Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Planck distribution of phonons in a Bose-Einstein condensate   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.

12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
26
The categories of this paper are: bayes-an
28
The categories of this paper are: alg-geom
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
25
The categories of this paper are: patt-sol
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
6
The categories of this paper are: astro-ph
18
The categories of this paper are: econ
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th