Adriana Gavidia, Observational Cosmology / Caltech

Galaxy clusters are the largest and most recently formed objects in the universe, making them rich laboratories to test the astrophysics of structure formation and cosmology. Within the current standard cosmological framework, structure formation is pictured as a hierarchical merging process, where clusters form from the gravitational collapse of the most dense peaks in the initial primordial density field. A triaxial collapse is a direct prediction of structure growth driven by the self-gravity of Gaussian density fluctuations. As large scale structure evolves, tidal forces from the most massive peaks guide the surrounding matter into massive filamentary structures, connecting the clusters to one another to form a foam-like structure called the “cosmic web”. Numerical simulations indicate that new matter is accreted onto clusters preferentially along these filaments, leading to an alignment between the major axis of the cluster mass halo and the large-scale filament, giving clusters an intrinsically aspherical morphology. This prediction is supported by evidence from various observational probes such as X-ray, Sunyaev-Zeldovich Compton-y, and gravitational lensing. Despite this, until recently studies have taken the infamous “spherical cow” approach to model cluster morphology, as data quality and the lack of availability of multi-wavelength data sets limited the ability to perform triaxial analyses. To obtain less biased measurements of the cluster properties used to constrain cosmology, it is crucial we upgrade to the “triaxial cow” model. In this talk, I will present our triaxial modeling technique enabled by the high quality multi-probe data set collected for the CHEX-MATE collaboration and the science we can extract from these measurements.

Guido Da Re, TAPIR, SXS / Caltech

In the conventional framework of relativistic quantum field theory, the vacuum state is considered to be the same for all inertial observers. That is, if one inertial observer measures zero temperature using a thermometer, all other inertial observers will find the same result. But what happens when we extend this analysis to observers in arbitrary states of motion? In this seminar, we will explore the simplest case of non-inertial motion—uniform proper acceleration in special relativity—and uncover a surprising effect: for accelerated observers, the vacuum is no longer perceived as empty. Instead, they experience a thermal bath, feeling a temperature! This phenomenon, known as the Unruh effect, is a fundamental relativistic quantum effect with far-reaching implications. We will derive the Unruh temperature starting from a scalar field theory, revealing how this seemingly simple model intertwines with the equivalence principle. This connection offers valuable insights into Hawking radiation near the horizon of a black hole. Moreover, the analysis highlights the complexities that arise when attempting to construct a quantum field theory that fully respects diffeomorphism invariance, posing intriguing challenges for our understanding of fundamental physics.

Yoonsoo Kim, TAPIR, SXS / Caltech

TBD

Andrew Laeuger, TAPIR / Caltech

From the Green Bank Observatory in the radio band to the Fermi Space Telescope in the gamma-ray band, modern astronomers have thrived on our ability to observe electromagnetic waves with frequencies spanning a staggering 16 orders of magnitude. It’s easy to take for granted the centuries of work between the first optical telescope and today’s extraordinary breadth of observational capabilities, but in the field of gravitational wave (GW) astronomy, the expansion of the frequency range we can access with our detectors is happening in real time. So far, GWs have only been convincingly detected in the 10 Hz - 1 kHz band (by the LIGO/Virgo interferometers) and in the nHz band (by the NANOGrav pulsar timing array), but there are many planned future experiments which will target different frequency bands, allowing us to uncover and test new physical phenomena. Over the past few years, my research has considered both pathways for detecting GWs outside of the LIGO and pulsar timing bands and possibilities for studying astrophysical environments and the fundamental nature of gravity itself using such waves. In this talk, I aim to provide a unique mixture of theoretical and experimental perspectives on the future of gravitational wave astronomy by highlighting upcoming detector developments along with some of my personal contributions to this rapidly expanding field.

Rhiannon Udall, LIGO / Caltech

One of the most interesting events in the third LVK observing run was GW191109, which was found to have spins which were confidently anti-aligned with the orbital angular momentum - one of only two such cases which has been released - and high masses. This implies GW191109 may be of dynamical origin, making it our first probe of this formation channel. However, GW191109 was coincident with a transient noise source known as a “glitch,” which may affect these conclusions dramatically. I will discuss the origins and modelling of the glitch in question, and work I have done to fully characterize the astrophysical properties of GW191109 in the presence of this glitch. If time allows I will also discuss additional aspects of glitch modeling, and statistical tests to identify the impact of glitches on parameter estimation.

