arXiv subjects
Publications and source records attributed to Dongsu Bak.
We introduce a nonabelian surface holonomy that is constructed from a one-form gauge potential that takes values in a loop algebra of the $U(N)$ gauge group. The surface holonomy parallel transports a nonabelian string. Although it is not manifest in our formulation, we will see that our nonabelian surface holonomy is invariant under reparametrizations of the surface.
We explore a possible mechanism for the formation of supermassive black hole (SMBH) seeds at the centers of ultralight dark matter (ULDM) halos in the early Universe. We investigate the conditions under which high-redshift baryonic gas, strongly confined by central solitonic cores of the halos, undergoes direct and monolithic collapse. The solitonic core sets characteristic mass and length scales for the confined baryons. Once the confined gas becomes self-gravitating, rapid inflow and shock heating may drive it into a high-temperature and high-density regime favorable for suppressing molecular cooling, without requiring a strong external UV background. We present semi-analytic scaling relations for the halo mass, soliton mass, baryonic core radius, and characteristic thermodynamic state of the gas, parametrizing the possible effects of baryonic contraction. These relations provide order-of-magnitude estimates of the characteristic range of SMBH seed masses as a function of redshift. In this framework, pristine gas clouds satisfying the adopted thermal criterion may avoid efficient fragmentation and undergo rapid central collapse, potentially forming massive black hole seeds with characteristic masses of order $10^5 M_\odot$, while systems below the threshold may form compact star clusters instead. The ULDM particle mass required to reproduce the inferred seed mass scale, $m \simeq O(10^{-22}){\rm eV}$, lies in a range favored by galactic-scale observations, suggesting a possible connection between the characteristic scales of galactic cores and early SMBH seeds. Our estimates indicate that favorable conditions for SMBH seed formation may arise at redshifts $z \gtrsim 10$. Such conditions may be relevant to the young SMBHs inferred in some little red dots, which appear to be embedded in compact, dense, ionized gas.
In Jackiw-Teitelboim gravity, the naive Schwarzian quantum mechanics leads to a continuous bulk spectrum, in apparent contradiction with the finite entropy of the black hole, which requires a discrete spectrum with level spacing of order $e^{-S_0}$. It was recently shown that restoring spectral discreteness with random statistics requires the introduction of a left confining potential that becomes relevant when the renormalized wormhole length reaches order $e^{S_0}$. In this work, we show how the known perturbative results of JT gravity are recovered within this modified framework. More importantly, we demonstrate that this modification has a direct dynamical consequence: it resolves the black-hole singularity. The confining potential generates a repulsive force at exponentially large wormhole length, preventing the indefinite growth that would otherwise lead to a singularity. We explain in detail how this turnaround arises and explore its implications for late-time bulk gravitational dynamics, the disappearance of horizons, and possible observational consequences.
We study how a machine based on deep learning algorithms learns Krylov spread complexity in quantum systems with N x N random Hamiltonians drawn from the Gaussian unitary ensemble. Using thermofield double states as initial conditions, we demonstrate that a convolutional neural network-based algorithm successfully learns the Krylov spread complexity across all timescales, including the late-time plateaus where states appear nearly featureless and random. Performance strongly depends on the basis choice, performing well with the energy eigenbasis or the Krylov basis but failing in the original basis of the random Hamiltonian. The algorithm also effectively distinguishes temperature-dependent features of thermofield double states. Furthermore, we show that the system time variable of state predicted by deep learning is an irrelevant quantity, reinforcing that the Krylov spread complexity well captures the essential features of the quantum state, even at late times.
We reformulate the abelian tensor multiplet on a curved spacetime with at least two supercharges in a cohomological form where all the bosonic and fermionic fields become tensor fields. These tensor fields are rewritten as fields in loop space by a transgression map. There are two lightlike conformal Killing vectors. By decomposing the spacetime tensor fields in transverse and parallel components to these Killing vectors, we obtain the equations of motion in loop space by closing the supersymmetry variations on-shell. We generalize to nonabelian gauge groups. By closing supersymmetry variations we obtain nonabelian fermionic equations of motion in loop space.
We argue that a discrete bulk spectrum with random statistics appears naturally in the Lorentzian description of Jackiw-Teitelboim (JT) gravity if an extra confining potential is introduced in the region where the renormalized geodesic length becomes of order $e^{S_0}$. The existence of such an extra confining potential may be inferred from the late behavior of complexity and also from the Saad-Shenker-Stanford (SSS) duality between JT gravity and the matrix model. We derive the explicit form of the extra confining potential from the well-established density of states obtained in the Euclidean approach to JT gravity. This extra potential is implicitly determined by the solution of the Abel's integral equation which turns out to be identical to the string equation of the matrix model in the SSS duality formulation of JT gravity. Thanks to the extra confining potential and the random nature of the spectrum, the time evolution of the Krylov complexity, which is identified with the renormalized geodesic length, naturally exhibits four phases, namely a ramp, a peak, a slope, and a plateau.
