Scientific Series

In this talk, I will review the recent advances in the understanding of intriguing infinite-dimensional symmetries that emerge at the boundary of asymptotically flat spacetime and in the vicinity of black hole horizons. These symmetries play an important role in the description of many phenomena such as gravitational memory effects and scattering amplitudes, and have paved the way to a holographic description of gravity in asymptotically flat spacetimes in terms of correlators on the celestial sphere.

We study the real-time dynamics of a small bubble of "false vacuum'' in a quantum spin chain near criticality, where the low-energy physics is described by a relativistic (1+1)-dimensional quantum field theory. Such a bubble can be thought of as a confined kink-antikink pair (a meson). We carefully construct bubbles so that particle production does not occur until the walls collide. To achieve this in the presence of strong correlations, we extend a Matrix Product State (MPS) ansatz for quasiparticle wavepackets [Van Damme et al., arXiv:1907.02474 (2019)] to the case of confined, topological quasiparticles. By choosing the wavepacket width and the bubble size appropriately, we avoid strong lattice effects and observe relativistic kink-antikink collisions. We use the MPS quasiparticle ansatz to identify scattering outcomes: In the Ising model, with transverse and longitudinal fields, we do not observe particle production despite nonintegrability (supporting recent numerical observations of nonthermalizing mesonic states). With additional interactions, we see production of confined and unconfined particle pairs. Although we simulated these low-energy, few-particle events with moderate resources, we observe significant growth of entanglement with energy and with the number of collisions, suggesting that increasing either will ultimately exhaust our methods. Quantum devices, in contrast, are not limited by entanglement production, and promise to allow us to go far beyond classical methods. We anticipate that kink-antikink scattering in 1+1 dimensions will be an instructive benchmark problem for relatively near-term quantum devices.

The talk will focus on the spectrum of near-extremal black holes in gravity and near-BPS black holes in supergravity. For concreteness, we will study cases in asymptotically four-dimensional flat space and three-dimensional Anti-de Sitter. This will be done by analyzing quantum effects near the horizon captured by an emergent Jackiw-Teitelboim mode at low temperatures. This will allow us to systematically study questions such as the extremal degeneracy and the size of the gap in the black hole spectrum, which can be compared to some string theory constructions.

Recent years new concepts of symmetries have been developed such as higher form symmetries, and categorical symmetries. The higher form symmetries can be either explicit in a Hamiltonian, or inexplicit as a dual of an ordinary symmetry. The behavior of higher form symmetries are easy to evaluate in phases with gaps. But at quantum criticalities their behaviors are more nontrivial. We evaluate the behaviors of higher form symmetries (either explicit or inexplicit) at various quantum critical points, and demonstrate that for many quantum critical points a universal logarithmic contribution arises, which is analogous to the quantum entanglement entropy. This logarithmic contribution is related to the universal conductance at the quantum critical points, and in some cases can be computed exactly using duality between CFTs developed in last few years. We also evaluate the behavior of categorical symmetries for more exotic cases with subsystem symmetries.

I will review the analysis of boundary symmetries in first order 3d gravity, and explain how the study of the boundary current algebra and the Sugawara construction actually leads to two dual notions of diffeomorphism charges. This provides a new understanding of the relationship between the second order and first order formulations, and of the existence of finite distance asymptotic symmetries (as strange as this sounds) in topological theories. This analysis is performed on the most general theory of first order 3d gravity, which also enables to understand the duality between curvature and torsion, as well as the relationship with chiral and massive gravity.

We construct, for the first time, the time-domain gravitational-wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy emitted as a function of width, radius and string tension Gμ. We find that the mass normalized gravitational-wave energy displays a strong dependence on the inverse of the string tension E_GW / M_0 ∝ 1/Gμ, with E_GW / M_0 ∼ O(1)% at the percent level, for the regime where Gμ ~ 1e-3 . Conversely, we show that the efficiency is only weakly dependent on the initial string width and initial string radii. Using these results, we argue that gravitational wave production is dominated by kinematical instead of geometrical considerations.

