Elastic Properties and Density of Helium up to 20 GPa

Using Brillouin scattering, we have measured the sound velocity in fluid and solid helium 4 at room temperature up to 20 GPa. Our results show that the pair potentials currently used to describe helium are inadequate. The density of the fluid is determined as a function of pressure up to the fusion point. In the solid, the sound velocity and the elastic anisotropy are compared with lattice dynamics calculations.