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Register a new userScaling pair count to next galaxy surveys
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Added by
Stephane Plaszczynski
Publication date
2020
fields
Physics
and research's language is
English
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The counting of pairs of galaxies or stars according to their distance is at the core of all real-space correlation analyzes performed in astrophysics and cosmology. The next stage upcoming ground (LSST) and space (Euclid) surveys will measure properties of billions of galaxies and tomographic shells will contain hundreds of millions of objects. The combinatorics of the pair count challenges our ability to perform such counting in a minute-scale time which is the order of magnitude useful for optimizing analyses through the intensive use of simulations. The problem is not CPU intensive and is only limited by an efficient access to the data, hence it belongs to the big data category. We use the popular Apache Spark framework to address it and design an efficient high-throughput algorithm to deal with hundreds of millions to billions of input data. To optimize it, we revisit the question of nonhierarchical sphere pixelization based on cube symmetries and develop a new one that we call the Similar Radius Sphere Pixelization (SARSPix) with square-like pixels. It provides the most adapted sphere packing for all distance-related computations. Using LSST-like fast simulations, we compute autocorrelation functions on tomographic bins containing between a hundred million to one billion data points. In all cases we achieve the full construction of a classical pair-distance histogram in about 2 minutes, using a moderate number of worker nodes (16 to 64). This is typically two orders of magnitude higher than what is achieved today and shows the potential of using these new techniques in the field of astronomy on ever-growing datasets. The method presented here is flexible enough to be adapted to any medium size cluster and the software is publicly available from https://github.com/LSSTDESC/SparkCorr.
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In the next decade, new ground-based Cosmic Microwave Background (CMB) experiments such as Simons Observatory (SO), CCAT-prime, and CMB-S4 will increase the number of detectors observing the CMB by an order of magnitude or more, dramatically improving our understanding of cosmology and astrophysics. These projects will deploy receivers with as many as hundreds of thousands of transition edge sensor (TES) bolometers coupled to Superconducting Quantum Interference Device (SQUID)-based readout systems. It is well known that superconducting devices such as TESes and SQUIDs are sensitive to magnetic fields. However, the effects of magnetic fields on TESes are not easily predicted due to the complex behavior of the superconducting transition, which motivates direct measurements of the magnetic sensitivity of these devices. We present comparative four-lead measurements of the critical temperature versus applied magnetic field of AlMn TESes varying in geometry, doping, and leg length, including Advanced ACT (AdvACT) and POLARBEAR-2/Simons Array bolometers. Molybdenum-copper bilayer ACTPol TESes are also tested and are found to be more sensitive to magnetic fields than the AlMn devices. We present an observation of weak-link-like behavior in AlMn TESes at low critical currents. We also compare measurements of magnetic sensitivity for time division multiplexing SQUIDs and frequency division multiplexing microwave rf-SQUIDs. We discuss the implications of our measurements on the magnetic shielding required for future experiments that aim to map the CMB to near-fundamental limits.
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For nearly a century, imaging and spectroscopic surveys of galaxies have given us information about the contents of the universe. We attempt to define the logical endpoint of such surveys by defining not the next galaxy survey, but the final galaxy survey at NIR wavelengths; this would be the galaxy survey that exhausts the information content useful for addressing extant questions. Such a survey would require incredible advances in a number of technologies and the survey details will depend on the as yet poorly constrained properties of the earliest galaxies. Using an exposure time calculator, we define nominal surveys for extracting the useful information for three science cases: dark energy cosmology, galaxy evolution, and supernovae. We define scaling relations that trade off sky background, telescope aperture, and focal plane size to allow for a survey of a given depth over a given area. For optimistic assumptions, a 280m telescope with a marginally resolved focal plane of 20 deg$^2$ operating at L2 could potentially exhaust the cosmological information content of galaxies in a 10 year survey. For galaxy evolution (making use of gravitational lensing to magnify the earliest galaxies) and SN, the same telescope would suffice. We discuss the technological advances needed to complete the last galaxy survey. While the final galaxy survey remains well outside of our technical reach today, we present scaling relations that show how we can progress toward the goal of exhausting the information content encoded in the shapes, positions, and colors of galaxies.
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