  The Planck distribution of photons emitted by a black body led to the
development of quantum theory. An analogous distribution of phonons should
exist in a Bose-Einstein condensate. We observe this Planck distribution of
thermal phonons in a 3D condensate. This observation provides an important
confirmation of the basic nature of the condensate's quantized excitations. In
contrast to the bunching effect, the density fluctuations are seen to increase
with increasing temperature. This is due to the non-conservation of the number
of phonons. In the case of rapid cooling, the phonon temperature is out of
equilibrium with the surrounding thermal cloud. In this case, a Bose-Einstein
condensate is not as cold as previously thought. These measurements are enabled
by our in situ k-space technique.
