Scientific Series

Recently, there has been much interest in black hole echoes, based on the idea that there may be some mechanism (e.g., from quantum gravity) that waves/fields falling into a black hole could partially reflect off of an interface before reaching the horizon. There does not seem to be a good understanding of how to properly model a reflecting surface in numerical relativity, as the vast majority of the literature avoids the implementation of artificial boundaries, or applies transmitting boundary conditions. Here, we present a framework for reflecting a scalar field in a fully dynamical spherically symmetric spacetime, and implement it numerically. We study the evolution of a wave packet in this situation and its numerical convergence, including when the location of a reflecting boundary is very close to the horizon of a black hole. This opens the door to model exotic near-horizon physics within full numerical relativity.

I introduce a unified framework for interpreting neural network classifiers tailored toward automated scientific discovery. In contrast to neural network-based regression, for classification, it is in general impossible to find a one-to-one mapping from the neural network to a symbolic equation even if the neural network itself bases its classification on a quantity that can be written as a closed-form equation. In this paper, I embed a trained neural network into an equivalence class of classifying functions that base their decisions on the same quantity. I interpret neural networks by finding an intersection between this equivalence class and human-readable equations defined by the search space of symbolic regression. The approach is not limited to classifiers or full neural networks and can be applied to arbitrary neurons in hidden layers or latent spaces or to simplify the process of interpreting neural network regressors.

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I’ll give a review of extended phase space in gauge theories and/or diffeomorphism-invariant theories, beginning with its original conception up to our most recent work. The primary focus in a classical theory is on the proper representation of symmetries and gauge symmetries on phase space. Correspondingly, this is understood in the quantum theory through the crossed product construction and conditional expectation. I’ll discuss several examples.

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The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently reported evidence for the background of low-frequency gravitational waves using an array of rapidly rotating, highly stable radio pulsars distributed across the galaxy. Each pulsar emits a radio beam that passes our line of sight and is observed as regular, pulsed emission. We measure the times of arrival of pulses and compare with a model that includes: the rotational motion of the pulsar, orbital motions, and interstellar propagation delays; random timing noise from pulsars themselves and from the interstellar medium; and finally a correlated gravitational-wave signal recently reported on in a suite of papers. I will highlight our recent results, including implications for the dynamics of supermassive black hole binary at the centers of merging galaxies, searches for individual supermassive black hole binary systems, and searches of gravitational wave signals from new physics. I will end by highlighting future prospects for radio facilities that will advance our look into this newly opened window to the Universe.

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The principle of information causality, proposed as a generalization of no signaling principle, has efficiently been applied to outcast beyond quantum correlations as unphysical. In this talk, we show that this principle, when utilized properly, can provide physical rationale toward structural derivation of multipartite quantum systems. In accordance with the no signaling condition, the state and effect spaces of a composite system can allow different possible mathematical descriptions, even when descriptions for the individual systems are assumed to be quantum. While in one extreme, namely, the maximal tensor product composition, the state space becomes quite exotic and permits composite states that are not allowed in quantum theory, the other extreme - minimal tensor product composition - contains only separable states, and the resulting theory allows only Bell local correlation. As we show, none of these compositions is commensurate with information causality, and hence cannot be the bona-fide description of nature. Information causality therefore promises an information-theoretical derivation of self duality of the state and effect cones for composite quantum systems.

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The Sunyaev-Zel’dovich Effect—the Doppler boost of low-energy Cosmic Microwave Background photons scattering off free electrons—is an excellent probe of ionized gas residing in distant galaxies. Its two main constituents are the kinematic SZ effect (kSZ), where electrons have a non-zero line-of-sight (LOS) velocity and which probes the electron line-of-sight momentum, and the thermal SZ effect (tSZ), where electrons have high energies due to their temperature, and which probes the electron integrated pressure. These two effects provide complementary information to constrain the thermodynamic profile of gas residing in distant galaxies, which can be further used to understand feedback processes, a necessary ingredient to describe the evolution of the large-scale structure in our Universe. Both tSZ and kSZ can be measured in cross-correlation with large-scale structure. In this talk, I will discuss my past and ongoing measurements of the SZ-galaxy cross-correlation with unWISE galaxies, where to measure the kSZ effect I use the projected-fields estimator. unWISE is a galaxy catalog containing over 500 million galaxies on the full sky and consists of three subsamples of mean redshifts z=0.5, 1.1, 1.5, whose halo occupation distribution I have already constrained. If time permits, I will also present my work on mitigating foregrounds in the SZ cross-correlations, particularly the Cosmic Infrared Background (CIB).

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The recent breakthrough in the detection of gravitational waves (GWs) from merging black hole (BH) and neutron star (NS) binaries by advanced LIGO/Virgo has generated renewed interest in understanding the formation mechanisms of merging compact binaries, from the evolution of massive stellar binaries and triples in the galactic fields, dynamical interactions in dense star clusters to binary mergers in AGN disks. I will review different aspects of the dynamical formation channels, and discuss how observations of spin-orbit misalignments, eccentricities, masses and mass ratios in a sample of merging binaries by aLIGO can constrain these formation channels. The important roles of space-borne gravitational wave detectors will also be discussed.

