Physics Colloquium Spring 2025

Thursdays 4:00 p.m. 104 Physics.
Colloquium organizer: Dr. Simeon Mistakidis smystakidis@mst.edu

(Link to main colloquium page)

Title: “Unveiling New Physics beyond the Standard Model of Cosmology with Galaxies”
Abstract: “The standard model of cosmology has been a tremendous success, providing a robust framework for understanding the Universe. However, it leaves several fundamental questions unresolved. Growing tensions and anomalies in current datasets suggest the possibility of new physics beyond this paradigm. The distribution of galaxies encodes critical information about the origin and dynamics of the Universe. Ongoing and upcoming galaxy surveys offer unprecedented data volumes and number densities, opening exciting opportunities to search for tensions and anomalies. Fully exploiting this data requires advanced algorithms that go beyond the widely used 2-point statistics. In this talk, I will provide examples of how higher-order statistics can extract additional information and uncover unexpected features from the data. Anomalies such as parity violation on cosmological scales could offer new insights into the matter-antimatter asymmetry and the dynamics of the early Universe.” 
Forging a Multi-Messenger View of Supermassive Black Hole Binaries
Supermassive black hole binaries lurk deep within the cores of post-merger galaxies, and their identification represents the only key to unlock previously impossible probes of gravity, galaxy evolution, and the structure of the cosmos. While electromagnetic signatures probe plasma and gas in the environment around a binary, only pulsar timing arrays are currently sensitive to the low-frequency gravitational waves emitted directly by these slow-evolving giants. Pulsar timing array experiments have reached a critical turning point, and have recently announced that they have at last uncovered evidence of the stochastic gravitational wave background. The window to the gravitational-wave universe has been widened as we are now able to expand our view to include nanohertz gravitational wave frequencies, and will now turn our eye towards gravitational waves emitted directly by individual binary systems. Simultaneously, our electromagnetic capabilities to study the variable universe are on the brink of a new paradigm that will be opened by Rubin, Roman, and their dedicated surveys. In this talk, I will give an overview of electromagnetic, gravitational wave-, and multi-messenger studies of supermassive black hole binaries thus far. Then I will describe the possibilities for the road ahead, methods under development, and the next steps necessary to achieve a multi-messenger detection of a supermassive black hole binary and uncover the secrets of the variable universe.

Title: Decoding the cosmos with gravitational waves

 
Our current understanding of the universe is shaped by a blend of observations and unverified theories, including the origins of black holes, the nature of dark matter, and the dynamics of accretion-driven phenomena. Until a decade ago, our ability to observe the universe was largely limited to electromagnetic waves. However, the first detection of gravitational waves from a merging black hole binary marked the beginning of a new era, allowing us to explore the cosmos through an entirely different messenger. A diverse array of current and upcoming observatories across both gravitational wave and electromagnetic astronomy presents an exciting opportunity to decode long-standing astrophysical mysteries, and bring us closer to groundbreaking discoveries. In this talk, I will highlight the strengths of gravitational wave astronomy, its complementarity with electromagnetic observations, and how the scientific community is preparing to fully harness the potential of these next-generation observatories.
Title: A Story of Tracing the Origins of Cosmic Rays through Gamma-Ray Observations in a Multi-Messenger Era
Abstract: Cosmic rays are among the most energetic particles in the universe, yet their origins remain a long-standing mystery. Where do they come from, and how are they accelerated to such extreme energies? In this talk, I will explore the journey of cosmic rays, from their acceleration in powerful astrophysical environments like supernova remnants, binary systems, and active galactic nuclei (AGNs) to the production of gamma rays by interacting with the interstellar medium. Gamma-ray observations play a key role in this investigation, offering critical directional information that enables us to trace cosmic rays back to their sources. By utilizing observational data from ground-based detectors such as the High Altitude Water Cherenkov (HAWC) Observatory, we can study high-energy gamma-ray emissions and uncover crucial clues about the origin of cosmic rays. I will highlight recent discoveries from the HAWC observatory, from detecting gamma-ray emissions in extragalactic AGNs to revealing unexpected gamma-ray signals within our own galaxy—ranging from microquasars to the Sun. Finally, I will discuss how a multi-wavelength and multi-messenger approach is helping us piece together the puzzle of these energetic phenomena.

