Seminars archive 2024

Gravitational waves exhibit the unique signature of their spin-2 nature in processes of wave scattering, due to the interaction between spin and a background spacetime. Since the spin effect is more pronounced for longer wavelengths and gravitational waves sourced by binaries have a long wavelength, it may become an important effect in addition to the wave effect. In this talk, I will discuss the spin effect on the propagation of gravitational waves lensed by a Kerr black hole, employing spin-optics formalism and the Teukolsky formalism.

I will present a method to introduce a measurement postulate in Quantum Field Theory (QFT) by Unruh-DeWitt detectors and algebraic tools. Beginning with a review of measurements in quantum mechanics and the detector-based measurements for QFT by J. Polo-Gomez, J. J. Garay, and E. Martin-Martinez (Phys. Rev. D 105, 2022), I will show how the latter can be extended to induce localised Kraus operators and selective measurements within the framework of Haag-Kastler's QFT axioms. This extension leads to a more robust measurement postulate for (A)QFT in 3+1 flat spacetime. Additionally, I will discuss the feasibility of defining detector-based measurements in more general spacetimes, and their applications in open quantum systems, quantum simulations, and quantum computation. [slides]

We often encounter a situation where linear wave equations around a black hole solution can be regarded as continuous deformations of simpler ones, or modifications from the general relativity case by continuous parameters. We discuss a general framework to compute high-order perturbative corrections to quasinormal mode frequencies in such deformed problems. Our method has many applications, and it allows us to compute numerical values of the high-order corrections very accurately. For several examples, we perform this computation explicitly and discuss analytic properties of the quasinormal mode frequencies for deformation parameters. We also discuss recent developments.

Reference:

arXiv:2307.16626

In recent years, the protocol known as entanglement harvesting has attracted great interest. Entanglement harvesting utilizes multiple Unruh-DeWitt (UDW) detectors to extract (or ‘harvest’) entanglement pre-existed in a quantum field. The extracted entanglement is influenced by the geometry of spacetime and the trajectories of UDW detectors. Additionally, various other correlations can also be extracted under this protocol, which we refer to as correlation harvesting. 

In this talk, I will focus on the influence of Unruh and Hawking phenomena on entanglement harvesting. Specifically, I will present: (i) the Unruh temperature of uniformly accelerating detectors prevents the detectors from extracting any correlations at the high temperatures; (ii) high black hole temperatures also prevent the detectors from harvesting correlations, and this is no exception even for tripartite entanglement; and (iii) freely falling detectors in a black hole spacetime are less affected by this, and they have no trouble extracting correlations from the field even when detectors are causally disconnected by an event horizon.

Abstract:

The gravitational waves emitted just after a binary black hole collision can be well approximated by the superposition of quasinormal modes (QNMs). In general relativity (GR), the QNM of a black hole is completely determined by its mass and angular momentum due to the uniqueness theorem. Thus, we can test GR by checking whether the quasinormal frequencies in future observations are consistent with the GR prediction or not. In this talk, we discuss the parametrized black hole quasinormal ringdown formalism, which is a robust framework for analyzing QNMs in systems that are close to the GR case, for higher overtones. We find that larger deviations from the GR case typically appear in the quasinormal frequencies for the higher overtones. This growing tendency for higher overtones can be understood using an analytical method. Our results suggest that we can impose a strong constraint on gravity theories by considering the overtones of QNMs.

References:

arXiv:1901.01265

arXiv:2001.09613

arXiv:2404.09672