Traditional weak-lensing mass reconstruction techniques suffer from various artifacts, including noise amplification and the mass-sheet degeneracy. In Hong et al. (2021), we demonstrated that many of these pitfalls of traditional mass reconstruction can be mitigated using a deep learning approach based on a convolutional neural network (CNN). In this paper, we present our improvements and report on the detailed performance of our CNN algorithm applied to next-generation wide-field observations. Assuming the field of view ($3°.5 \times 3°.5$) and depth (27 mag at $5σ$) of the Vera C. Rubin Observatory, we generated training datasets of mock shear catalogs with a source density of 33 arcmin$^{-2}$ from cosmological simulation ray-tracing data. We find that the current CNN method provides high-fidelity reconstructions consistent with the true convergence field, restoring both small and large-scale structures. In addition, the cluster detection utilizing our CNN reconstruction achieves $\sim75$% completeness down to $\sim 10^{14}M_{\odot}$. We anticipate that this CNN-based mass reconstruction will be a powerful tool in the Rubin era, enabling fast and robust wide-field mass reconstructions on a routine basis.
When two galaxies merge, they often produce a supermassive black hole binary (SMBHB) at their center. Numerical simulations with stars and cold dark matter show that SMBHBs typically stall out at a distance of a few parsecs apart and take billions of years to coalesce. This is known as the final parsec problem. We suggest that ultralight dark matter (ULDM) halos around SMBHBs can generate dark matter waves due to dynamical friction. These waves can effectively carry away orbital energy from the black holes, rapidly driving them together. To test this hypothesis, we performed numerical simulations of black hole binaries inside ULDM halos. Due to gravitational cooling and quasi-normal modes, the loss-cone problem can be avoided. The decay time scale gives lower bounds on masses of the ULDM particles and SMBHBs comparable to observational data. Our results imply that ULDM waves can lead to the rapid orbital decay of black hole binaries.
We show that the (1,0) tensor and hypermultiplet supersymmetry variations can be uplifted to loop space. Upon dimensional reduction we make contact with abelian five-dimensional super Yang-Mills, which has a nonabelian generalization that we subsequently uplift back to loop space where we conjecture a nonabelian generalization of the (1,0) supersymmetry variations and demonstrate their on-shell closure.
We consider a time-dependent $\mathcal{O}(1/G)$ deformation of pure de Sitter (dS) space in dS gravity coupled to a massless scalar field. It is the dS counterpart of the AdS Janus deformation and interpolates two asymptotically dS spaces in the far past and the far future with a single deformation parameter. The Penrose diagram can be elongated along the time direction indefinitely as the deformation becomes large. After studying the classical properties of the geometry such as the area theorem and the fluctuation by a matter field, we explore the algebraic structure of the field operators on the deformed spacetime. We argue that the algebra is a von Neumann factor of type II$_\infty$ for small deformations, but there occurs a transition to type I$_\infty$ as the deformation increases so that the neck region of the deformed space becomes a Lorentzian cylinder.
We consider Jackiw-Teitelboim gravity with a massless matter field and turn on bulk excitations leading to a nontrivial vev of the corresponding dual boundary operator. To leading order, we realize the corresponding deformation of thermofield double state by explicitly identifying their Hilbert space. The deformed state can be prepared with an operator insertion at the mid-point of the Euclidean time evolution in the context of Hartle-Hawking construction. We show that the inserted operators form an SL(2,{\bf R}) representation. We construct a specific orthonormal basis that is directly related to the operator basis of the vev deformations. If we include the higher order corrections, the bulk geometry is no longer left-right symmetric. We argue that, classically, the mode coefficients in the bulk deformation cannot be fully recovered from the data collected along the boundary cutoff trajectories. Then the bulk seems to contain more information than the cutoff boundary, and this might be responsible for nontrivial behind-horizon degrees of freedom.
We study canonical quantization of Jackiw-Teibelboim (JT) gravity coupled to a massless scalar field. We provide concrete expressions of matter SL(2,{\,\bf R}) charges and the boundary matter operators in terms of the creation and annihilation operators in the scalar field. The matter charges are represented in the form of an oscillator (Jordon-Schwinger) realization of the SL(2,{\,\bf R}) algebra. We also show how the gauge constraints are implemented classically, by matching explicitly classical solutions of Schwarzian dynamics with bulk solutions. We introduce $n$-point transition functions defined by insertions of boundary matter operators along the two-sided Lorentzian evolution, which may fully spell out the quantum dynamics in the presence of matter. For the Euclidean case, we proceed with a two-sided picture of the disk geometry and consider the two-sided $2$-point correlation function where initial and final states are arranged by inserting matter operators in a specific way. For some simple initial states, we evaluate the correlation function perturbatively. We also discuss some basic features of the two-sided correlation functions with additional insertions of boundary matter operators along the two-sided evolution.
