08/20/2020 12:00 PM - Challenges to LCDM, and How We Explore Beyond

Angela Chen, Ph.D. Candidate, Department of Physics

LCDM is the most successful cosmology model in the past decades, proven by multiple independent observations including CMB, Supernovae, Large scale structures, etc. LCDM states that our flat universe (mostly) consists of dark matter and dark energy. However, as the precision of cosmological probes increases, some anomalies that cannot be perfectly explained by LCDM start to show up. Among them the most statistically significant one is the 4-5 \sigma discrepancy between early and late universe Hubble constant. In this talk, I will briefly summarize the facts we know about Hubble constant measurements, and present the current discussions in the field on the Hubble tension. After that, I will use my recent effort on resolving this tenison by decaying dark matter cosmology as an example, to demonstrate where such anomalies lead us to and how we explore beyond LCDM.

08/13/2020 12:00 PM - Reduced Order Models using Graph Theoretic Approaches for Physical Systems

Matthew Duschenes, Ph.D. Candidate, Department of Applied Physics/Mechanical Engineering

Physical systems are conventionally studied using experiments, that may be costly, exceedingly difficult, or infeasible. Alternatively, various computational approaches, have proven rigorous and successful, but still suffer from being generally resource intensive and occasionally difficult to interpret. Particularly for classes of systems that can be described by partial differential equations, such as for example multi-component crystalline solids undergoing mechanical deformations, and changes in chemical potential, high-fidelity solutions generally contain up to tens of millions of degrees of freedom. New methods must therefore be developed that represent this information in a lower dimensional space, allowing for more efficient, and intuitive computations.

In this talk, I will be introducing a novel graph theoretic approach for reduced order modelling of a wide class of systems, allowing for more efficient and effective data-driven simulations. Concepts in graph theory will be introduced, including a rigorous non-local discrete calculus, and I will describe how states of a system and their relationships can be represented using this formalism. I will then discuss the numerical methods for computing a reduced order model for quantities of interest, and will show some preliminary results, indicating the validity and possible exciting future applications of this general framework.

08/06/2020 12:00 PM - Branching Rules and LieART 2.0

Robert Saskowski, Ph.D. Pre-Candidate, Department of Physics

There is a natural connection between particle physics and representation theory. In particular, we often embed the Standard Model in a larger gauge group (such as in Grand Unified Theories) and spontaneously break the symmetry back down to the Standard Model at low energies. Particles naturally live in representations of this larger gauge group, and the branching rules of these representations tell us how GUT representations break into Standard Model representations. Hence, the computation of these branching rules tells us much about the physics of these theories.

In this talk, I will first give an introduction to the necessary details about Lie algebras, discuss their internal structure and representations, and explain what precisely we mean by "branching rules" and how one would compute these rules. The second part of the talk will focus on my previous work at Vanderbilt on the LieART (Lie Algebras and Representation Theory) Mathematica program, extending it to compute almost all branching rules.

07/30/2020 12:00 PM - Dark Matter Detection in LZ

Chami Amarasinghe, Ph.D. Candidate, Department of Physics

Dark matter is a mysterious substance that pervades the universe and affects its evolution via gravity. The particulate nature of dark matter is unknown, although several theoretical candidates have been proposed. One such candidate is the Weakly Interacting Massive Particle (WIMP), which several experiments over the last decade have sought to discover. Among the forthcoming generation of these experiments is LUX-ZEPLIN (LZ), an underground tonne-scale detector employing liquid xenon to search for faint dark matter interactions. This talk will go over the operating principle of the detector, its sensitivity to physics beyond WIMPs, hardware challenges to achieve these science goals, and the simulation and analysis tasks in preparation for LZ's first data.

07/23/2020 12:00 PM - Eliminating Artifacts in Astronomical Images Using Deep Learning

Dhruv Paranjpye, Graduate Student, Electrical and Computer Engineering

Polarization measurements done using Imaging Polarimeters such as the Robotic Polarimeter are very sensitive to the presence of artifacts in images. Artifacts can range from internal reflections in a telescope to satellite trails that could contaminate an area of interest in the image. With the advent of wide-field polarimetry surveys, it is imperative to develop methods that automatically flag artifacts in images. In this paper, we implement a Convolutional Neural Network to identify the most dominant artifacts in the images. We find that our model can successfully classify sources with 98% true positive and 97% true negative rates. Such models, combined with transfer learning, will give us a running start in artifact elimination for near-future surveys like the Wide Area Linear Optical Polarimeter (WALOP). This work was done at the Astronomy department of the California Institute of Technology in the US with Dr. Ashish Mahabal, Prof. Anthony Readhead, Prof. Kieran Cleary, Prof. A. N. Ramaprakash and Dr. Gina Panapoulou. This was a collaborative work with the PASIPHAE group which stands for Polar-Areas Stellar-Imaging in Polarization High-Accuracy Experiment. Pasiphae is an international collaboration between the University of Crete, Caltech, the South African Astronomical Observatory, the Inter-University Center for Astronomy and Astrophysics, and the University of Oslo. This experiment aims to map, with unprecedented accuracy, the polarization of millions of stars at areas of the sky away from the Galactic plane, in both the Northern and the Southern hemispheres.

