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Hideo Furugori

Publications and source records attributed to Hideo Furugori.

10 recordsLinked to original sources

Generalized Black Holes with Fully Non-aligned Electromagnetic Fields

We develop a Plebański--Demiański-adapted parametrization of the general Ovcharenko--Podolský solution and construct new families of generalized black holes in four-dimensional Einstein--Maxwell theory. These Petrov type~D spacetimes possess non-null, fully non-aligned electromagnetic fields. A restriction imposed in the previous parametrization is not required by the field equations in shifted coordinates. Removing it retains an independent charge parameter $q$, which may be real or purely imaginary. For $q^2\geq0$, both the metric and electromagnetic field admit a smooth aligned limit to the charged Plebański--Demiański solution with $Λ=0$ and $e^2+g^2=q^2$. We impose angular conditions in the Griffiths--Podolský form allowing Killing horizon cross sections with spherical topology. The resulting eight-parameter class extends the seven-parameter class of Ovcharenko and Podolský [arXiv:2508.04850]. The remaining normalization condition is generically quartic, defining four algebraic branches. We give explicit metrics and electromagnetic fields for the non-twisting and $l=0$ twisting families and analyze their black hole and acceleration horizons, extremality, conicity, and further limits. We determine the effective-NUT-free branches of the latter family and their degenerate cases. Using the genuine Griffiths--Podolský form, we establish an explicit correspondence between the non-accelerating effective-NUT-free subclass and the Kerr--Newman--Bertotti--Robinson family and express its electromagnetic flux charges in our parameters. The $q^2<0$ sector includes the genuine uncharged Kerr--Bertotti--Robinson solution recently identified by Ovcharenko and Podolský [arXiv:2608.21672].

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Quasinormal modes of Kerr-Newman-de Sitter black holes from Heun connection formulae

We study quasinormal modes governed by the Teukolsky equation in Kerr-Newman-de Sitter spacetime, formally treating the spin weight and field charge as arbitrary parameters. We use exact Heun connection formulae expressed through classical conformal blocks. We analytically derive the quasinormal mode spectra in the small cosmological constant, near-extremal, and near-Nariai limits. In the small cosmological constant limit, we obtain the leading charged de Sitter spectrum together with its subleading corrections. In both the small cosmological constant and near-extremal limits, solving the full connection condition beyond the leading pole approximation gives rise to generically noninteger-power corrections to the quasinormal mode frequencies, whereas no analogous correction appears in the near-Nariai limit. We also analyze the angular eigenvalue problem as an illustration of the same connection-formula method.

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Multi-rotating black holes with non-aligned angular momenta in 5D Kaluza-Klein theory

We present an exact solution describing multi-rotating black holes in 4D Einstein-Maxwell-dilaton theory, which can be obtained from 5D Kaluza--Klein theory via dimensional reduction. The solution represents a multi-centered configuration of rotating black holes carrying both electric and magnetic charges, with each black hole possessing a non-aligned angular momentum. This work generalizes our previous solution for black holes with aligned angular momenta to the more general case of non-aligned angular momenta. It includes, as special cases, the Majumdar--Papapetrou solution, the recent multi-centered rotating black hole solutions of Teo and Wan, and our previous solution with unequal electric and magnetic charges. The resulting spacetimes are free of curvature singularities and closed timelike curves, both on and outside the horizons, provided that the magnitude of the spin angular momentum of each black hole remains below a certain upper bound.

hep-th↗

Multi--black holes in Bertotti--Robinson spacetime

We construct a new class of exact solutions describing multi-black holes in the Bertotti--Robinson spacetime, using the monodromy-matrix formalism associated with integrable sigma models. Starting from the extremal Reissner--Nordström black hole in the Bertotti--Robinson background, we derive the corresponding coset and monodromy matrices and show that they are governed by nilpotent algebraic structures. This property enables an explicit factorization of the monodromy matrix, allowing for a systematic reconstruction of the underlying gravitational solutions. We extend this construction to multi-center configurations by introducing multiple poles in the monodromy matrix, leading to Majumdar--Papapetrou--type solutions with Bertotti--Robinson asymptotics. Each center is shown to correspond to a regular extremal black hole with an $\mathrm{AdS}_2 \times S^2$ near-horizon geometry, and the asymptotic end likewise approaches a Bertotti--Robinson geometry. We further generalize the framework to stationary configurations in the Bertotti--Robinson spacetime, as well as to a broader class of Israel--Wilson--Perjés-type solutions, by considering more general nilpotent elements. Our results demonstrate that the monodromy-matrix approach provides a powerful and systematic framework for constructing multi-black hole solutions in nontrivial backgrounds, and suggest a promising route toward more general configurations.

hep-th↗

Trapped Surface as a Cosmic Censor

We formulate a local geometric criterion for weak cosmic censorship in black hole overcharging and overspinning thought experiments. Under the null convergence and generic conditions, matter injection turns a horizon cross section into a closed trapped surface. Any final spacetime unable to accommodate this surface is ruled out. This trapped surface criterion excludes superextremal Reissner-Nordström, Reissner-Nordström-de Sitter, and Kerr-Newman final states, as well as Weyl-class naked singularities. Our criterion does not rely on asymptotic charges or on an extremal condition characterizing naked singularities.

