Friday Seminar

Date: 10/03/25

Speaker: Bruno Bucciotti

Host: Sang-Eon Bak 

Title:  The quadratic quasi-normal modes of a Schwarzschild black hole

Abstract:  Black hole spectroscopy is a program that aims at testing black hole quasi-normal modes (QNMs), generated during the ringdown phase of a merger. Next-generation gravitational wave detectors will not only be sensitive to many linear QNMs, but also to non-linearities named quadratic QNMs. I will show that quadratic QNMs can be fully understood using black hole perturbation theory, which allows very clear and insightful computations. While the frequencies of quadratic QNMs have long been known, their amplitudes and phases had remained poorly understood. I will introduce new master scalars regular at infinity and the black hole horizon that allow to compute amplitudes and phases for any parity, overtone and angular momentum of the perturbations for a non-rotating black hole. I will confirm our predictions against existing numerical relativity waveforms, while also predicting new phenomenologically interesting modes and numerically studying the large angular momentum limit.

Date: 2/21/25

Speaker: Phil Levin 

Institution:  ASU

Faculty Host: Sang-Eon Bak 

Title: Three cosmological mysteries

Abstract: Three questions have troubled theoretical physics in the last century. What is the nature of the black hole singularity? What happens at the end of the evaporation of a black hole? And what is dark matter? Remarkably, all three questions can be connected by a surprisingly simple hypothesis: dark matter consists of the remnants of nonsingular black holes formed during the primordial universe. It is commonly assumed that low-mass primordial black holes cannot constitute a significant fraction of the dark matter due to their predicted short lifetimes arising from the conventional Hawking evaporation process. However, the curing of the singularity – likely due to quantum gravity – leads to a class of nonsingular black holes which have modified thermodynamic properties which include a maximum finite temperature and slow cooling phase which leads to either low-mass black holes or remnant states, thereby circumventing traditional evaporation constraints. As a proof of concept, we demonstrate how a nonsingular black hole arising in two-dimensional dilaton gravity forms a remnant with a mass proportional to the fundamental length scale, likely the Planck mass. We show how slowly evaporating low-mass nonsingular black holes, or remnants, can comprise all of the dark matter in the observable universe today.

Date: 10/24/25

Speaker: Carlos Perez-Pardavilla

Institution: ASU 

Faculty Host: Sang-Eon Bak 

Title:   Entanglement Islands in Braneworlds

Abstract:  The information paradox has been a long-standing problem in black hole physics. Recent proposals suggest that a region called an "island" contributes to the entanglement entropy of black hole radiation. Lower-dimensional models demonstrate that the inclusion of this region reproduces the so-called Page curve for the entropy of radiation. However, in higher dimensions, all models attempting to reproduce these results include a massive graviton. In this talk, I will discuss these higher-dimensional models, presenting the associated issues and how they suggest the use of tools from holography to construct swampland criteria for braneworlds as effective field theories.  

Date: 10/31/25

Speaker: Shahriyar Jafarzade (ASU)

Institution: ASU

Host: Sang-Eon Bak 

Title:  Modeling of Light Exotica: Tetraquarks and Glueballs

Abstract: The light hadron spectrum exemplifies the complexity of nonperturbative QCD, where gluonic and multiquark configurations coexist with conventional mesons. We study light exotic states in the 1.8–2.6 GeV region, focusing on hidden-strange resonances such as ϕ(2170), f₂(2340), and η(2370). Using the dynamical diquark model in both adiabatic and diabatic formulations, we find these states to be predominantly compact tetraquarks rather than meson–meson molecules. As several of them have also been proposed as glueball candidates, we examine their possible gluonic admixtures through decay-rate analyses within the extended linear sigma model, which incorporates the approximate chiral symmetry of QCD. Together, these studies offer complementary perspectives on the internal structure of light exotic mesons.

