<<

INTERNATIONAL INSTITUTE No 7, May 2001

Published by the Association Pro ISSI

InIn SearchSearch ofof thethe DarkDark MatterMatter inin thethe UniverseUniverse

Dark constitutes the vast majority of matter in the . Recent research sheds on what it might be. Editorial

Dark matter. Dark . In March 2000 Professor Pretzl Impressum Dark is beautiful! reported on his institute’s fascinat- ing search for to an But it is not so easy to find the interested Pro-ISSI audience. We dark. It requires charting the un- are very grateful to Klaus Pretzl SPATIUM known as was done some six hun- for his kind permission to publish Published by the dred years ago with the terra herewith his lecture. Association Pro ISSI incognita. Vague contours, how- twice a year ever, are known. Dark matter manifests itself by gravitational ef- fects on visible matter, giving a Dark is beautiful. INTERNATIONAL SPACE general direction for those who SCIENCE want to move on in the dark. But INSTITUTE much more remains to be found Association Pro ISSI out. Hansjörg Schlaepfer Hallerstrasse 6, CH-3012 Bern Bern, May 2001 Phone ++41 31 631 48 96 Dark matter is a playground for Fax ++41 31 631 48 97 creative scientists. Imagination is required to define the unknown President and formulate hypotheses, which Prof. Hermann Debrunner, will most likely be thrown out University of Bern once sufficient scientific progress Publisher has been made. Karl Popper, Dr. Hansjörg Schlaepfer, the great Austrian philosopher, legenda schläpfer wort & bild, showed that rejection – not con- Winkel firmation – of a theory is the cre- Layout ative step when it is followed by Marcel Künzi, marketing · kom- the formulation of a more com- munikation, CH-8483 Kollbrunn prehensive theory. Printing Druckerei Peter + Co dpc Dark matter is full of surprises. CH-8037 Zurich Dark matter is everywhere. Dark matter may even be found in the dark underworld of Bern. The author of this issue of Spatium, Professor Klaus Pretzl, head of the Laboratory for High Energy of the University of Bern, knows where to find it. He is in charge of the ORPHEUS experi- ment, now in its final stage of im- plementation some thirty meters below the University of Bern. Front Cover This complex -up seeks to in- Spiral NGC 1232 vestigate a specific class of dark Copyright: European Southern matter , the weakly inter- Observatory ESO, PR Photo acting massive particles. 37/e/98 (23 September 1998)

SPATIUM 7 2 In Search of the Dark Matter in the Universe *)

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

Introduction How was matter tinguishable from each other. However, while the universe was created? expanding rapidly this symmetry was quickly broken and , The story of the dark matter start- the strong and the electro-weak ed in 1933, when the Swiss as- appear today as separate tronomer published forces with vastly different astonishing results from his stud- The very first instants strength and reach. ies of galactic clusters. In his paper he concluded that most of the Our knowledge about the begin- The enormous and ho- matter in clusters is totally invisi- ning of the universe is rather ob- mogeneity of the universe, which ble. Zwicky argued that gravity scure, since it was born out of a we encounter when looking at the must keep the in the clus- state which is not describable by distribution of galaxies through- ter together, since otherwise they any physical law we know of. We out space, and which we obtain would move apart from each simply call its indescribable mo- from measurements of the cosmic other due to their own . By ments of birth the . After background determining the speed of motion the Big Bang the universe rapidly (CMB), is very puzzling. This is of many galaxies within a cluster expanded starting from an incred- because there are regions in the from the measurement of the ibly small region with expanding universe which have Doppler-shift of the spectral lines of the order of 10–33 cm and an un- never been in causal contact with he could infer the required gravi- thinkably high energy of each other, i.e. light never had suf- tational pull and thereby the total 1094 g/cm3. Grand Unified Theo- ficient since the Big Bang to in the cluster. Much to his ries (GUT), which aim to describe travel far enough to transmit in- surprise the required mass by far the physical laws at this young age formation from one to the other exceeded the visible mass in the of the universe, tell us that physics of these regions. To solve this puz- cluster. His results were received was much simpler at that time, zle the astrophysicists A. Guth and with great scepticism by most as- since there was only one rul- A. Linde introduced a scenario ac- tronomers at that time. It took an- ing . Today, however, cording to which the universe ex- other 60 years until he was proven we distinguish four fundamental perienced an exponentially rapid to be right. forces: the force of gravity, which expansion during a very short pe- attracts us to the and the riod of time between 10–36 to 10–34 How much of this mysterious to the , the electromag- seconds after the Big Bang. Dur- dark mass is there in the universe? netic force, which keeps the nega- ing this period the universe was Where do we find it? What is tively charged in the rapidly growing in size by about its real ? Will we be able to outer shell of an attracted to a factor of 3 x 1043. Within this directly detect it, since it does the positively charged nucleus in scenario of it becomes not radiate light or particles? It the center, the weak force, which possible to causally connect re- demonstrates its presence only by is responsible for the radioactivity gions of the universe which seem its gravitational pull on visible we observe when unstable nuclei to be otherwise disconnected from matter. These are the questions decay, and the strong force, which each other. Nevertheless, after that which intrigue , cos- holds the and short period of inflation the uni- mologists, and nuclear together in the nucleus. All these verse continued its expansion alike. In this talk I will forces were unified in a single with retarded speed. Inflation is try to give a short account of what force at this early time. Physicists very much favoured by most cos- we have learned about this myste- call this a state of symmetry, since mologists and strongly supported rious kind of matter which domi- the forces all have the same by the recent of the nates our universe. strength and are therefore indis- cosmic microwave background

