VSL theories and the Doppler peak

We discuss constraints on so-called 'varying speed of light theories' coming from the physics of the early universe. specifically, we consider the position of the first acoustic peak of the CMB angular power spectrum, showing that the recent determination of its position by various CMB experiments, in particular BOOMERanG/NA, can he used to constrain the value of the speed of light c after the epoch of last scattering. Specifically, we find that far a flat universe and a standard density of baryonic matter a variation in c of up to 4% is consistent with the current observations, The position of the Doppler peal; is fairly sensitive to changes in c, and future observations should dramatically improve this bound. On the other hand, we also find that the maximum baryonic density allowed in VSL theories is about Ohm(B)h(2)similar to 0.06 Ohm(0). We comment on the significance of these results. (C) 2000 Elsevier Science B.V. All rights reserved.