Kerr-de Sitter Spacetime Research Articles

Stationarity and Fredholm Theory in Subextremal Kerr-de Sitter Spacetimes

In a recent paper, we proved that solutions to linear wave equations in a subextremal Kerr-de Sitter spacetime have asymptotic expansions in quasinormal modes up to a decay order given by the normally hyperbolic trapping, extending the results of Vasy (2013). One central ingredient in the argument was a new definition of quasinormal modes, where a non-standard choice of stationary Killing vector field had to be used in order for the Fredholm theory to be applicable. In this paper, we show that there is in fact a variety of allowed choices of stationary Killing vector fields. In particular, the horizon Killing vector fields work for the analysis, in which case one of the corresponding ergoregions is completely removed.

Wave equations in the Kerr–de Sitter spacetime: The full subextremal range

We prove that solutions to linear wave equations in a subextremal Kerr–de Sitter spacetime have asymptotic expansions in quasinormal modes up to a decay order given by the normally hyperbolic trapping, extending the result of the second named author (2013). The main novelties are a different way of obtaining a Fredholm setup that defines the quasinormal modes and a new analysis of the trapping of lightlike geodesics in the Kerr–de Sitter spacetime, both of which apply in the full subextremal range. In particular, this reduces the question of decay for solutions to wave equations to the question of mode stability.

Universality of the quantum energy flux at the inner horizon of asymptotically de Sitter black holes

Recently, it was found that the energy flux of a free scalar quantum field on a Reissner–Nordström–de Sitter spacetime has a quadratic divergence towards the inner horizon of the black hole. Moreover, the leading divergence was found to be state independent as long as the spectral gap of the wave equation on the spacetime is sufficiently large. In this work, we show that the latter result can be extended to all subextremal Reissner–Nordström–de Sitter and subextremal Kerr–de Sitter spacetimes with a positive spectral gap.

Test particle motion along equatorial circular orbits in the revisited Kerr–de Sitter spacetime

Test particle motion along equatorial circular orbits in the revisited Kerr–de Sitter spacetime

Dominant spacetime in three dimensional de Sitter gravity

In three dimensions, Kerr-de Sitter spacetime as a solution of Einstein gravity with positive cosmological constant has a single cosmological horizon. The flat-space limit (zero cosmological constant limit) of this spacetime is well-defined and yields the flat-space cosmological solution which is a significant spacetime in the context of flat-space holography. In this paper, we calculate the free energy of this spacetime and compare it with the free energy of the three-dimensional de Sitter spacetime. We investigate which one of these two spacetimes will dominate in the semi-classical approximation for estimating the partition function. It is shown that for the same temperature of cosmological horizon of two spacetimes this is the de Sitter spacetime which is always dominant. Hence, contrary to asymptotically flat and asymptotically AdS spacetimes, there is no phase transition in three dimensional de Sitter gravity.

Stationary observers in Kerr-de Sitter spacetimes

In this paper, we present a study of basic classes of stationary observers orbiting in stationary and axially symmetric Kerr-de Sitter spacetimes. Along with proper definitions of these classes, we focus on their possible matching on particular circular orbits. Especially, we determine a number of such orbits in a given spacetime characterized by its rotational and cosmological parameters that leads to a comprehensive classification of the Kerr-de Sitter spacetimes. Studied profiles of orbital velocities of the observers and of forces keeping them on their orbits directly reflect complex structure of the Kerr-de Sitter spacetimes that is caused by a unique interplay of attraction and rotation of a central object and an ambient cosmic repulsion.

Analyticity of Quasinormal Modes in the Kerr and Kerr–de Sitter Spacetimes

We prove that quasinormal modes (or resonant states) for linear wave equations in the subextremal Kerr and Kerr–de Sitter spacetimes are real analytic. The main novelty of this paper is the observation that the bicharacteristic flow associated to the linear wave equations for quasinormal modes with respect to a suitable Killing vector field has a stable radial point source/sink structure rather than merely a generalized normal source/sink structure. The analyticity then follows by a recent result in the microlocal analysis of radial points by Galkowski and Zworski. The results can then be recast with respect to the standard Killing vector field.