th *) Pro ISSI lecture, Bern, March 15 2000 SPATIUM 7 3 radiation by the Boomerang and was abruptly ended by the cre- and the electrons, and their Maxima experiments (see also ation of matter and radiation . Nevertheless, most below). about 10–34 seconds after the Big of the energy of the universe Bang. At that time the universe resided in radiation, mainly pho- According to this scenario the in- contained all the basic building tons and , etc... How- flationary of the universe blocks of matter, the , the ever, as the universe cooled by ex- pansion, radiation lost its energy faster than matter and when the universe became about ten thou- years old the energy balance shifted in favour of matter.

The phase of matter ended about 10–6 seconds after the Big Bang, when the universe cooled to a temperature of 2 x 1012 . At that temperature a from a quark gluon to a nucleonic phase of matter took place (Figure 1), where the protons and neutrons were formed. In this process three quarks of different flavour (so- called up-quarks and down- quarks) combine together to form a (two up-quarks and one down-quark) or a (two down-quarks and one up-quark) and similarly (two an- tiup-quarks and one antidown- quark) and antineutrons (two antidown-quarks and one antiup- quark). The gluons were given their name because they provide the glue for holding the quarks to- gether in the nucleons. After this phase transition one would expect to end up with the same of nucleons and antinucleons, which annihilate each other after creation, leaving us not a chance to exist. Fortunately this was not Figure 1 the case. The that we live Temperatures of about 2 x 1012 were reached in this collision experiment at in a of matter with no anti- CERN. A target was bombarded with highly relativistic lead nuclei. At that tem- matter is due to a very subtle ef- perature, the quark gluon plasma occurs. The picture shows the outburst of many particles which looks typical when transitions from the quark gluon to the nucleonic fect, which treats matter and anti- phase takes place. (NA49 experiment at CERN). matter in a different way during

SPATIUM 7 4 25 thousand million years

1thousand million years

300 thousand years

3 minutes

1 second

10 –10 seconds

10 –34 seconds

10 –43 seconds

1032 degrees

1027 degrees

1015 degrees

1010 degrees

109 degrees

6000 degrees radiation (anti-) particles proton neutron heavy particles 18 degrees carrying the weak force quark anti-quark 3 degrees K electron

Figure 2 The of the universe. and experimental observations document the history of the universe from an incremental fraction of time after the Big Bang 15 billions of years ago up to its present state. Dark matter is seen today as having played a key role in the formation of and galaxies. (© CERN Publications, July 1991) the phase of creation. This effect, After further expansion the uni- formed many million years later, which is known as CP-violation verse cooled to a temperature of mainly during formation and (Charge conjugation and Parity 10 9 Kelvin, when protons and explosions. After their violation), was first discovered in neutrons started to hang on to formation the light nuclei had an accelerator experiment by V. each other to form the light ele- hundred thousand years of time to Fitch and J. Cronin in 1964, for ments like helium, deuterium, catch electrons to build . which they got the Nobel prize in lithium and . This phase 1980, and was used by A. Sakha- of nucleosynthesis began a few rov to explain the matter antimat- seconds after the Big Bang. The ter asymmetry in the universe. heavier elements were only

