The Atacama Cosmology Telescope: Cosmological Parameters from the 2008 Power Spectra (1009.0866v1)

Abstract: We present cosmological parameters derived from the angular power spectrum of the cosmic microwave background (CMB) radiation observed at 148 GHz and 218 GHz over 296 deg^2 with the Atacama Cosmology Telescope (ACT) during its 2008 season. ACT measures fluctuations at scales 500<l<10000. We fit a model for the lensed CMB, Sunyaev-Zel'dovich (SZ), and foreground contribution to the 148 GHz and 218 GHz power spectra, including thermal and kinetic SZ, Poisson power from radio and infrared point sources, and clustered power from infrared point sources. The power from thermal and kinetic SZ at 148 GHz is estimated to be B_3000 = 6.8+-2.9 uK^2, where B_l=l(l+1)C_l/2pi. We estimate primary cosmological parameters from the 148 GHz spectrum, marginalizing over SZ and source power. The LCDM cosmological model is a good fit to the data, and LCDM parameters estimated from ACT+WMAP are consistent with the 7-year WMAP limits, with scale invariant n_s = 1 excluded at 99.7% CL (3sigma). A model with no CMB lensing is disfavored at 2.8sigma. By measuring the third to seventh acoustic peaks, and probing the Silk damping regime, the ACT data improve limits on cosmological parameters that affect the small-scale CMB power. The ACT data combined with WMAP give a 6sigma detection of primordial helium, with Y_P = 0.313+-0.044, and a 4sigma detection of relativistic species, assumed to be neutrinos, with Neff = 5.3+-1.3 (4.6+-0.8 with BAO+H0 data). From the CMB alone the running of the spectral index is constrained to be dn/dlnk = -0.034 +- 0.018, the limit on the tensor-to-scalar ratio is r<0.25 (95% CL), and the possible contribution of Nambu cosmic strings to the power spectrum is constrained to string tension Gmu<1.6 \times 10^-7 (95% CL).

• The paper presents key cosmological parameter estimations from ACT’s 2008 CMB power spectra across multipoles 500–10,000.
• It quantifies SZ effects with an amplitude of 6.8 ± 2.9 μK² and constrains contributions from infrared and radio point sources.
• The study reinforces the ΛCDM model—with a spectral index of 0.962 ± 0.013—and sets tight limits on primordial helium and effective neutrino numbers.

Overview of "The Atacama Cosmology Telescope: Cosmological Parameters from the 2008 Power Spectra"

The paper, authored by J. Dunkley et al., details the cosmological parameter estimations derived from data collected during the 2008 observation cycle of the Atacama Cosmology Telescope (ACT). The objective was to analyze the angular power spectrum of the Cosmic Microwave Background (CMB) radiation at 148 GHz and 218 GHz over a sky area of 296 square degrees. The paper focuses on scales ranging from multipoles $\ell = 500$ to $\ell = 10,000$. The observational data were used to derive a lensed CMB model along with contributions from the Sunyaev-Zel'dovich (SZ) effects and extragalactic foreground sources.

Key Findings and Numerical Results

The fitting model incorporated various components:

• Primary CMB: The primary CMB accounts for about half of the power at $\ell=3000$ in the 148 GHz spectrum.
• SZ Effects: The amplitude of SZ effects was quantified, providing insights into cluster physics. The detected SZ power, including both the thermal and kinetic contributions, was about ${\cal B}_{3000}^{\rm SZ} = 6.8 \pm 2.9 \mu K^2$ at $\ell=3000$, showing consistency across different SZ models.
• Foreground Sources: The paper constrained the infrared (IR) point sources to ${\cal B}_{3000} = 7.8 \pm 0.7 \mu K^2$ for the Poisson power at 148 GHz, while a clustered component was necessary to fit the data at 218 GHz. Meanwhile, radio sources, modeled through a prior, accounted for part of the additional power detected at 148 GHz.

Cosmological Implications

The paper also discusses the broader implications for cosmological models:

• Concordance Model Verification: The $\Lambda$CDM model remains a good fit. The spectral index ($n_s$) showed a deviation from unity with $n_s = 0.962 \pm 0.013$, disfavoring scale invariance at the $3\sigma$ level.
• Impact on Extensions of $\Lambda$CDM: Data from ACT combined with the seven-year WMAP results strengthened constraints on several extended cosmological models:
  • A $6\sigma$ detection of primordial helium with $Y_P=0.313 \pm 0.044$.
  • A $4\sigma$ detection of relativistic species, resulting in an effective number of neutrinos $N_{\rm eff} = 5.3 \pm 1.3$.
  • Running of the scalar index estimated at $dn_s/d \ln k = -0.034 \pm 0.018$.
  • Upper limits set for the tensor-to-scalar ratio (r < 0.25) and cosmic strings tension ($G\mu < 1.6 \times 10^{-7}$).

Future Directions

The research highlights the significance of detailed high-resolution CMB observations in improving cosmological parameter constraints and understanding the early universe's physics. The data underscore the importance of SZ and point source modeling in deriving precise cosmological parameters, which remains an area for future refinement as observational techniques and theoretical models advance. Moving forward, combining ACT data with other cosmological surveys will refine our understanding of dark matter, dark energy, and the inflationary epoch, helping clarify the cosmological standard model's standing and any potential necessary modifications.