Smooth Energy: Cosmological Constant or Quintessence?

For a flat universe presently dominated by smooth energy, either cosmological constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed mass fraction as function of the present ratio of smooth energy to matter energy R0. Identifying the normalized collapsed fraction as a conditional probability for habitable galaxies, we observe that the observed present ratio R0 ∼ 2 is likely in LCDM, but more likely in QCDM. Inverse application of Bayes’ Theorem makes the Anthropic Principle a predictive scientific principle: the data implies that the prior probability forR0 must be essentially flat over the anthropically allowed range. Interpreting this prior as a distribution over theories lets us predict that any future theory of initial conditions must be indifferent to R0. This application of the Anthropic Principle does not demand the existence of other universes. 1. A Flat Low-Density Universe In the absence of a recognized symmetry principle protecting its value, no theoretical reason for making the cosmological constant zero or small has been found. Inflation makes the universe appear flat, so that, at present, the vacuum or smooth energy density ΩQ0 = 1 − Ωm0 < 1, is 10 times smaller than would be expected on current particle theories. To explain this small but non-vanishing present value, a dynamic vacuum energy, quintessence, has been invoked: a background scalar field whose potential energy dominates its kinetic energy, so that the fluid pressure P and its ratio to energy density wQ ≡ P/ρ < 0. (When we speak of a static vacuum energy or cosmological constant, we mean the limiting Assisted in part by U.S. Department of Energy grant DE-FGO2-95ER40893.