SPATIUM 7 5 The cosmic background radiation

About three hundred thousand years after the Big Bang radiation had not enough energy left to interact with matter and the uni- verse became transparent for elec- tromagnetic radiation. This radia- tion from the early universe was first discovered by R. Wilson and A. Penzias in 1965. They received the Nobel prize for this finding in 1978. Their discovery was made by chance, since they were on a mission from Bell Laboratories to test new microwave receivers to relay telephone calls to earth-or- biting communication satellites. No matter in what direction they pointed their antenna, they always measured the same noise. At first this was rather disappointing to them. But they happened to learn of the work of the astronomers R. Dicke and P. Peebles and they re- alised that the noise they were measuring was finally not the noise of the receiver, but rather the cooled down cosmic mi- crowave background radiation (CMB) from the Big Bang. From the spectrum and ’s law of radia- tion the temperature of CMB was derived to be 2.7 Kelvin. Regard- less which direction the cosmic ra- diation was received from, the temperature came out to be the same everywhere, demonstrating the enormous of the universe. The most accurate CMB measurements come from Figure 3 the Cosmic Background Explorer Full 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) satellite (COBE), which was sent temperature variation of 0.01% unveil matter density fluctuations in the very early into orbit in 1989. They found universe (bottom). (© Physics Today)

SPATIUM 7 6 temperature variations only at a How much ago in an unthinkably small vol- level of one part in a hundred ume with an unthinkably high en- thousand. matter is in the ergy density, the so-called Big Bang, and is expanding ever since. universe? However, this expansion is coun- teracted by the gravitational pull of the matter in the universe. De- Where does dark matter pending on how much matter come from? At first this question seems to be there is, the expansion will contin- highly academic. It is not. The fate ue forever or come to a halt, of our universe depends on its which subsequently could lead to After this short description of the mass and its expansion . a collapse of the universe ending history of the universe and the in a , the opposite of creation of matter we ask our- the Big Bang. The matter density selves: What about the dark The destiny of the universe needed to bring the expansion of matter? When and how was it the universe to a halt is called the created? A partial answer to this In the 1920s the famous as- critical mass density, which today question is given to us by the tronomer demon- would be roughly the equivalent COBE cosmic microwave back- strated that all galaxies are moving of 10 hydrogen atoms per cubic ground radiation measurements. away from us and from each meter. This seems incredibly They show islands of lower and other. His discovery was the foun- small, like a , when com- higher temperatures appearing on dation stone of modern cosmolo- pared to the density of our earth the map of the universe which are gy, which claims that the universe and planets, but seen on a cosmic due to density fluctuations of mat- originated about 15 scale it represents a lot of matter. ter (Figure 3). They were already present at the time radiation de- coupled from matter, three hun- dred thousand years after the Big NGC 3198 Bang, long before matter was 150 clumping to form galaxies and clusters of galaxies. We have rea- sons to believe that these density Orbital velocity (km/s) fluctuations are due to the dark 100 matter, which was probably creat- Kepler’s law ed from fluctuations during the inflationary phase of 50 the universe. These tiny fluctua- tions expanded first through in- flation and then retarded their expansion due to gravitational 0 0 5 10 15 20 25 30 35 binding forces. They then formed the gravitational potential wells Radial distance (kpc) into which ordinary matter fell to Figure 4 form billion years later galaxies The of constant orbital of stars around the and stars. All galaxies and clusters (here the NGC 3198) as a function of the radial distance provides con- vincing evidence for the presence of an extended halo of dark matter surrounding of galaxies seem to be embedded the galaxy. The expected curve from Kepler’s law if there would be no dark matter is into halos of dark matter. also shown.

