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Sunyaev–Zel'dovich Array
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<p>The Sunyaev–Zeldovich Array (SZA) in California is an array of eight 3.5 meter telescopes that was operated as part of the now-closed <a href="/facts/Combined_Array_for_Research_in_Millimeter-wave_Astronomy/bTz78Bqx">Combined Array for Research in Millimeter-wave Astronomy</a> (CARMA). Its initial goals were to survey the <a href="/facts/Cosmic_microwave_background/ThB2HkLS">cosmic microwave background</a> (CMB) in order to measure its fine-scale <a href="/facts/Anisotropy/YMqTQWVB">anisotropies</a> and to find clusters of galaxies. The survey was completed in 2007, and the array is now used primarily to characterize clusters via the <a href="/facts/Sunyaev%25E2%2580%2593Zeldovich_effect/5o66v730">Sunyaev–Zeldovich effect</a>. Observations commenced at the SZA in April 2005. </p><p>One of the most important developments of the last few years has been the detection, through observations of the CMB and <a href="/facts/Supernova/Pfo2J23l">supernova</a> studies, of a form of energy that is accelerating the expansion of the universe. Dubbed <a href="/facts/Dark_energy/6bWIYewA">dark energy</a> by analogy with dark matter, it is believed to account for roughly 70% of the universe's energy content. </p><p>While dark energy cannot be observed directly, its basic properties can be inferred from its effect on structure formation in the universe. Just as an ecologist can learn about the food supply by studying how animal populations evolve with time, physicists can learn about dark energy by studying the population statistics of the universe's inhabitants—in this case, <a href="/facts/Galaxy_clusters/BhIEgbx8">galaxy clusters</a>. </p><p>The SZA gets its name from the means by which it measures galaxy clusters: the scattering of CMB light as it passes through the hot ionized cluster gas, known as the Sunyaev–Zeldovich effect (SZ effect). In short, the CMB is used as a backlight against which galaxy clusters can be seen by the shadows they cast. Since the SZA sees the shadow rather than the light emitted by the cluster itself, it can be used to measure sufficiently large clusters nearly independently of their <a href="/facts/Redshift/Qfl4VjDF">redshift</a>, back to the epoch at which clusters first began to form. </p>