  Physical research looks for clues to quantum properties of the gravitational
field. On the basis of the common Schr\"odinger theory, a simple model of the
quantization of a Friedmann universe comprising dust and radiation is
investigated. With regard to energy quantization, the result suggests a
universal limitation of the energy spacing between neighbouring quantum states
by the Planck energy. Applied to black-body radiation, a modified Planck
radiation law follows. If this could be verified in the laboratory, it would
provide a direct hint at quantum properties of the space-time manifold.
