Colloquia This Term

We utilize transport systems daily to commute, e.g. via road networks, or bring energy to our houses through the power grid. Our body needs transport networks, such as the lymphatic, arterial or venous system, to distribute nutrients and remove waste. If the transported quantity is information, for example carried by an electrical signal, then even the internet and the brain can be thought of as members of this broad class of webs. Despite our daily exposure to transport networks, their function and physics can still surprise us. This is exemplified by the Braess paradox, where the addition of an extra road in a network worsens rather than improves traffic contrary to a naïve prediction.

In this talk we will explore two cases that highlight the importance of time in load transmission in transport networks. In the first problem, we will discuss how short timescale dynamics in the flow alters the topology of the network in longer timescales, and shapes its morphology. We will first present the system of phenomenological adaptation equations that govern the structural evolution of vascular networks. We will then demonstrate how implicit of explicit dynamics in the boundary conditions (the power supply, or heart) can drastically alter the network topology, and discuss the implications for the development and function of human circulation. Moving to a larger system, will provide evidence that the similar dynamical developmental rules to the ones that are thought to control vascular remodeling in humans also shape tidal delta geomorphology. 

In the second problem, we consider stochastic transport in geometrically embedded graphs. Intuition suggests that providing a shortcut between a pair of nodes improves the time it takes to randomly explore the entire graph. Counterintuitively, we find a Braess' paradox analog. For regular diffusion, shortcuts can worsen the overall search efficiency of the network, although they bridge topologically distant nodes. We propose an optimization scheme under which each edge adapts its conductivity to minimize the graph's search time. The optimization reveals a relationship between the structure and diffusion exponent and a crossover from dense to sparse graphs as the exponent increases.

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The Electron-Ion Collider (EIC) is a pioneering new particle accelerator that will be built on the current site of the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. It will provide high energy collisions of polarized electrons with polarized protons and ions, allowing for experiments that probe the nature of strong interactions to unprecedented precision. The EIC Project has grown and evolved rapidly since the official launch by the U.S. Department of Energy in 2020. This talk will discuss the primary physics themes driving the EIC effort, the recent milestones achieved by the project and the efforts to establish two complementary detectors at adjacent interaction regions. 

Olivia Mostow:  The Core-Cusp Problem in Low-Mass Galaxies: Is One Burst Enough? (15 minutes)

We present a novel method for assessing the ability of a single burst of star formation to transform dark matter cusps into cores in low-mass galaxies. Following the approach of Rose et al. 2022, we manually add a contribution to the potential that accounts for a centrally concentrated baryon component within an otherwise dark matter only simulation. This approach allows us to maintain control over how and when these bursts occur. We demonstrate that this method can reproduce the established result of core formation for systems that undergo multiple episodes of bursty outflows. In contrast, we find that equivalent models that undergo only a single (or small number) of burst episodes do not form cores with the same efficacy. This is important because many low-mass galaxies in the local universe are observed to have tightly constrained star formation histories that are best described by a single, early burst of star formation. Using a suite of cosmological, zoom-in simulations, we identify the regimes in which single bursts can and cannot form a cored density profile, and therefore, can or cannot resolve the core-cusp problem.

Sam Crowe:  Unveiling Massive Star Formation: Near-Infrared Observations of Protostellar Jets (15 Minutes)

Massive stars are significant throughout the universe, as they impact their surroundings from the early stages of their formation until they die in the form of supernova. Observations in the near-infrared (NIR) of the bright and large-scale (~parsec) jets that young stars ubiquitously produce during their formation process can place important constraints on the phenomenon of massive star formation. Here, I present a detailed NIR view of two massive star-forming complexes at opposite ends of the sky, AFGL 5180 and Sagittarius C, utilizing extremely high-resolution imaging from the Large Binocular Telescope, Hubble Space Telescope, and James Webb Space Telescope. In AFGL 5180, the data reveal several multidirectional outflows indicative of highly clustered star formation, confirmed by the detection of over a dozen compact sub-millimeter sources using data from the Atacama Large Millimeter/Submillimeter Array (ALMA). By sampling the number density of young stellar objects in the vicinity of the central massive (~12 Msun) protostar, and comparing with recent numerical simulations, we present a novel method for directly distinguishing between theories of massive star formation in situ. Conversely, in Sagittarius C, located in the turbulent and chaotic center of our Milky Way, we find relatively ordered and isolated massive star formation, evidenced by collimated and undisturbed NIR jets. We report the discovery of a new star-forming region ~1 arcminute to the west of Sagittarius C, hosting two prominent bow shocks visible in the NIR imaging, and characterize the most luminous protostar in each neighboring complex via ancillary ALMA data and Spectral Energy Distribution fitting, shedding light on how the process of massive star formation is unfolding in this extreme region.

