Katsuki Aoki
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View article: Quantum Effects for Black Holes with On-Shell Amplitudes
Quantum Effects for Black Holes with On-Shell Amplitudes Open
We develop a framework based on modern amplitude techniques to analyze emission and absorption effects in black hole physics, including Hawking radiation. We first discuss quantum field theory on a Schwarzschild background in the Boulware …
View article: Effective field theory of coupled dark energy and dark matter
Effective field theory of coupled dark energy and dark matter Open
We formulate an effective field theory (EFT) of coupled dark energy (DE) and dark matter (DM) interacting through energy and momentum transfers. In the DE sector, we exploit the EFT of vector-tensor theories with the presence of a preferre…
View article: On-shell approach to black hole mergers
On-shell approach to black hole mergers Open
A bstract We develop an on-shell approach to study black hole mergers. Since, asymptotically, the initial and final states can be described by point-like spinning particles, we propose a massive three-point amplitude for the merger of two …
View article: On-Shell Approach to Black Hole Mergers
On-Shell Approach to Black Hole Mergers Open
We develop an on-shell approach to study black hole mergers. Since, asymptotically, the initial and final states can be described by point-like spinning particles, we propose a massive three-point amplitude for the merger of two Schwarzsch…
View article: Anomalous thresholds for the S-matrix of unstable particles
Anomalous thresholds for the S-matrix of unstable particles Open
A bstract In this work, we study the analytic properties of S-matrix for unstable particles, which is defined as the residues on the unphysical sheets where unstable poles reside. We demonstrate that anomalous thresholds associated with UV…
View article: Gravitational positivity for phenomenologists: Dark gauge boson in the swampland
Gravitational positivity for phenomenologists: Dark gauge boson in the swampland Open
The gravitational positivity bound gives quantitative “swampland” constraints on low-energy effective theories inside theories of quantum gravity. We give a comprehensive discussion of this bound for those interested in applications to phe…
View article: Cosmology and the classical limit of the S-matrix
Cosmology and the classical limit of the S-matrix Open
A bstract We investigate the relationships between classical observables in cosmology and the classical limit of quantum scattering amplitudes. We first look at the relation between Bogoliubov transformations and the notion of classical li…
View article: Exploring the spin of ultralight dark matter with gravitational wave detectors
Exploring the spin of ultralight dark matter with gravitational wave detectors Open
We propose a novel method for distinguishing the spin of ultralight dark matter (ULDM) using interferometric gravitational wave detectors. ULDM can be a bosonic field of spin-0, 1, or 2, and each induces distinctive signatures in signals. …
View article: CMB spectrum in unified EFT of dark energy: scalar-tensor and vector-tensor theories
CMB spectrum in unified EFT of dark energy: scalar-tensor and vector-tensor theories Open
We study the cosmic microwave background (CMB) radiation in the unified description of the effective field theory (EFT) of dark energy that accommodates both scalar-tensor and vector-tensor theories. The boundaries of different classes of …
View article: Effective field theory of black hole perturbations in vector-tensor gravity
Effective field theory of black hole perturbations in vector-tensor gravity Open
We formulate the effective field theory (EFT) of vector-tensor gravity for perturbations around an arbitrary background with a timelike vector profile, which can be applied to study black hole perturbations. The vector profile spontaneousl…
View article: Cosmology and the classical limit of the S-matrix
Cosmology and the classical limit of the S-matrix Open
We investigate the relationships between classical observables in cosmology and the classical limit of quantum scattering amplitudes. We first look at the relation between Bogoliubov transformations and the notion of classical limit. Then,…
View article: Anomalous Thresholds for the S-matrix of Unstable Particles
Anomalous Thresholds for the S-matrix of Unstable Particles Open
In this work, we study the analytic properties of S-matrix for unstable particles, which is defined as the residues on the unphysical sheets where unstable poles reside. We demonstrate that anomalous thresholds associated with UV physics a…
View article: Effective Field Theory of Black Hole Perturbations in Vector-Tensor Gravity
Effective Field Theory of Black Hole Perturbations in Vector-Tensor Gravity Open
We formulate the effective field theory (EFT) of vector-tensor gravity for perturbations around an arbitrary background with a ${\it timelike}$ vector profile, which can be applied to study black hole perturbations. The vector profile spon…
View article: Cosmological Perturbation Theory in Metric-Affine Gravity
Cosmological Perturbation Theory in Metric-Affine Gravity Open
We formulate cosmological perturbation theory around the spatially curved FLRW background in the context of metric-affine gauge theory of gravity which includes torsion and nonmetricity. Performing scalar-vector-tensor decomposition of the…
View article: Exploring spin of ultralight dark matter with gravitational wave detectors
Exploring spin of ultralight dark matter with gravitational wave detectors Open
We propose a novel method for distinguishing the spin of ultralight dark matter (ULDM) using interferometric gravitational wave detectors. ULDM can be a bosonic field of spin-0, 1, or 2, and each induces distinctive signatures in signals. …
