arXiv subjects

Shinji Mukohyama

Publications and source records attributed to Shinji Mukohyama.

At least 55 records · Page 3Linked to original sources

arXiv2024

Even- and odd-parity stabilities of black holes in Einstein-Aether gravity

In Einstein-Aether theories with a timelike unit vector field, we study the linear stability of static and spherically symmetric black holes against both even- and odd-parity perturbations. For this purpose, we formulate a gauge-invariant black hole perturbation theory in the background Aether-orthogonal frame where the spacelike property of hypersurfaces orthogonal to the timelike Aether field is always maintained even inside the metric horizon. Using a short-wavelength approximation with large radial and angular momenta, we show that, in general, there are three dynamical degrees of freedom arising from the even-parity sector besides two propagating degrees of freedom present in the odd-parity sector. The propagation speeds of even-parity perturbations and their no-ghost conditions coincide with those of tensor, vector, and scalar perturbations on the Minkowski background, while the odd sector contains tensor and vector modes with the same propagation speeds as those in the even-parity sector (and hence as those on the Minkowski background). Thus, the consistent study of black hole perturbations in the Aether-orthogonal frame on static and spherically symmetric backgrounds does not add new small-scale stability conditions to those known for the Minkowski background in the literature.

gr-qc↗

arXiv2024

Black holes, multiple propagation speeds, and energy extraction

The Standard Model of particle physics predicts the speed of light to be a universal speed of propagation of massless carriers. However, other possibilities exist -- including Lorentz-violating theories -- where different fundamental fields travel at different speeds. Black holes are interesting probes of such physics, as distinct fields would probe different horizons. Here, we build an exact spacetime for two interacting scalar fields which have different propagation speeds. One of these fields is able to probe the black hole interior of the other, giving rise to energy extraction from the black hole and a characteristic late-time relaxation. Our results provide further stimulus to the search for extra degrees of freedom, black hole instability, and extra ringdown modes in gravitational-wave events.

gr-qc↗

arXiv2024

(In)stability of the black hole greybody factors and ringdowns against a small-bump correction

Recently, it has been proposed that the black hole greybody factors can be important to model ringdown spectral amplitudes. We study the stability of greybody factors against a small-bump correction in the perturbation equation. We find (I) that the greybody factor is stable in the frequency region relevant to ringdown and (II) that it is destabilized at higher frequencies, especially for a sharper bump correction. This behavior is similar to the case of higher overtones, which is also very sensitive to a small correction. We clarify this (in)stability with the WKB analysis. As the greybody factor is stable at the frequency region relevant to the main part of ringdown, we conclude that the greybody factor is suitable to model ringdown amplitude. In order to investigate a bump correction in a self-consistent manner, we consider the small-bump correction that can be realized in the general framework of effective field theory of black hole perturbations.

gr-qc↗

arXiv2024

Are $f(R, {\rm Matter})$ theories really relevant to cosmology?

We examine $f(R, \text{Matter})$ theories that directly couple the curvature $R$ or $R_{μν}$ with the matter sector in the action, in addition to the universal coupling. We argue that if the matter sector includes the Standard Model (SM), such theories are either inconsistent or already excluded by experiments unless they are a rewriting of $f(R)$ gravity or general relativity. If these theories genuinely couple the SM to curvature, they suffer from the presence of ghost states at energies within their domain of application for cosmological purposes. Therefore, we raise questions about their relevance to cosmology. Moreover, if such theories do not include the SM, they should just be seen as scalar-tensor, vector-tensor, ..., theories, depending on the additional degrees of freedom. They should thus be studied accordingly.}

gr-qc↗

arXiv2024

Revisiting the stochastic QCD axion window: departure from equilibrium during inflation

If dark matter is made of QCD axions, its abundance is determined by the vacuum expectation value acquired by the axion field during inflation. The axion is usually assumed to follow the equilibrium distribution arising from quantum diffusion during inflation. This leads to the so-called stochastic window under which the QCD axion can make up all the dark matter. It is characterised by $10^{10.4}\mathrm{GeV}\leq f\leq 10^{17.2}\mathrm{GeV}$ and $H_{\mathrm{end}}>10^{-2.2}\mathrm{GeV}$, where $f$ is the axion decay constant and $H_{\mathrm{end}}$ is the Hubble expansion rate at the end of inflation. However, in realistic inflationary potentials, we show that the axion never reaches the equilibrium distribution at the end of inflation. This is because the relaxation time of the axion is much larger than the typical time scale over which $H$ varies during inflation. As a consequence, the axion acquires a quasi-flat distribution as long as it remains light during inflation. This leads us to reassessing the stochastic axion window, and we find that $ 10^{10.3}\mathrm{GeV}\leq f\leq 10^{14.1}\mathrm{GeV}$ and $H_{\mathrm{end}}>10^{-13.8}\mathrm{GeV}$.