Joonhwi Kim, Burke Institute for Theoretical Physics / Caltech

In 1964, Newman and Janis made a curious observation that the Kerr metric can be obtained by performing a “complex coordinate transformation” on the Schwarzschild solution, describing an imaginary displacement into “complex spacetime.” This method, nowadays referred to as the Newman-Janis Algorithm (NJA), holds the very historical significance as the idea that facilitated the discovery of the Kerr-Newman black hole solution. However, it is often dismissed as an ad-hoc trick or “fluke” due to the lack of any established physical or geometrical basis. Contrary to this belief, this talk explains how the recent work [arXiv:2412.19611] has eventually refined the NJA into a rigorous and systematic derivation of spinning black hole solutions, after sixty years of mysterious status. Intriguingly, this demystification reveals that spinning black holes can be regarded as “molecules” of magnetic monopoles. This idea will be realized in a precise and quantitative manner in terms of the classical double copy correspondence, which borrows insights from the study of scattering amplitudes in quantum field theory. Important lemmas are the Kerr-Schild metric established in [arXiv:2405.09518] and a theorem concerning nonlinear superpositions of self-dual and anti-self-dual spacetimes. Finally, physical implications of this “molecule” picture will be outlined, with applications to making all-orders-in spin predictions in the post-Minkowskian, self-force, or black hole perturbation theory programs. In particular, a dynamical implementation of the NJA implies that the equations of motion of a Kerr probe describe the Wick rotation of the geodesic deviation equation in a sense, from which all-orders-in-spin conserved quantities are derived.

Nicholas Rui, TAPIR / Caltech

White dwarfs are the compact end products of stellar evolution in the vast majority of stars. They are highly dense Earth-sized, solar-mass objects supported by electron degeneracy pressure. I will first describe the essential physics and phenomenology needed to understand white dwarfs. Subject to time, I will then briefly describe the state of research into white dwarfs, which includes puzzles in magnetism, pulsations, close binarity, detonation, radio emission, and compositional evolution.

Aaron Steiger, Observational Cosmology / Caltech

The theory of cosmic inflation posits that the universe underwent a period of rapid expansion in the first fractions of a second following the Big Bang. Inflation solves multiple issues in modern cosmology, but the search for direct evidence is ongoing. The detection of B-mode polarizations in the cosmic microwave background would serve as strong evidence of inflation and constrain the theory space of different inflation models. These B-modes are the target of BICEP Array, the latest in the BICEP/Keck series of experiments, which observes the CMB from the South Pole with an array of small aperture receivers. In this talk, I discuss the theoretical motivation for inflation, the BICEP/Keck observing strategy and results to date, and the current state of BICEP Array.

Isabella Pretto, TAPIR, SXS / Caltech

The ringdown phase of a binary black hole merger encodes key information about the remnant’s properties through its quasinormal mode (QNM) spectrum. Extracting these modes accurately is crucial for testing general relativity and understanding binary black hole dynamics. In this talk, I’ll explore ringdown physics and introduce an automated algorithm for efficient QNM fitting. I’ll discuss its application to simulated waveforms, highlighting the expected physics of asymmetric mergers and mode excitation. These results have important implications for future observational tests of black hole structure and potential beyond-GR signatures.

Annika Dugad, TAPIR / Caltech

I will be talking about our attempts to resolve the measurement problem by allowing the emitted/detected particles in a system to travel both forwards and backwards in time. I will discuss the issue of causality, which may or may not be violated depending on the model. This research is primarily influenced by the Wheeler-Feynman absorber theory and John Cramer’s Transactional Interpretation of quantum mechanics.

Taylor Knapp, TAPIR, SXS, LIGO / Caltech

Since 2015, we have detected gravitational waves from over a hundred compact binary coalescences (CBCs) using the LIGO interferometers. These CBCS include binary black holes, binary neutron stars, and black hole-neutron star mergers. Once we subtract these louder CBC foreground signals from the LIGO interferometer data, we are left with a background. This background, referred to as the stochastic gravitational wave background (GWB), is the assumed isotropic, stationary, and unpolarized superposition of quieter sources. Some proposed components of the GWB include white dwarf binaries, supermassive binaries, cosmic strings, and first order phase transitions. The better we can resolve and understand the GWB, the more information we can extract about the universe and stellar populations. I will discuss the current state of research on the GWB and what we anticipate seeing in next generation detectors such as Pulsar Timing Array, LISA, Cosmic Explorer, and Einstein Telescope. I will also touch on my own research, which exemplifies how resolving the GWB is a statistical analysis problem.