Alex Rosenthal:  Mass-Loss Rates for Massive Stars from Stellar Bowshocks (15 Minutes)

Massive stars lose a significant portion of their mass through stellar winds over the course of their lifetime, and understanding the rate of mass-loss is critical for understanding stellar evolution and compact object genesis. Traditional methods of determining mass-loss rates rely on UV observations and parameterizing a “clumping” factor, which varies significantly and results in a two-order-of-magnitude difference between prediction and observation for stars with weak winds. We intend to address this “weak-wind problem” using a novel method to measure mass-loss rates of massive stars powering stellar bowshocks using optical spectroscopy of the central stars, far infrared measurements of the bowshock nebulae, and space velocities calculated from GAIA DR3 proper motions. This method utilizes the geometry of the bowshock and the principle of balancing the momentum flux between stellar winds and ambient interstellar material to make a mass-loss rate determination. We observed late-O and early-B type stars with bowshocks with the Apache Point Observatory 3.5m telescope with the KOSMOS long-slit spectrograph and the Wyoming Infrared Observatory’s 2.3m telescope with an optical spectrograph. We used the emcee package in Python and interpolated between models from the PoWR OB-I grid to fit their spectra to find temperatures and surface gravities. We found that our sample spanned a range of stellar parameters, with temperatures varying from 16,000-38,000 K and the log of surface gravity ranging from 2.8-4.1 dex. Using these parameters and photometric data, we calculated predicted mass-loss rates. This work is supported by the National Science Foundation under REU grant AST 1852289

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We report on a precise measurement of the electron-antineutrino angular correlation (the “a” coefficient) in free neutron beta-decay. The “a” coefficient is inferred from the recoil energy spectrum of the protons which are detected in the aSPECT spectrometer. The measurement is substantially more precise than what has been achieved previously, and its implications will be discussed. An alternative analysis is presented that allows for Beyond Standard Model Physics, specifically a non-zero value for the Fierz term “b”. The result constitutes the most precise determination of “b” in free neutron beta decay, and is consistent with the Standard Model prediction. In the Standard Model, however, the aSPECT result for “a” is inconsistent by about 3 sigma with the value for the beta asymmetry “A” recommended by the Particle Data Group. A combined analysis of the results from PERKEO III, which dominates the beta asymmetry average, and aSPECT, now allowing for a non-zero Fierz term, reconciles the two. It finds a more precise value for the Fierz term, consistent with the one obtained from aSPECT data alone, but about 3 sigma away from the Standard Model value, and suggests scalar and tensor interactions in free neutron beta decay.

The particle nature of dark matter is an outstanding mystery at the intersection of particle physics and cosmology.  The axion, originally proposed to explain a purely particle physics problem, is one of the most compelling candidates for dark matter.  In the last decade, the field of axion searches has undergone a renaissance of new ideas and improvements to experimental sensitivities.  I will give a brief overview of the field of dark matter axion searches and a more detailed look into the leading axion dark matter search, ADMX.

In 1964, when Thorne was a student, there were hints that our universe might have a Warped Side:  Objects and phenomena made from warped space and warped time instead of from matter.  Thorne and his colleagues have spent these past sixty years turning those hints into clear understanding.  They have explored the Warped Side through theory (using mathematics and computer simulations to probe what the laws of physics predict) and through astronomical observations (primarily with gravitational waves). In this lecture he will describe what they have learned about Warped-Side phenomena:  black holes, wormholes, gravitational waves, our universe’s big-bang birth, and the possibility of time travel.

The quantum laws governing atoms and other tiny objects seem to defy common sense, and information encoded in quantum systems has weird properties that baffle our feeble human minds. John Preskill will explain why he loves quantum entanglement, the elusive feature making quantum information fundamentally different from information in the macroscopic world. By exploiting quantum entanglement, quantum computers should be able to solve otherwise intractable problems, with far-reaching applications to cryptology, materials, and fundamental physical science. Preskill is less weird than a quantum computer, and easier to understand.

Inspired by the work of Hermann von Helmholtz, William Thomson proposed in 1867 that atoms could be vortices in the aether. While subsequent experiments put this proposal out of business, the concept of topological solitons as fundamental building blocks or artificial atoms remains enticing. Recent developments since the 1960s have revealed ample evidence that nature offers updated versions of the aether concept. In the realm of quantum magnets, the aether manifests as the vector field of magnetic moments, whose topological solitons can be seen as emergent mesoscale atoms. Analogous to real atoms, these solitons organize into periodic arrays or crystals governed by principles of symmetry, anisotropy, and competing microscopic interactions. These magnetic textures generate an effective magnetic field, coupled to the orbital degrees of freedom of conduction electrons, capable of reaching astronomical magnitudes. We will explore how these topological magnetic structures manifest in real materials and how the quantum mechanical nature of spins can give rise to more intricate skyrmion textures than those observed thus far.

The pyramids of ancient Egypt and of pre-Hispanic Mesoamerica have fascinated people since the cultures that built them vanished into the annals of history.  How were they built?  What were they used for?  Are there unknown internal substructures, perhaps hidden chambers that have yet to be discovered?  Using the detector technology we developed for a particle physics experiment at Fermilab, we intend to perform non-invasive searches for hidden structures at the Great Pyramid of Khufu, in Egypt, and at the Temple of Kukulkán at Chichén Itzá.  The apparatus will detect cosmic-ray muons produced high in the atmosphere that course through the pyramids to produce a tomographic image of their interiors. I will review the status of both projects, describe in detail the technique we intend to use, present recent simulation results and detector prototype results.