astro-ph.CO↗

arXiv2024

Angular and radial stabilities of spontaneously scalarized black holes in the presence of scalar-Gauss-Bonnet couplings

We study the linear stability of spontaneously scalarized black holes (BHs) induced by a scalar field $ϕ$ coupled to a Gauss-Bonnet (GB) invariant $R_{\rm GB}^2$. For the scalar-GB coupling $ξ(ϕ)=(η/8) (ϕ^2+αϕ^4)$, where $η$ and $α$ are constants, we first show that there are no angular Laplacian instabilities of even-parity perturbations far away from the horizon for large multipoles $l \gg 1$. The deviation of angular propagation speeds from the speed of light is largest on the horizon, whose property can be used to put constraints on the model parameters. For $α\gtrsim -1$, the region in which the scalarized BH is subject to angular Laplacian instabilities can emerge. Provided that $α\lesssim -1$ and $-1/2<αϕ_0^2<-0.1155$, where $ϕ_0$ is the field value on the horizon with a unit of the reduced Planck mass $M_{\rm Pl}=1$, there are scalarized BH solutions satisfying all the linear stability conditions throughout the horizon exterior. We also study the stability of spontaneously scalarized BHs in scalar-GB theories with a nonminimal coupling $-βϕ^2 R/16$, where $β$ is a positive constant and $R$ is a Ricci scalar. As the amplitude of the field on the horizon approaches an upper limit $|ϕ_0|=4/\sqrtβ$, one of the squared angular propagation speeds $c_{Ω-}^2$ enters the instability region $c_{Ω-}^2<0$. So long as $|ϕ_0|$ is smaller than a maximum value determined for each $β$ in the range $β>5$, however, the scalarized BHs are linearly stable in both angular and radial directions.

gr-qc↗

arXiv2024

CMB spectrum in unified EFT of dark energy: scalar-tensor and vector-tensor theories

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 theories are universally parameterised by a new EFT parameter $α_V$ characterising the vectorial nature of dark energy and a set of consistency relations associated with the global/local shift symmetry. After implementing the equations of motion in a Boltzmann code, as a demonstration, we compute the CMB power spectrum based on the $w$CDM background with the EFT parameterisation of perturbations and a concrete Horndeski/generalised Proca theory. We show that the vectorial nature generically prevents modifications of gravity in the CMB spectrum. On the other hand, while the shift symmetry is less significant in the perturbation equations unless the background is close to the $Λ$CDM, it requires that the effective equation of state of dark energy is in the phantom region $w_{\rm DE}<-1$. The latter is particularly interesting in light of the latest result of the DESI+CMB combination as the observational verification of $w_{\rm DE}>-1$ can rule out shift-symmetric theories including vector-tensor theories in one shot.

astro-ph.CO↗

arXiv2024

Effective Field Theory of Black Hole Perturbations in Vector-Tensor Gravity

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 spontaneously breaks both the time diffeomorphism and the $U(1)$ symmetry, leaving their combination and the spatial diffeomorphism as the residual symmetries in the unitary gauge. We derive two sets of consistency relations which guarantee the residual symmetries of the EFT. Also, we provide the dictionary between our EFT coefficients and those of generalized Proca (GP) theories, which enables us to identify a simple subclass of the EFT that includes the GP theories as a special case. For this subclass, we consider the stealth Schwarzschild(-de Sitter) background solution with a constant temporal component of the vector field and study the decoupling limit of the longitudinal mode of the vector field, explicitly showing that the strong coupling problem arises due to vanishing sound speeds. This is in sharp contrast to the case of gauged ghost condensate, in which perturbations are weakly coupled thanks to certain higher-derivative terms, i.e., the scordatura terms. This implies that, in order to consistently describe this type of stealth solutions within the EFT, the scordatura terms must necessarily be taken into account in addition to those already included in the simple subclass.

hep-th↗

arXiv2024

Singularity at the demise of a black hole

We consider a class of quasiregular singularities characterized by points possessing two future-directed light cones and two past-directed light cones. Such singularities appear in the $1+1$ trousers spacetime and the Deutsch-Politzer spacetime. We argue that these singularities are relevant for describing the end point of an evaporating black hole, and show that a class of emergent Lorentz signature theories can provide a microscopic description for these singularities.

gr-qc↗

arXiv2024

On spin optics for gravitational waves lensed by a rotating object

We study gravitational lensing of gravitational waves taking into account the spin of a graviton coupled with a dragged spacetime made by a rotating object. We decompose the phase of gravitational waves into helicity-dependent and independent components with spin optics, analyzing waves whose wavelengths are shorter than the curvature radius of a lens object. We analytically confirm that the trajectory of gravitational waves splits depending on the helicity, generating additional time delay and elliptical polarization onto the helicity-independent part. We exemplify monotonic gravitational waves lensed by a Kerr black hole and derive the analytical expressions of corrections in phase and magnification. The corrections are enhanced for longer wavelengths, potentially providing a novel probe of rotational properties of lens objects in low-frequency gravitational-wave observations in the future.

