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In Search of the Dark Matter in the Universe *) 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. Editorial Dark matter. Dark energy. In March 2000 Professor Pretzl Impressum Dark is beautiful! reported on his institute’s fascinat- ing search for dark matter 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 Physics 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 set-up seeks to in- Spiral Galaxy NGC 1232 vestigate a specific class of dark Copyright: European Southern matter particles, 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 gravity, The story of the dark matter start- the strong and the electro-weak ed in 1933, when the Swiss as- forces appear today as separate tronomer Fritz Zwicky 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 isotropy 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 galaxies 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 motion. By ments of birth the Big Bang. After microwave background radiation 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 dimensions 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 density of each other, i.e. light never had suf- tational pull and thereby the total 1094 g/cm3. Grand Unified Theo- ficient time since the Big Bang to mass 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 force rul- A. Linde introduced a scenario ac- tronomers at that time. It took an- ing everything. 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 earth and the riod of time between 10–36 to 10–34 How much of this mysterious planets to the sun, 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 electrons in the rapidly growing in size by about its real nature? Will we be able to outer shell of an atom attracted to a factor of 3 x 1043. Within this directly detect it, since it does the positively charged nucleus in scenario of inflation 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 astronomers, cos- holds the protons and neutrons short period of inflation the uni- mologists, particle and nuclear together in the nucleus. All these verse continued its expansion physicists 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 observations 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- gluons and the electrons, and their Maxima experiments (see also ation of matter and radiation antiparticles. 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 neutrinos, etc... How- flationary phase of the universe blocks of matter, the quarks, the ever, as the universe cooled by ex- pansion, radiation lost its energy faster than matter and when the universe became about ten thou- sand years old the energy balance shifted in favour of matter. The quark gluon phase of matter ended about 10–6 seconds after the Big Bang, when the universe cooled to a temperature of 2 x 1012 Kelvin. At that temperature a phase transition from a quark gluon plasma 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 proton (two up-quarks and one down-quark) or a neutron (two down-quarks and one up-quark) and similarly antiprotons (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 number 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 reason that we live Temperatures of about 2 x 1012 were reached in this collision experiment at in a world of matter with no anti- CERN. A lead 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 positron (anti-electron) particles proton neutron heavy particles 18 degrees carrying the meson weak force hydrogen quark deuterium anti-quark helium 3 degrees K electron lithium Figure 2 The evolution of the universe.
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