We examine the possibility that Friedman-Robertson-Walker evolution is governed by an effective ( rather then by the actual ) energy density .
A concrete example is provided by \Lambda = 0 Regge-Teitelboim cosmology , where critical cosmology only requires subcritical matter density ( \Omega _ { m } < 1 ) , and the age of a flat matter dominated Universe gets enhanced by a factor of \frac { 9 } { 8 } .
Dual to the mature dilute Universe is the embryo Universe , the evolution of both is governed by P _ { eff } = - \frac { 1 } { 9 } \rho _ { eff } .