  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).

Cosmology and Nongalactic Astrophysics