Browsing by Department "University of Illinois at Urbana-Champaign"

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    Bayesian structure learning for dynamic brain connectivity
    (PMLR, 2018-01-01) Andersen, Michael Riis; Winther, Ole; Hansen, Lars Kai; Poldrack, Russell; Koyejo, Oluwasanmi; Department of Computer Science; Danmarks Tekniske Universitet; Stanford University; University of Illinois at Urbana-Champaign
    Human brain activity as measured by fMRI exhibits strong correlations between brain regions which are believed to vary over time. Importantly, dynamic connectivity has been linked to individual differences in physiology, psychology and behavior, and has shown promise as a biomarker for disease. The state of the art in computational neuroimaging is to estimate the brain networks as relatively short sliding window covariance matrices, which leads to high variance estimates, thereby resulting in high overall error. This manuscript proposes a novel Bayesian model for dynamic brain connectivity. Motivated by the underlying neuroscience, the model estimates covariances which vary smoothly over time, with an instantaneous decomposition into a collection of spatially sparse components – resulting in parsimonious and highly interpretable estimates of dynamic brain connectivity. Simulated results are presented to illustrate the performance of the model even when it is mis-specified. For real brain imaging data with unknown ground truth, in addition to qualitative evaluation, we devise a simple classification task which suggests that the estimated brain networks better capture the underlying structure.
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    Can CMB surveys help the AGN community?
    (2017-08-30) Partridge, Bruce; Bonavera, Laura; López-Caniego, Marcos; Datta, Rahul; Gonzalez-Nuevo, Joaquin; Gralla, Megan; Herranz, Diego; Lähteenmäki, Anne; Mocanu, Laura; Prince, Heather; Vieira, Joaquin; Whitehorn, Nathan; Zhang, Lizhong; Haverford College; University of Oviedo; European Space Astronomy Centre; NASA Goddard Space Flight Center; University of Arizona; Universidad de Cantabria; Metsähovi Radio Observatory; University of Chicago; Princeton University; University of Illinois at Urbana-Champaign; University of California Los Angeles; Department of Electronics and Nanoengineering
    Contemporary projects to measure anisotropies in the cosmic microwave background (CMB) are now detecting hundreds to thousands of extragalactic radio sources, most of them blazars. As a member of a group of CMB scientists involved in the construction of catalogues of such sources and their analysis, I wish to point out the potential value of CMB surveys to studies of AGN jets and their polarization. Current CMB projects, for instance, reach mJy sensitivity, offer wide sky coverage, are "blind" and generally of uniform sensitivity across the sky (hence useful statistically), make essentially simultaneous multi-frequency observations at frequencies from 30 to 857 GHz, routinely offer repeated observations of sources with interesting cadences and now generally provide polarization measurements. The aim here is not to analyze in any depth the AGN science already derived from such projects, but rather to heighten awareness of their promise for the AGN community.
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    Customer Liquidity Provision: Implications for Corporate Bond Transaction Costs
    (Institute for Operations Research and the Management Sciences (INFORMS), 2024-01) Choi, Jaewon; Huh, Yesol; Shin, Sean; University of Illinois at Urbana-Champaign; Federal Reserve Board; Department of Finance
    The convention when calculating corporate bond trading costs is to estimate bid–ask spreads that customers pay, implicitly assuming that dealers always provide liquidity to customers. We show that, contrary to this assumption, customers increasingly provide liquidity following the adoption of post-2008 banking regulations, and thus, conventional bid–ask spread measures underestimate the cost of dealers’ liquidity provision. Among large trades wherein dealers use inventory capacity, customers pay 40%–60% wider spreads than before the crisis. Customers’ balance-sheet capacity and their trading relationships with dealers are important determinants of customer liquidity provision.
