Periodic Report Summary - ACT (Testing fundamental physics with the Atacama cosmology telescope)
The Atacama cosmology telescope (ACT) is a six-metre telescope on Cerro Toco in the Atacama Desert in the north of Chile. It is designed to make high-resolution, microwave-wavelength surveys of the sky in order to study the Cosmic microwave background radiation (CMB). Erected in the (austral) autumn of 2007, ACT saw first light on 22 October 2007 and completed its first season in December of 2007. In began its second season of observations in June of 2008. Unlike most telescopes which track the rotating sky during observation, the ACT observes a strip of sky, by scanning back and forth in azimuth at the relatively rapid rate of two degrees per second. The rotating portion of the telescope weighs approximately 32 tonnes, creating a substantial engineering challenge. A ground screen surrounding the telescope minimises contamination from radiation emitted by the ground.
Observations are made at resolutions of about an arcminute (1/60th of a degree) in three frequencies: 145 GHz, 215 GHz and 280 GHz. Each frequency is measured by a 3 cm x 3 cm (1.2" x 1.2"), 1 024-element array, for a total of 3 072 detectors. The detectors are superconducting transition-edge sensors, a new technology able to achieve the required high sensitivity. A system of cryogenic helium refrigerators keep the detectors a third of a Kelvin above absolute zero. Because water vapour in the atmosphere emits microwave radiation which contaminates measurements of the CMB, the telescope benefits from its arid, high-altitude site.
Measurements of the CMB by experiments such as COBE, Boomerang, WMAP, CBI and many others, have greatly advanced our knowledge of cosmology. It is expected that higher resolution CMB observations will not only improve the precision of current knowledge, but will also allow new types of measurements. At ACT resolutions, the Sunyaev-Zeldovich effect (SZ), by which galaxy clusters leave an imprint on the CMB, should be prominent. The power of this method of detection is that it is a redshift-independent measurement of the mass of the clusters, meaning that very distant, ancient clusters are as easy to detect as nearby clusters. It is expected that ACT will detect on the order of hundreds such clusters.
The ACT survey is complemented by a follow up program called the Southern cosmology survey. Its goal is to enable a multi-national observational program to survey the ACT sky region with multiple telescopes: optical telescopes based in South Africa and Chile, space-based telescopes ranging from ultraviolet to X-rays. The project will greatly enhance the scientific return from the ACT project. Together with its follow-up measurements, ACT would provide a picture of the evolution of structure in the universe since the Big Bang. Among other things, this would improve our understanding of the nature of the mysterious dark energy which seems to be a dominant component of the universe.
ACT released results measuring the statistical properties of the temperature of the CMB in January 2010. It found signals that were consistent with unresolved point sources and the SZ effect. It also measured accurately the damping tail of the primary CMB. Preliminary results with galaxy clusters observed through the SZ effect (some previously known clusters and some previously unknown ones) were also presented.
ACT has also observed in details the SZ signature the famous Bullet Cluster (previously studied in the optical and X-ray wavelengths).
It is expected that final maps can be used not only to study the primary CMB at small angular scale but also to study the secondary CMB. The intervening cosmological structures leave their imprint on the CMB fluctuations through several effects (e.g. Rees-Sciama, thermal and kinetic Sunyaev-Zeldovich effects, gravitational lensing). These signals can be used to constrain cosmological parameters such as neutrino mass, primordial power spectrum spectral slope, or the nature of dark energy.
Observations are made at resolutions of about an arcminute (1/60th of a degree) in three frequencies: 145 GHz, 215 GHz and 280 GHz. Each frequency is measured by a 3 cm x 3 cm (1.2" x 1.2"), 1 024-element array, for a total of 3 072 detectors. The detectors are superconducting transition-edge sensors, a new technology able to achieve the required high sensitivity. A system of cryogenic helium refrigerators keep the detectors a third of a Kelvin above absolute zero. Because water vapour in the atmosphere emits microwave radiation which contaminates measurements of the CMB, the telescope benefits from its arid, high-altitude site.
Measurements of the CMB by experiments such as COBE, Boomerang, WMAP, CBI and many others, have greatly advanced our knowledge of cosmology. It is expected that higher resolution CMB observations will not only improve the precision of current knowledge, but will also allow new types of measurements. At ACT resolutions, the Sunyaev-Zeldovich effect (SZ), by which galaxy clusters leave an imprint on the CMB, should be prominent. The power of this method of detection is that it is a redshift-independent measurement of the mass of the clusters, meaning that very distant, ancient clusters are as easy to detect as nearby clusters. It is expected that ACT will detect on the order of hundreds such clusters.
The ACT survey is complemented by a follow up program called the Southern cosmology survey. Its goal is to enable a multi-national observational program to survey the ACT sky region with multiple telescopes: optical telescopes based in South Africa and Chile, space-based telescopes ranging from ultraviolet to X-rays. The project will greatly enhance the scientific return from the ACT project. Together with its follow-up measurements, ACT would provide a picture of the evolution of structure in the universe since the Big Bang. Among other things, this would improve our understanding of the nature of the mysterious dark energy which seems to be a dominant component of the universe.
ACT released results measuring the statistical properties of the temperature of the CMB in January 2010. It found signals that were consistent with unresolved point sources and the SZ effect. It also measured accurately the damping tail of the primary CMB. Preliminary results with galaxy clusters observed through the SZ effect (some previously known clusters and some previously unknown ones) were also presented.
ACT has also observed in details the SZ signature the famous Bullet Cluster (previously studied in the optical and X-ray wavelengths).
It is expected that final maps can be used not only to study the primary CMB at small angular scale but also to study the secondary CMB. The intervening cosmological structures leave their imprint on the CMB fluctuations through several effects (e.g. Rees-Sciama, thermal and kinetic Sunyaev-Zeldovich effects, gravitational lensing). These signals can be used to constrain cosmological parameters such as neutrino mass, primordial power spectrum spectral slope, or the nature of dark energy.