View article: Coupled vector Gauss-Bonnet theories and hairy black holes
Coupled vector Gauss-Bonnet theories and hairy black holes Open
We study vector-tensor theories in which a 4-dimensional vector field Aμ is coupled to a vector quantity Jμ, which is expressed in terms of Aμ and a metric tensor gμν. The divergence of Jμ is equivalent to a Gauss-Bonnet (GB) term. We show…
View article: Gravitational Positivity for Phenomenologists: Dark Gauge Boson in the Swampland
Gravitational Positivity for Phenomenologists: Dark Gauge Boson in the Swampland Open
The gravitational positivity bound gives quantitative "swampland'' constraints on low-energy effective theories inside theories of quantum gravity. We give a comprehensive discussion of this bound for those interested in applications to ph…
View article: Cosmological gravity probes: Connecting recent theoretical developments to forthcoming observations
Cosmological gravity probes: Connecting recent theoretical developments to forthcoming observations Open
Since the discovery of the accelerated expansion of the present universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality…
View article: Unitarity and unstable-particle scattering amplitudes
Unitarity and unstable-particle scattering amplitudes Open
Unitarity is the fundamental property of the S-matrix, while its usage for a scattering of unstable particles has been subtle as unstable particles do not appear in the asymptotic states. Defining unstable-particle amplitudes as residues o…
View article: Coupled vector Gauss-Bonnet theories and hairy black holes
Coupled vector Gauss-Bonnet theories and hairy black holes Open
We study vector-tensor theories in which a 4-dimensional vector field $A_μ$ is coupled to a vector quantity ${\cal J}^μ$, which is expressed in terms of $A_μ$ and a metric tensor $g_{μν}$. The divergence of ${\cal J}^μ$ is equivalent to a …
View article: Some disquisitions on cosmological 2-form dualities
Some disquisitions on cosmological 2-form dualities Open
In this work we study different aspect of self-interacting 2-form fields with special emphasis in their cosmological applications. We provide the explicit construction of how massless 2-forms are compatible with the cosmological principle …
View article: Cosmological gravity probes: connecting recent theoretical developments to forthcoming observations
Cosmological gravity probes: connecting recent theoretical developments to forthcoming observations Open
Since the discovery of the accelerated expansion of the present Universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality…
View article: Highly compact Proca stars with quartic self-interactions
Highly compact Proca stars with quartic self-interactions Open
We study self-gravitating bound states of a complex vector field, known as Proca stars, with a new type of quartic-order self-interaction which does not exist in the case of either a complex scalar field or a real vector field. Depending o…
View article: Unitarity and unstable-particle scattering amplitudes
Unitarity and unstable-particle scattering amplitudes Open
Unitarity is the fundamental property of the S-matrix while its usage for a scattering of unstable particles has been subtle as unstable particles do not appear in the asymptotic states. Defining unstable-particle amplitudes as residues of…
View article: Spin-2 dark matter from anisotropic Universe in bigravity
Spin-2 dark matter from anisotropic Universe in bigravity Open
Bigravity is one of the natural extensions of general relativity and contains an additional massive spin-2 field which can be a good candidate for dark matter. To discuss the production of spin-2 dark matter, we study fixed point solutions…
View article: Effective field theory of gravitating continuum: solids, fluids, and aether unified
Effective field theory of gravitating continuum: solids, fluids, and aether unified Open
We investigate the relativistic effective field theory (EFT) describing a non-dissipative gravitating continuum. In addition to ordinary continua, namely solids and fluids, we find an extraordinary more symmetric continuum, aether. In part…
View article: Resolving the pathologies of self-interacting Proca fields: A case study of Proca stars
Resolving the pathologies of self-interacting Proca fields: A case study of Proca stars Open
It has been argued that a self-interacting massive vector field is pathological due to a dynamical formation of a singular effective metric of the vector field, which is the onset of a gradient or ghost instability. We discuss that this si…
View article: Effective Field Theory of Gravitating Continuum: Solids, Fluids, and Aether Unified
Effective Field Theory of Gravitating Continuum: Solids, Fluids, and Aether Unified Open
We investigate the relativistic effective field theory (EFT) describing a non-dissipative gravitating continuum. In addition to ordinary continua, namely solids and fluids, we find an extraordinary more symmetric continuum, aether. In part…
View article: The effective field theory of vector-tensor theories
The effective field theory of vector-tensor theories Open
We investigate a systematic formulation of vector-tensor theories based on the effective field theory (EFT) approach. The input of our EFT is that the spacetime symmetry is spontaneously broken by the existence of a preferred timelike dire…
View article: Shift-symmetric $SO(N)$ multi-Galileon
Shift-symmetric $SO(N)$ multi-Galileon Open
A Poincarè invariant, local scalar field theory in which the Lagrangian and the equation of motion contain only up to second-order derivatives of the fields is called generalized Galileon. The covariant version of it in four dimensions is …