SPATIUM 7 7 and galaxies were the only matter Envelope in the universe, the universe Luminous Galaxy of Dark Matter would expand forever. We neg- lected here the amount of matter in 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 when and her team found out in the 1970s that the visible stars are not the only objects mak- ing up the mass of the galaxies. They measured the orbital speeds of stars around the center of spiral galaxies and found that they move with a constant velocity inde- pendent of their radial distance from the center (Figure 4). This is in apparent disagreement with Kepler's law, which says that the Sun velocity should decrease as the 150,000 Light-years distance of the star from the galac- Figure 5 tic center increases, provided that A galaxy as seen schematically from a distant point in the galactic . Dark all mass is concentrated at the cen- matter forms a large halo extending far outside the outer edges of the galaxy. ter of the galaxy, which seems to

(kiloparsecs) How can we find out how much 0 4 8 12162024283236 matter there is? When estimating M 31 the visible matter in the universe, 400 From Rubin and astronomers look in a very wide Ford (1970) 21 centimeters and very deep region in space and 21’ Beam count the number of galaxies. 300 10’ Beam Typical galaxies containing hun- dreds of billions of luminous stars have a brightness which is propor- 200 tional to their mass. Thus, by sim- ply counting galaxies over a large Optical volume in space and by assuming 100 Center that galaxies are evenly distrib- SW

uted over the entire universe, one Rotational velocity (kilometers per second) NE can estimate the total mass they 0 0 20406080100120140160180 contribute in form of visible mass to the universe. However, it turns Distance to center (arc minutes) out to be only 1% of the critical Figure 6 Experimental analysis of the rotational velocities in the mass of the universe. Therefore, if M31 from optical observations (V. Rubin et al.) and radio observations at 21cm - the visible matter in form of stars length (B. Roberts et al.).

SPATIUM 7 8 be the case if only the luminous matter is considered. If however star Kepler’s law, which describes the (true position) orbital motion of the planets in our solar very correctly, is valid everywhere in the universe, then the rotational velocities of star the stars can only be explained if (apparent the mass of the galaxy is increasing position) with the radial distance from its center. Numerical calculations show that there must be at least an more matter in the galaxies than is visible. From their measurements, which Earth they repeated on hundreds of dif- Sun ferent galaxies, they conjectured that each galaxy must be embed- ded in an enormous halo of dark Figure 7 matter, which reaches out even Space is curved by gravity. The light rays from a distant star are bent by the grav- ity of the sun. The distant star therefore appears at a different position. beyond the visible diameter of the (Spektrum der Wissenschaft) galaxy (Figure 5). Spiral galaxies are surrounded also by of neu- already typical for distances be- means for the most direct deter- tral hydrogen, which themselves tween neighbouring galaxies with- mination of the mass of very large do not contribute considerably to in galactic clusters. galactic clusters, including dark the mass of the galaxy, but which matter. But it took more than serve as tracers of the orbital mo- 50 years until his suggestion was tion beyond the optical limits of Determining the mass finally realized and his early deter- the galaxies. The hydrogen atoms in the universe mination of the mass of the in the clouds are emitting a char- , in 1933, was con- acteristic radiation with a wave- The effect of gravitational lensing firmed. With the Hubble tele- length of 21 cm, which is due to is a consequence of Einstein's gen- scope in space and the Very Large a hyperfine interaction between eral relativity. It was first observed Telescopes (VLT) at the Southern the electron and the proton in in 1919, when an apparent angular Observatory in Chile astronomers the hydrogen atom and which can shift of the close now have very powerful tools, be detected. These measurements to the solar limb was measured which allow them not only to ex- show that the ex- during a total solar eclipse. This plore the visible, but also the dark tends far beyond the optical limits was the first, important proof for side of the universe with gravita- of the galaxies (Figure 6). But, how the validity of Einstein’s theory, tional lensing. far does it really reach out? Very according to which light coming recently gravitational lensing ob- from a distant star is bent when An observer sees a distorted mul- servations seem to indicate that grazing a massive object due to tiple image of a light source in the the dark matter halo of galaxies the space caused by the far background, when the deflect- may have dimensions larger than gravity of the object (Figure 7). It ing massive object in the fore- ten the optical diameter. It was Fritz Zwicky in 1937 who re- ground is close to the line of sight. is quite possible that the dark alized that the effect of gravita- The light source appears to be a halos have dimensions which are tional lensing would provide the ring, the so-called Einstein-Ring,