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Apparent Horizons Associated with Dynamical Black Hole Entropy

We define entropic marginally outer trapped surfaces (E-MOTSs) as a generalization of apparent horizons. We then show that, under first-order perturbations around a stationary black hole, the dynamical black hole entropy proposed by Hollands, Wald, and Zhang, defined on a background Killing horizon, can be expressed as the Wall entropy evaluated on an E-MOTS associated with it. Our result ensures that the Hollands-Wald-Zhang entropy reduces to the standard Wald entropy in each stationary regime of a dynamical black hole, thereby reinforcing the robustness of the dynamical entropy formulation.

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Bridging dS/CFT and Celestial Holography via Ward-Takahashi Identities

In 2507.17558, we provide a map from a scalar theory on $(D+2)$-dimensional Minkowski spacetime to a scalar theory with a continuous mass spectrum on $(D+1)$-dimensional de Sitter spacetime, and propose a link between celestial amplitudes and cosmological correlators (the cosmological-celestial dictionary). We extend the construction to fields with spin 1 and 2, and find that massless spin fields map to spin fields with continuous mass spectra. In this construction, we identify the de Sitter counterparts of the Nambu-Goldstone modes associated with the asymptotic symmetries in Minkowski spacetime. For $U(1)$ gauge theories, the counterpart is restricted to the massless sector within the continuous Proca spectrum, while for linearized gravity supertranslations are encoded in the partially massless sector and superrotations in the strictly massless sector. Using the identification, we reveal that the associated Ward-Takahashi identities of the cosmological correlators reproduce the conformally soft photon and graviton theorems via the cosmological-celestial dictionary. In particular, the celestial stress tensor is derived from the asymptotic limit of gravitons in de Sitter spacetime.

hep-th↗

Celestial Holography meets dS/CFT

We provide a concrete link between celestial amplitudes and cosmological correlators. We first construct a map from quantum field theories (QFTs) in $(D+2)$-dimensional Euclidean space to theories on the $(D+1)$-dimensional sphere, through a Weyl rescaling and a Fourier transformation. An analytic continuation extends this map to a relation between QFTs in Minkowski spacetime $\text{M}_{D+2}$ and in de Sitter spacetime $\text{dS}_{D+1}$ with the Bunch-Davies vacuum. Combining this relation with celestial holography, we show that the extrapolated operators in de Sitter space can be represented by operators on the celestial sphere $S^{D}$. Our framework offers a systematic route to transfer computational techniques and physical insights between celestial holography and the dS/CFT correspondence.

hep-th↗

Celestial two-point functions and rectified dictionary

A naive celestial dictionary causes massless two-point functions to take the delta-function forms in the celestial conformal field theory (CCFT). We rectify the dictionary, involving the shadow transformation so that the two-point functions follow the standard power-law. In this new definition, we can smoothly take the massless limit of the massive dictionary. We also compute a three-point function using the new dictionary and discuss the OPE in CCFT.

hep-th↗

Dressed-Asymptotic States and QED Infrared Physics

The dressed state formalisms, which incorporate interactions of soft particles into an asymptotic state, are known as the prescriptions expected to solve the problem of infrared (IR) divergence in the quantum field theory (QFT). A particularly famous example is the dressed state formalism proposed by Kulish and Faddeev in quantum electrodynamics (QED). As pointed out by Hirai and Sugishita, however, this formalism has problems in gauge invariance and the IR divergence. These problems are mainly caused by the existence of ghosts or unphysical photon modes. Therefore, we start by studying the asymptotic states in the Coulomb gauge, which excludes ghosts and/or unphysical photon modes. In this paper, we propose a formalism to construct the asymptotic states directly from the interaction of the theory by setting a sufficiently large time scale $T$. In this dressed state formalism, we define the asymptotic interaction remaining at $|t|>T$ in terms of some fixed order of $1/T$, and we are performing all calculations according to that order. We study the asymptotic states in QED specifically, but we can formally apply the dressed state formalism proposed in this paper to any perturbative QFT. We show that, at least in QED, we can construct divergence-free and unitary $S$-matrix using dressed states proposed in this paper. Furthermore, we discuss the transition rate to show that we can predict experimental results. We also show that the $\mathrm{U}(1)$ gauge symmetry of $S$-matrix leads to the QED large gauge symmetry, and deviation of the expectation values of the vector potential between initial and final spacelike hypersurfaces emerges as a QED memory effect. The dressed state formalism in this paper may give a unified and new insight into IR physics, including asymptotic symmetries, memory effects, and unitarity of the state evolution.

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