The Atacama Cosmology Telescope: Cosmological Parameters From the 2008 Power Spectrum

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 < < 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).

Planck2013 results. XVI. Cosmological parameters

“…In the final 9-year WMAP analysis no significant running was seen using WMAP data alone, with dn s /dln k = −0.019 ± 0.025 (68% confidence; Hinshaw et al 2012. Combining WMAP data with the first data releases from ACT and SPT, Hinshaw et al (2012) found a negative running at nearly the 2σ level with dn s /dln k = −0.022 ± 0.012 (see also Dunkley et al 2011 andKeisler et al 2011 for analysis of ACT and SPT with earlier data from WMAP). The ACT 3-year release, which incorporated a new region of sky, gave dn s /dln k = −0.003 ± 0.013 ) when combined with WMAP 7 year data.…”

“…In the Standard Model, N eff = 3.046, due to noninstantaneous decoupling corrections (Mangano et al 2005). However, there has been some mild preference for N eff > 3.046 from recent CMB anisotropy measurements (Komatsu et al 2011;Dunkley et al 2011;Keisler et al 2011;Archidiacono et al 2011;Hinshaw et al 2012;Hou et al 2014). This is potentially interesting, since an excess could be caused by a neutrino/anti-neutrino asymmetry, sterile neutrinos, and/or any other light relics in the Universe.…”

“…The tSZ template used here is similar in shape to the Battaglia et al (2010) template that has been used extensively in the analysis of ACT and SPT data. The effects of varying tSZ and kSZ templates on high-resolution CMB experiments have been investigated by Dunkley et al (2011), Reichardt et al (2012b, and Dunkley et al (2013) who find very little effect on cosmological parameters.…”

Planck2013 results. XV. CMB power spectra and likelihood

“…Further theoretical investigation (Scott & White 1999;Haiman & Knox 2000) was stimulated by the detection of the infrared background in the COBE data (Puget et al 1996;Fixsen et al 1998), and the detection of bright "sub-millimetre" galaxies in SCUBA data (Hughes et al 1998). Subsequently, the clustering has been detected at 160 microns , at 250, 350 and 500 microns by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST, Viero et al 2009;Hajian et al 2012) and at 217 GHz by SPT and ACT (Hall et al 2010;Dunkley et al 2011). Recent Planck measurements of the CIB (Planck Collaboration XVIII 2011) have extended the measurements at 217 GHz, 353 GHz, and 545 GHz to larger scales, and recent Herschel measurements (Viero et al 2013) have improved on the BLAST measurements and extended them to smaller angular scales.…”

“…We develop a model for these foregrounds, designed to allow the Planck likelihood to be combined with high resolution data from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). We combine frequencies and model unresolved foregrounds in a physical way, as in e.g., Shirokoff et al (2011);Dunkley et al (2011);Reichardt et al (2012), performing component separation at small scales at the power spectrum level. On large scales, < 50, Galactic contamination is more significant.…”