 Title: Where are the supermassive black holes measured by PTAs?

Abstract: Pulsar timing arrays (PTAs) consist of a set of regularly monitored millisecond pulsars with extremely stable rotational periods. The arrival time of pulses can be altered by the passage of gravitational waves (GWs) between them and the Earth, thus serving as a galaxy-wide GW detector. Evidence for the first detection of low-frequency (~nHz) gravitational waves has recently been reported across multiple PTA collaborations, opening a new observational window into the Universe. Although the origin of the GW signal is yet to be determined, the dominant sources are expected to be inspiralling supermassive black holes. I will discuss what we are learning from mapping the nano-hertz GW sky, focusing on a recent work in which we compare the GW detections by PTAs with the expected signal implied by existing electromagnetic observations in a simple but robust manner. We highlight that there is a simple upper limit to the GWB amplitude and that the currently measured GW amplitude is somewhat larger than expected. I will then show that additional information regarding the typical number of sources contributing to the background can already be inferred from current PTA data. 

Title: What’s wrong with QED?

 

Abstract:  Quantum electrodynamics (QED) is the extraordinary relativistic quantum theory of electromagnetic interactions of particles.  It is extraordinary because it makes incredibly accurate predictions that all appear to be consistent with experiment.  For example, QED gives the ratio of the magnetic moment of the electron to the Bohr magneton with an uncertainty of less than 2 parts in 1013 or with about 14 significant figures.  It would thus appear that nothing is wrong with the theory and we can skip this talk.  However, despite its overwhelming success, QED is deeply flawed, at least in principle.  The problem is that to get such extraordinarily accurate results, it is necessary to do calculations with expressions that are actually infinite and to subtract another slightly different expression that is also infinite.  If things are done according to a well-defined set of rules, the answer comes out to be finite and, perhaps surprisingly, also correct, meaning in agreement with experiment.  One could take the position that this is just an inconvenience because there are practitioners who know how to sweep the infinities under the rug, and many share this point of view.  However, being one of those practitioners, I, along with many other physicists find this to be unsatisfactory.  In this talk, you will see videos of Paul Dirac and Richard Feynman, two of the most important contributors to the development of QED, expressing the same concern.  Also in the talk, I will point out general features of QED that may be hiding the problem.  Even though this sounds like a very technical topic, I will attempt to make it clear to people who may have no familiarity with QED.

Title: Chemistry without Chemical: An Introduction to Computational Quantum

Chemistry through Applications to Hydrogen Bonding & Concerted Proton Transfer

 

Abstract:  The subjects of solvation, molecular recognition and supramolecular self-assembly provide some of the motivation and impetus for the work that is the focus of the talk. Convergent approaches to quantum mechanical (QM) ab initio electronic structure calculations have provided tremendous insight into the structures, energetics and spectroscopic signatures of molecular clusters held together by relatively weak, non-covalent interactions (London dispersion forces, hydrogen bonding, halogen bonding, π-stacking, etc.). Unfortunately, the computational demands associated with the most accurate and reliable QM methods often prohibit their application to large molecular systems. Part of this talk will focus on strategies that systematically converge toward exact numerical solutions of the electronic Schrödinger equation via methodical application of correlated wave function methods and Gaussian atomic orbital basis sets. That will set the stage for an overview of computational techniques for non-covalent clusters that take advantage of the many-body expansion (MBE) of the total energy. A layered, ONIOM-like approach to the MBE is one such technique that we have developed to extend demanding QM electronic structure computations, such as the CCSD(T) method, to larger systems. If time permits, some additional applications will be discussed that examine concerted proton transfer processes in cyclic hydrogen-bonded clusters composed of H2O,

HF and HCl.

