Frequency-dependent viscosity near the critical point: the scale to two-loop order.

The recent accurate measurements of Berg, Moldover, and Zimmerli [Phys. Rev. Lett. 82, 920 (1999); Phys. Rev. E 60, 4079 (1999)] of the viscoelastic effect near the critical point of xenon has shown that the scale factor involved in the frequency scaling is about twice the scale factor obtained theoretically. We show that this discrepancy is a consequence of using first order perturbation theory. Including two-loop contribution goes a long way towards removing the discrepancy.