Date: 11/07/25

Speaker: Glennys Farrar

Institution: ASU

Faculty Host: Sang-Eon Bak 

Title:   Gamma Rays and Galactic CRs

Abstract: 

Date: 11/14/25

Speaker: Hayley Williams

Institution: ASU 

Faculty Host: Sang-Eon Bak 

Title:  Highly magnified individual stars at cosmological distances: nine massive stars detected as transient events in the Warhol galaxy at z = 0.94

Abstract: Without the aid of gravitational lensing, individual stars can only be resolved in our closest neighbor galaxies (< 40 Mpc). Massive stars in strongly-lensed galaxies at cosmological distances can be individually resolved and detected during caustic-crossing and microlensing transient events, where the magnification of a star in a background galaxy lying close to the lensing cluster's critical curve is temporarily greatly boosted. Pre-JWST, a handful of magnified stars had been found in lensed galaxies at z ~ 1, and JWST's infrared sensitivity has dramatically improved our ability to detect these events. Highly magnified individual stars have a variety of science applications, including measuring the distribution of dark matter subhalos, constraining the high-redshift stellar initial mass function, improving our ability to characterize and model distant stellar populations, and potentially discriminating between dark matter models. In this talk, I will present the discovery and analysis of nine highly magnified individual stars in a single strongly-lensed galaxy at z = 0.94. These stars were discovered as transient events in multi-epoch JWST NIRCam imaging of the Hubble Frontier Fields cluster MACS J0416 from the PEARLS and CANUCS programs. One of the transient sources is likely a binary stellar system consisting of a red supergiant and a ~10,000 K companion star. 

Date: 2/14/25

Speaker: Garv Chauhan

Institution: ASU 

Faculty Host: Omer 

Title:  Probing Sterile Neutrinos with Supernovae

Abstract:  Sterile neutrinos represent one of the simplest solutions for explaining the origin of neutrino masses and can be readily produced in the hot, dense environment of a core-collapse supernova (SN). Standard constraints on their interactions come from SN1987A’s energy-loss arguments. I will first discuss the physics underlying the cooling bound, followed by a novel bound arising from energy deposition and the observed population of under luminous Type IIP supernovae (SN-IIP). This new bound can probe parameter space up to two orders of magnitude lower than that of the SN1987A energy-loss argument. Additionally, heavy sterile neutrinos may escape the stellar envelope and decay into photons, detectable by $\gamma$-ray telescopes. I will also revisit $\gamma$-ray constraints derived from the non-detection of $\gamma$-rays from SN1987A and, finally, discuss the sensitivity of current and near-future $\gamma$-ray telescopes for a potential future Galactic core-collapse supernova.

Date: 2/21/25

Speaker: Nicolas Fernandez 

Institution:  Rutgers

Faculty Host: Kevin Croker 

Title: Exploring the Early Universe with Rotating Axion Fields and Gravitational Waves

Abstract: The early universe holds key insights into fundamental physics beyond the Standard Model, with gravitational waves offering a powerful window into its dynamics. In this talk, I present a novel cosmological scenario in which the rotation of an axion field in field space plays a pivotal role in shaping the evolution of the universe. This rotation naturally gives rise to the observed dark matter abundance and baryon asymmetry while also sourcing distinctive gravitational wave signatures. Notably, the axion’s kinetic energy dominance leads to an era of kination following an early phase of matter domination, with no entropy production at the transition. This allows for the possibility of these epochs occurring even after Big Bang Nucleosynthesis. I will discuss the observational implications of this framework, including its impact on the primordial gravitational wave spectrum, potential signatures in upcoming gravitational wave detectors, and its relevance to structure formation and the Hubble tension.

Date: 2/28/25

Speaker: Lars Aalsma 

Institution: ASU 

Faculty Host: Omer Albayrak

Title:  Quantum Gravity Fluctuations in Causal Diamonds and Cosmology

Abstract: Given that the Planck length is 10^{-35} meters, directly measuring quantum gravity effects is a tremendous challenge. For this reason, finding physical systems where quantum fluctuations are enhanced is crucial to put theoretical ideas about quantum gravity to the test. Work by Verlinde and Zurek in 2019 has suggested that an enhancement of geometrical fluctuations indeed takes place in a ‘causal diamond’: the causally accessible region to an observer in a finite time interval. This has been dubbed the ‘VZ-effect’. If correct, this leads to a quantum gravitational background noise that can be measured by future dedicated interferometry experiments. In this talk, I will explain the status of this proposal and highlight some of its controversial aspects. To address these controversies, I will show that the fluctuations proposed by VZ have a natural interpretation in inflationary cosmology. This clarifies the assumptions that need to be satisfied for the ‘VZ-effect' to be observable. Based on upcoming work with Sang-Eon Bak.

