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The gravitational wave contribution to CMB anisotropies and the amplitude of mass fluctuations from COBE results

A stochastic background of primordial gravitational waves may substantially contribute, via the Sachs--Wolfe effect, to the large--scale Cosmic Microwave Background (CMB) anisotropies recently detected by COBE. This implies a {\it bias} in any resulting determination of the primordial amplitude of density fluctuations. We consider the constraints imposed on $n<1$ (``tilted") power--law fluctuation spectra, taking into account the contribution from both scalar and tensor waves, as predicted by power--law inflation. The gravitational--wave contribution to CMB anisotropies generally reduces the required {\it rms} level of mass fluctuation, thereby increasing the linear {\it bias parameter}, even in models where the spectral index is close to the Harrison--Zel'dovich value $n=1$. This ``gravitational--wave bias" helps to reconcile the predictions of CDM models with observations on pairwise galaxy velocity dispersion on small scales.