0 Vacuum Energy : “ If Not Now , Then When ? ”

For a flat universe presently dominated by static or dynamic vacuum energy, cosmological constant (LCDM) or quintessence (QCDM), we calculate the asymp-totic collapsed mass fraction as function of the present ratio of smooth energy to matter energy R 0 = (1 − Ω m0)/Ω m0. Identifying these collapsed fractions as anthropic probabilities, we find the observed present ratio R 0 ∼ 2 to be likely in LCDM, but most likely in QCDM. Absent a known symmetry principle protecting its value, no theoretical reason for making the cosmological constant zero or small has been found. Inflation makes the universe flat, so that, at present, the vacuum or smooth energy density Ω Q0 = 1 − Ω m0 < 1, is 10 120 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, which obeys the equation of state w Q ≡ P/ρ < 0. (The limiting case, w Q = −1, a static vacuum energy or Cosmological Constant, is homogeneous on all scales.) The evidence for a flat low-density universe come from [1, 2]: (1) The location of the first Doppler peak in the CBR anisotroy at l ∼ 200: Ω m + Ω Q = 1 ± 0.2; (2) The slow evolution of rich clusters, the mass power spectrum, the CBR anisotropy, the cosmic flow:Ω m0 = 0.3 ± 0.05; (3) Curvature in the SNIa Hubble diagram, dynamic age, height of first Doppler peak, cluster evolution: Ω Q0 = 1 − Ω m0 ∼ 2/3. Of these, the SNIa evidence is most subject to systematic errors due to precursor intrinsic evolution and the possibilty of grey dust extinction. The combined data nevertheless implies a flat, low-density universe with Ω m0 ∼ 1/3 and a smooth energy component with present energy density Ω Q0 ∼ 2/3 and negative pressure −1 ≤ w Q ≤ −1/2.