SPATIUM 7 9 when the object is exactly in the line of sight (Figure 8). If one knows Star true the distance of the light source position and the object to the observer one is able to infer the mass of the ob- ject from the lensing image. With this method it was possible to de- termine the mass of galactic clus- Sun ters, which turned out to be much larger than the luminous matter. It seems that the gravitational pull of Observed Earth huge amounts of dark matter is pre- position venting individual galaxies from moving away from each other and Star true is keeping them bound together position in large clusters, like for example the famous Coma cluster.By adding the total matter (dark and lumi- Star matter) in galaxies and clus- ters of galaxies one ends up with a total mass which corresponds to about 30% of the critical mass of the universe. With only 1% lumi- Observed nous mass this would mean that position there is 30 times more dark mass in the universe. In addition, the true universe would be growing forev- position Observed er, since its total mass is subcritical positions to bring the expansion to a halt. Galaxy But as we will see, this seems not to be the full story.

The discovery of

Observed The big surprise came in 1998 from positions a supernovae type1a survey per- formed by the Super Project (SCP) and the High z-Su- pernova Search (HZS) groups. Su- pernovae type1a are a hundred Figure 8 thousand times brighter than or- Gravitational lensing occurs when the gravity field of a massive celestial object dinary stars. They are still visible at bends the path of light emitted by a distant source. Einstein predicted the deflection very great distances for which their of starlight by the sun (top) and the ring that would appear if the star and the celestial light needed several million years body were aligned perfectly (center). Lens found to date result from the alignment of extra-galactic and galaxies (bottom). (© , to travel until it reached us.In prin- July 1988) ciple we experience now super-

SPATIUM 7 10 0.00001 logical constant to keep the uni- verse in a steady state. At that 0.0001 early time all observations seemed to favour a steady state universe Accelerating with no evolution and no knowl- 0.001 Expansion edge about its beginning and its Decelerating Expansion end. When Einstein learned about 0.01 the Hubble expansion of the uni- verse in 1920 he discarded the

0.1 Relative Distance by admit- ting that it was his biggest blunder. Relative Intensitiy of Light 1 For a long time cosmologists as- sumed the cosmological constant to be negligibly small and set its high low near far 0.01 0.1 1 value to zero, since it did not seem (z) to be of any importance in de- Figure 9 scribing the evolution of the uni- Recent supernovae distance measurements show that the expansion of the uni- verse is accelerating rather than decelerating as assumed before. This observation verse. This has changed very re- suggests the presence of dark energy. (© Scientific American, January 1999) cently, since we know about the accelerated expansion of the uni- novae explosions which happened may be provided by some uniden- verse. However, there remain several million years ago. Since in tified form of dark energy. burning questions like why is the every supernova type1a explosion cosmological constant so constant there is always the same total This ubiquitous dark energy over the lifetime of the universe amount of energy released, they amounts to 70% of the critical and did not change similar to the all have the same brightness and mass of the universe and has the matter density, and what fixes its therefore they qualify as standard strange feature that its gravitation- value. Besides the cosmological candles in the . Their dis- al force does not attract, on the constant, other forms of dark en- tance from us can then be inferred contrary it repels. This is hard to ergy are also discussed by cosmol- from the measurement of their ap- imagine since our everyday expe- ogists, like for example vacuum parent brightness. By probing space rience and Newton’s law of gravi- energy, which consists of quantum and its expansion with supernovae ty tell us that matter is gravitation- fluctuations providing negative distance measurements astrophysi- ally attractive. In Einstein’s law of pressure, or , an ener- cists learned that the universe has gravity, however, the strength of gy source which, unlike vacuum not been decelerating, as assumed gravity depends not only on mass energy and the cosmological con- so far, but has rather been expand- and other forms of energy, but stant, can vary in space and time. ing with (Figure 9). also on pressure. From the Ein- More measurements are still need- stein equation, which describes In contrast to dark matter, which is ed to corroborate these astonish- the state of the universe, it follows gravitationally attractive, dark en- ing findings of the supernovae that gravitation is repulsive if the ergy cannot clump. Therefore it is survey. But it already presents a pressure is sufficiently negative the dark matter which is responsi- surprising new feature of our uni- and it is attractive if the pressure is ble for the in verse, which revolutionizes our positive. In order to provide the universe. Although the true previous views and leaves us with enough negative pressure to nature of the dark energy and the a new puzzle. In order to speed up counterbalance the attractive dark matter is not known, the lat- the expansion of the universe a force of gravity, Einstein originally ter can eventually be directly de- negative pressure is needed, which introduced the so-called cosmo- tected, while the former cannot.

