Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

The high and dry desert of the Chilean Andes is a magnificent site from which to observe the universe.  The Atacama Cosmology Telescope (ACT) made measurements of the cosmic microwave background (CMB) from there for fifteen years, with a series of three cameras.  ACT was designed to complement Planck, the latest and most sensitive CMB satellite.  To compete with the powerful data set provided from a  major space mission requires years of observations from bespoke arrays of  thousands of detectors; for ACT these were cooled to 100 mK and  deployed on the eponymous special-purpose 6 m telescope,  on a plateau at 5190 m.  The last major data release from ACT is DR6.  Analysis of the DR6 CMB power spectra complemented by Planck CMB data leads to the most precise LCDM  model of the universe yet obtained.  With or without combining with BAO data, the resulting LCDM model does not predict the H0 estimated from SNe calibrated with Cepheids.  The data set allows improved constraints on a range of extensions to the LCDM model as well.   How the ACT instrument worked and how DR6 informs our understanding of the universe will be discussed, capped with a quick look at future instrumentation.