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SEVEN-YEARWILKINSON MICROWAVE ANISOTROPY PROBE(WMAP) OBSERVATIONS: COSMOLOGICAL INTERPRETATION

Why this mattered

This paper mattered because it turned the ΛCDM model from a successful framework into a precision-tested baseline. The seven-year WMAP analysis showed that a simple six-parameter cosmology could fit the microwave background while also agreeing with baryon acoustic oscillation and local H0 distance information. Its measurement of the scalar spectral index, n_s = 0.968 ± 0.012, was especially important: by excluding an exactly scale-invariant primordial spectrum at 99.5% confidence, it strengthened the case that the early universe carried the slight scale dependence expected in many inflationary models, rather than merely fitting a featureless Harrison-Zel’dovich spectrum.

The paper also expanded what the CMB could be used to test. WMAP’s improved constraints on neutrino mass, effective relativistic species, primordial helium, parity-violating polarization rotation, and the Sunyaev-Zel’dovich signal made cosmology a laboratory for particle physics, big bang nucleosynthesis, and cluster astrophysics. The reported temperature-polarization patterns around hot and cold spots gave a visually and statistically direct confirmation of the acoustic physics at recombination and of predominantly adiabatic scalar perturbations.

Its longer-term significance was as a bridge between first-generation precision cosmology and the higher-resolution era of Planck, ACT, SPT, and large-scale structure surveys. WMAP seven-year results fixed the reference cosmological model tightly enough that later experiments could focus on smaller deviations: lensing, neutrino physics, inflationary parameters, cluster pressure profiles, and possible tensions among probes. In that sense, the paper did not overthrow ΛCDM; it made ΛCDM the standard that subsequent breakthroughs had to refine, stress-test, or explain.

Abstract

(Abridged) The 7-year WMAP data and improved astrophysical data rigorously test the standard cosmological model and its extensions. By combining WMAP with the latest distance measurements from BAO and H0 measurement, we determine the parameters of the simplest LCDM model. The power-law index of the primordial power spectrum is n_s=0.968+-0.012, a measurement that excludes the scale-invariant spectrum by 99.5%CL. The other parameters are also improved from the 5-year results. Notable examples of improved parameters are the total mass of neutrinos, sum(m_nu)<0.58eV, and the effective number of neutrino species, N_eff=4.34+0.86-0.88. We detect the effect of primordial helium on the temperature power spectrum and provide a new test of big bang nucleosynthesis. We detect, and show on the map for the first time, the tangential and radial polarization patterns around hot and cold spots of temperature fluctuations, an important test of physical processes at z=1090 and the dominance of adiabatic scalar fluctuations. With the 7-year TB power spectrum, the limit on a rotation of the polarization plane due to potential parity-violating effects has improved to Delta(alpha)=-1.1+-1.4(stat)+-1.5(syst) degrees. We report significant detections of the SZ effect at the locations of known clusters of galaxies. The measured SZ signal agrees well with the expected signal from the X-ray data. However, it is a factor of 0.5 to 0.7 times the predictions from "universal profile" of Arnaud et al., analytical models, and hydrodynamical simulations. We find, for the first time in the SZ effect, a significant difference between the cooling-flow and non-cooling-flow clusters (or relaxed and non-relaxed clusters), which can explain some of the discrepancy. This lower amplitude is consistent with the lower-than-theoretically-expected SZ power spectrum recently measured by the South Pole Telescope collaboration.

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