SPATIUM 7 11 What is the nature of the dark matter?

Baryonic dark matter Luminousity Before speculating with , the obvious thing is to Large look for non-luminous or very Magellanic faint ordinary matter in form of planetary objects like or brown dwarfs for example. If these objects represent the dark matter, our must be abundantly populated by them. Since they may not be visible even

if searched for with the best tele- bdw-grafik scopes, B. Paczynski suggested 0 20 40 60 80 100 120 Time (days) to look for them by observing millions of individual stars in the Large and the Small Magellan- ic Cloud to see whether their brightness changes with time due MACHO to gravitational lensing when a massive dark object is moving through their line of sight (Figure 10). Several research groups, so- called MACHO, EROS and OGLE followed Paczynski's idea – Paczynski himself was member Our Galaxy of the OGLE group – to look for these so-called Massive Astro- physical Compact Halo Objects (MACHOs) using gravitational Earth lensing. They found some of these dark objects with smaller than the solar mass, but by far not enough to explain the dark matter in the halo of our galaxy. Other Figure 10 objects like black holes or neutron Massive dark objects (Massive Astrophysical Compact Halo Objects MACHOs) moving through the line of sight between the observer and a distant star in the Large stars could also have been detect- Magellanic Cloud cause the apparent luminosity to change. (© Bild der Wissenschaft, ed by this method, but we knew 2/1997)

SPATIUM 7 12 already that there are not many of mental attempts to determine the the hypothesis of various non- them in the galactic halo. mass of the neutrinos ended in candidates. providing only upper limits. Two main categories are distin- Do we know how much ordinary However, in 1998 an under- guished, namely the so-called hot matter exists in the universe? ground detector with the name and dark matter. Neutrinos Under ordinary matter or so- SUPER-Kamiokande in Japan ob- would qualify under the category called baryonic matter (barys served anomalies in the atmos- , since their veloc- meaning strong or heavy in an- pheric flux which is ities were very large when they cient Greek) we understand mat- highly suggestive that neutrinos decoupled from matter, a few mil- ter in form of chemical elements have indeed a mass. These obser- liseconds after the Big Bang. Be- consisting of protons, neutrons vations will have to be corroborat- cause of their speed they were not and electrons. About 3 minutes ed by planned accelerator experi- able to clump on small, typical after the Big Bang the light ele- ments, like K2K in Japan, MINOS galactic scales, but their gravita- ments, like hydrogen, deuterium in the USA and OPERA in Eu- tional force would still allow for and helium, were produced via rope. The OPERA experiment clustering on very large, typical nucleosynthesis. From the meas- will be constructed in the under- scales. Thus in a hot urement of their present abun- ground Gran Sasso laboratory, dark matter dominated universe dances we can estimate the total which is located about 100 km only the formation of large scale amount of the baryonic matter north east of Rome. For this ex- would be favoured. density in the universe. This periment, a neutrino beam will be In such a model superclusters amounts to not more than 6% of sent from CERN to the Gran would fragment into smaller clus- the critical mass density of the Sasso laboratory. If neutrinos have ters at a later time. Hence galaxy universe. It shows that most of the a mass they would change their formation would be a relatively baryonic matter is invisible and flavour during their journey over recent phenomenon, which how- most of the dark matter must be the 735 km distance from CERN ever is in contrast to observation. of non-baryonic nature. to the Gran Sasso. They would candidates, on start as -neutrinos at CERN the other hand, would have small and would arrive as -neutrinos velocities at early phases and Non-baryonic dark matter at the Gran Sasso. This change of therefore would be able to aggre- flavour can be detected by the gate into bound systems at all The most obvious candidates for OPERA experiment, in which our scales. A cold dark matter domi- non-baryonic matter would be group in Bern is also participating. nated universe would therefore the neutrinos, if they had a mass. The future will show whether allow for an early formation of Neutrinos come in three flavours. neutrinos will be able to con- galaxies in good agreement with If the heaviest neutrino had a mass tribute significantly to the missing observations, but it would over- of approximately 10–9 times the mass in the universe. Massive populate the universe with small mass of a hydrogen atom, it would neutrinos may also provide the so- scale structures, which does not fit qualify to explain the dark matter. lution to the puzzle of the missing our observations. Questions like, This looks like an incredibly small neutrinos from our sun. how much hot and how much mass, but the neutrinos belong to cold or only cold dark matter, the most abundant particles in the are still not answered. Some com- universe and outnumber the A cocktail of non-baryonic puter models yield results which by a factor of 1010. For a dark matter come closest to observations when long time it was assumed that using a cocktail of 30% hot and neutrinos have no mass. The stan- Computer models allow us to 70% cold dark matter. dard model of in- study the development of small cludes this assumption. All experi- and large scale structures under

