|Prof. Dr. Dr. h.c. Reinhard Stock
Institut für Kernphysik
Data on the Big Bang evolution from relativistic nuclear collision studies at CERN and Brookhaven
Our universe is expanding from an initial phase of near infinite energy density and temperature. The only "data" we have about the very beginning is the very existence of the expanding universe. A subsequent phase which begins before the femtosecond era and is believed to have prevailed until about 10 microseconds was equally hypothetical up to now: The quark-gluon plasma phase represents a state of quasi-free, deconfined quarks - the most primitive form of matter governed by the strong force. The fundamental theory of the strong interaction, Quantum Chromodynamics (QCD), leads us to expect such a state of deconfined quarks, owing to its prediction of a phase transition at very high temperature. This means a drastic reduction of the extremely strong attraction mediated by gluon exchange in low temperature hadron states. The transition occurs at about 2 times 10**12 degrees Kelvin whence the thermal wavelength is about equal to the mesonic sizes, of about 0.5 fermi. Quark, or colour deconfinement is thus expected to set in once the energy density steps well above this limit: confined quarks in protons, neutrons, mesons give way to a colour conducting quark gluon plasma.For a more detailed understanding of the cosmological plasma phase one would require data detailing its moment of creation from a preceding "Grand Unification" era (which is supposed to consist of super-energetic X-particles experiencing the electromagnetic and weak interactions in as strong a manner as the strong force itself), as well as the point at which the plasma freezes out into confined protons and neutrons;this occurs at about 5 microseconds after the "bang". Also the equation of state - the relation between pressure and energy density - would be important for the detailed cosmological dynamics. QCD makes only semi-qualitative predictions here (confinement - deconfinement belongs to the less well established non-perturbative sectors of QCD). However there are advances toward these topics in modern non-perturbative QCD theory on the lattice.CERN, the European laboratory for particle physics in Geneva, has until 2000 undertaken a program of relativistic nucleus-nucleus-collision experiments, which have provided the first data concerning the existence of a new deconfined phase of matter, and also about the energy density and temperature that characterise the phase transition point from deconfined plasma to confined hadrons. According to the results, which confirm the estimates provided by QCD lattice theory, the phase transition takes place at a temperature T = 165 MeV and an energy density of about 1 GeV per cubic fermi. A "classical" translation of these conditions results in an estimate of the pressure corresponding to the hadronization point: about 10**32 kg/cm² (i.e. about 50 solar masses per cm²)! Thus cosmology is pushed an entire era backwards toward the beginning with the emerging experimental data, and the fundamental QCD theory gains new momentum in its non-perturbative sector.Seven experiments at the CERN SPS 33 TeV Pb beam facilities (NA44/45/49/50/52 and WA 97/98) have contributed complementary aspects of these overall findings, which were presented in a public seminar and press conference held at CERN in February 1999. The CERN program continues at the newly constructed LHC Collider since 2011, at more than 100-fold higher energy. The physicists are also engaged at present in a round of quark gluon plasma study, by collider experiments at the BNL RHIC Collider in Brookhaven ( STAR, PHENIX, PHOBOS and BRAHMS). At the 10- fold higher cm. energies provided here, the intricate details of QCD matter and its equation of state have already become accessible, with the result that quark-gluon-matter exhibits quite an unexpected behaviour at 300 MeV temperature: as a strongly coupled liquid of minimal (shear-)viscosity -- quite different from the idea of a gas of free quarks and gluons that was postulated at the beginning of this research field.