Natsuko Yamaguchi, Astronomy / Caltech

Binary interaction is among the most important unsolved problems in stellar astrophysics. Close white dwarf (WD) + main sequence (MS) binaries are the end products of mass transfer processes that occurred when the WD progenitor was a red giant, making them key tools for improving our understanding of these processes. In this presentation, I will provide an overview of the existing literature, going into some detail about the sample of WD + MS binaries discovered using Gaia DR3, and describe a few of my completed and ongoing projects related to this.

Elijah Kane, Observational Cosmology / Caltech

Infrared emission lines are powerful probes of the physical conditions within dust-obscured regions, such as protoplanetary disks, molecular clouds, and active galactic nuclei. In this talk I will describe the physics behind this line emission and show examples of how measurements of atomic and molecular lines have been used to help us understand the dusty ISM.

Jiaxi Wu, TAPIR, SXS / Caltech

General-relativistic magnetohydrodynamics (GRMHD) simulation is one of the most important tools to study the evolution of compact objects and high-energy phenomena in the universe. I will give an overview of the formalism of GRMHD simulations coupled to dynamical spacetime evolution, and the implementation of code. Following this, I will talk about the current limitation of numerical simulation and possible solutions to overcome this.

Dongze Sun, TAPIR, SXS / Caltech

I’ll give an overview of the Post Newtonian framework to model the two body gravitational waveform. Following this, I’ll introduce the additional results from post-Minkowskian approach and Effective Field Theory. Finally, I’ll compare the post-Newtonian framework with numerical relativity.

Talya Klinger, TAPIR, SXS / Caltech

I present a broad overview of extreme mass ratio inspirals with a particular focus on what makes them so challenging to model and detect. We’ll look at a range of theoretical and numerical approaches, the astrophysical population, and upcoming GW detectors poised to observe these unique systems. I’ll also discuss my own work on tidal heating, a phenomenon whose importance to late-stage, high-spin EMRIs could help us distinguish exotic compact objects from black holes.

Nicholas Rui, TAPIR / Caltech

Observations from asteroseismology suggest that stellar cores rotate more slowly than predicted by standard physics. This implies an additional mechanism of angular momentum transport, still highly uncertain. I summarize a few key works on the problem, namely Zahn, et al. (1997), on transport by gravity waves, and Fuller, et al. (2019), on the modified Tayler–Spruit instability.

Isabel Sands, TAPIR / Caltech

The nature of dark matter remains one of the great outstanding mysteries in the fields of astrophysics, cosmology, and particle physics. Observations have determined the amount of dark matter in the universe, but how that exact amount came into being is an active topic of theoretical work. In this blackboard talk, I will give a brief overview of the primary mechanisms commonly invoked to set the dark matter relic abundance in the universe: freeze-out, freeze-in, and the axion misalignment mechanism (if time permits).

Tony Rodríguez, Astronomy / Caltech

Astronomy is the archetypal “discovery-driven” field of science. So what do you do with nearly 10 billion sources in the major optical astronomy catalogs of today? In some ways, making new discoveries actually seems harder than ever with such a large, almost intractable dataset. I will present a plot similar to the Hertzsprung-Russell diagram, which I have dubbed the “X-ray Main Sequence”. With that diagram, one can use X-ray catalogs (which are much smaller than optical catalogs), to supplement the optical data and pick out accreting compact objects harboring a white dwarf, neutron star, or black hole. I will explain how this simple diagram works and some theoretical reasons for the apparent clustering of objects into various classes. However, I would like audience input on how to extend this to other wavelengths in astronomy, such as radio, gamma rays, and the infrared, potentially multi-messenger astronomy as well. I would like to explore how supervised machine learning models could aid in the classification of sources based on this diagram, but will argue that in some cases it may not be necessary. No previous data experience is required to listen in on this talk — the more outside perspective the better!