Many of previous approaches for the firewall puzzle rely on a hypothesis that interior partner modes are embedded on the early radiation of a maximally entangled black hole. Quantum information theory, however, casts doubt on this folklore and suggests a different tale; the outgoing Hawking mode will be decoupled from the early radiation once an infalling observer, with finite positive energy, jumps into a black hole. In this talk, I will provide counterarguments against current mainstream proposals and present an alternative resolution of the firewall puzzle which is consistent with predictions from quantum information theory. My proposal builds on the fact that interior operators can be constructed in a state-independent manner once an infalling observer is explicitly included as a part of the quantum system. Hence, my approach resolves a version of the firewall puzzle for typical black hole microstates as well on an equal footing.
 

The primordial non-Gaussianity (PNG) is a key feature to screen various inflationary models and it is one of the main targets in the next generation galaxy surveys. In particular, the local-type of PNG makes the galaxy bias scale-dependent in large-scales, which is known as the scale-dependent bias, allowing to constrain the local-type PNG from galaxy surveys. In this talk, I will present the galaxy shape correlation, called the ``intrinsic alignment'', can explore the angular-dependent PNG. Using N-body simulations, we show the angular-dependent PNG induces the scale-dependent bias in the intrinsic alignment, just as the angular-independent PNG leads to the scale-dependent bias in the galaxy number density correlation. By combining photometric and spectroscopic surveys to measure the intrinsic alignment, future galaxy surveys are potentially capable of constraining the angular-dependent PNG better than CMB experiments.

How to deal with diffeomorphism symmetries is one of the difficult problems in general relativity. Because of the diffeomorphism symmetries, we need to consider diffeomorphism invariant operators and gravitational dressing. In this work, we consider a special gravitational dressing which is to locate the operator by shooting geodesic from the spatial boundary. We try to use Peierls bracket to study the commutator between this gravitational dressing operator and the ADM energy operator. We found the ADM energy increase/decrease when the extra created out-going particle is in front of/behind the event horizon. Our result strengthens the Marolf-Polchinski firewall argument in some sense. In the talk, I will first briefly review the Peierls bracket, which is a linear response interpretation of bracket computation in covariant phase space formalism. We also illustrate the Peierls bracket with several examples. After that, we use Peierls bracket to study the Hamiltonian flow of the gravitational dressing operator, from which we can read out the commutator between the gravitational dressing operator with the ADM energy operator.

The entanglement pattern of a quantum many-body system can be characterized by quasiparticles and emergent gauge fields, much like those found in Maxwell's theory. My talk begins with the basic aspects of symmetry fractionalization and emergent gauge fields in strongly correlated systems. I will further extend this paradigm into a new type of quantum many-body state, dubbed "fracton phase," from a quantum melting transition of plaquette paramagnetic crystals. These exotic states contain fractionalized sub-dimensional quasiparticles with constraint motion and emergent higher-rank gauge fields. Such constraint dynamics of the quasiparticles bring about an intriguing Bose metal phase with quasi-long range order and yields non-local quantum entanglement. In particular, the key peculiarities of this phase is the UV/IR mixing, where the short wavelength physics controls the low energy theory and hence challenges the standard notion of the renormalization group perspective.

If time is described by a fundamental process rather than a coordinate, it

interacts with any physical system that evolves in time. The resulting
dynamics has recently been shown to be consistent provided the fundamental
period of the time system is sufficiently small. A strong upper bound T_C
< 10^{-33}s of the fundamental period of time, several orders of magnitude
below any direct time measurement, can be obtained from bounds on
dynamical variations of the period of a lab system evolving in time.
Possible cosmological implications will be discussed.

Two of the dominant channels to produce black-hole binary mergers are believed to be the isolated evolution of stellar binaries in the field and dynamical formation in star clusters. Pair instabilities prevent stellar collapse from generating black holes more massive than about 45-60 solar mass. This  “mass gap” only applies to the field formation scenario: repeated mergers in clusters can fill the gap. A similar reasoning applies to the binary’s spin parameters. If black holes are born slowly rotating, the high-spin portion of the parameter space (the “spin gap”) can only be filled by black-hole binaries that are assembled dynamically. I will discuss how such signatures are a smoking gun for the hierarchical origin, and how recent detections (GW190521 and GW190412) fit in this context. I will also talk about how we may be able to reconstruct the properties of progenitors of second-generation black holes.