Aula
The standard model of cosmology is based on the Friedmann-Robertson-Walker (FRW) metric. Often written in terms of co-moving coordinates, this elegant and highly practical solution to Einstein's equations is based on the Cosmological principal and Weyl's postulate. But not all of the physics behind such symmetries has yet been recognized. We invoke the fact that the co-moving frame also happens to be in free fall to demonstrate that the FRW metric is apparently valid only for a medium with zero active mass. In other words, the application of FRW appears to require an equation-of-state rho+3p = 0, in terms of the total energy density rho and total pressure p. Though the standard model is not framed in these terms, the optimization of its parameters brings it ever closer to this constraint as the precision of the observations continues to improve. For example, the latest high-precision BAO measurements rule out the standard model at better than the 99.34% C.L. if the zero active mass condition is ignored.