## The Atacama Cosmology Telescope: cosmological parameters from three seasons of data

We present constraints on cosmological and astrophysical parameters from high-resolution microwave background maps at 148 GHz and 218 GHz made by the Atacama Cosmology Telescope (ACT) in three seasons of observations from 2008 to 2010. A model of primary cosmological and secondary foreground parameters is fit to the map power spectra and lensing deflection power spectrum, including contributions from both the thermal Sunyaev-Zeldovich (tSZ) effect and the kinematic Sunyaev-Zeldovich (kSZ) effect, Poisson and correlated anisotropy from unresolved infrared sources, radio sources, and the correlation between the tSZ effect and infrared sources. The power ℓ2*C*ℓ/2π of the thermal SZ power spectrum at 148 GHz is measured to be3.4±1.4 μK2 at ℓ = 3000, while the corresponding amplitude of the kinematic SZ power spectrum has a 95% confidence level upper limit of 8.6 μK2. Combining ACT power spectra with the WMAP 7-year temperature and polarization power spectra, we find excellent consistency with the LCDM model. We constrain the number of effective relativistic degrees of freedom in the early universe to be *N*eff = 2.79±0.56, in agreement with the canonical value of *N*eff = 3.046 for three massless neutrinos. We constrain the sum of the neutrino masses to be Σ*m*ν < 0.39 eV at 95% confidence when combining ACT and WMAP 7-year data with BAO and Hubble constant measurements. We constrain the amount of primordial helium to be *Y**p* = 0.225±0.034, and measure no variation in the fine structure constant α since recombination, withα/α0 = 1.004±0.005. We also find no evidence for any running of the scalar spectral index, *dn**s*/*d*ln *k* = −0.004±0.012.