Atacama Cosmology Telescope

Atacama Cosmology Telescope is a six-meter diameter telescope designed for high-sensitivity, arcminute-resolution observations of the cosmic microwave background (CMB). Located at an extremely high and dry site in the Andes, it operates at millimeter and submillimeter wavelengths to map the CMB's faint temperature and polarization anisotropies. The project is a collaboration led by Princeton University, the University of Pennsylvania, and the National Science Foundation, with significant contributions from other international institutions.

Overview

The facility was constructed on Cerro Toco in the Atacama Desert of northern Chile, a premier site for astronomy due to its high altitude, low precipitation, and stable atmosphere. It began scientific operations in 2007, building upon the legacy of earlier CMB experiments like the Cosmic Background Explorer and the Wilkinson Microwave Anisotropy Probe. Its primary mission is to probe the physics of the early universe, test models of cosmic inflation, and constrain fundamental parameters like the Hubble constant. The telescope's design emphasizes a wide field of view and the ability to conduct deep, high-fidelity surveys of the sky.

Design and instrumentation

The telescope employs a compact, off-axis Gregorian design with a six-meter primary mirror, providing excellent control over systematic errors and stray light. It is housed within a custom-built, ground-screen enclosure to minimize contamination from terrestrial radiation. The initial instrument was the Millimeter Bolometer Array Camera (MBAC), which used transition-edge sensor bolometers cooled by a He-3/He-4 dilution refrigerator. This was later succeeded by more advanced polarimeter arrays, known as ACTPol and then Advanced ACTPol, which added the capability to measure the CMB's B-mode polarization. These instruments were developed with key technology from the National Institute of Standards and Technology and funding from the NASA.

Scientific goals and discoveries

A central goal has been to measure the gravitational lensing of the CMB by large-scale structure, providing a map of the distribution of dark matter across cosmic history. The collaboration has produced highly precise measurements of the CMB's damping tail, leading to constraints on the number of effective neutrino species and the sum of neutrino masses. Its data have been used to create a catalog of thousands of galaxy clusters via the Sunyaev–Zel'dovich effect, testing models of dark energy and structure formation. Notably, observations contributed to ongoing debates about the Hubble tension, providing an early-universe measurement independent of the Cepheid variable and Type Ia supernova distance ladder.

Collaboration and operations

The project is an international partnership originally spearheaded by Lyman Page of Princeton University and Mark Devlin of the University of Pennsylvania. Major funding has been provided by the National Science Foundation through the AST Division, with additional support from the U.S. Department of Energy and contributions from institutions in Canada, the United Kingdom, Chile, and South Africa. Operations and data analysis are coordinated through the ACTPol collaboration, with data processing centers at Princeton University and the University of Pennsylvania. The telescope ceased routine observations in 2022, transitioning to a final data release phase.

Data products and legacy

The collaboration has released multiple public data sets, including temperature and polarization maps, a Sunyaev–Zel'dovich effect cluster catalog, and cosmological parameter constraints. These products have been cross-correlated with optical surveys like the Dark Energy Survey and the Sloan Digital Sky Survey to study galaxy evolution and baryon acoustic oscillations. The telescope's instrumental and analysis techniques directly informed the design of the Simons Observatory and the upcoming CMB-S4 project. Its legacy solidifies the role of ground-based, specialized telescopes in precision cosmology alongside space missions like the Planck satellite.

Category:Astronomical observatories in Chile Category:Cosmic microwave background experiments Category:Radio telescopes