Date: 3/7/25

Speaker: Kimberly Boddy

Institution: UT Austin 

Faculty Host: Matt Baumgart 

Title:  Searching for New Physics with Pulsar Timing Arrays

Abstract: Pulsar timing array (PTA) experiments aim to detect nHz-frequency gravitational waves using high-precision timing of millisecond pulsars. Multiple PTA collaborations have recently reported evidence for a stochastic gravitational wave background (SGWB), expected to arise predominantly from a population of inspiraling supermassive black hole binaries. In this talk, I will discuss how PTA experiments characterize the expected signal of the SGWB and present a new analysis technique that may better facilitate tests of general relativity. I will also discuss how timing data can be used to search for metric perturbations due to the presence of ultralight dark matter in the Galaxy.

Date: 3/21/25

Speaker: Evita Verheijden

Institution: Harvard University and MIT 

Faculty Host: Lars Aalsma

Title:  Quantum Cosmic Censorship

Abstract: The classical formulation of the weak cosmic censorship conjecture (WCCC) – the statement that singularities resulting from gravitational collapse are generically hidden behind event horizons – is most probably false. However, there is compelling evidence that some version of it should be true in quantum gravity. Working towards a quantum gravitational formulation of the WCCC, I will first prove “Cryptographic Censorship”, a theorem that provides a general condition for the formation of event horizons in holography: sufficiently (pseudo)random boundary dynamics. I will then move on to define a semiclassical version of global hyperbolicity, “effective predictability”. I will argue that this can distinguish the types of singularities that are forbidden by the WCCC but should be allowed in quantum gravity (like the endpoint of black hole evaporation) from the ones that should actually be ruled out in quantum gravity.

Date: 4/11/25

Speaker: Tzvetelina Dimitrova

Institution: ASU 

Faculty Host: Omer

Title:  

Abstract: 

Date: 8/23/2024

Speaker: George Zahariade

Institution: Jagiellonian University in Krakow

Faculty Host: Tanmay 

Title: Semiclassical Backreaction in Quantum Mechanics

Abstract: The backreaction of quantum degrees of freedom on classical backgrounds is a poorly understood topic in theoretical physics. Most often it is treated within the semiclassical approximation with the help of various ad hoc prescriptions accounting for the effect of quantum excitations on the dynamics of the background. In this talk I will focus on two popular ones: (i) the mean-field approximation whereby quantum degrees of freedom couple to the classical background via their quantum expectation values; (ii) the stochastic method whereby the fully coupled system is evolved using classical equations of motion for various randomly sampled initial conditions of the quantum degree of freedom, and a statistical average is done posteriori. I will evaluate the performance of each method in a simple toy model against a fully quantum mechanical treatment, identify its regime of validity, and interpret the results in terms of quantum entanglement and loss of classicality of the background.

Date: 8/30/2024

Speaker: Giovanni Ferrami

Institution: University of Melbourne

Faculty Host: Tim Carleton and Rogier Windhorst 

Title: Galaxy-galaxy strong lensing statistics

Abstract: Photometric surveys are expected to detect 10^5 galaxy scale lenses in the next few years. This wealth of newly discovered lensing systems will enable many science cases that were so far limited by low-number statistics. In this talk, I will present a model to estimate the number and distributions in redshift and the size of strong lenses in surveys as a function of the galaxy mass function, the survey parameters and the method used to detect the lenses. I will then introduce a lens search I am leading within the PEARLS team on the NEP field observed with JWST and compare the initial results with the yield expected from my model. Finally, I will show how the statistics of strong galaxy scale lenses can constrain the evolution of the Velocity Dispersion Function (VDF, a proxy for the halo mass function), and potentially alleviate a current tension between two methods of estimating the VDF at redshift ~1.

Date: 9/27/2024

Speaker: Sang-Eon Bak

Institution: ASU

Faculty Host: Omer Albayrak

Title: Quantum Information Tools for Gravity - Renyi reflected entropy in AdS/BCFT

Abstract: Holographic duality has been studied as a quantum information tool for understanding classical and quantum gravity. In this talk, I will focus on quantum information theoretic aspects of AdS/BCFT (Anti-de Sitter/Boundary Conformal Field Theory). AdS/BCFT describes the holographic duality between gravitational theory in anti-de Sitter space (AdS) with end-of-the-world branes and the conformal field theory (CFT) on a manifold with boundaries. First, I will introduce the Ryu-Takayanagi formula, which shows the correspondence between the area of an extremal surface in AdS and entanglement entropy in CFT. I will then present one of the motivations for studying AdS/BCFT by reviewing what we can learn about the black hole information loss problem from the Ryu-Takayanagi formula. Then, I will discuss reflected entropy in quantum field theory, which was newly proposed to consider entanglement in bipartite mixed states or entanglement within subregions. I will conclude by discussing the gravity dual of the Renyi reflected entropy, which involves gravitational backreaction in AdS/BCFT.