gr-qc↗

arXiv2024

Effective Field Theory of Black Hole Perturbations with Timelike Scalar Profile: Formulation

We formulate the Effective Field Theory (EFT) of perturbations within scalar-tensor theories on an inhomogeneous background. The EFT is constructed while keeping a background of a scalar field to be $\textit{timelike}$, which spontaneously breaks the time diffeomorphism. We find a set of consistency relations that are imposed by the invariance of the EFT under the 3d spatial diffeomorphism. This EFT can be generically applied to any inhomogeneous background metric as long as the scalar profile is everywhere timelike. For completeness, we report a dictionary between our EFT parameters to those of Horndeski theories. Finally, we compute background equations for a class of spherically symmetric, static black hole backgrounds, including a stealth Schwarzschild-de Sitter solution.

hep-th↗

arXiv2024

Spherical scalar collapse in a type-II minimally modified gravity

We investigate the spherically-symmetric gravitational collapse of a massless scalar field in the framework of a type-II minimally modified gravity theory called VCDM. This theory propagates only two local physical degrees of freedom supplemented by the so-called instantaneous (or shadowy) mode. Imposing asymptotically flat spacetime in the standard Minkowski time slicing, one can integrate out the instantaneous mode. Consequently, the equations of motion reduce to those in general relativity (GR) with the maximal slicing. Unlike GR, however, VCDM lacks 4D diffeomorphism invariance, and thus one cannot change the time slicing that is preferred by the theory. We then numerically evolve the system to see if and how a black hole forms. For small amplitudes of the initial scalar profile, we find that its collapse does not generate any black hole, singularity or breakdown of the time slicing. For sufficiently large amplitudes, however, the collapse does indeed result in the formation of an apparent horizon in a finite time. After that, the solution outside the horizon is described by a static configuration, i.e. the Schwarzschild geometry with a finite and time-independent lapse function. Inside the horizon, on the other hand, the numerical results indicate that the lapse function keeps decreasing towards zero so that the central singularity is never reached. This implies the necessity for a UV completion of the theory to describe physics inside the horizon. Still, we can conclude that VCDM is able to fully describe the entire time evolution of the Universe outside the black hole horizon without knowledge about such a UV completion.

gr-qc↗

arXiv2024

Hartle-Hawking No-boundary Proposal and Hořava-Lifshitz Gravity

We study the Hartle-Hawking no-boundary proposal in the framework of Hořava-Lifshitz gravity. The former is a prominent hypothesis that describes the quantum creation of the universe, while the latter is a potential theory of quantum gravity that ensures renormalizability and unitarity, at least in the so-called projectable version. For simplicity, we focus on a global universe composed of a set of local universes each of which is closed, homogeneous and isotropic. Although applying the no-boundary proposal to Hořava-Lifshitz gravity is not straightforward, we demonstrate that the proposal can be formulated within the Hořava-Lifshitz gravity utilizing the Lorentzian path integral formulation of quantum gravity. In projectable Hořava-Lifshitz gravity, the no-boundary wave function of the global universe inevitably contains entanglement between different local universes induced by ``dark matter as integration constant''. On the other hand, in the non-projectable version, the no-boundary wave function of the global universe is simply the direct product of wave functions of each local universe. We then discuss how the no-boundary wave function is formulated under Dirichlet and Robin boundary conditions. For the Dirichlet boundary condition, we point out that its on-shell action diverges due to higher-dimensional operators, but this problem can in principle be ameliorated by taking into account the renormalization group flow. However, utilizing the Picard-Lefschetz theory to identify the relevant critical points and performing the complex lapse integration, we find that only the tunneling wave function can be obtained, as in the case of general relativity. On the other hand, for the Robin boundary condition with a particular imaginary Hubble expansion rate at the initial hypersurface, the no-boundary wave function can be achieved in the Hořava-Lifshitz gravity.

gr-qc↗

arXiv2023

Observational bounds on extended minimal theories of massive gravity: New limits on the graviton mass