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    Mechanical Modelling of Asphalt Concrete Using Grid Division
    (Taylor and Francis Ltd., 2020-07-02) Castillo, Daniel; Al-Qadi, Imad; Mineral Based Materials and Mechanics; University of Illinois at Urbana-Champaign; Department of Civil Engineering
    In this paper, a simple method is introduced for the computational modelling of multiphase materials, and for the approximation of their mechanical response. The two-dimensional microstructures of six asphalt concrete specimens are selected; three of the specimens have ‘low’-, and three have ‘high’ aggregate fraction. A grid is used to divide the surface of each microstructure into square cells. The procedure of grid division is applied from 1 up to 100 divisions per side (i.e., up to 10,000 cells in the grid-divided specimen). To obtain an approximation of the mechanical response of the microstructure, the properties of the cells are estimated using three simple interpolation rules between the properties of the two phases, i.e. asphalt matrix and rock aggregates. It is found that the interpolation rules can yield reasonably representative results depending on the aggregate fraction of the microstructures and the number of divisions/cells in the grid. The grid-divided specimens allow approximating the overallmechanical response of the microstructures, and characteristics such as strain concentrations, overall deformations, and resulting force.
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    Planck 2015 results
    (2016-10-01) Ade, P. A R; Aghanim, N.; Arnaud, M.; Ashdown, M.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Bartolo, N.; Battaner, E.; Battye, R.; Benabed, K.; Benoît, A.; Benoit-Lévy, A.; Bernard, J. P.; Bersanelli, M.; Bielewicz, P.; Bock, J. J.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bucher, M.; Burigana, C.; Butler, R. C.; Calabrese, E.; Cardoso, J. F.; Catalano, A.; Challinor, A.; Chamballu, A.; Chary, R. R.; Chiang, H. C.; Chluba, J.; Christensen, P. R.; Church, S.; Clements, D. L.; Colombi, S.; Colombo, L. P L; Combet, C.; Coulais, A.; Crill, B. P.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; De Bernardis, P.; De Rosa, A.; De Zotti, G.; Delabrouille, J.; Désert, F. X.; Di Valentino, E.; Dickinson, C.; Diego, J. M.; Dolag, K.; Dole, H.; Donzelli, S.; Doré, O.; Douspis, M.; Ducout, A.; Dunkley, J.; Dupac, X.; Efstathiou, G.; Elsner, F.; Enßlin, T. A.; Eriksen, H. K.; Farhang, M.; Fergusson, J.; Finelli, F.; Forni, O.; Frailis, M.; Fraisse, A. A.; Franceschi, E.; Frejsel, A.; Galeotta, S.; Galli, S.; Ganga, K.; Gauthier, C.; Gerbino, M.; Ghosh, T.; Giard, M.; Giraud-Héraud, Y.; Giusarma, E.; Gjerløw, E.; González-Nuevo, J.; Górski, K. M.; Gratton, S.; Gregorio, A.; Gruppuso, A.; Gudmundsson, J. E.; Hamann, J.; Hansen, F. K.; Hanson, D.; Harrison, D. L.; Helou, G.; Henrot-Versillé, S.; Hernández-Monteagudo, C.; Herranz, D.; Hildebrandt, S. R.; Hivon, E.; Hobson, M.; Holmes, W. A.; Hornstrup, A.; Hovest, W.; Huang, Z.; Huffenberger, K. M.; Hurier, G.; Jaffe, A. H.; Jaffe, T. R.; Jones, W. C.; Juvela, M.; Keihänen, E.; Keskitalo, R.; Kisner, T. S.; Kneissl, R.; Knoche, J.; Knox, L.; Kunz, M.; Kurki-Suonio, H.; Lagache, G.; Lähteenmäki, A.; Lamarre, J. M.; Lasenby, A.; Lattanzi, M.; Lawrence, C. R.; Leahy, J. P.; Leonardi, R.; Lesgourgues, J.; Levrier, F.; Lewis, A.; Liguori, M.; Lilje, P. B.; Linden-Vørnle, M.; López-Caniego, M.; Lubin, P. M.; Maciás-Pérez, J. F.; Maggio, G.; Maino, D.; Mandolesi, N.; Mangilli, A.; Marchini, A.; Maris, M.; Martin, P. G.; Martinelli, M.; Martínez-González, E.; Masi, S.; Matarrese, S.; Mcgehee, P.; Meinhold, P. R.; Melchiorri, A.; Melin, J. B.; Mendes, L.; Mennella, A.; Migliaccio, M.; Millea, M.; Mitra, S.; Miville-Deschênes, M. A.; Moneti, A.; Montier, L.; Morgante, G.; Mortlock, D.; Moss, A.; Munshi, D.; Murphy, J. A.; Naselsky, P.; Nati, F.; Natoli, P.; Netterfield, C. B.; Nørgaard-Nielsen, H. U.; Noviello, F.; Novikov, D.; Novikov, I.; Oxborrow, C. A.; Paci, F.; Pagano, L.; Pajot, F.; Paladini, R.; Paoletti, D.; Partridge, B.; Pasian, F.; Patanchon, G.; Pearson, T. J.; Perdereau, O.; Perotto, L.; Perrotta, F.; Pettorino, V.; Piacentini, F.; Piat, M.; Pierpaoli, E.; Pietrobon, D.; Plaszczynski, S.; Pointecouteau, E.; Polenta, G.; Popa, L.; Pratt, G. W.; Prézeau, G.; Prunet, S.; Puget, J. L.; Rachen, J. P.; Reach, W. T.; Rebolo, R.; Reinecke, M.; Remazeilles, M.; Renault, C.; Renzi, A.; Ristorcelli, I.; Rocha, G.; Rosset, C.; Rossetti, M.; Roudier, G.; Rouillé