  Amorphous solids, and many disordered lattices, exhibit a remarkable
qualitative and quantitative universality in their acoustic properties at
temperature $\lesssim 3$K. This phenomenon is attributed to the existence of
tunneling two level systems (TTLSs), characterized by a homogenous density of
states (DOS) at energies much lower than the disorder energy ($\approx 0.1$eV).
Here we calculate numerically, from first principles, the DOS of KBr:CN glass,
the archetypal disordered lattice showing universality. In contrast to the
standard tunneling model, we find that the DOS diminishes abruptly at $\approx
3$ K, and that tunneling states differ essentially by their symmetry under
inversion. This structure of the TTLSs dictates the low temperature below which
universality is observed, and the quantitative universality of the acoustic
properties in glasses. Consequences to the properties of glasses at
intermediate temperatures, as well as to the microscopic structure of amorphous
solids, are discussed.
