Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation (1001.4538v3)

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.

• The paper refines the ΛCDM model using 7-year WMAP data combined with BAO and H0 measurements, excluding the Harrison-Zel'dovich spectrum at 99.5% CL.
• The analysis constrains neutrino mass (Σ mν < 0.58 eV) and suggests an effective neutrino species count (N_eff ≈ 4.34), hinting at potential new physics.
• The study limits primordial non-Gaussianity and the tensor-to-scalar ratio (r < 0.24), thereby strengthening our understanding of inflationary dynamics.

Cosmological Interpretation of the 7-Year WMAP Observations

The Wilkinson Microwave Anisotropy Probe (WMAP) has provided profound insights into the cosmology of our universe through its measurement of the cosmic microwave background (CMB). This essay discusses the key findings and implications from the 7-year data collected by WMAP, as articulated in the work by Komatsu et al.

Summary of Key Results

The WMAP 7-year data in conjunction with other astrophysical measurements, such as Baryon Acoustic Oscillations (BAO) and measurements of the Hubble Constant ($H_0$), offer a stringent test of the $\Lambda$CDM cosmological model and constrain its parameters with unprecedented precision.

• Spectral Index and Gravitational Waves: The power-law index of the primordial power spectrum, $n_s = 0.968 \pm 0.012$, excludes the Harrison-Zel'dovich-Peebles spectrum with high confidence (99.5% CL). The data also yield an upper limit on the tensor-to-scalar ratio, $r < 0.24$, thus placing constraints on models that produce significant gravitational waves during inflation.
• Neutrino Mass and Relativistic Species: Improved constraints on the total mass of neutrinos, $\sum m_\nu < 0.58 \, \text{eV}$ (95% CL), have implications for particle physics and cosmological structure formation. The effective number of neutrino species, $N_{\rm eff} = 4.34^{+ 0.86}_{- 0.88}$, suggests a slight deviation from the standard model expectation, hinting at possible physics beyond the standard model.
• Dark Energy and Spatial Curvature: The WMAP data combined with BAO and $H_0$ measurements tightly constrain the dark energy equation of state parameter, $w = -1.10 \pm 0.14$, for a flat universe. The curvature parameter, $\Omega_k$, is also well constrained, offering no substantial evidence for deviation from spatial flatness.
• Primordial Non-Gaussianity: The analysis of non-Gaussianity reveals limits of $-10 < \fnlKS < 74$ for local-type primordial non-Gaussianities, indicating that the simplest models of inflation remain consistent with observations.

Implications for Cosmology

The results from the WMAP 7-year data continue to uphold the standard model of cosmology—the $\Lambda$CDM model—while refining our understanding of its parameters and providing insights into the early universe. The high precision with which the parameters of this model are determined allows for more stringent tests of physics beyond the standard model, such as the properties of dark matter and dark energy, the possible existence of extra neutrino species, and the dynamics of the early universe during inflation.

The constraints on these parameters also highlight the interplay between various components of our cosmological model, such as the interaction between matter and radiation in the primordial power spectrum and the role of neutrinos in shaping large-scale structures.

Future Prospects

These results pave the way for further studies with upcoming CMB experiments, such as the Planck satellite, which aim to provide even higher resolution data. Future experiments will refine the measurement of $n_s$ and $r$, potentially uncovering small deviations that could point to new physics. Moreover, as our understanding of the CMB's polarization improves, new insights into the physics of the early universe, particularly regarding inflation and the nature of primordial fluctuations, are expected.

In summary, the 7-year WMAP observations offer invaluable data that confirm the robustness of the $\Lambda$CDM model while opening up avenues for exploring new physics through precise cosmological measurements.