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Published December 1, 2002 | Published
Book Section - Chapter Open

The Cosmic Microwave Background & Inflation, Then & Now


The most recent results from the Boomerang, Maxima, DASI, CBI and VSA CMB experiments significantly increase the case for accelerated expansion in the early universe (the inflationary paradigm) and at the current epoch (dark energy dominance). This is especially so when combined with data on high redshift supernovae (SN1) and large scale structure (LSS), encoding information from local cluster abundances, galaxy clustering, and gravitational lensing. There are "7 pillars of Inflation" that can be shown with the CMB probe, and at least 5, and possibly 6, of these have already been demonstrated in the CMB data: (1) the effects of a large scale gravitational potential, demonstrated with COBE/DMR in 1992–96; (2) acoustic peaks/dips in the angular power spectrum of the radiation, which tell about the geometry of the Universe, with the large first peak convincingly shown with Boomerang and Maxima data in 2000, a multiple peak/dip pattern shown in data from Boomerang and DASI (2nd, 3rd peaks, first and 2nd dips in 2001) and from CBI (2nd, 3rd, 4th, 5th peaks, 3rd, 4th dips at 1-sigma in 2002); (3) damping due to shear viscosity and the width of the region over which hydrogen recombination occurred when the universe was 400000 years old (CBI 2002); (4) the primary anisotropies should have a Gaussian distribution (be maximally random) in almost all inflationary models, the best data on this coming from Boomerang; (5) secondary anisotropies associated with nonlinear phenomena subsequent to 400000 years, which must be there and may have been detected by CBI and another experiment, BIMA. Showing the 5 "pillars" involves detailed confrontation of the experimental data with theory; e.g., (5) compares the CBI data with predictions from two of the largest cosmological hydrodynamics simulations ever done. DASI, Boomerang and CBI in 2002, AMiBA in 2003, and many other experiments have the sensitivity to demonstrate the next pillar, (6) polarization, which must be there at the ~ 7% level. A broad-band DASI detection consistent with inflation models was just reported. A 7th pillar, anisotropies induced by gravity wave quantum noise, could be too small to detect. A minimal inflation parameter set, {ω_b, ω_(cdm), Ω_(tot), Ω_Q, W_Q, n_s, τ_C, σ_8}, is used to illustrate the power of the current data. After marginalizing over the other cosmic and experimental variables, we find the current CMB+LSS+SN1 data give Ω_(tot) = 1.00_(-.03_^(+.07), consistent with (non-baroque) inflation theory. Restricting to Ω_(tot) = 1, we find a nearly scale invariant spectrum, n_s = 0.97_(-.05)^(+.06). The CDM density, ω_(cdm) = Ω_(cdm)h^2 = .12_(-.01)^(+.01), and baryon density, ω_b ≡ Ω_bh^2 = .022_(-.002)^(+.003), are in the expected range. (The Big Bang nucleosynthesis estimate is 0.019 ± 0.002.) Substantial dark (unclustered) energy is inferred, Ω_Q ≈ 0.68 ± 0.05, and CMB+LSS Ω_Q values are compatible with the independent SN1 estimates. The dark energy equation of state, crudely parameterized by a quintessence-field pressure-to-density ratio W_Q, is not well determined by CMB+LSS (W_Q < −0.4 at 95% CL), but when combined with SN1 the resulting W_Q < −0.7 limit is quite consistent with the W_Q=−1 cosmological constant case.

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© 2002 American Institute of Physics. Issue Date: 1 December 2002.

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