From its start by the Big Bang the history of the Universe can be marked by subsequent phase transitions. In the laboratory it is possible to study some of these phase transitions in the reverse order i.e. from low to high temperatures. The subject of this lecture concentrates on the phases which can be studied by collisions of heavy ion nuclei at high energies namely the liquid to gas phase transition for nuclei and the ultimate transitions to quark gluon phase. During the year 2000 the Relativistic Heavy Ion Collider at Brookhaven National Laboratory (RHIC) started its operation. The first experiments concentrated on Au + Au collision at the C.M. energies per nucleon pair \sqrt {s_NN}=56, 130, and 200 GeV. Four detectors: STAR, PHOBOS, BRAHMS, and PHENIX produced first results concerning particle production and properties of hadronic matter at high excitations. The present paper describes in more detail the construction and operation of the PHOBOS detector. The main results obtained by PHOBOS during the first runs of RHIC can be listed as follows: (1) Energy dependence of the charged particle pseudorapidity density near midrapidity for central collisions (dN_ch/d\eta )_{\delimiter "026A30C \eta \delimiter "026A30C <1}. (2) Centrality dependence of (dN_ch/d\eta )_{\delimiter "026A30C \eta \delimiter "026A30C <1}. (3) Charged particle density distribution in full the range of \eta values \delimiter "026A30C \eta \delimiter "026A30C <5.4. (4) Azimuthal anisotropy of events. (5) Ratios of \overline {p}/p, K^-/K⁺, and \pi ^-/\pi ⁺ measured in the midrapidity region within the acceptance of the spectrometer.

PACS numbers: 25.75.--q