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Full-Text Articles in Physics
Fisher Formalism For Anisotropic Gravitational-Wave Background Searches With Pulsar Timing Arrays, Y. Ali-Haïmoud, Tristan L. Smith, C. M. F. Mingarelli
Fisher Formalism For Anisotropic Gravitational-Wave Background Searches With Pulsar Timing Arrays, Y. Ali-Haïmoud, Tristan L. Smith, C. M. F. Mingarelli
Physics & Astronomy Faculty Works
Pulsar timing arrays (PTAs) are currently the only experiments directly sensitive to gravitational waves with decade-long periods. Within the next five to ten years, PTAs are expected to detect the stochastic gravitational-wave background (SGWB) collectively sourced by inspiraling supermassive black hole binaries. It is expected that this background is mostly isotropic, and current searches focus on the monopole part of the SGWB. Looking ahead, anisotropies in the SGWB may provide a trove of additional information on both known and unknown astrophysical and cosmological sources. In this paper, we build a simple yet realistic Fisher formalism for anisotropic SGWB searches with …
Robustness Of Baryon Acoustic Oscillation Constraints For Early-Universe Modifications Of Λcdm Cosmology, J. L. Bernal, Tristan L. Smith, K. K. Boddy, M. Kamionkowski
Robustness Of Baryon Acoustic Oscillation Constraints For Early-Universe Modifications Of Λcdm Cosmology, J. L. Bernal, Tristan L. Smith, K. K. Boddy, M. Kamionkowski
Physics & Astronomy Faculty Works
Baryon acoustic oscillations (BAO) provide a robust standard ruler and can be used to constrain the expansion history of the Universe at low redshift. Standard BAO analyses return a model-independent measurement of the expansion rate and the comoving angular diameter distance as a function of redshift, normalized by the sound horizon at radiation drag. However, this methodology relies on anisotropic distance distortions of a fixed, precomputed template (obtained in a given fiducial cosmology) in order to fit the observations. Therefore, it may be possible that extensions to the consensus ΛCDM add contributions to the BAO feature that cannot be captured …
The Genetic Algorithm: Using Biology To Compute Liquid Crystal Director Configurations, Shouzhuo Yang , '23, Peter J. Collings
The Genetic Algorithm: Using Biology To Compute Liquid Crystal Director Configurations, Shouzhuo Yang , '23, Peter J. Collings
Physics & Astronomy Faculty Works
The genetic algorithm is an optimization routine for finding the solution to a problem that requires a function to be minimized. It accomplishes this by creating a population of solutions and then producing “offspring” solutions from this population by combining two “parental” solutions in much the way that the DNA of biological parents is combined in the DNA of offspring. Strengths of the algorithm include that it is simple to implement, no trial solution is required, and the results are fairly accurate. Weaknesses include its slow computational speed and its tendency to find a local minimum that does not represent …
Temperature-Dependent Phonon Lifetimes And Thermal Conductivity Of Silicon By Inelastic Neutron Scattering And Ab Initio Calculations, D. S. Kim, O. Hellman, N. Shulumba, C. N. Saunders, J. Y. Y. Lin, Hillary L. Smith, J. E. Herriman, J. L. Niedziela, D. L. Abernathy, C. W. Li, B. Fultz
Temperature-Dependent Phonon Lifetimes And Thermal Conductivity Of Silicon By Inelastic Neutron Scattering And Ab Initio Calculations, D. S. Kim, O. Hellman, N. Shulumba, C. N. Saunders, J. Y. Y. Lin, Hillary L. Smith, J. E. Herriman, J. L. Niedziela, D. L. Abernathy, C. W. Li, B. Fultz
Physics & Astronomy Faculty Works
Inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors →Q and frequencies ω in the first Brillouin zone. Thermal broadenings of the phonon peaks were obtained by fitting and by calculating with an iterative ab initio method that uses thermal atom displacements on an ensemble of superlattices. Agreement between the calculated and experimental broadenings was good, with possible discrepancies at the highest temperatures. Distributions of phonon widths versus phonon energy had similar shapes for computation and experiment. …
Thermodynamic Stability And Contributions To The Gibbs Free Energy Of Nanocrystalline Ni₃ Fe, S. H. Lohaus, M. B. Johnson, P. F. Ahnn, C. N. Saunders, Hillary L. Smith, M. A. White, B. Fultz
Thermodynamic Stability And Contributions To The Gibbs Free Energy Of Nanocrystalline Ni₃ Fe, S. H. Lohaus, M. B. Johnson, P. F. Ahnn, C. N. Saunders, Hillary L. Smith, M. A. White, B. Fultz
Physics & Astronomy Faculty Works
The heat capacities of nanocrystalline Ni₃ Fe and control materials with larger crystallites were measured from 0.4–300K. The heat capacities were integrated to obtain the enthalpy, entropy, and Gibbs free energy and to quantify how these thermodynamic functions are altered by nanocrystallinity. From the phonon density of states (DOS) measured by inelastic neutron scattering, we find that the Gibbs free energy is dominated by phonons and that the larger heat capacity of the nanomaterial below 100 K is attributable to its enhanced phonon DOS at low energies. Besides electronic and magnetic contributions, the nanocrystalline material has an additional contribution at …
Teaching The Electrical Origins Of The Electrocardiogram: An Introductory Physics Laboratory For Life Science Students, Catherine Hirshfeld Crouch, J. W. Hirshfeld Jr.
