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The measurement of the baryon density is now a highly active area of research. In the coming years, we anticipate that higher accuracy measurements of the baryon density, from the CMB, clusters of galaxies, and the Lyalpha forest, will give a new rigorous test of BBN [11]. This test can be viewed from two directions. First, we can use the baryon density to fix the last free parameter in BBN, and second, we can compare the different baryon density measurements, which should be identical if SBBN is correct, and all baryons are counted in the measurements made at later times.

In addition to BBN, the baryon density is measured in four ways: in the IGM, in clusters of galaxies, using simulations of galaxy formation, and directly from the CMB. All agree with the value from SBBN using low D/H, but today they are each about an order of magnitude less accurate.