Title: Strategic Crystal Growth of f-electron Intermetallics in a Quantum Landscape

Abstract: The growth, characterization, and physical properties of new systems and architectures of novel classes of materials are essential for advancing the field of highly correlated systems. Solid-state synthesis remains challenging due to the unpredictability of phase formation, temperature profiles, and reaction ratios. However, solid-state chemists have developed chemical heuristics, such as periodic trends, the Zintl-Klemm concept, Pauling’s rules, valence electron concentration, and geometrical constraints, to predict outcomes.

Title: Nonclassical quantum light and matter state transfer and noise-enhanced excitation transport in a cavity

Abstract: Collective behavior and quantized EM field are trademarks of strong matter coupling in a cavity. I will discuss several paradigmatic models of matter interaction with a vacuum cavity and describe ways that nonclassical states of light and matter can be generated and exchanged between quantum matter and quantum light. I will also touch upon the spread of excitations in a multimode cavity exhibiting diffusive and ballistic transport.  

Title: Two-channel Kondo Physics: from intrinsic criticality to unconventional orders

Abstract: The interactions between local moments and itinerant electrons gives rise to a rich array of ``Kondo'' physics, from heavy Fermi liquids to unconventional superconductivity and quantum criticality.  Here, I'll introduce the even richer two-channel Kondo physics, where two symmetry-related channels of itinerant electrons compete to screen the same local moments.  This physics leads to intrinsic quantum criticality with local Majorana fermions in the impurity, and unusual spinorial orders in the lattice.  I will discuss our recent insights into these unconventional orders, including the prevalence in one dimension; the experimentally and numerically detectable consequences of the spinorial order; how it can be realized in intermetallic materials based on non-Kramers ions like Pr, U and Tm; and how topological defects can be engineered to host mobile Majorana zero modes.

Title: Do All Galaxies Form Stars the Same Way?

Abstract: Current techniques for analyzing large photometric catalogs are generally forced to assume a single, universal stellar initial mass function (IMF). However, the IMF is predicted to depend upon the temperature of gas in star-forming molecular clouds and thus should be expected to vary depending upon conditions within a star-forming galaxy. The introduction of an additional parameter into photometric template fitting allows galaxies to be fit with a range of different IMFs. Three surprising new features appear: (1) most star-forming galaxies are best fit with a bottom-lighter IMF than the Milky Way; (2) most star-forming galaxies at fixed redshift are fit with a very similar, but non-Milky Way IMF; and (3) the lowest-mass star-forming galaxies appear to exhibit a distinct relationship between IMF and star formation rate, possibly hinting at distinct feedback mechanisms in the earliest stages of star formation.  Finally, this points to the possibility that most stars are made not in blue, luminous galaxies as previously thought, but in a previously undiscovered population of intrinsically red star-forming galaxies which are only capable of forming low-mass stars.

Title: Self-patterning in a dilute gas of ten thousand atoms

Abstract: Once cooled to temperatures as low as one nano-Kelvin, the quantum mechanical wave nature of cold atoms becomes apparent. By controlling the interaction between these atoms, a dilute gas of merely ten thousand atoms—about a million times less dense than air—can exhibit novel nonlinear wave phenomena and fascinating self-patterning dynamics, some of which may be difficult to observe in other nonlinear systems. In this talk, I will discuss the preparation, control, and measurement of quantum matter waves trapped in an optical box. I will demonstrate how triggering instabilities in these nonlinear matter waves can induce self-patterning dynamics, forming spatially or temporally ordered structures and density defects such as vortex dipoles and soliton lasers. As quantum fluctuations in the matter waves primarily seed these pattern-forming instabilities, I will explore how quantum entanglement may arise from the underlying dynamics. One concrete example is the quasiparticle pair production and the formation of Einstein–Podolsky–Rosen pairs of entangled phonons within a matter wave.