In this work, we derive for the first time observational constraints on the extended Minimal Theory of Massive Gravity (eMTMG) framework in light of Planck-CMB data, geometrical measurements from Baryon Acoustic Oscillation (BAO), Type Ia supernovae from the recent Pantheon+ samples, and also using the auto and cross-correlations cosmic shear measurements from KIDS-1000 survey. Given the great freedom of dynamics choice for the theory, we consider an observationally motivated subclass in which the background evolution of the Universe goes through a transition from a (positive or negative) value of the effective cosmological constant to another value. From the statistical point of view, we did not find evidence of such a transition, i.e. deviation from the standard $Λ$CDM behavior, and from the joint analysis using Planck + BAO + Pantheon+ data, we constrain the graviton mass to $< 6.6 \times 10^{-34}$ eV at 95% CL. We use KIDS-1000 survey data to constrain the evolution of the scalar perturbations of the model and its limits for the growth of structure predicted by the eMTMG scenario. In this case, we find small evidence at 95% CL for a non-zero graviton mass. We interpret and discuss these results in light of the current tension on the $S_8$ parameter. We conclude that, within the subclass considered, the current data are only able to impose upper bounds on the eMTMG dynamics. Given its potentialities beyond the subclass, eMTMG can be classified as a good candidate for modified gravity, serving as a framework in which observational data can effectively constrain (or confirm) the graviton mass and deviations from the standard $Λ$CDM behavior.

astro-ph.CO↗

arXiv2023

Exploring spin of ultralight dark matter with gravitational wave detectors

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. We find that the finite-time traveling effect causes a dominant signal for spin-0 and spin-1 ULDM, while not for spin-2. By using overlap reduction functions (ORF) of multiple detectors, we can differentiate between the spins of ULDM. Furthermore, we point out that the current constraint on the coupling constant of spin-1 ULDM to baryons becomes 30 times weaker when the finite-time light-travel effect on the ORF is taken into account.

hep-ph↗

arXiv2023

Multi-scalar theories of gravity with direct matter couplings and their parametrized post-Newtonian parameters

We study theories of gravity including, in addition to the metric, several scalar fields in the gravitational sector. The particularity of this work is that we allow for direct couplings between these gravitating scalars and the matter sector, which can generally be different for the source and the probe of gravity, in addition to the universal interactions generated by the Jordan frame metric. The weak gravity regime of this theory, which would describe solar-system experiments, is studied using the parametrized post-Newtonian (PPN) formalism. We derive the expression of the ten parameters of this formalism. Among them, $ζ_3$ and $ζ_4$ are modified with respect to their values in the theories without direct couplings. This fact holds even after eliminating the direct couplings between the gravitating scalars and the energy density of the source, by redefinition of the Jordan frame. All other PPN parameters are insensitive to the direct couplings once in the correctly identified Jordan frame. When direct couplings are different for the source and the probe of gravity, they make non-relativistic probes deviate from the geodesics of the PPN metric in this frame, already at Newtonian order. Such couplings would thus be directly detectable and would have excluded by experiments. This shows that, contrary to the claims in the recent literature, it is impossible to screen the presence of gravitating scalars relying only on a curved target space and direct couplings to matter.

gr-qc↗

arXiv2023

Quasinormal Modes from EFT of Black Hole Perturbations with Timelike Scalar Profile

The Effective Field Theory (EFT) of perturbations on an arbitrary background geometry with a timelike scalar profile was recently constructed in the context of scalar-tensor theories. In this paper, we use this EFT to study quasinormal frequencies of odd-parity perturbations on a static and spherically symmetric black hole background. Keeping a set of operators that can accommodate shift-symmetric quadratic higher-order scalar-tensor theories, we demonstrate the computation for two examples of hairy black holes, of which one is the stealth Schwarzschild solution and the other is the Hayward metric accompanied by a non-trivial scalar field. We emphasize that this is the first phenomenological application of the EFT, opening a new possibility to test general relativity and modified gravity theories in the strong gravity regime.

gr-qc↗

arXiv2023

Instability of scalarized compact objects in Einstein-scalar-Gauss-Bonnet theories

We investigate the linear stability of scalarized black holes (BHs) and neutron stars (NSs) in the Einstein-scalar-Gauss-Bonnet (GB) theories against the odd- and even-parity perturbations including the higher multipole modes. We show that the angular propagation speeds in the even-parity perturbations in the $\ell \to \infty$ limit, with $\ell$ being the angular multipole moments, become imaginary and hence scalarized BH solutions suffer from the gradient instability. We show that such an instability appears irrespective of the structure of the higher-order terms in the GB coupling function and is caused purely due to the existence of the leading quadratic term and the boundary condition that the value of the scalar field vanishes at the spatial infinity.~This indicates that the gradient instability appears at the point in the mass-charge diagram where the scalarized branches bifurcate from the Schwarzschild branch. We also show that scalarized BH solutions realized in a nonlinear scalarization model also suffer from the gradient instability in the even-parity perturbations. Our result also suggests the gradient instability of the exterior solutions of the static and spherically-symmetric scalarized NS solutions induced by the same GB coupling functions.

gr-qc↗