Skip to content

First‐Year Wilkinson Microwave Anisotropy Probe ( WMAP ) Observations: Determination of Cosmological Parameters

Why this mattered

This paper helped turn cosmology from a largely parameter-fitting and model-discriminating field into a precision science with a tightly specified baseline model. Before WMAP, evidence for a flat, dark-energy-dominated universe with cold dark matter and nearly scale-invariant primordial fluctuations had accumulated from COBE, balloon CMB experiments, supernovae, galaxy surveys, and nucleosynthesis. The first-year WMAP parameter paper made that picture quantitatively hard to evade: a six-parameter, spatially flat ΛCDM model fit the CMB and also predicted independent large-scale-structure and smaller-scale CMB measurements. Its headline values, including an age of about 13.7 billion years, baryon and matter densities, spectral index, optical depth, and fluctuation amplitude, became the reference coordinates for modern cosmology.

The paradigm shift was not merely better error bars. WMAP showed that the early-universe fluctuation field could be treated as a measurable physical relic, not just a qualitative clue. Its agreement with nearly scale-invariant, adiabatic, Gaussian initial perturbations strongly supported inflationary-style initial conditions, while its temperature-polarization signal gave direct evidence for early reionization and opened CMB polarization as a cosmological probe. The paper also demonstrated the power of combining CMB data with galaxy surveys, Lyman-alpha forest measurements, and other astronomical observations to constrain curvature, dark energy, and neutrino masses within a common statistical framework.

Subsequent breakthroughs built directly on this foundation. Later WMAP releases, Planck, baryon acoustic oscillation surveys, weak-lensing measurements, and large galaxy redshift surveys all treated ΛCDM as the benchmark model to refine, stress-test, or extend. The paper therefore mattered both as a measurement and as a standardization event: it supplied the parameter set, methodology, and evidentiary threshold against which later claims about inflation, neutrino physics, dark energy, curvature, and possible tensions in H_0 and structure growth would be judged.

Abstract

WMAP precision data enables accurate testing of cosmological models. We find that the emerging standard model of cosmology, a flat Lambda-dominated universe seeded by nearly scale-invariant adiabatic Gaussian fluctuations, fits the WMAP data. With parameters fixed only by WMAP data, we can fit finer scale CMB measurements and measurements of large scle structure (galaxy surveys and the Lyman alpha forest). This simple model is also consistent with a host of other astronomical measurements. We then fit the model parameters to a combination of WMAP data with other finer scale CMB experiments (ACBAR and CBI), 2dFGRS measurements and Lyman alpha forest data to find the model's best fit cosmological parameters: h=0.71+0.04-0.03, Omega_b h^2=0.0224+-0.0009, Omega_m h^2=0.135+0.008-0.009, tau=0.17+-0.06, n_s(0.05/Mpc)=0.93+-0.03, and sigma_8=0.84+-0.04. WMAP's best determination of tau=0.17+-0.04 arises directly from the TE data and not from this model fit, but they are consistent. These parameters imply that the age of the universe is 13.7+-0.2 Gyr. The data favors but does not require a slowly varying spectral index. By combining WMAP data with other astronomical data sets, we constrain the geometry of the universe, Omega_tot = 1.02 +- 0.02, the equation of state of the dark energy w < -0.78 (95% confidence limit assuming w >= -1), and the energy density in stable neutrinos, Omega_nu h^2 < 0.0076 (95% confidence limit). For 3 degenerate neutrino species, this limit implies that their mass is less than 0.23 eV (95% confidence limit). The WMAP detection of early reionization rules out warm dark matter.

Sources