Stationary vacuum bubble in a Kerr–de Sitter spacetime

We study false vacuum decay in a black hole (BH) spacetime with an angular momentum. Considering the false vacuum region described by a Kerr--de Sitter geometry, under the thin wall approximation, we can obtain the stationary configuration of the vacuum bubble seen from the outside false vacuum region without specifying the geometry inside the domain wall. Then, assuming the true vacuum region is described by a Kerr geometry, we can fix the mass and the spin parameter for the Kerr geometry by imposing the first junction conditions and conservation of the angular momentum. Although the assumption of the Kerr geometry inside the domain wall cannot be fully consistent with the second junction conditions, we can roughly evaluate the error associated with this inconsistency by calculating the Brown-York quasilocal energy on the domain wall. Then the decay rate can be estimated by using the obtained parameters for the inside Kerr geometry and the Brown-York quasilocal energy. Our results support the statement that the BH spin suppresses the false vacuum decay in a BH spacetime.

Construction of the Unruh State for a Real Scalar Field on the Kerr-de Sitter Spacetime

The study of physical effects of quatum fields in black hole spacetimes, which is related to questions such as the validity of the strong cosmic censorship conjecture, requires a Hadamard state describing the physical situation. Here, we consider the theory of a free scalar field on a Kerr-de Sitter spacetime, focussing on spacetimes with sufficiently small angular momentum of the black hole and sufficiently small cosmological constant. We demonstrate that an extension of the Unruh state, which describes the expected late-time behaviour in spherically symmetric gravitational collapse, can be rigorously constructed for the free scalar field on such Kerr-de Sitter spacetimes. In addition, we show that this extension of the Unruh state is a Hadamard state in the black hole exterior and in the black hole interior up to the inner horizon. This provides a physically motivated Hadamard state for the study of free scalar fields in rotating black hole spacetimes.

Covariant classification of conformal Killing vectors of locally conformally flat n -manifolds with an application to Kerr–de Sitter spacetimes

We obtain a coordinate independent algorithm to determine the class of conformal Killing vectors of a locally conformally flat $n$-metric $\gamma$ of signature $(r,s)$ modulo conformal transformations of $\gamma$. This is done in terms of endomorphisms in the pseudo-orthogonal Lie algebra $\mathfrak{o}(r+1,s+1)$ up to conjugation of the its group $O(r+1,s+1)$. The explicit classification is worked out in full for the Riemannian $\gamma$ case ($r = 0, s = n$). As an application of this result, we prove that the set of five dimensional, $(\Lambda>0)$-vacuum, algebraically special metrics with non-degenerate optical matrix, previously studied by Bernardi de Freitas, Godazgar and Reall, is in one-to-one correspondence with the metrics in the Kerr-de Sitter-like class. This class exists in all dimensions and its defining properties involve only properties at $\mathscr{I}$. The equivalence between two seemingly unrelated classes of metrics points towards interesting connections between the algebraically special type of the bulk spacetime and the conformal geometry at null infinity

Hidden conformal symmetries in Kerr–de Sitter spacetime revisited

We study the holographic picture for the Kerr–de Sitter spacetime with multiple horizons. Motivated by the studies of hidden conformal symmetry that associates to the black hole horizon, we show that such symmetry can also exist for the cosmological horizon that can lead us to the holographic calculation for the cosmological horizon entropy. To support the holography, we provide the scalar scattering calculation in the corresponding near region of each horizon by using the dual CFT formula. As one would expect, the cosmological horizon entropy and the associated scalar scattering can be understood by using a two dimensional conformal field theory.

Islands in Kerr–de Sitter spacetime and their flat limit

We use quantum extremal island method to study the information paradox on certain cosmological setups known as three dimensional Kerr-de Sitter spacetimes. To do so, we couple an auxiliary flat bath system to this spacetime in timelike singularity and measure entropy of hawking radiation in its asymptotic regions where the gravity is weak. We show that adding island regions to the entanglement wedge of radiation causes its entropy to obey Page curve. The boundary of island i.e. quantum extremal surface is located outside the cosmological horizon in the region connected directly to the bath. Taking the flat-space limit from the location of island and its related calculation in the Kerr-de Sitter sapcetime results in the flat-space cosmology (FSC) island, scrambling time and also Page time that were obtained in our previous paper. We repeat the same calculation for the pure de Sitter spacetime and show that our setup which neglects the effect of backreaction, leads also to a quantum extremal surface outside the cosmological horizon. We calculate the scrambling time and confirm the idea that pure de Sitter spacetime is a fast scrambler.