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The current progress in gravitational wave detection opened a new exciting window in cosmology. It is natural to ask ourselves how we can best use this new tool to explore physics beyond the standard model. With this idea in mind, my collaborators and I asked what we could learn from Cosmological Gravitational Wave Backgrounds if they were to be detected to a certain accuracy. By drawing comparison to the cosmic microwave background, we investigate the impact of elastic scattering on any cosmological background. Specifically, we focus on quantifying spectral distortions in the energy density spectrum of CGWB attributed to interaction with beyond-the-standard-model particles. We will also explore the effect that elastic scattering of graviton of Primordial Black holes would have on CGWB in the regime where PBHs account for all the dark matter.

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Zoom link https://pitp.zoom.us/j/98460268383?pwd=RytzWHd5dU1lenRhWG1NYXM3OVJpQT09

For a noncommutative algebra $A$ and an antilinear automorphism $\rho$ of $A$ there is a notion of positive trace. On the physics side, positive traces are related to quantizations of superconformal field theories. On the mathematical side, positive traces are connected to spherical unitary representations of complex Lie groups. We will discuss the classification of positive traces in the case when the algebra $A$ is a deformation or a $q$-deformation of a Kleinian singularity of type A.

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Incorporating time poses a challenge for all quantum gravity programs primarily build around Euclidean spacetimes. This talk surveys how the gravitational asymptotic safety program may address this challenge by encoding the gravitational degrees of freedom via the Arnowitt-Deser-Misner (ADM) decomposition of the spacetime metric. This formulation equips spacetime with a foliation structure singling out a preferred direction. This structure may then take the role of time in the Lorentzian framework. Within this setting, we will outline recent results related to the Wilsonian renormalization group flow of the graviton two-point function including the infrared attractors rendering the graviton massless. Furthermore, we will explain how these results generalize to Lorentzian signature spacetimes and briefly comment on potential applications in the context of cosmology.

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Zoom link https://pitp.zoom.us/j/98382991491?pwd=SmtpMTRTK2NUMEpuN3laa0pIWWs2UT09

Spinning supermassive black holes (BHs) in active galactic nuclei (AGN) can launch relativistic collimated outflows, or jets, by coiling up the magnetic field lines that cross the event horizon on the scale of BH gravitational radius, Rg. Sustained jet launching and propagation to kpc-scales are thought to require non-zero angular momentum to form an accretion disk, keep the magnetic field lines on the BH, and ensure stable jet propagation. Past 3D general relativistic magnetohydrodynamic (GRMHD) work predicted that absent gas angular momentum, magnetized jets are expected to become unstable to the kink instability already at 800 Rg, i.e., orders of magnitudes smaller distances than the BH sphere of influence, or the Bondi radius, RB (e.g., for Sgr A* and M87, RB~10^[5-6] Rg). This raises the question: Do the jets need gas angular momentum to survive the kink instability and make it out to large distances, outside of Rb? I will present 3D GRMHD simulation results for a rapidly spinning BH immersed into uniform zero angular momentum gas threaded by a weak vertical magnetic field, with the largest simulated Bondi radius to date, RB = 1000 Rg. I will show that the BH accumulates dynamically-important magnetic field, enters the magnetically-arrested disk state, and produces powerful jets that make it out of Rb. I will also show how the jets get twisted into knots and dissipate their energy after the MAD state ends.

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Zoom link https://pitp.zoom.us/j/99925612004?pwd=QkhIUmpKRkVKUURpclhPekJST2NIQT09

The field of indefinite order in quantum theory was born from an attempt to construct a theory of quantum gravity, where the first step is to construct a generalized quantum theory in which events could have an indefinite order [1]. It is expected that such a theory would lead us more naturally to the construction of a quantum gravity theory. One way to explore this topic operationally is to consider that two agents Alice and Bob apply operations A and B on a given target system and that quantum mechanics holds locally for each agent [2].  The quantum switch is the simplest example of a task with indefinite order, where the order of operations applied by two agents on a target system is entangled with the state of a quantum control system. In particular, in the gravitational quantum switch, the order of these operations is entangled with the state of a quantum spacetime [3].

In this talk, I will present a recent result, where we propose a distinct protocol for performing a gravitational quantum switch [4]. One of the agents crosses the interior region of massive spherical shells in a superposition of different radii and becomes entangled with their geometry. This entanglement is used as a resource to control the order of operation in the implementation of the quantum switch. Novel features of the protocol include: i) the superposition of nonisometric geometries; ii) the existence of a region with a definite geometry; iii) the fact that the agent that experiences the superposition of geometries is in free fall, preventing information on the global geometry to be obtained by this agent.

[1] Hardy, J. Phys. A: Math. Theor. 40, 3081 (2007);
[2] Chiribella, D’Ariano, Perinotti, Valiron, PRA 88, 022318 (2013); Oreshkov, Costa, Brukner, Nat. Commun. 3, 1092 (2012).
[3] Zych, Costa, Pikovski, Brukner, Nat. Commun. 10, 3772 (2019).
[4] Móller, Sahdo, Yokomizo, arXiv:2306.10984 (2023).

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Zoom link https://pitp.zoom.us/j/96568644174?pwd=ak9qM0Y1R29WdXRXREIxdTFtcDhYUT09