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 < < 10,000. 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. At = 3000, about half the power at 148 GHz comes from primary CMB after masking bright radio sources. The power from thermal and kinetic SZ is estimated to be B 3000 = 6.8 ± 2.9 μK 2 , where B ≡ ( + 1)C /2π . The IR Poisson power at 148 GHz is B 3000 = 7.8 ± 0.7 μK 2 (C = 5.5 ± 0.5 nK 2 ), and a clustered IR component is required with B 3000 = 4.6 ± 0.9 μK 2 , assuming an analytic model for its power spectrum shape. At 218 GHz only about 15% of the power, approximately 27 μK 2 , is CMB anisotropy at = 3000. The remaining 85% is attributed to IR sources (approximately 50% Poisson and 35% clustered), with spectral index α = 3.69 ± 0.14 for flux scaling as S(ν) ∝ ν α . We estimate primary cosmological parameters from the less contaminated 148 GHz spectrum, marginalizing over SZ and source power. The ΛCDM cosmological model is a good fit to the data (χ 2 /dof = 29/46), and ΛCDM parameters estimated from ACT+Wilkinson Microwave Anisotropy Probe (WMAP) are consistent with the seven-year WMAP limits, with scale invariant n s = 1 excluded at 99.7% confidence level (CL) (3σ ). A model with no CMB lensing is disfavored at 2.8σ . 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 6σ detection of primordial helium, with Y P = 0.313 ± 0.044, and a 4σ detection of relativistic species, assumed to be neutrinos, with N eff = 5.3 ± 1.3 (4.6 ± 0.8 with BAO+H 0 data). From the CMB alone the running of the spectral index is constrained to be dn s /d ln k = −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 Gμ < 1.6 × 10 −7 (95% CL).

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