The Mirage of w=-1

We demonstrate that cosmic microwave background observations consistent with a cosmological constant universe predict in a well-defined sense that lower redshift measures will nearly automatically deliver w=-1 for the dark energy equation of state value unless they are sensitive to w(z). Thus low redshift data pointing to w=-1 does not truly argue for a cosmological constant. Even the simplest question of whether the equation of state of dark energy is equal to the cosmological constant therefore requires experiments able to sensitively constrain time variation w(z) and not merely a constant w. We also note a number of issues regarding parametrization of w(z), demonstrating that the standard form w(z)=w_0+w_a z/(1+z) is robust but use of high order polynomials and cutting off the high redshift behavior can be pathological.