The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectra at 98 and 150 GHz (2007.07289v2)

Abstract: We present the temperature and polarization angular power spectra of the CMB measured by the Atacama Cosmology Telescope (ACT) from 5400 deg$^2$ of the 2013-2016 survey, which covers $>$15000 deg$^2$ at 98 and 150 GHz. For this analysis we adopt a blinding strategy to help avoid confirmation bias and, related to this, show numerous checks for systematic error done before unblinding. Using the likelihood for the cosmological analysis we constrain secondary sources of anisotropy and foreground emission, and derive a "CMB-only" spectrum that extends to $\ell=4000$. At large angular scales, foreground emission at 150 GHz is $\sim$1% of TT and EE within our selected regions and consistent with that found by Planck. Using the same likelihood, we obtain the cosmological parameters for $\Lambda$CDM for the ACT data alone with a prior on the optical depth of $\tau=0.065\pm0.015$. $\Lambda$CDM is a good fit. The best-fit model has a reduced $\chi^2$ of 1.07 (PTE=0.07) with $H_0=67.9\pm1.5$ km/s/Mpc. We show that the lensing BB signal is consistent with $\Lambda$CDM and limit the celestial EB polarization angle to $\psi_P =-0.07^{\circ}\pm0.09^{\circ}$. We directly cross correlate ACT with Planck and observe generally good agreement but with some discrepancies in TE. All data on which this analysis is based will be publicly released.

• The paper presents detailed measurements of the CMB power spectra using ACT data that robustly constrain key parameters of the ΛCDM model.
• It utilizes a rigorous blinding strategy and extensive systematic tests, including cross-comparisons with Planck observations.
• Key findings include a Hubble constant measurement of 67.9±1.5 km/s/Mpc and consistent baryon and matter density estimates, reinforcing the standard cosmological paradigm.

Overview of the Atacama Cosmology Telescope's Measurement of the Cosmic Microwave Background

The paper provides detailed results from the Atacama Cosmology Telescope (ACT), focusing on measurements of the Cosmic Microwave Background (CMB) at frequencies of 98 GHz and 150 GHz across a broad swathe of the sky. The primary data were collected from the 2013 to 2016 observing seasons. This discussion revolves around the angular power spectra derived from the CMB observations, which are paramount for understanding the fundamental parameters of cosmology within the framework of the $\Lambda$CDM model.

The analysis utilized a blinding strategy to preclude confirmation bias, a commendable practice that ensures the integrity and reliability of the derived cosmological parameters. This approach, alongside rigorous checks for systematic errors, contributes to a robust cosmological inference from the ACT data. The data analysis explores temperature and polarization components of the CMB, corroborating the six-parameter $\Lambda$CDM standard model of cosmology.

Key Numerical Results

A pivotal result from ACT's data is the determination of the Hubble constant ($H_0$), with ACT's measurement yielding $67.9 \pm 1.5$ km/s/Mpc. This value is notably consistent with the results from {\sl Planck}, though it diverges from local measurements of $H_0$. The ACT data also tightly constraints other parameters like the matter density and the baryon density in the universe, with measured values for $100\Omega_b h^2$ and $100\Omega_c h^2$ fitting well within theoretical expectations.

The paper upholds the $\Lambda$CDM model as a good fit to the data, demonstrated by the reduced chi-square statistic of 1.07 with a probability to exceed (PTE) of 0.07. The lensing B-mode signal aligns with $\Lambda$CDM predictions, bolstering the model's accuracy across CMB temperature, E-mode, and B-mode spectra.

Systematic Considerations and Null Tests

Several systematic error checks were rigorously applied to ensure data fidelity. These included cross-comparisons with Planck data, assessment of time constant variations across detectors, and comparisons between observations made at different elevation angles. Moreover, differences in atmospheric conditions (quantified by Precipitable Water Vapor levels) and maps derived from varying seasonal observations were analyzed to confirm consistency.

Notably, the polarization angle calibration was refined through both modeling and comparison with null EB cross-spectra, establishing the instrumental angle with an accuracy better than $0.1^\circ$. These systematic treatments and null tests further cement the reliability of the dataset.

Implications and Future Prospects

The data and methodology presented herein fortify the foundation for future cosmological endeavors, notably highlighting the potency of ground-based telescopes like ACT in mapping the CMB with precision comparable to space missions. The implications for cosmic birefringence constraints and the inferred limits on axion-like particles are profound, asserting ACT's role in probing fundamental physics within the field of cosmology.

Looking ahead, increased sensitivity from forthcoming ACT datasets and integration with advanced instruments like the Simons Observatory anticipates even stricter constraints on $\Lambda$CDM and insights into potential physics beyond the standard model. Consequently, these observations not only affirm established cosmological paradigms but invigorate future theoretical and observational pursuits in understanding the universe's grand design.