The Atacama Cosmology Telescope

The Atacama Cosmology Telescope (ACT) project is described. This multi-institution collaboration aims to produce arcminute-resolution and micro-Kelvin sensitivity maps of the microwave background temperature over 200 square degrees of the sky in three frequency bands. We give a brief overview of the scientific motivations for such a map, followed by a design outline of our six-meter custom telescope, an overview of our proposed bolometer array detector technology, and site considerations and scan strategy. We also describe associated optical and X-ray galaxy cluster surveys. SCIENTIFIC MOTIVATION With results from WMAP in hand, it is clear that the near-term future of microwave background measurements will be primarily a push towards smaller angular scales and polarization. As described at this meeting, many small-scale experiments are currently underway, under construction, or in the planning stage. This paper describes an ambitious proposed collaboration, the Atacama Cosmology Telescope, which aims to combine new bolometer array technology with a custom-designed six-meter telescope to produce an instrument with a notable combination of sensitivity, angular resolution, and control of systematic errors. Current information about the experiment is available at http://www.hep.upenn.edu∼angelica/act/act.html. Before detailing the experimental and observational aspects of this effort, we give a brief summary of the main scientific questions we aim to address, which are covered in more detail elsewhere in these proceedings (see also [1]). At angular scales smaller than around 4 arcminutes, corresponding to multipoles l > 3000, nonlinear contributions begin to dominate the total microwave background temperature anisotropies. The major sources of temperature fluctuations on these scales include the Sunyaev-Zeldovich effect, the Ostriker-Vishniac effect, and gravitational lensing. All of these effects arise both from individual clusters of galaxies and from the large-scale matter distribution. The largest amplitude signal will come from the thermal SZ galaxy cluster distortions. We expect to compile a large catalog of clusters selected by their SZ signals, which provides a cleaner cluster selection criterion than flux-limited optical or X-ray surveys. This catalog will be well-suited to measuring the cluster number density as a function of mass and redshift, which is a sensitive probe of the growth rate of structure since redshift z = 1; in turn, the structure growth rate constrains dark energy and neutrino masses. We also expect to place significant constraints on cluster masses and peculiar velocities via their kinematic SZ and gravitational lensing signatures. For the diffuse signals, we aim to detect gravitational lensing of the microwave background and construct a projected mass map on scales of 15 arcminutes. We expect to have sufficient sensitivity to detect the Ostriker-Vishniac effect, which is sensitive to the redshift and spatial variation of reionization, and the Rees-Sciama effect, which is sensitive to the non-linear evolution of gravitational potentials. ACT will provide a measurement of the power spectrum on all scales from l = 200 to l = 10000, probing the primordial fluctuation spectrum for departures from a power law or for features; either could arise from non-minimal models of inflation. Having a single experiment which probes a wide range of angular scales with good control of systematics is crucial for uncovering small departures from perfect power law primordial spectra. ACT’s scan strategy results in a significant area of the survey being in the galactic plane. A variety of interesting topics in galactic astrophysics can be addressed with such a map, particularly properties and distribution of dust.