D'orfeuil, B.; Rowan-Robinson, M.; Rubinõ-Martín, J. A.; Rusholme, B.; Said, N.; Salvatelli, V.; Salvati, L.; Sandri, M.; Santos, D.; Savelainen, M.; Savini, G.; Scott, D.; Seiffert, M. D.; Serra, P.; Shellard, E. P S; Spencer, L. D.; Spinelli, M.; Stolyarov, V.; Stompor, R.; Sudiwala, R.; Sunyaev, R.; Sutton, D.; Suur-Uski, A. S.; Sygnet, J. F.; Tauber, J. A.; Terenzi, L.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Trombetti, T.; Tucci, M.; Tuovinen, J.; Türler, M.; Umana, G.; Valenziano, L.; Väliviita, J.; Van Tent, F.; Vielva, P.; Villa, F.; Wade, L. A.; Wandelt, B. D.; Wehus, I. K.; White, M.; White, S. D M; Wilkinson, A.; Yvon, D.; Zacchei, A.; Zonca, A.; Cardiff University; CNRS/IN2P3; Service d'Astrophysique CEA; Kavli Institute for Cosmology Cambridge; University of Cambridge; International School for Advanced Studies; IRAP; Universite de Toulouse; Instituto de Física de Cantabria (CSIC-Universidad de Cantabria); Jet Propulsion Laboratory, California Institute of Technology; AstroParticule et Cosmologie; Università Degli Studi di Padova; Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy; University of Granada; University of Manchester; UMR7095; CNRS; University College London; INAF/IASF Milano; Università degli Studi di Milano; Nicolaus Copernicus Astronomical Center; California Institute of Technology; University of Toronto; University of California at Berkeley; Lawrence Berkeley National Laboratory; Universite Paris Sorbonne - Paris IV; Institut d 'Astrophysique de Paris; INAF/IASF Bologna; Università di Ferrara; INFN, Sezione di Bologna; University of Oxford; UMR 5141; LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres; Laboratoire AIM, Service d’Astrophysique, DSM\IRFU, CEA\Saclay; Institut d'Astrophysique Spatiale; Princeton University; University of KwaZulu-Natal; Johns Hopkins University; Niels Bohr Institute; Stanford University; Imperial College London; University of Southern California; Universidad de Cantabria; Università La Sapienza; INAF, Osservatorio Astronomico di Padova; UMR 7095; Ludwig Maximilian University of Munich; Max-Planck-Institut für Astrophysik; Institut Universitaire de France; European Space Agcy, European Space Agency, ESAC, Planck Sci Off; University of Oslo; Shahid Beheshti University; Osservatorio Astronomico di Trieste; University of Chicago; National Taiwan University; Stockholms universitet; NORDITA; University of Warsaw; Università Degli Studi di Trieste; Istituto Nazionale di Fisica Nucleare; CERN; University of Sydney; McGill University; Centro de Estudios de la Física del Cosmos de Aragón; Technical University of Denmark; Florida State University; University of Helsinki; European Southern Observatory Santiago; ALMA Santiago Central Offices; University of California; Université de Genève; African Institute for Mathematical Sciences; Helsinki Institute of Physics; Aix Marseille Universite; Department of Radio Science and Engineering; Metsähovi Radio Observatory; INFN, Sezione di Ferrara; Centro de Gestão e Estudos Estratégicos; RWTH Aachen University; University of Sussex; INFN, Sezione di Padova; University of California, Santa Barbara; INAF, Osservatorio Astronomico di Trieste; Universite Paris-Sud; INFN, Sezione di Roma 1; University of Heidelberg; Gran Sasso Science Institute; CEA Saclay, CEA, DSM Irfu SPP; Inter-University Centre for Astronomy and Astrophysics; CNRS Centre National de la Recherche Scientifique; University of Nottingham; National University of Ireland; University of Copenhagen; ASI Science Data Center; RAS - Pn Lebedev Physics Institute; Haverford College; INAF, Osservatorio Astronomico di Roma; Institute