General Relativistic Aberration Equation and Measurable Angle of Light Ray in Kerr–de Sitter Spacetime

As an extension of our previous paper, instead of the total deflection angle α, we will mainly focus on the discussion of measurable angle of the light ray ψP at the position of observer P in Kerr–de Sitter spacetime, which includes the cosmological constant Λ. We will investigate the contribution of the radial and transverse motion of the observer which are connected with radial velocity vr and transverse velocity bvϕ (b is the impact parameter) as well as the spin parameter a of the central object which induces the gravito-magnetic field or frame dragging and the cosmological constant Λ. The general relativistic aberration equation is employed to take into account the influence of motion of the observer on the measurable angle ψP. The measurable angle ψP derived in this paper can be applicable to the observer placed within the curved and finite-distance region in the spacetime. The equation of light trajectory will be obtained in such a sense that the background is de Sitter spacetime instead of Minkowski one. As an example, supposing the cosmological gravitational lensing effect, we assume that the lens object is the typical galaxy and the observer is in motion with respect to the lensing object at a recession velocity vr=bvϕ=vH=H0D (where H0 is a Hubble constant and D means the distance between the observer and the lens object). The static terms O(Λbm,Λba) are basically comparable with the second order deflection term O(m2), and they are almost one order smaller that the Kerr deflection −4ma/b2. The velocity-dependent terms O(Λbmvr,Λbavr) for radial motion and O(Λb2mvϕ,Λb2avϕ) for transverse motion are at most two orders of magnitude smaller than the second order deflection O(m2). We also find that even when the radial and transverse velocity have the same sign, asymptotic behavior as ϕ approaches 0 is different from each other, and each diverges to opposite infinity.

Skew-symmetric endomorphisms in : a unified canonical form with applications to conformal geometry

We show the existence of families of orthonormal, future directed bases which allow to cast every skew-symmetric endomorphism of () in a single canonical form depending on a minimal number of parameters. This canonical form is shared by every pair of elements in differing by an orthochronous Lorentz transformation, i.e. it defines the orbits of the orthochronous Lorentz group O +(1, n) under the adjoint action on its algebra. Using this form, we obtain the quotient topology of . From known relations between and the conformal Killing vector fields (CKVFs) of the sphere , a canonical form for CKVFs follows immediately. This form is used to find adapted coordinates to an arbitrary CKVF that covers all cases at the same time. We do the calculation for even n and obtain the case of odd n as a consequence. Finally, we employ the adapted coordinates to obtain a wide class of TT-tensors for n = 3, which provide Cauchy data at conformally flat null infinity . Specifically, this class of data is characterized for generating Λ > 0-vacuum spacetimes with two-symmetries, one of which axial, admitting a conformally flat . The class of data is infinite dimensional, depending on two arbitrary functions of one variable as well as a number of constants. Moreover, it contains the data for the Kerr–de Sitter spacetime, which we explicitly identify within.

Divergent part of the stress-energy tensor for Maxwell\u2019s theory in curved space-time: a systematic derivation

In this paper the Feynman Green function for Maxwell’s theory in curved space-time is studied by using the Fock–Schwinger–DeWitt asymptotic expansion; the point-splitting method is then applied, since it is a valuable tool for regularizing divergent observables. Among these, the stress-energy tensor is expressed in terms of second covariant derivatives of the Hadamard Green function, which is also closely linked to the effective action; therefore one obtains a series expansion for the stress-energy tensor. Its divergent part can be isolated, and a concise formula is here obtained: by dimensional analysis and combinatorics, there are two kinds of terms: quadratic in curvature tensors (Riemann, Ricci tensors and scalar curvature) and linear in their second covariant derivatives. This formula holds for every space-time metric; it is made even more explicit in the physically relevant particular cases of Ricci-flat and maximally symmetric spaces, and fully evaluated for some examples of physical interest: Kerr and Schwarzschild metrics and de Sitter space-time.

Dirac fermion, cosmological event horizons, and quantum entanglement

We discuss the field quantisation of a free massive Dirac fermion in the two causally disconnected static patches of the de Sitter spacetime, by using mode functions that are normalisable on the cosmological event horizon. Using this, we compute the entanglement entropy of the vacuum state corresponding to these two regions, for a given fermionic mode. Further extensions of this result to more general static spherically symmetric and stationary axisymmetric spacetimes are discussed. For the stationary axisymmetric Kerr-de Sitter spacetime in particular, the variations of the entanglement entropy with respect to various eigenvalues and spacetime parameters are depicted numerically. We also comment on such variations when instead we consider the non-extremal black hole event horizon of the same spacetime.