Teaching The Electrical Origins Of The Electrocardiogram: An Introductory Physics Laboratory For Life Science Students, Catherine Hirshfeld Crouch, J. W. Hirshfeld Jr.
Physics & Astronomy Faculty Works
We present the design, pedagogical logic, and assessment of a laboratory and supporting materials that integrate a clinical academic cardiologist's understanding of the origins of the electrocardiogram (ECG) with a physics educator's insights into how to teach the underlying physics at the introductory level to life science students. In this article, we explain the choices made throughout the design process, connect a more advanced treatment of the physics to our approach, and present our assessment of the curriculum. Before the laboratory, students learn the cellular origins of the electric dipole potential produced by the heart on the body's surface, including …
Structured Randomness: Jamming Of Soft Discs And Pins, Prairie Wentworth-Nice , '18, S. A. Ridout, Brian Jenike , '20, Ari Liloia , '21, Amy Lisa Graves
Structured Randomness: Jamming Of Soft Discs And Pins, Prairie Wentworth-Nice , '18, S. A. Ridout, Brian Jenike , '20, Ari Liloia , '21, Amy Lisa Graves
Physics & Astronomy Faculty Works
Simulations are used to find the zero temperature jamming threshold, ϕj, for soft, bidisperse disks in the presence of small fixed particles, or “pins”, arranged in a lattice. The presence of pins leads, as one expects, to a decrease in ϕj. Structural properties of the system near the jamming threshold are calculated as a function of the pin density. While the correlation length exponent remains ν = 1/2 at low pin densities, the system is mechanically stable with more bonds, yet fewer contacts than the Maxwell criterion implies in the absence of pins. In addition, as …
Oscillating Scalar Fields And The Hubble Tension: A Resolution With Novel Signatures, Tristan L. Smith, V. Poulin, M. A. Amin
Oscillating Scalar Fields And The Hubble Tension: A Resolution With Novel Signatures, Tristan L. Smith, V. Poulin, M. A. Amin
Physics & Astronomy Faculty Works
We present a detailed investigation of a subdominant oscillating scalar field [“early dark energy” (EDE)] in the context of resolving the Hubble tension. Consistent with earlier work, but without relying on fluid approximations, we find that a scalar field frozen due to Hubble friction until log10(zc)∼3.5, reaching ρEDE(zc)/ρtot∼10% and diluting faster than matter afterwards, can bring cosmic microwave background (CMB), baryonic acoustic oscillations, supernovae luminosity distances, and the late-time estimate of the Hubble constant from the SH0ES Collaboration into agreement. A scalar field potential that scales as V(ϕ)∝ϕ2n with 2≲n≲3.4 around the minimum is preferred at the 68% confidence level, …
Taylor State Merging At Ssx: Experiment And Simulation, Michael R. Brown, Kaitlin Gelber , '20, Matiwos Mebratu , '21
Taylor State Merging At Ssx: Experiment And Simulation, Michael R. Brown, Kaitlin Gelber , '20, Matiwos Mebratu , '21
Physics & Astronomy Faculty Works
We describe experiments and simulations of dynamical merging with two Taylor state plasmas in a Swarthmore Spheromak Experiment (SSX) device. Taylor states are formed by magnetized plasma guns at opposite ends of the device. We performed experiments with Taylor states of both senses of magnetic helicity (right-handed twist or left-handed twist). We present results of both counter-helicity merging (one side left-handed, the other right-handed) and co-helicity merging (both sides left-handed). Experiments show significant ion heating, consistent with magnetic reconnection. We suggest that the merged, warm state could be a suitable target for future magneto-inertial fusion experiments. Magnetohydrodynamic simulations of these …
How Sound Are Our Ultralight Axion Approximations?, J. Cookmeyer, D. Grin, Tristan L. Smith
How Sound Are Our Ultralight Axion Approximations?, J. Cookmeyer, D. Grin, Tristan L. Smith
Physics & Astronomy Faculty Works
Ultralight axions (ULAs) are a promising dark-matter candidate. ULAs may have implications for small-scale challenges to the ΛCDM model and arise in string scenarios. ULAs are already constrained by cosmic microwave background (CMB) experiments and large-scale structure surveys, and will be probed with much greater sensitivity by future efforts. It is challenging to compute observables in ULA scenarios with sufficient speed and accuracy for cosmological data analysis because the ULA field oscillates rapidly. In past work, an effective fluid approximation has been used to make these computations feasible. Here this approximation is tested against an exact solution of the ULA …