  We study the effect of a violation of the strong equivalence principle (SEP)
on the cosmic microwave background (CMB). Such a violation would modify the
weight of baryons in the primordial gravitational potentials and hence their
impact in the establishment of the photon-baryon plasma acoustic oscillations
before recombination. This cosmological Nordtvedt effect alters the odd peaks
height of the CMB temperature anisotropy power spectrum. A gravitational
baryonic mass density of the universe may already be inferred at the first peak
scale from the analysis of WMAP data. Experimental constraints on a primordial
SEP violation are derived from a comparison with the universe's inertial
baryonic mass density measured either in a full analysis of the CMB, or in the
framework of the standard big bang nucleosynthesis (BBN).