Constrained field theories on Kerr backgrounds

We analyze the constraints of gauge theories on Kerr and Kerr-de Sitter spacetimes, which contain one or more horizons. We find that the constraints are modified on such backgrounds through the presence of additional surface terms at the horizons. As a concrete example, we consider the Maxwell field and find that the Gauss law constraint involves surface corrections at the horizons. These surface contributions correspond to induced surface charges and currents on the horizons, which agree with those found within the membrane paradigm. The modification of the Gauss law constraint also influences the gauge fixing and Dirac brackets of the theory.

Particle creation by de Sitter black holes revisited

Creation of thermal distribution of particles by a black hole is independent of the detail of gravitational collapse, making the construction of the eternal horizons suffice to address the problem in asymptotically flat spacetimes. For eternal de Sitter black holes however, earlier studies have shown the existence of both thermal and non-thermal particle creation, originating from the non-trivial causal structure of these spacetimes. Keeping this in mind we consider this problem in the context of a quasistationary gravitational collapse occurring in a $(3+1)$-dimensional eternal de Sitter, settling down to a Schwarzschild- or Kerr-de Sitter spacetime and consider a massless minimally coupled scalar field. There is a unique choice of physically meaningful `in' vacuum here, defined with respect to the positive frequency cosmological Kruskal modes localised on the past cosmological horizon ${\cal C^-}$, at the onset of the collapse. We define our `out' vacuum at a fixed radial coordinate `close' to the future cosmological horizon, ${\cal C^+}$, with respect to positive frequency outgoing modes written in terms of the ordinary retarded null coordinate, $u$. We trace such modes back to ${\cal C^-}$ along past directed null geodesics through the collapsing body. Some part of the wave will be reflected back without entering it due to the greybody effect. We show that these two kind of traced back modes yield the two temperature spectra and fluxes subject to the aforementioned `in' vacuum. Since the coordinate $u$ used in the `out' modes is not well defined on a horizon, estimate on how `close' we might be to ${\cal C^+}$ is given by estimating backreaction. We argue no other reasonable choice of the `out' vacuum would give rise to any thermal spectra. Our conclusions remain valid for all non-Nariai class black holes, irrespective of the relative sizes of the two horizons.

Asymptotics for the wave equation on differential forms on Kerr–de Sitter space

We study asymptotics for solutions of Maxwell's equations, in fact of the Hodge-de Rham equation $(d+\delta)u=0$ without restriction on the form degree, on a geometric class of stationary spacetimes with a warped product type structure (without any symmetry assumptions), which in particular include Schwarzschild-de Sitter spaces of all spacetime dimensions $n\geq 4$. We prove that solutions decay exponentially to $0$ or to stationary states in every form degree, and give an interpretation of the stationary states in terms of cohomological information of the spacetime. We also study the wave equation on differential forms and in particular prove analogous results on Schwarzschild-de Sitter spacetimes. We demonstrate the stability of our analysis and deduce asymptotics and decay for solutions of Maxwell's equations, the Hodge-de Rham equation and the wave equation on differential forms on Kerr-de Sitter spacetimes with small angular momentum.

On causality violation on a Kerr-de Sitter spacetime

The causal properties of the family of Kerr-de Sitter spacetimes are analyzed and compared to those of the Kerr family. At first we show that a Kerr-de Sitter spacetime can be viewed as an assembly of Carter's blocks i.e. four dimensional spacetime regions contained within Killing horizons or a Killing horizon and the asymptotic de Sitter region. From this perspective and leaving aside topological identifications, the causal properties of a Kerr de Sitter spacetime are determined by the causal properties of the individual Carter's blocks viewed as spacetimes in their own right. We show that any Carter's block is stably causal except for the blocks that contain the ring singularity. The latter are vicious sets: any two events within such block can be connected by a future (respectively past) directed timelike curve. This behavior is identical to the causal behavior of the Boyer-Lindquist blocks that contains the Kerr ring singularity which are also vicious sets. On the other hand, while for the case of a naked Kerr singularity the entire spacetime is a vicious set and thus closed timelike curves pass through any event including events lying in the asymptotic region, for the case of a Kerr-de Sitter spacetime the cosmological horizons imply that the asymptotic de Sitter region is causally well behaved. In that regard a positive cosmological constant appears to improve the causal behavior of the underlying spacetime.