  We study anharmonic effects in MgB2 by comparing Inelastic X-ray and
Ramanscattering together with ab-initio calculations. Using high statistics and
high q resolution measurements we show that the E2g mode linewidth is
independent of temperature along Gamma-A. We show, contrary to previous claims,
that the Raman-peak energy decreases as a function of increasing temperature, a
behaviour inconsistent with all the anharmonic ab-initio calculations of the
E2g mode at Gamma available in literature. These findings and the excellent
agreement between the X-ray measured and ab-initio calculated phonon spectra
suggest that anharmonicity is not the main mechanism determining the
temperature behaviour of the Raman-peak energy. The Raman E2g peak position and
linewidth can be explained by large dynamical effects in the phonon
self-energy. In light of the present findings, the commonly accepted
explanation of the reduced isotope effect in terms of anharmonic effects needs
to be reconsidered.