SPATIUM 7 13

Exotic particles like neutralinos are among the most favoured cold dark matter candidates. Neutrali- nos are stable elementary particles which are predicted to exist by Super Symmetry (SUSY), a theo- ry which is an extension of the of elementary particles. Thus if they exist, they would solve two problems at the same time: namely the dark mat- ter as well as SUSY, which is a prerequisite for the unification of all forces in nature, the so called grand unification theory (GUT). Experiments at the Large Figure 11 Collider (LHC) at CERN, which The 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- is under construction and will be conducting state. An interacting WIMP may generate enough in a granule operational in 2005, will also to cause a phase transition from the superconducting to the normal conducting state. search for these particles. This phase transition of the granule can be measured with a sensitive read-out system.

Weakly interacting particles which would be equivalent to one rounding the detector, which hydrogen atom in 3 cm3. Since yield similar signals in the detec- If the dark matter consisted of neu- they would be bound to our tor as the WIMPs, the experiment tralinos, which would have been galaxy allowing for an average ve- must be carried out deep under- produced together with other par- locity of 270 km /s, their flux ground, where cosmic rays cannot ticles in the early universe and (density times velocity) would be penetrate, and must be shielded which would have escaped recog- very large. However, because they locally against the rest radioactivi- nition because they only weakly only weakly interact with matter, ty of materials and the radioactivi- interact with ordinary matter, spe- the predicted rates are typically ty in the rock. cial devices would have to be built less than one per day per for their detection. These detec- kilogram detector material. tors would have to be able to The race for WIMPs measure very tiny which WIMPs can be detected by meas- these particles transfer in elastic uring the nuclear recoil energy in Several groups in the USA, in Eu- scattering processes with the de- the rare events when one of these rope and in Japan are searching tector material. Because of the particles interacts with a nucleus for WIMPs employing different very weak coupling to ordinary of the detector material. It is like techniques. However, in order to matter these particles are also measuring the speed of a billiard be sensitive to very small nuclear called WIMPs, for Weakly Inter- ball sitting on a pool table after it recoil energies, innovative new acting Massive Particles. They has been hit by another ball. Be- cryogenic detectors have been de- would abundantly populate the cause of the background coming veloped. At temperatures near the halo of our galaxy and would have from the cosmic rays and the ra- absolute zero point, at –273 °C, a local density in our dioactivity of the material sur- where the heat capacity is very