07/16/2020 12:00 PM - Are you listening to your gut? A study on the stomach-brain interaction

Cherry Cao, Graduate Student, Biomedical Engineering

As humans, we eat and drink every day, and our stomachs help to digest food continuously. The stomach is essential for high-quality life, but it works so quietly that we hardly notice it. Does our stomach work independently? Does our brain watch out for and control the digesting process? To answer these questions, I will talk about current understandings on the stomach-brain interaction and introduce our work about how the brain intrinsically interacts with the stomach.

07/06/2020 12:00 PM - The Blending Problem in Cosmology

Ismael Mendoza, Ph.D. Pre-Candidate, Department of Physics

The mass distribution in our universe gravitationally distorts light from galaxies, making galaxy shapes a powerful probe of cosmology. Visually overlapping galaxies along the line of sight, a problem known as blending, will negatively impact shape measurements taken from the Rubin Observatory Legacy Survey of Space and Time (LSST), the most powerful ground-based optical cosmology survey to date.

In this talk, I will discuss the blending problem in the context of LSST and the challenges that it represents for the collaboration. Then, I will survey state-of-the-art solutions that have been developed via algorithms known as deblenders. Finally, I will describe how these approaches fit together in the broader context and try to anticipate what the future of the blending problem looks like.

06/25/2020 12:00 PM - Axion Cosmology and Detection

Joshua Foster, Ph.D. Candidate, Department of Physics

Evidenced by astrophysical and cosmological observation, dark matter comprises the dominant fraction of matter in the universe. However, its particle nature is as of yet unknown, representing arguably the most tantalizing hint for searches for new fundamental physics beyond the Standard Model. In this talk, I will review the status of searches for a particle dark matter known as the axion, which is theoretically well-motivated and largely experimentally unprobed, with emphasis on its unique cosmology and ongoing efforts towards its indirect detection. I will also briefly discuss new results from the Xenon1T experiment which report a 3.5 sigma detection of a solar axion.

06/18/2020 12:00 PM - Holographic tests of quantum gravity

Marina David, Ph.D. Candidate, Department of Physics

String theory provides the most consistent framework for quantum gravity and to investigate the quantum effects on gravity within this theory, I focus on the holographic principle. The most successful realization of holography is the duality relating certain quantum theories and gravity theories. This duality gives a dictionary translating concepts from one theory to that of the other, providing a new methodology to study physical systems that was not possible prior to this correspondence. I utilize the strength of holography by developing tools for string theory using quantum theory guidance. It is in this setting that new methods are established for gravity to match with the expected quantum prediction. I will discuss some recent examples where the expressions I obtain on the gravity side agree with the quantum side, providing another check on holography while further deepening our understanding of string theory.

06/11/2020 12:00 PM - Ultrafast charge and energy transfer in TMD heterostructures using collinear multi-dimensional coherent spectroscopy

Torben Purz, Ph.D. Candidate, Department of Physics

Two-dimensional transition metal dichalcogenides (TMDs) are the most prominent group of optically active van-der-Waals materials and show promising properties such as the potential for high carrier mobility, atomic thickness, and ultrafast charge transfer. The implications of fast charge and energy transfer for optoelectronic, energy-, and light-harvesting applications has made this group of materials one of the most well-studied ones over the past years. However, previous work employed techniques not uniquely suited to study the processes in these materials, especially with respect to the temporal resolution, yielding only insufficient information about the physics of the system.

In this talk, I will give an overview about the technique called multi-dimensional coherent spectroscopy (MDCS) that we employ to study the charge and energy transfer in a MoSe2/WSe2 heterostructure. Using this technique, we are able to infer information about the dominance of charge transfer on a sub-picosecond time-scale in these samples, the underlying time-scales of other processes occurring in the material such as fast decay into dark states, and the high spatial inhomogeneity of these dynamics. We are also able to clearly distinguish between energy and charge transfer dynamics. I will explain how our implementation of MDCS can be combined with existing imaging techniques and how we plan to use this in the future to advance the study of these promising materials for electronic applications.

06/04/2020 12:00 PM - The Degree of Fine Tuning in our Universe -- and Others

Fred C. Adams, Ta You-Wu Collegiate Professor of Physics

The fundamental constants of nature must fall within a range of values in order for the universe to develop structure and ultimately support life. This talk considers current constraints on these quantities and assesses the degree of tuning required for the universe to be viable. In the realm of particle physics, the relevant parameters are the strengths of the fundamental forces and the particle masses. Additional astrophysical parameters include the cosmic energy density, the cosmological constant, the abundances of ordinary matter and dark matter, and the amplitude of primordial density fluctuations. These quantities are constrained by the necessity that the universe lives for a long time, emerges from its early epochs with an acceptable chemical composition, and successfully produces galaxies. On smaller scales, stars and planets must be able to form and function. The stars must have sufficiently long lifetimes and hot surface temperatures. We also consider potential fine-tuning related to the triple alpha reaction that produces carbon, the case of unstable deuterium, and the possibility of stable diprotons. For all of these issues, the goal is to delineate the range of parameter space for which universes can remain habitable. In spite of its biophilic properties, our universe is not optimized for the emergence of life, in that the proper variations could result in more galaxies, stars, and potentially habitable planets.