  Future metrology standards will be partly based on physical quantities
computed from first principles rather than measured. In particular, a new
pressure standard can be established if the dynamic polarizability of helium
can be determined from theory with an uncertainty smaller than 0.2 ppm. We
present calculations of the frequency-dependent part of this quantity including
relativistic effects with full account of leading nuclear recoil terms and
using highly optimized explicitly correlated basis sets. A particular emphasis
is put on uncertainty estimates. At the He-Ne laser wavelength of 632.9908 nm,
the computed polarizability value of 1.391 811 41 a.u. has uncertainty of 0.1
ppm that is two orders of magnitude smaller than those of the most accurate
polarizability measurements. We also obtained an accurate expansion of the
helium refractive index in powers of density.