SPATIUM 7 14 small, a very tiny recoil energy can Conclusions matter and dark energy will deter- be transformed into a measurable mine the destiny of our universe. signal. One of these innovative Despite many interesting candi- From all we have learned so far, it detection systems has been devel- dates, we still do not know what looks as if the universe will not oped by our group at the Univer- the dark matter is made of. Inten- end in a Big Crunch, instead it sity of Bern. The detector is called sive experimental work is going will expand forever. ORPHEUS in analogy to Greek on by trying to directly detect this mythology. However, we the ex- abundant matter in our own In the coming years we are eager- perimentalists spend a lot more galaxy or by learning about it ly awaiting new insights into the time underground than Orpheus through indirect methods. Recent dark side of the universe. I would ever did, because our detector is studies of the Cosmic Background like to close with a quote from situated in an underground labo- Radiation lead us to believe that Shakespeare: There is no dark- ratory 30 meters below the Uni- the dark matter was present long ness, only ignorance. versity of Bern. The detector con- before the chemical elements sists of billions of small super- were created and was responsible conducting tin grains with a diam- for the fascinating architecture of Acknowledgements eter of a few micrometers, which our , provid- are cooled down to –273 °C where ing also the necessary conditions I would like to thank Dr. Hansjörg they are in a superconducting state for to develop. Without the Schlaepfer for his valuable contri- (Figure 11). When an incoming attractive pull of the dark matter, butions and for enjoyable discus- WIMP hits a granule it can lead to the universe would be extremely sions. I am very grateful to Prof. a minute temperature increase of dull today. There would be no Gerhard Czapek, Prof. Peter Min- the granule, which can be just galaxies, no stars, no planets and kowski and Irene Neeser for the sufficient to cause a phase transi- no life. The total amount of dark critical reading of the manuscript. tion from the superconducting to the normal state. This phase tran- sition of an individual granule can be detected by a pick-up loop which measures the magnetic flux change due to the disappearance of the Meissner effect. The OR- PHEUS detector, consisting of about 0.5 kg of superconducting tin granules, just started to be operational (Figure 12). It will however take some time before the background is sufficiently understood and true WIMP sig- nals can be detected. Neverthe- less, there is still the possibility that the dark matter does not con- sist of WIMPs, in which case we would not find any signals and other strategies would have to be Figure 12 Cross sectional view of the ORPHEUS dark matter detector. developed in order to disclose the The detector is horizontally connected via a side axis with a dilution refrigerator, which cools it mystery of the dark matter in our down to a temperature of –273 °C. It is surrounded by shielding material and scintil- universe. lation counters to protect against background from particles other than WIMPs.

SPATIUM 7 15 The author

His interest turned from nuclear at DESY (Deutsches Elektron physics to elementary particles, Synchrotron), in Hamburg, with the fundamental building blocks the so-called DASP (Double Arm of matter. For this reason he went Spectrometer) experiment, which to CERN in Geneva, where he lead to the discovery of the char- met Prof. W. Paul (Nobel Prize monium states. Later he per- winner 1989), who offered him formed several experiments at to work on an experiment to CERN studying QCD (Quantum study meson nucleon backward Chromo Dynamic) processes. In scattering at high energies. This 1982 he became interested in the work was the subject of his PhD topic of dark matter and started to thesis, which he submitted to the study new concepts for its direct faculty of the University of Mu- detection. nich in 1968. In 1988 he became professor of Klaus Pretzl was born in Munich, After receiving his PhD, Klaus physics and head of the Laborato- Germany, in 1940. From 1941 to Pretzl went to the USA for ry for High Energy Physics at the 1956 his family lived in Schliersee, 4 years, where he worked at Bern University. There he started south of Munich in the Bavarian Fermi-Lab, near Chicago, under a new group with the aim to de- Alps, where he also went to pri- Prof. R. Wilson on the construc- velop cryogenic detectors for a mary school. In 1956 he returned tion of the world’s largest particle dark matter experiment, which is to Munich, where he made his accelerator at that time. During now in operation in the Bern Matura at the Wilhelms-Gymna- the construction time of the ac- Underground Laboratory and is sium. celerator, Klaus Pretzl conducted called ORPHEUS. Klaus Pretzl several experiments studying and his group are also involved in His fascination for physics was in- processes at the the ATLAS experiment at the spired by public lectures about nearby Argonne National Labo- (LHC) at quantum physics given by Profes- ratory. When the construction of CERN and in the long baseline sor Auer, a former director of the the Fermi-Lab accelerator was experiment Deutsches Museum in Munich. completed, he was involved in OPERA at the Gran Sasso Labora- As a result he studied physics at the first experiments to explore tory in Italy. the Technische Hochschule in physics at the very highest ener- Munich and wrote his diploma gies. thesis in nuclear physics under Prof. H. Meyer Leibnitz and Prof. In 1973 Klaus Pretzl returned to P. Kienle. He also gained a schol- Europe and joined the Max arship from EURATOM to work Planck Institute for Physics in at the Centre des Recherches Nu- Munich. There he first worked at cléaires in Strasbourg. the e+ - e– - storage ring DORIS INTERNATIONAL SPACE SCIENCE INSTITUTE No 7, May 2001

Published by the Association Pro ISSI

InIn SearchSearch ofof thethe DarkDark MatterMatter inin thethe UniverseUniverse

Dark matter constitutes the vast majority of matter in the universe. Recent research sheds light on what it might be.