Date: 10/04/24

Speaker: Cliff Cheung

Institution: Caltech

Faculty Host: Matt Baumgart

Title:  Generalized Symmetry in Dynamical Gravity

Abstract: The concept of symmetry in quantum field theory has broadened considerably in the past decade.  A particularly well-studied example is higher-form symmetry, which acts inherently on extended objects.  Working in lockstep analogy with gauge theory, we explore one-form symmetry in the effective field theory context of dynamical, nonlinear gravity.  Notably, the interplay between line operators and symmetry defect operators in gravity has a natural geometric interpretation in terms of classical general relativity.

Date: 10/11/24

Speaker: Tucker Manton 

Institution: Brown University 

Faculty Host: Damien

Title:  Dark matter and beyond GR effects in gravitational waves

Abstract: The existence of stable timelike orbits around Kerr black holes (BHs) implies that dark matter densities spike in the vicinity of BH-BH mergers. Thus, observations of gravitational waves from merger processes offer a unique opportunity to probe dark matter phenomena in strong gravitational fields. Moreover, in these regimes, any beyond General Relativity (GR) physics connected to dark matter (such as parity violation) will be enhanced, which may be expected through the lens of effective field theory. In this talk, we will first introduce a model independent parameterization of the gravitational wave strains which account for interactions of this type, before examining three specific models which include non-minimal couplings between the (dark) matter sector and gravity. The gravitational waveforms exhibit gentle departures from GR, and the non-observation of deviations from GR can be used to constrain the models. Finally, we point out that these models harbor regimes where the beyond GR effects are 'shielded' at certain energy scales and/or cosmological epochs.This complicates the problem but also offers a new technique with which to constrain the many dark matter models on the market.
 

Date: 11/08/24

Speaker: Nikhil Garuda

Institution: University of Arizona  

Faculty Host: Rogier/Tim 

Title: Arc 11: The Missing Link in the Supernova H0pe Story

Abstract: PLCK G165.7+67.0 (G165) is a massive galaxy cluster that was discovered using the Planck telescope by its rest-frame far-infrared colors. James Webb Space Telescope (JWST)  images uncovered dozens of giant arcs and even examples of single galaxy images that are gravitationally-lensed by the cluster  into multiple locations in the image plane. One such example is the triply-imaged galaxy “Arc 2,” which hosted a Type Ia Supernova “H0pe” at z = 1.783. Differences in the arrival times to the three different images of this same galaxy yielded a value for the current expansion rate for the universe, H0, with 9 \% uncertainty for this single target.  One way to achieve higher accuracy on H0 is to refine the lens model. Arc 11 is another one of the  gravitationally lensed galaxies in the G165 cluster field that is strategically positioned close to SN H0pe, yet lacks a spectroscopic redshift or even a reliable photometric redshift owing to significant cluster background light. Due to its close projected proximity to supernova H0pe,  an accurate redshift for Arc 11 will play an outsized role in refining the cluster’s lens model. This study presents a detailed methodology to achieve a reliable redshift measurement for Arc 11, to do a spatially-resolved study of its physical properties, and to search for additional and previously undetected transients.

Date: 11/15/24

Speaker: Joseph Shields

Institution: University of Arizona 

Faculty Host: Rogier Windhorst 

Title: LBT Updates and Future Prospects as a Discovery Machine

Abstract: As the first of the new generation of Extremely Large Telescopes, the Large Binocular Telescope remains the largest optical telescope in existence as measured by collecting area and angular resolution delivered by mirrors on a common mount.  The LBT has historically had an important role in demonstrating and implementing innovative technical advances that open new possibilities for cosmic discovery.  In this presentation I will discuss the current state of the Observatory, examples of recent important science results, and emerging opportunities resulting from the second generation instruments SHARK-NIR, SHARK-VIS, and iLocater.   The telescope’s capabilities continue to grow in importance for enabling distinctive measurements at the forefront of astronomy.