for Space Sciences; Université Pierre and Marie Curie; Radboud University Nijmegen; Universities Space Research Association; Instituto Astrofisico de Canarias; CSIC; Universidad de La Laguna; Università di Roma Tor Vergata; Department of Applied Physics; ROTA – Topological superfluids; University of British Columbia; Special Astrophysical Observatory, Russian Academy of Sciences; Kazan Federal University; Space Research Institute, Russian Academy of Sciences; ESTEC - European Space Research and Technology Centre; Università degli Studi e-Campus; Universidad de Oviedo; Trinity College Dublin; INAF, Osservatorio Astrofisico di Catania; University of Illinois at Urbana-Champaign
    This paper presents cosmological results based on full-mission Planck observations of temperature and polarization anisotropies of the cosmic microwave background (CMB) radiation. Our results are in very good agreement with the 2013 analysis of the Planck nominal-mission temperature data, but with increased precision. The temperature and polarization power spectra are consistent with the standard spatially-flat 6-parameter ΛCDM cosmology with a power-law spectrum of adiabatic scalar perturbations (denoted "base ΛCDM" in this paper). From the Planck temperature data combined with Planck lensing, for this cosmology we find a Hubble constant, H0 = (67.8 ± 0.9) km s-1Mpc-1, a matter density parameter Ωm = 0.308 ± 0.012, and a tilted scalar spectral index with ns = 0.968 ± 0.006, consistent with the 2013 analysis. Note that in this abstract we quote 68% confidence limits on measured parameters and 95% upper limits on other parameters. We present the first results of polarization measurements with the Low FrequencyInstrument at large angular scales. Combined with the Planck temperature and lensing data, these measurements give a reionization optical depth of τ = 0.066 ± 0.016, corresponding to a reionization redshift of \hbox{$z-{\rm re}=8.8{+1.7}-{-1.4}$}. These results are consistent with those from WMAP polarization measurements cleaned for dust emission using 353-GHz polarization maps from the High Frequency Instrument. We find no evidence for any departure from base ΛCDM in the neutrino sector of the theory; for example, combining Planck observations with other astrophysical data we find Neff = 3.15 ± 0.23 for the effective number of relativistic degrees of freedom, consistent with the value Neff = 3.046 of the Standard Model of particle physics. The sum of neutrino masses is constrained to â'mν < 0.23 eV. The spatial curvature of our Universe is found to be very close to zero, with | ΩK | < 0.005. Adding a tensor component as a single-parameter extension to base ΛCDM we find an upper limit on the tensor-to-scalar ratio of r0.002< 0.11, consistent with the Planck 2013 results and consistent with the B-mode polarization constraints from a joint analysis of BICEP2, Keck Array, and Planck (BKP) data. Adding the BKP B-mode data to our analysis leads to a tighter constraint of r0.002 < 0.09 and disfavours inflationarymodels with a V(φ) φ2 potential. The addition of Planck polarization data leads to strong constraints on deviations from a purely adiabatic spectrum of fluctuations. We find no evidence for any contribution from isocurvature perturbations or from cosmic defects. Combining Planck data with other astrophysical data, including Type Ia supernovae, the equation of state of dark energy is constrained to w =-1.006 ± 0.045, consistent with the expected value for a cosmological constant. The standard big bang nucleosynthesis predictions for the helium and deuterium abundances for the best-fit Planck base ΛCDM cosmology are in excellent agreement with observations. We also constraints on annihilating dark matter and onpossible deviations from the standard recombination history. In neither case do we find no evidence for new physics. The Planck results for base ΛCDM are in good agreement with baryon acoustic oscillation data and with the JLA sample of Type Ia supernovae. However, as in the 2013 analysis, the amplitude of the fluctuation spectrum is found to be higher than inferred from some analyses of rich cluster counts and weak gravitational lensing. We show that these tensions cannot easily be resolved with simple modifications of the base ΛCDM cosmology. Apart from these tensions, the base ΛCDM cosmology provides an excellent description of the Planck CMB observations and many other astrophysical data sets.
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    Slit-Strip Ising Boundary Conformal Field Theory 1: Discrete and Continuous Function Spaces
    (SPRINGER, 2022-12) Ameen, Taha; Kytölä, Kalle; Park, S. C.; Radnell, David; University of Illinois at Urbana-Champaign; Algebra and Discrete Mathematics; Korea Institute for Advanced Study; Department of Mathematics and Systems Analysis
    This is the first in a series of articles about recovering the full algebraic structure of a boundary conformal field theory (CFT) from the scaling limit of the critical Ising model in slit-strip geometry. Here, we introduce spaces of holomorphic functions in continuum domains as well as corresponding spaces of discrete holomorphic functions in lattice domains. We find distinguished sets of functions characterized by their singular behavior in the three infinite directions in the slit-strip domains, and note in particular that natural subsets of these functions span analogues of Hardy spaces. We prove convergence results of the distinguished discrete holomorphic functions to the continuum ones. In the subsequent articles, the discrete holomorphic functions will be used for the calculation of the Ising model fusion coefficients (as well as for the diagonalization of the Ising transfer matrix), and the convergence of the functions is used to prove the convergence of the fusion coefficients. It will also be shown that the vertex operator algebra of the boundary conformal field theory can be recovered from the limit of the fusion coefficients via geometric transformations involving the distinguished continuum functions.
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    Stabbing Rectangles by Line Segments - How Decomposition Reduces the Shallow-Cell Complexity
    (Schloss Dagstuhl--Leibniz-Zentrum für Informatik, 2018) Chan, Timothy M.; Dijk, Thomas C. van; Fleszar, Krzysztof; Spoerhase, Joachim; Wolff, Alexander; University of Illinois at Urbana-Champaign; University of Würzburg; Max Planck Institute for Informatics; Department of Computer Science
    We initiate the study of the following natural geometric optimization problem. The input is a set of axis-aligned rectangles in the plane. The objective is to find a set of horizontal line segments of minimum total length so that every rectangle is stabbed by some line segment. A line segment stabs a rectangle if it intersects its left and its right boundary. The problem, which we call Stabbing, can be motivated by a resource allocation problem and has applications in geometric network design. To the best of our knowledge, only special cases of this problem have been considered so far. Stabbing is a weighted geometric set cover problem, which we show to be NP-hard. While for general set cover the best possible approximation ratio is (log n), it is an important field in geometric approximation algorithms to obtain better ratios for geometric set cover problems. Chan et al. [SODA’12] generalize earlier results by Varadarajan [STOC’10] to obtain sub-logarithmic performances for a broad class of weighted geometric set cover instances that are characterized by having low shallow-cell complexity. The shallow-cell complexity of Stabbing instances, however, can be high so that a direct application of the framework of Chan et al. gives only logarithmic bounds. We still achieve a constant-factor approximation by decomposing general instances into what we call laminar instances that have low enough complexity. Our decomposition technique yields constant-factor approximations also for the variant where rectangles can be stabbed by horizontal and vertical segments and for two further geometric set cover problems.
