  The elliptic flow of emitted particles and fragments observed in heavy-ion
reactions at high energy has become an important observable reflecting the
pressure generated in the dense collision zone. More recently, the strength of
the nuclear symmetry energy has been investigated by measuring the ratios or
differences of the elliptic flows exhibited by neutrons and charged particles
in 197Au+197Au collisions at 400 MeV/nucleon incident energy at the GSI
laboratory. A moderately soft to linear dependence on density was deduced for a
range of densities shown to reach beyond twice the saturation value in these
experiments. The known sources of uncertainties and possible model dependencies
were thoroughly studied with transport models of the UrQMD and Tuebingen QMD
type. A new source of information on the nuclear equation of state at high
density has opened up with the observation of the first LIGO and Virgo GW170817
gravitational wave signal from a neutron star merger. The quantitative
comparison of terrestrial and celestial results on the basis of measured or
inferred neutron star radii or core pressures, including those obtained from
X-ray observations, reveals a rather satisfactory agreement. Depending on the
precision that can be achieved with future measurements and observations, it
will thus become possible to assess the validity of the applied models and
methods. The perspectives for improved experiments at FAIR using the NeuLAND
and KRAB detection systems are outlined.
