  Exploring quantum phenomena in a curved spacetime is an emerging
interdisciplinary area relating many fields in physics such as general
relativity, thermodynamics, and quantum information. One famous prediction is
the Hawking-Unruh thermal radiation, the manifestation of Minkowski vacuum in
an accelerating reference frame. We simulate the radiation by evolving a
parametrically driven Bose-Einstein condensate of $\approx 10^5$ atoms, which
radiates coherent pairs of atoms with opposite momenta. We observe a matterwave
field which follows a Boltzmann distribution for a local observer. The
extracted temperature and entropy from the atomic distribution are in agreement
with Unruh's predictions. We further observe the long-distance phase coherence
and temporal reversibility of emitted matter-waves, hallmarks that distinguish
Unruh radiations from classical counterparts. Our results may lead to further
insights regarding the nature of the Hawking and Unruh effects and behaviors of
quantum physics in a curved spacetime.
