Committee: Alexandra Gade (Chairperson), Sean Liddick (Thesis Director), Tyce DeYoung, Alyssa Gaiser, Heiko Hergert

Current observations of binary black-hole (BBH) merger events show support for a feature in the primary BH-mass distribution at ∼ 35 M , previously interpreted as a signature of pulsational pair-instability (PPISN) supernovae. Such supernovae are expected to map a wide range of pre-supernova carbon-oxygen (CO) core masses to a narrow range of BH masses, producing a peak in the BH mass distribution. However, recent numerical simulations place the mass location of this peak above 50 M. Motivated by uncertainties in the progenitor’s evolution and explosion mechanism, we explore how modifying the distribution of BH masses resulting from PPISN affects the populations of gravitational-wave (GW) and electromagnetic (EM) transients. To this end, we simulate populations of isolated BBH systems and combine them with cosmic star-formation rates. Our results are the first cosmological BBH-merger predictions made using the binary_c rapid population synthesis framework. We find that our fiducial model does not match the observed GW peak. We can only explain the 35 M peak with PPISNe by shifting the expected CO core-mass range for PPISN downwards by ∼ 15 Msun. Apart from being in tension with state-of-the art stellar models, we also find that this is likely in tension with the observed rate of hydrogen-less super-luminous supernovae. Conversely, shifting the mass range upward, based on recent stellar models, leads to a predicted third peak in the BH mass function at ∼ 64 Msun. The shape and height of this peak is sensitive details of the PPISN mechanism, nuclear physics and the maximum stellar mass, and, as such, uncovering a peak feature here may inform us about these properties. This upward shift is supported by recent detailed stellar evolution models and softens the tension with the observed transient rates. Upcoming observation runs will increase detections in the high-mass range and may indeed uncover this newly proposed peak. As such, they will improve our understanding of the details of the PPISN mechanism.

Committee: Jaideep Singh (Chairperson), Alyssa Gaiser, Hironori Iwasaki, Ryan LaRose, Johannes Pollanen

Committee: Christopher Wrede (Chairperson), Sean Couch, Stephen Hsu, Witold Nazarewicz, Artemis Spyrou

The foundations of physics are typically associated with interpretations of quantum mechanics and the search for a theory of everything. Working on our project Assumptions of Physics, which aims to find a minimal set of physical starting points from which the laws of physics can be rederived, gave us a different perspective. In this talk we will discuss seven ideas that most physicists take for granted without realizing and which, as it turns out, steer us in the wrong direction. Surpassing these misconceptions leads us to a new understanding of what the laws of physics are, the relationship between physics and mathematics, and what the ultimate goal of the foundations of physics should be. Christine A. Aidala and Gabriele Carcassi Physics Department, University of Michigan -------------------- Christine Aidala received her Bachelor's degree in Physics and in Music from Yale University in 1999 and her Ph.D. in Physics from Columbia University in 2005. After a postdoctoral position with the University of Massachusetts, Amherst, she was a Frederick Reines Distinguished Postdoctoral Fellow and then staff scientist with Los Alamos National Lab until joining the faculty at the University of Michigan in 2012. In addition to her theoretical work on the foundations of physics, she performs experimental research in high-energy nuclear physics, studying the internal structure of the proton. Gabriele Carcassi received his degree in Informatics Engineering in 2000 from the Politecnico di Milano, Italy. He worked for twenty years in software and computing in support of large scale accelerator facilities and experiments, from control systems and databases, to wide area network data management and security. For the past decade he has had an increasing focus on foundational issues in physics, which he now pursues full time. He has been a researcher in the Physics Department at the University of Michigan since 2012. He is particularly interested in the boundaries between foundations of physics, foundations of mathematics and philosophy of science.

The study of the Milky Way's (MW) stellar halo offers a window into the cosmic past because halo stars tend to be very old, with some born in the MW itself (in-situ halo) and others born in ancient satellite galaxies which have long since merged with the MW or been torn apart by tidal forces (accreted halo). In this talk, I will present existing techniques for recovering such substructures in the halo with a primary focus on chemical tagging. This method, akin to a "DNA fingerprint" for stars, involves matching the unique chemical abundance patterns found in stars to their original birth place. These patterns are imprinted by nucleosynthesis processes that occurred early in the galaxy's history, leaving behind distinct elemental signatures. By outlining the key elements and nucleosynthesis channels that contribute to elemental diversity, I will examine how successful chemical hyperplanes, such as [α/Fe] – [Fe/H], and [Mg/Mn] – [Al/Fe], differentiate between the thick and thin discs and accreted halo. Additionally, I will discuss the promise of neutron-capture abundances for distinguishing between the various metal-poor components of the MW, leveraging the extensive chemodynamical data provided by GALAH DR3. By constructing a chemical hyperplane using Principal Component Analysis (PCA), we utilise the information of 19 different abundances to determine which of these contribute the most to the chemical variance. We then employ a Gaussian mixture method called Extreme Deconvolution (XD) to disentangle chemically distinct groups in the resultant PCA-transformed plane, probing the metal-poor tail of the disc as well as the remnants of Gaia-Enceladus-Sausage, a dwarf galaxy which merged approximately 8-11 Gyr ago. Finally, I will discuss which novel abundance ratios may offer a physical interpretability when distinguishing stellar components which PCA lacks.