The Zero Active Mass Condition in Friedmann-Robertson-Walker Cosmologies

Understand

Many cosmological measurements today suggest that the Universe is expanding at a constant rate.

• This is inferred from the observed age versus redshift relationship and various distance indicators, all of which point to a cosmic equation of state (EoS) p=-rho/3, where rho and p are, respectively, the total energy density and pressure of the cosmic fluid.
• It has recently been shown that this result is not a coincidence and simply confirms the fact that the symmetries in the Friedmann-Robertson-Walker (FRW) metric appear to be viable only for a medium with zero active mass, i.e., rho+3p=0.
• In their latest paper, however, Kim, Lasenby and Hobson have provided what they believe to be a counter argument to this conclusion.