We perform a numerical comparison between the fuzzy dark matter model and the cold dark matter model, focusing on formation of satellite galaxy planes around massive galaxies. Such galactic dynamics with controlled initial subhalo configurations are investigated using GADGET2 for the cold dark matter and PyUltraLight for the fuzzy dark matter, respectively. We demonstrate that satellite galaxies in the fuzzy dark matter side have a tendency to form more flattened and corotating satellite systems than in the cold dark matter side mainly due to the dissipation by the gravitational cooling effect of the fuzzy dark matter. Our simulations with the fuzzy dark matter typically show the minor-to-major axis ratio $c/a$ of the satellite galaxy planes to be $0.21 \sim 0.30$; This well matches the current observed value for the Milky Way.
We study Python's lunch geometries in the two-dimensional Jackiw-Teitelboim model coupled to a massless scalar field in the semiclassical limit. We show that all extrema including the minimal quantum extremal surface, bulges and appetizers lie inside the horizon. We obtain fully back-reacted general bulk solutions with a massless scalar field, which can be understood as deformations of black holes. The temperatures of the left/right black holes become in general different from each other. Moreover, in the presence of both state and source deformations at the same time, the asymptotic black hole spacetime is further excited from that of the vacuum solution. We provide information-theoretic interpretation of deformed geometries including Python's lunches, minimal quantum extremal surface and appetizers according to the entanglement wedge reconstruction hypothesis. By considering the restricted circuit complexity associated with Python's lunch geometries and the operator complexity of the Petz map reconstructing a code space operation, we show that the observational probability of Python's lunch degrees of freedom from the boundary is exponentially suppressed. Thus, any bulk causality violation effects related with Python's lunch degrees are suppressed nonperturbatively.
We introduce a novel method for reconstructing the projected matter distributions of galaxy clusters with weak-lensing (WL) data based on convolutional neural network (CNN). Training datasets are generated with ray-tracing through cosmological simulations. We control the noise level of the galaxy shear catalog such that it mimics the typical properties of the existing ground-based WL observations of galaxy clusters. We find that the mass reconstruction by our multi-layered CNN with the architecture of alternating convolution and trans-convolution filters significantly outperforms the traditional reconstruction methods. The CNN method provides better pixel-to-pixel correlations with the truth, restores more accurate positions of the mass peaks, and more efficiently suppresses artifacts near the field edges. In addition, the CNN mass reconstruction lifts the mass-sheet degeneracy when applied to our projected cluster mass estimation from sufficiently large fields. This implies that this CNN algorithm can be used to measure cluster masses in a model-independent way for future wide-field WL surveys.
We numerically study the movement of two colliding fuzzy dark matter solitons without explicit self-interaction and find the effective cross section of dissipative change in velocity. The cross section turns out to be inversely proportional to the velocity cubed, and we present its analytic interpretation. Using the result we roughly estimate spatial offsets during head-on collisions of two fuzzy dark matter halos, which can be related to the spatial offsets between stars and dark matter in collisions of some galaxy clusters. We also show that the gravitational cooling plays an important role during the collisions.
We explore the entanglement evolution of boundary intervals in eternal Janus black holes that can be embedded consistently into string theory in the low-energy limit. By studying the geodesics we show that there is a transition in the entanglement characteristic around the Page time, which manifests the unitarity of the evolution. We reproduce and reinterpret these bulk results from two different lower-dimensional perspectives: first as an interface CFT in the usual AdS/CFT correspondence and second as an effective gravity theory in one lower dimension coupled to a radiation background. In the limit where the number of interface degrees of freedom becomes large, we obtain an effective theory on appropriate branes that replace the deep interior region in the bulk, coined the shadow region. In this effective theory, we also identify the island of the radiation entanglement wedge and verify the newly proposed quantum extremization method. Our model clarifies that double holography with gravity in two higher dimensions can be realized in a concrete and consistent way and that the occurrence of islands is natural in one higher dimension. Furthermore, our model reveals that there can be a transitional behavior of the Page curve before the Page time, which is related to the emergence of new matter degrees of freedom on the branes.
We propose possible probes which could be used to demonstrate experimentally the existence of the bulk and the formation of a traversable wormhole purely in terms of boundary operations only. In the two-dimensional Einstein-dilaton gravity, the traversable wormhole is realized by turning on a double trace interaction which couples the two boundaries of the AdS$_2$ black hole. Signals can propagate in the traversable wormhole through two different channels. The boundary channel is direct and instantaneous, while the bulk channel respects the bulk causality and takes a certain amount of time to complete signaling. In the latter case, we show that the signal frequency detected on the other side is highly modulated in general. The time delay as well as the frequency-modulation pattern could then be clear indications that the signal comes out through the bulk channel. We discuss the characteristics of the observed signal more explicitly for simple transitional configurations of the black hole from/to the eternal traversable wormhole.