped by our group at the Univer-sity of Bern. The detector is calledORPHEUS in analogy to Greekmythology. However, we the ex-perimentalists spend a lot moretime underground than Orpheusever did, because our detector issituated in an underground labo-ratory 30 meters below the Uni-versity of Bern. The detector con-sists of billions of small super-conducting tin grains with a diam-eter of a few micrometers, whichare cooled down to –273 °C wherethey are in a superconducting state(Figure 11). When an incomingWIMP hits a granule it can lead toa minute temperature increase ofthe granule, which can be justsufficient to cause a phase transi-tion from the superconducting tothe normal state. This phase tran-sition of an individual granule can be detected by a pick-up loopwhich measures the magnetic fluxchange due to the disappearanceof the Meissner effect. The OR-PHEUS detector, consisting ofabout 0.5 kg of superconductingtin granules, just started to beoperational (Figure 12). It willhowever take some time beforethe background is sufficientlyunderstood and true WIMP sig-nals can be detected. Neverthe-less, there is still the possibilitythat the dark matter does not con-sist of WIMPs, in which case wewould not find any signals andother strategies would have to bedeveloped in order to disclose themystery of the dark matter in ouruniverse.

Conclusions

Despite many interesting candi-dates, we still do not know whatthe dark matter is made of. Inten-sive experimental work is goingon by trying to directly detect thisabundant matter in our owngalaxy or by learning about itthrough indirect methods. Recentstudies of the Cosmic BackgroundRadiation lead us to believe thatthe dark matter was present longbefore the chemical elementswere created and was responsiblefor the fascinating architecture ofour observable universe, provid-ing also the necessary conditionsfor life to develop. Without theattractive pull of the dark matter,the universe would be extremelydull today. There would be nogalaxies, no stars, no planets andno life. The total amount of dark

matter and dark energy will deter-mine the destiny of our universe.From all we have learned so far, itlooks as if the universe will notend in a Big Crunch, instead itwill expand forever.

In the coming years we are eager-ly awaiting new insights into thedark side of the universe. I wouldlike to close with a quote fromShakespeare: There is no dark-ness, only ignorance.

Acknowledgements

I would like to thank Dr. HansjörgSchlaepfer for his valuable contri-butions and for enjoyable discus-sions. I am very grateful to Prof.Gerhard Czapek, Prof. Peter Min-kowski and Irene Neeser for thecritical reading of the manuscript.

Figure 12Cross sectional view of the ORPHEUS dark matter detector. The detector ishorizontally connected via a side axis with a dilution refrigerator, which cools itdown to a temperature of –273 °C. It is surrounded by shielding material and scintil-lation counters to protect against background from particles other than WIMPs.

The author

His interest turned from nuclearphysics to elementary particles,the fundamental building blocksof matter. For this reason he wentto CERN in Geneva, where hemet Prof. W. Paul (Nobel Prizewinner 1989), who offered himto work on an experiment tostudy meson nucleon backwardscattering at high energies. Thiswork was the subject of his PhDthesis, which he submitted to thefaculty of the University of Mu-nich in 1968.

After receiving his PhD, KlausPretzl went to the USA for 4 years, where he worked atFermi-Lab, near Chicago, underProf. R. Wilson on the construc-tion of the world’s largest particleaccelerator at that time. Duringthe construction time of the ac-celerator, Klaus Pretzl conductedseveral experiments studyingstrong interaction processes at thenearby Argonne National Labo-ratory. When the construction ofthe Fermi-Lab accelerator wascompleted, he was involved inthe first experiments to explorephysics at the very highest ener-gies.

In 1973 Klaus Pretzl returned toEurope and joined the MaxPlanck Institute for Physics inMunich. There he first worked atthe e+ - e– - storage ring DORIS

at DESY (Deutsches ElektronSynchrotron), in Hamburg, withthe so-called DASP (Double ArmSpectrometer) experiment, whichlead to the discovery of the char-monium states. Later he per-formed several experiments atCERN studying QCD (QuantumChromo Dynamic) processes. In1982 he became interested in thetopic of dark matter and started tostudy new concepts for its directdetection.

In 1988 he became professor ofphysics and head of the Laborato-ry for High Energy Physics at theBern University. There he starteda new group with the aim to de-velop cryogenic detectors for adark matter experiment, which isnow in operation in the BernUnderground Laboratory and iscalled ORPHEUS. Klaus Pretzland his group are also involved inthe ATLAS experiment at theLarge Hadron Collider (LHC) atCERN and in the long baselineneutrino oscillation experimentOPERA at the Gran Sasso Labora-tory in Italy.

Klaus Pretzl was born in Munich,Germany, in 1940. From 1941 to1956 his family lived in Schliersee,south of Munich in the BavarianAlps, where he also went to pri-mary school. In 1956 he returnedto Munich, where he made hisMatura at the Wilhelms-Gymna-sium.

His fascination for physics was in-spired by public lectures aboutquantum physics given by Profes-sor Auer, a former director of theDeutsches Museum in Munich.As a result he studied physics atthe Technische Hochschule inMunich and wrote his diplomathesis in nuclear physics underProf. H. Meyer Leibnitz and Prof.P. Kienle. He also gained a schol-arship from EURATOM to workat the Centre des Recherches Nu-cléaires in Strasbourg.

Published by the Association Pro ISSI

No 7, May 2001

INTERNATIONAL SPACESCIENCEINSTITUTE

In Search of the Dark Matterin the Universe

In Search of the Dark Matterin the Universe

Dark matter constitutesthe vast majority ofmatter in the universe.Recent research sheds light on what itmight be.

Dark matter constitutesthe vast majority ofmatter in the universe.Recent research sheds light on what itmight be.