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    Superconducting phase transition in inhomogeneous chains of superconducting islands
    (American Physical Society, 2020-10-02) Ilin, Eduard; Burkova, Irina; Song, Xiangyu; Pak, Michael; Golubev, Dmitri S.; Bezryadin, Alexey; University of Illinois at Urbana-Champaign; United States Air Force Institute of Technology; Centre of Excellence in Quantum Technology, QTF; Department of Applied Physics
    We study one-dimensional chains of superconducting islands with a particular emphasis on the regime in which every second island is switched into its normal state, thus forming a superconductor-insulator-normal metal (S-I-N) repetition pattern. As is known since Giaever tunneling experiments, tunneling charge transport between a superconductor and a normal metal becomes exponentially suppressed, and zero-bias resistance diverges, as the temperature is reduced and the energy gap of the superconductor grows larger than the thermal energy. Here we demonstrate that this physical phenomenon strongly impacts transport properties of inhomogeneous superconductors made of weakly coupled islands with fluctuating values of the critical temperature. We observe a nonmonotonous dependence of the chain resistance on both temperature and magnetic field, with a pronounced resistance peak at temperatures at which some but not all islands are superconducting. We explain this phenomenon by the inhomogeneity of the chains, in which neighboring superconducting islands have slightly different critical temperatures. We argue that the Giaever's resistance divergence can also occur in the zero-temperature limit. Such quantum transition can occur if the magnetic field is tuned such that it suppresses superconductivity in the islands with the weaker critical field, while the islands with stronger energy gap remain superconducting. In such a field, the system acts as a chain of S-I-N junctions.
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    Visualizing Devices for Configuring Complex Phenomena in-the-Making
    (The Finnish Society for Science and Technology Studies, 2021-09-15) Karasti, Helena; Botero, Andrea; Saad-Sulonen, Joanna; Baker, Karen; IT University of Copenhagen; Department of Design; University of Illinois at Urbana-Champaign
    STS scholars are engaging in collaborative research in order to study extended socio-technical phenomena. This article participates in discussions on methodography and inventive methods by reflecting on visualizations used both internally by a team of researchers and together with study participants. We describe how these devices for generating and transforming data were brought to our ethnographic inquiry into the formation of research infrastructures which we found to involve unwieldy and evolving phenomena. The visualizations are partial renderings of the object of inquiry, crafted and informed by 'configuration' as a method of assemblage that supports ethnographic study of contemporary socio-technical phenomena. We scrutinize our interdisciplinary bringing together of visualizing devices - timelines, collages, and sketches - and position them in the STS methods toolbox for inquiry and invention. These devices are key to investigating and engaging with the dynamics of configuring infrastructures intended to support scientific knowledge production. We conclude by observing how our three kinds of visualizing devices provide flexibility, comprehension and in(ter)ventive opportunities for study of and engagement with complex phenomena in-the-making.
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    Vortices in trapped superfluid fermi gases
    (2001-09-03) Rodriguez, M.; Paraoanu, G. S.; Törmä, P.; University of Illinois at Urbana-Champaign; Department of Neuroscience and Biomedical Engineering
    The Ginzburg-Landau equation in a trapped geometry was used to define analytical estimates for the basic quantities describing a trapped superfluid Fermi gas. A striking difference to metallic superconductors was found in the temperature and system parameter dependence of the vortex core size/healing length. The results indicate that the effect of the confining geometry is essential for mesoscopic fermionic superfluids.