Date: 11/22/24

Speaker: Keaten Wood

Institution: ASU

Faculty Host: Omer Albayrak

Title: In-in de-Sitter to NLL and beyond

Abstract: De-sitter space is an interesting metric that is relevant not only to our universe's cosmological past via inflation, but also its dark-energy dominated future. However, calculations in de-sitter space have been found to be very sensitive to both IR and UV divergences, and in ways that are difficult to merely renormalize.

The Leading-Log dS description for light scalars is that of classical motion convolved with quantum noise - a picture that makes good intuitive sense. But will this picture still hold in the next leading log (NLL) order. What about NNLL? Will the IR and UV divergences still resum nicely, or will everything blow up?

During my presentation, I will present a novel technique that views the time derivative as a morphism that transforms an in-in Feynman diagram into a linear combination of other Feynman diagrams. This view allows us to construct a Fokker-Planck equation of motion for light scalars in de-sitter to NLL and beyond, while side-stepping the ugly divergences that come with it. To support this, I will additionally present improvements to the in-in formalism that allow for in-in diagrams to be sorted and computed more similarly to S-matrix elements.

Date: 1/19/2024

Speaker: Albin Joseph

Institution: ASU

Faculty Host: Simon Foreman 

Title: Dynamics of Dark Energy: Cosmological Trackers

Abstract: Two decades have passed since the groundbreaking discovery of cosmic acceleration, yet the mysteries surrounding the laws of physics on the largest scales persist. This remarkable phenomenon, initially revealed through the observation of type 1a supernovae, has revolutionized our comprehension of cosmology and serves as a paradigm shift in our understanding of the Universe. In this talk, I will discuss the cosmological models exhibiting tracker properties that have great significance in the context of dark energy as they can reach the present value of dark energy density from a wide range of initial conditions, thereby alleviating both the fine-tuning and the cosmic coincidence problem. In particular, I will discuss the Oscillatory Tracker Model, which belongs to the family of tracker dark energy models and their effect on various observational probes.

Date: 3/22/24

Speaker: Tarun Souradeep

Institution: Raman Research Institute

Faculty Host: Tanmay

Title:  Validating the Cosmological principle

Abstract: The Cosmological Principle, a fundamental tenet of the 'standard model of cosmology',   predicates a statistically isotropic distribution of fluctuations in the measured Cosmic Microwave Background (CMB)  temperature and polarisation sky maps. Enigmatic anomalies claimed in the WMAP and Planck CMB sky maps, and some other recent observations could  challenge the standard model. However, these claims need to be cast in an objective mathematical framework and established with statistical rigour. Bayesian inference of the underlying covariance structure of random fields on the sphere in the Bipolar Spherical Harmonic  (BipoSH) representation developed in our research program provides such a framework.  We review the recent inferences drawn from Planck data and dwell on the future prospects with proposed CMB observations.

Date: 3/29/24

Speaker: Damien Easson

Institution: ASU

Faculty Host: Matt

Title: Solving the initial singularity problem: eternal inflation and a world without end

Abstract: 

We discuss the possibility an eternal universe, a universe with no first moment and no end. 

The talk will focus on eternal inflation and the key role that inflation plays in resolving the initial big-bang singularity.

Along the way we, will describe how proposed no-go theorems, such as the famous theorem of Borde, Guth and Vilenkin (BGV)

are circumvented or obviated. Our exploration encompasses eternal inflating, loitering, and bouncing models, shedding light on 

the critical aspects that underpin geodesic completeness. This will include a detailed analysis of the classical energy conditions in General Relativity. 

Ignoring the intractable subtleties introduced by quantum considerations, such as rare tunneling events and

Boltzmann brains, we will argue that the universe need not have a beginning or an end.

Date: 4/12/24

Speaker: Yosef Verbin

Institution:

Faculty Host: Damien

Title: Palatini Nonlinear Electrodynamics

Abstract: The general subject of this talk is the formulation of electrodynamics and its non-linear generalizations, in an analogy with the Palatini formalism of General Relativity and its extensions and modifications. As in GR, the first order formulation of Maxwell theory is equivalent to the second order standard formulation since the field equations are identical. Moreover, for theories of nonlinear electrodynamics, the situation is similar to that of gravity and in certain cases, the first and second order formulations yield different field equations. Some of the initial results of the most basic implications of these Palatini non-linear electrodynamics (PNLED for short) will be presented – mainly as compact objects like solitons and black holes. If time permits, cosmological implications will be mentioned briefly.

Date: 4/19/24

Speaker: Francesc Ferrer 

Institution:

Faculty Host: Tanmay

Title: 

Abstract: