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Progress of Theoretical and Experimental Physics
, Volume Advance Article (5) – May 30, 2018

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13 pages

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- Oxford University Press
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- © The Author(s) 2018. Published by Oxford University Press on behalf of the Physical Society of Japan.
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- 2050-3911
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- 2050-3911
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- 10.1093/ptep/pty052
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Prog. Theor. Exp. Phys. 2018, 053E01 (13 pages) DOI: 10.1093/ptep/pty052 Asymptotically locally Euclidean/Kaluza–Klein stationary vacuum black holes in five dimensions 1 2 3,∗ Marcus Khuri , Gilbert Weinstein , and Sumio Yamada Department of Mathematics, Stony Brook University, Stony Brook, NY 11794, USA Physics Department and Department of Mathematics, Ariel University, Ariel, 40700, Israel Department of Mathematics, Gakushuin University, Tokyo 171-8588, Japan E-mail: yamada@math.gakushuin.ac.jp Received February 8, 2018; Revised March 27, 2018; Accepted April 12, 2018; Published May 30, 2018 ................................................................................................................... We produce new examples, both explicit and analytical, of bi-axisymmetric stationary vacuum black holes in ﬁve dimensions. A novel feature of these solutions is that they are asymptotically locally Euclidean, in which spatial cross-sections at inﬁnity have lens space L(p, q) topology, or asymptotically Kaluza–Klein so that spatial cross-sections at inﬁnity are topologically S × S . These are nondegenerate black holes of cohomogeneity 2, with any number of horizon components, where the horizon cross-section topology is any one of the three admissible types: 3 1 2 S , S × S ,or L(p, q). Uniqueness of these solutions is also established. Our method is to solve the relevant harmonic map problem with prescribed singularities, having target symmetric space SL(3, R)/SO(3). In addition, we analyze the possibility of conical singularities and ﬁnd a large family for which geometric regularity is guaranteed. ................................................................................................................... Subject Index B22, E01, E04 1. Introduction The study of higher-dimensional (D > 4) black holes has received substantial interest in recent years, primarily motivated by considerations in string theory. Some intriguing features of these objects, which separate them from their four-dimensional counterparts, include nontrivial horizon topologies and failure of the classical no-hair theorem [1]. Another curious attribute that has remained relatively unexplored is the possibility of nonstandard asymptotics, that is, the presence of ends that D−1,1 are not asymptotically ﬂat (approaching Minkowski space R ) or asymptotically Kaluza–Klein d,1 D−d−1 D−d−1 (approaching R ×T , where T is a torus). Trivial examples of such vacuum spacetimes may be constructed from the Schwarzschild–Tangherlini solution by replacing the round spheres D−2 D−2 S , which foliate the constant time slices, by quotients S /G where G is a discrete subgroup D−1,1 of the group of isometries O(D − 1). In this case the spacetime is asymptotic to R /G, and is neither asymptotically ﬂat nor asymptotically Kaluza–Klein; its horizon cross-section has topology D−2 S /G. Spacetimes with these asymptotics may be referred to as asymptotically locally Euclidean (ALE) [2] or asymptotically locally ﬂat (ALF) [3], although the later terminology is not consistent with deﬁnitions commonly used in the mathematics literature [4]. In Ref. [5], Lü, Mei, and Pope constructed explicit solutions of the stationary vacuum Einstein equations in D = 5 with a bi-axisymmetry. Thus, in contrast to the cohomogeneity 1 quotients of the Schwarzschild–Tangherlini spacetime above, these black holes are cohomogeneity 2. Moreover, in the static limit both the horizon topology and that of the spatial sections at inﬁnity are the lens space L(p, q) = S /Z , where p and q are relatively prime positive integers. These solutions are © The Author(s) 2018. Published by Oxford University Press on behalf of the Physical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 Funded by SCOAP by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. then ALE with nontrivial horizon topology. The general stationary metrics are characterized by four parameters: two independent angular momenta and one mass, as in the Myers–Perry solution, and one extra parameter that plays a role similar to a NUT charge. These solutions were found by following a method analogous to the procedure employed in the construction of general type D metrics in four dimensions [6,7]. See also Ref. [2] for other examples of ALE spacetimes. The purpose of the current paper is to introduce a new technique for constructing ALE as well as asymptotically Kaluza–Klein (AKK) stationary vacuum black holes in ﬁve dimensions. The methodology is versatile in that it allows for any combination of admissible horizon topology and spatial section at inﬁnity to coexist within the same spacetime. For example, a lens L(p, q) horizon 1 2 may be combined with a ring S × S spatial section at inﬁnity to yield an AKK lens black hole, or a ring horizon may be combined with a lens spatial section at inﬁnity to produce an ALE black ring. Furthermore, we are also able to assemble multiple component horizons of differing topologies, again with various spatial sections at inﬁnity. Our approach is based on solving the harmonic map problem with prescribed singularities which naturally arises from the dimensional reduction procedure for stationary bi-axisymmetric vacuum black holes. It is an extension of the work in Ref. [8], which treated the traditional asymptotically ﬂat case. More precisely, we will prove the existence and uniqueness of an axially symmetric harmonic map : R \ → SL(3, R)/SO(3) with prescribed singularities on a subset of the z-axis. The type of prescribed singularities determines not only the black hole topology but also the nature of the asymptotics at spatial inﬁnity. 2. Background and statement of main results Consider a stationary vacuum bi-axisymmetric ﬁve-dimensional spacetime M , and let ∂ , ∂ , i = 1, 2, denote the generators of the symmetry group R × U (1) . Under mild hypotheses, the orbit 5 2 space of the domain of outer communication M /[R × U (1) ] is known [9] to be homeomorphic to the right half-plane {(ρ, z) | ρ> 0}. This may be enhanced to cylindrical coordinates (ρ, z, φ) on R \{z − axis}, which plays the role of the domain for the relevant axisymmetric harmonic map. The boundary ρ = 0 of the right half-plane contains information concerning the topology of the horizon as well as the asymptotic structure at inﬁnity. In this regard the z-axis is broken into intervals called rods: =[z , ∞), =[z , z ], ... , =[z , z ], = (−∞, z ]. (2.1) 1 1 2 2 1 L L L−1 L+1 L On each interval a linear combination m ∂ + n ∂ vanishes, where m and n are integers that are 1 2 l l l l φ φ relatively prime whenever both are not zero. The tuple (m , n ) is referred to as the rod structure of l l the rod .A horizon rod is an interval on which no closed-orbit Killing ﬁeld degenerates, that is m = n = 0, and the remaining intervals are axis rods. End points of horizon rods are called poles, l l whereas the remaining interval end points are corners. The topology of a horizon component associated to a horizon rod may be identiﬁed from the rod structure as follows. Connect the adjacent rods via a semicircle in the right half-plane starting at and ending on , and enclosing . By turning on the U (1) symmetry, each point on l−1 l+1 l the interior of this semicircle represents a two-torus, and at each end point a one-cycle of the torus degenerates. Thus, we obtain a three-manifold with a singular foliation by tori, and the topology is determined by which one-cycles collapse at the end points. For example, if , have rod l−1 l+1 3 1 2 structures (1, 0), (0, 1) we obtain a sphere S , whereas (1, 0), (1, 0) yields a ring S × S , and (1, 0), (q, p) produces a lens L(p, q). Similarly, by foliating inﬁnity in the orbit space by such semicircles 2/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. connecting the two semi-inﬁnite rods and , the end within a constant time slice has topology 1 L+1 3 1 2 R × S (asymptotically ﬂat), R × S × S (AKK), and R × L(p, q) (ALE) respectively. Let p be a point on the z-axis where a corner is present, and let (m , n ) and (m , n ) be the rod l l l l+1 l+1 structures for the surrounding axis rods. In order to prevent orbifold singularities, the admissibility condition m n l l det =±1 (2.2) m n l+1 l+1 is imposed. A further hypothesis, referred to as the compatibility condition, will be needed for technical reasons arising from the harmonic map existence proof. To state this condition, let p l−1 and p be two consecutive corners, ﬂanked by axis rods , , and . It may be assumed l l−1 l l+1 without loss of generality that the determinants in Eq. (2.2) associated with the corners p and p l−1 l are both +1. We then require m m ≤ 0. (2.3) l−1 l+1 As is well known [3,10], the stationary bi-axisymmetric vacuum Einstein equations reduce to solving the harmonic map equations f km μ −1 μ lj τ :=f − f ∇ f ∇ f + f ∇ ω ∇ ω = 0, lj lm μ kj l μ j ω kl μ −1 lm μ τ :=ω − f ∇ f ∇ ω − f f ∇ f ∇ ω = 0, (2.4) j jl μ k lm μ j where the vector τ denotes the tension ﬁeld, F = (f ) isa2 × 2 symmetric positive deﬁnite matrix ij determining the rod structure, f = det F, and ω = (ω , ω ) are twist potentials. The spacetime 1 2 metric on M associated with these quantities is given in Weyl–Papapetrou coordinates by −1 2σ 2 2 −1 2 2 i i j j g = f e (dρ + dz ) − f ρ dt + f (dφ + v dt)(dφ + v dt), (2.5) ij where the v are obtained from the twist potentials by quadrature. From this we see that the rod structure may be interpreted as a vector (m , n ) lying in the (one-dimensional) kernel of the matrix l l F at an axis rod . Let be the union of all axis rods; then the relevant harmonic map : R \ → SL(3, R)/SO(3) may be constructed from (F, ω) and represented as a 3 × 3 symmetric positive deﬁnite unimodular matrix [11]. Boundary conditions for the potentials ω are given by constants c ∈ R on each axis rod , such that the values of the constants are the same on consecutive axis rods. Thus, these constants only change value across a horizon rod, and the difference is proportional to the angular momenta of the associated horizon component, as deﬁned by the Komar integrals (see Refs. [8,12]). We deﬁne a rod data set D to be the rods { } with rod structures {(m , n )}, and the potential constants {c }. l l l l Theorem 1. Given a rod data set D respecting the admissibility and compatibility conditions, there exists a unique harmonic map = (F, ω) : R \ → SL(3, R)/SO(3) which realizes the prescribed potential constants and rod structures of D. From this a stationary vacuum bi-axisymmetric black hole spacetime may be constructed with prescribed angular momenta, in which the topology of each 3 1 2 horizon component and spatial cross-section at inﬁnity is prescribed to be either S ,S × S ,or L(p, q). 3/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. This is analogous to the main result of Ref. [8], which treated the asymptotically ﬂat case. Here, in contrast, asymptotically Kaluza–Klein and asymptotically locally Euclidean black holes are produced 1 2 where the spatial cross-sections at inﬁnity are S × S and L(p, q), respectively. These solutions do not possess closed timelike curves due to the nature of their construction using the Weyl–Papapetrou form of the metric. It should also be noted that Theorem 1 yields essentially all possible black holes of this type. Two issues which are not immediately answered by the theorem are the questions of the analytic regularity of the metric coefﬁcients across the axes, and the possibility of conical singularities. Nevertheless, in Sects. 3 and 6 we are able to demonstrate that Theorem 1 produces geometrically regular (having no conical singularity) ALE black lenses and AKK black rings. These are the ﬁrst geometrically regular solutions to be constructed using a harmonic map approach. More traditional methods have previously been employed to ﬁnd explicit solutions in the AKK case, some of which coincide with the solutions produced here, as explained in Sect. 6. We refer the reader to Ref. [13] and the references therein for an account of explicit solutions. Theorem 2. The solutions produced in Theorem 1 have no conical singularity at spatial inﬁnity. In particular, the two semi-inﬁnite rods and are always void of conical singularities. 1 L+1 It is worth pointing out that this theorem applies to the asymptotically ﬂat case treated in Ref. [8], as this falls within the ALE setting. 3. Examples In this section we construct two classes of explicit solutions. The ﬁrst is a rotating lens black hole with an asymptotically locally Euclidean end, and the second is a static (non-rotating) ring black hole with an asymptotically Kaluza–Klein end. Both classes of spacetime will be used to construct model maps which play an important role in the proof of Theorem 1. The static example will be promoted to a fully rotating ring solution by application of Theorem 1, and will be shown to have no conical singularities in Sect. 6. The black lens examples will be geometrically regular by virtue of their construction. 3.1. Quotients of Myers–Perry Let M be a Myers–Perry black hole [14]. This is a three-parameter family of spherical black holes, parameterized by mass and two angular momenta. They are asymptotically ﬂat stationary vacuum 2 5 solutions with a bi-axisymmetry. Thus, the subgroup Z ⊂ U (1) acts on M by isometries of the form φ → φ +2π/p, φ → φ +2πq/p, where p and q are relatively prime positive integers. Since 1 1 2 2 this action is properly discontinuous the quotient M /Z is a smooth manifold, and with the quotient metric it is a solution of the vacuum Einstein equations. Cross-sections of the horizon now have the topology of the lens space L(p, q), and the asymptotic region has the topology R × R × L(p, q), making these solutions ALE. The quotient spacetime has three rods, =[a, ∞), =[−a, a], = (−∞, −a], (3.1) 1 2 3 and the corresponding rod structures are (1, 0), (0, 0), (q, p). Note that the northernmost and south- ernmost rods are axes, while the middle rod denotes a nondegenerate horizon for a > 0. Dimensional reduction yields a singular harmonic map : R \ → SL(3, R)/SO(3). These solutions are all regular. 4/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. A special case of these examples are the quotients of the Schwarzschild–Tangherlini black holes mentioned in the introduction. Moreover, in the limiting case where the potential constants vanish and a → 0 we obtain the quotients of Minkowski space by the same Z subgroups. Such solutions will have an orbifold singularity at the origin, but will be regular elsewhere. It is not difﬁcult to ﬁnd the corresponding harmonic map = (F , 0), namely lens lens r sin (θ /2) 0 10 F = h h , h = , (3.2) lens 0 r cos (θ /2) −q/p 1/p where (r, θ) are polar coordinates in the half-plane in which θ = 0 corresponds to the positive z-axis. These expressions will be used in the construction of model maps near inﬁnity in the next section. 3.2. Ring-like inﬁnity Consider the same set of three rods as in Eq. (3.1) and deﬁne e 0 F = , u = u + v , (3.3) ring a −a where 2 2 u = log(r − (z − a)) = log 2r sin (θ /2) , v = log(r + (z − a)) = log 2r cos (θ /2) . a a a a a a a a (3.4) Here, (r , θ ) denotes polar coordinates in the half-plane centered at the point z = a on the z-axis. a a Since the functions u and v are harmonic, and hence so is u, it follows that = (F , 0) is a a a ring ring harmonic map. The rod structure from north to south is (1, 0), (0, 0), (1, 0). The horizon topology 1 2 1 2 is then that of the ring S × S , and the asymptotic region has the topology R × R × S × S . This therefore gives rise to a static AKK black ring spacetime. Such solutions, often referred to as Schwarzschild black strings, have been found previously in Ref. [15] via a different approach. These will also be used in the construction of model maps in the next section. In order to construct the spacetime metric in Eq. (2.5), it remains to ﬁnd σ . After a series of standard calculations [8]weget ∂ σ =−ρT , ∂ σ = ρT , (3.5) ρ zz z ρz where 1 2 1 2 1 (∂ u) − (∂ u) ∂ u∂ u ρ z ρ z dρ 4 4 2 i j 1 2 T dx ⊗ dx = (dρ dz) , x = ρ, x = z. ij 1 1 2 1 2 dz ∂ u∂ u − (∂ u) + (∂ u) ρ z ρ z 2 4 4 (3.6) The harmonic map equations guarantee that the right-hand sides of Eq. (3.5) form a closed one-form, guaranteeing the existence of σ . Using the formula 2 2 2 2 u = log ρ + (z − a) − (z − a) ρ + (z + a) + (z + a) , (3.7) 5/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. an expression for σ may then be found by integrating the differential dσ . In the limiting case when 3,1 1 a → 0wehave u = 2 log ρ and σ = log ρ, so that the spacetime is R × S with ﬂat metric 2 2 1 2 2 2 2 2 g =−dt + ρ (dφ ) + (dφ ) + dρ + dz . (3.8) 4. The model maps In this section we construct the model map : R \ → SL(3, R)/SO(3) used to prescribe the singular behavior of the desired harmonic map near the axis , as well as the asymptotics at inﬁnity. The requirement on this map is that it has uniformly bounded tension |τ( )| < C, and −α decays appropriately: |τ( )|= O(r ), α> 2. The harmonic maps produced in Theorem 1 will be asymptotic to the model map in the sense that the distance d(, ) in the target SL(3, R)/SO(3) will remain bounded near , and the distance will be asymptotic to zero at inﬁnity. The model map may be thought of as an approximate solution to the singular harmonic map problem on which the exact solution will be built. Before proceeding with the construction, we ﬁrst collect a few formulas from Ref. [8] that are needed to aid the computations. Using the same parameterization (F, ω) of the target manifold as described in the previous section, the symmetric space metric takes the form 2 ij 1 df 1 1 f dω dω 1 1 i j ij kl −1 2 −1 −1 + f f df df + = [Tr(F dF )] + Tr(F dF F dF ) ik jl 4 f 4 2 f 4 4 t −1 1 dω F dω + , (4.1) 2 f where f = det F. From this and the harmonic map equations in Eq. (2.4), the norm of the tension is found to be 1 1 1 2 2 t |τ | = [Tr(div H + G)] + Tr [(div H + G)(div H + G)] + f (div K ) F (div K ), (4.2) 4 4 2 where −1 −1 −1 2 −1 −1 H = F ∇F, G = f F (∇ω) , K = f F ∇ω. (4.3) 4.1. Model map for ALE solutions For the sake of having a speciﬁc example in mind, consider a conﬁguration with a spherical S horizon cross-section topology and a lens L(p,1) spatial cross-section topology at inﬁnity. The rod structure from north to south should then be (1, 0), (0, 0), (0, 1), (1, p), as in Fig. 1. The construction of the model map differs from that in Ref. [8] only in the exterior region. Thus, on the region interior to the large ball in Fig. 1 all the desired properties of the model map are known to be valid. Outside the large ball in Fig. 1 deﬁne = (F, ω(θ)), where F = F is as in Sect. 3.1 and ω(θ) is independent of r. 0 lens −1 Note that since div(F ∇F ) = 0, whenever ω is constant this map is harmonic because G = K = 0. Therefore ω is chosen to be the required potential constants on the intervals [0, ]∪[π − , π ], and to smoothly connect these two constants on [, π − ]. It follows that |τ( )|= 0 in a neighborhood of the north and south axes, and is a smooth function elsewhere in the exterior region. It remains to show that the tension has the required fall-off at inﬁnity. Observe that direct computation yields t −7 f (div K ) F (div K ) = O(r ), (4.4) 6/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. Fig. 1. Model map construction. and −5 G = O(r ). (4.5) The exact expressions for these quantities are similar to those in Eqs. (4.6) and (4.7) below. Although it may appear that difﬁculties arise due to the negative powers of cos(θ /2) and sin(θ ), such expressions are always multiplied by derivatives of ω, and since these derivatives vanish near θ = 0, π the stated estimates hold. Now, using Eq. (4.2) and the fact that div H = 0, it follows that the tension decays −7/2 like O(r ). 4.2. Model map for AKK solutions Once again for the sake of having a speciﬁc example in mind, consider a rod structure 3 1 2 (1, 0), (0, 0), (0, 1), (1, 0). This corresponds to an S horizon cross-section with ring S × S spatial cross-sections at inﬁnity. As above, we need only give the construction outside the large ball since the prescription for the model map in the remaining portion of the domain is given in Ref. [8]. In the exterior region we deﬁne = (F, ω(θ)), where F = F is as in Sect. 3.2 and ω(θ) is as in 0 ring Sect. 4.1. Then div H = 0 in this region, and ω = 0 near the axes. Furthermore, calculations show that 4 6 16 csc θ sin (θ /2) t 6 f (div K ) F (div K ) = r csc (θ /2) ω − (csc θ + 2 cot θ)ω 1 1 +16 csc θ ω + (csc θ − 2 cot θ)ω 2 2 −8 = O(r ), (4.6) 7/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. and ⎛ ⎞ 2 2 2 ω cot (θ /2) ω ω cot (θ /2) 1 1 ⎜ ⎟ 4 4 −3 r r ⎜ ⎟ G = = O(r ). (4.7) ⎝ 2 2 2 ⎠ ω ω cos (θ /2) ω cos (θ /2) 1 2 3 3 r r −3 From Eq. (4.2) it follows that |τ|= O(r ). 5. Existence and uniqueness With the model map in hand, the proof of Theorem 1 may now be carried out following the now standard techniques originally developed in Ref. [16]. For the sake of completeness we sketch the arguments here. Given a rod data set D satisfying the hypotheses of Theorem 1, let be a corresponding model map constructed as in the previous section. It will be shown that there is a unique harmonic map : R \ → SL(3, R)/SO(3) which is asymptotic to . Recall that two such maps are said to be asymptotic if d(, ) remains bounded near , and d(, ) → 0as 0 0 r →∞, where d(, ) represents the distance in SL(3, R)/SO(3). As is shown in Ref. [8], two maps which are asymptotic give rise to the same rod structure, and thus the spacetime resulting from the harmonic map will have prescribed topology for the horizon cross-sections and spatial cross-sections at inﬁnity, as well as prescribed angular momenta for each horizon component. Consider ﬁrst the uniqueness portion of the result. Suppose that there are two harmonic maps , : R \ → SL(3, R)/SO(3). Due to the fact that the target space is nonpositively curved, a 1 2 computation yields 1 + d( , ) ≥− (|τ( )|+|τ( )|) = 0. (5.1) 1 2 1 2 If the two maps and are asymptotic to the model map then these maps are asymptotic to 1 2 0 each other. Therefore there is a uniform bound for the distance d( , ) ≤ C. Since the set is of 1 2 codimension 2, 1 + d( , ) is weakly subharmonic and the maximum principle applies (see 1 2 2 2 Lemma 8 of Ref. [16]). As 1 + d( , ) → 1 at inﬁnity, it follows that 1 + d( , ) ≤ 1. 1 2 1 2 Consequently, = . 1 2 The proof of existence proceeds as follows. Let ={x ∈ R : |x| < 1/, dist(x,)>}, and let : → SL(3, R)/SO(3) be the unique harmonic map with = on ∂ . Due to the boundedness and decay of |τ( )|, there exists a positive smooth function w on R satisfying w ≤−|τ( )| and w → 0 at inﬁnity. With the help of Eq. (5.1) we then have 2 2 1 + d( , ) − w ≥ 0, 1 + d( , ) − w ≤ 1on ∂ . (5.2) 0 0 The maximum principle again applies to yield a uniform L estimate for d( , ). This leads to a local pointwise energy estimate as in Sect. 6 of Ref. [8]. These bounds form the basis from which a bootstrap procedure can be employed to control all higher-order derivatives of on compact subsets. Hence, this sequence of maps subconverges to a harmonic map asymptotic to . 6. Conical singularities In this section we will prove Theorem 2. That is, it will be shown that there are no conical singularities on the two semi-inﬁnite rods and for any of the solutions produced in the previous section. 1 L+1 In particular, solutions having a single black hole and no corner points in the rod structure are void 8/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. of conical singularities. This gives geometrically regular examples of black holes/rings with AKK asymptotics, as well as black lenses with ALE asymptotics. The latter have already been exhibited in Sect. 3 as quotients of Myers–Perry black holes, whereas the former have previously been found with an alternative approach in Ref. [17]. We expect the techniques developed here to be useful for the more interesting problem of determining conditions for the absence of conical singularities on ﬁnite axis rods. The absence of a conical singularity on a rod requires 2 −1 2σ ρ f e lim = 1, (6.1) i j ρ→0 f u u ij where u = (m , n ) is the rod structure for . This is equivalent to l l l b = 2P(z) − Q(z) = 0, (6.2) where b is the angle deﬁcit, which is known to be constant on each rod [2,18], and i j u u 1 f ij P(z) := lim σ − log f , Q(z) := lim log . (6.3) ρ→0 2 ρ→0 ρ Consider now the two semi-inﬁnite rods and . By the regularity of the harmonic map and 1 L+1 d(, ) ≤ C it may be shown that the limit Q(z) exists. Moreover, since d(, ) → 0as r →∞ 0 0 we ﬁnd that Q(∞) − Q(−∞) = 0 as this property holds for the model map; details for this argument can be found in the proof of Theorem 11 in Ref. [8]. Observe that since 2(P(z) − P(−z)) = b − b + Q(z) − Q(−z), (6.4) 1 L+1 we have r r 4 2 (P(z) − P(−z))dz = b − b + (Q(z) − Q(−z))dz. (6.5) 1 L+1 r r r/2 r/2 Thus, if it can be shown that (P(z) − P(−z))dz = o(r) as r →∞, (6.6) r/2 then b = b . Furthermore, since σ is obtained by quadrature it is only deﬁned up to a constant, 1 L+1 and by choosing this constant appropriately we may assume without loss of generality that b = 0. The desired conclusion b = b = 0 now follows. The rest of this section is dedicated to verifying 1 L+1 the claim in Eq. (6.6). Let γ(θ) = (r sin θ , r cos θ) be a large semicircle connecting to in the ρz-plane, so that 1 L+1 f f ρ z P(−z) − P(z) = σ − dρ + σ − dz. (6.7) ρ z 2f 2f 9/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. According to Ref. [10], 2 2 ij f − f f ρ 2f 4f ρ ρ z ρ ij kl α := σ − = + f f (f f − f f ) + (ω ω − ω ω ) − , ρ ρ ik,ρ jl,ρ ik,z jl,z i,ρ j,ρ i,z j,z 2f 8 f f ρf ij f ρ f f 2f 2f z ρ z z ij kl α := σ − = + f f f f + ω ω − , (6.8) z z ik,ρ jl,z i,ρ j,z 2f 4 f f ρf and therefore r r π (P(−z) − P(z))dz = (α cos θ − α sin θ)rdθdr ρ z r/2 r/2 0 1 cos θ sin θ = α − α dx. (6.9) ρ z 2π ρ ρ B \B r r/2 It turns out that this integral may be estimated in terms of a reduced (or renormalized) energy, which will be shown to have the appropriate asymptotics. Consider ﬁrst the ρ-term, and observe that it may be re-expressed as ij α f 2 ij 0ij 0 kl 0kl 0 8 = 4E () − 2(∂ log f ) − (f f − f f )(f f − f f ) − 4 ω ω z ik,z lj,z i,z j,z ik,z lj,z ρ f ij kl 0ij 0 kl 0kl 0 0ij 0kl 0 0 2 2 − f f f f + 2f f (f f − f f ) + f f f f −|∇ log ρ | , (6.10) ik,z jl,z lj,ρ ik,ρ lj,ρ ik,ρ jl,ρ where the energy density and reduced energy density are given by ij 1 1 1 f 2 ij kl E () = |∇ log f | + f f ∇f ·∇f + ∇ω ·∇ω , (6.11) ik jl i j 4 4 2 f ij 1 1 1 f 2 2 ij 0ij 0 kl 0kl 0 E () = |∇(log f − log ρ )| + (f ∇f − f ∇f ) · (f ∇f − f ∇f ) + ∇ω ·∇ω . ik lj i j ik lj 4 4 2 f (6.12) All the terms on the ﬁrst line of Eq. (6.10) are part of the reduced energy density. Let us now 0 0 estimate the last term on the second line. In the AKK case the model map matrix F = (f ) is ij diagonal [see Eq. (3.3)], and therefore a computation yields 0ij 0kl 0 0 0 2 0 2 f f f f = (∂ log f ) + (∂ log f ) ρ ρ ik,ρ jl,ρ 11 22 2 cos θ − cos θ 4 4a sin θ sin θ a −a = + = − + O . (6.13) 2 2 2 2 ρ ρ ρ rρ r ρ 0 0 −1 0 t −1 In the ALE case, although F is not necessarily diagonal [see Eq. (3.2)], F = h F (h ) is diagonal so that 0ij 0kl 0 0 0ij 0kl 0 0 ˜ ˜ ˜ ˜ f f f f = f f f f ik,ρ jl,ρ ik,ρ jl,ρ 0 2 0 2 ˜ ˜ = (∂ log f ) + (∂ log f ) ρ ρ 11 22 2 2 1 + cos θ 1 − cos θ 4 2 sin θ = + = − . (6.14) 2 2 ρ ρ ρ ρ 10/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. It follows that in both cases 0ij 0kl 0 0 2 2 f f f f −|∇ log ρ | cos θdx = O , (6.15) ik,ρ jl,ρ B \B r r/2 since the terms involving sin θ integrate to zero against cos θ . The ﬁrst term on the second line of Eq. (6.10) may be rewritten as ij kl ij 0ij 0 kl 0kl 0 f f f f = (f f − f f )(f f − f f ) ik,z jl,z ik,z lj,z ik,z lj,z 0ij 0 kl 0kl 0 0ij 0kl 0 0 + 2f f (f f − f f ) + f f f f . (6.16) lj,z ik,z lj,z ik,z jl,z Moreover, in a similar manner to the above calculations we ﬁnd that in the AKK case 2 2 1 1 a cos θ 1 0ij 0kl 0 0 0 2 0 2 f f f f = (∂ log f ) + (∂ log f ) = − = + O , (6.17) z z ik,z jl,z 11 22 4 5 r r r r a −a while in the ALE setting 0ij 0kl 0 0 0ij 0kl 0 0 0 2 0 2 ˜ ˜ ˜ ˜ ˜ ˜ f f f f = f f f f = (∂ log f ) + (∂ log f ) = . (6.18) z z ik,z jl,z ik,z jl,z 11 22 Hence, in both cases 0ij 0kl 0 0 f f f f cos θdx = O , (6.19) ik,z jl,z B \B r r/2 since the term on the right-hand side of Eqs. (6.17) and (6.18) integrates to zero against cos θ . Combining the above computations with Eq. (6.10) yields cos θ 1 0ij 0 kl 0kl 0 α dx ≤ 2 E ()dx + f f (f f − f f ) cos θdx ρ lj,ρ ik,ρ lj,ρ ρ 4 B \B B \B B \B r r/2 r r/2 r r/2 1 1 0ij 0 kl 0kl 0 + f f (f f − f f ) cos θdx + O . (6.20) lj,z ik,z lj,z 4 r B \B r r/2 The reduced energy may be estimated in two possible ways. One method exploits the fact that the target symmetric space SL(3, R)/SO(3) for the harmonic map has nonpositive curvature, and therefore the energy is naturally convex along geodesic deformations. Then, by connecting the harmonic map to its model map via a geodesic in the target space, and using that the energy is a convex function of the geodesic parameter, it can be shown that the reduced energy of is dominated by the reduced energy of . This provides the desired bounds for the reduced energy of . However, some care must be taken in implementing this procedure since the energy of (and of ) is inﬁnite. In order to prove that the reduced energy inherits the convexity property from the pure energy, a cut-off argument must be used near the axes. Such a procedure has been carried out successfully within the context of mass–angular momentum inequalities; see, for example, Refs. [19–22]. An alternate approach to estimating the reduced energy, which is more straightforward to carry out, consists of applying standard “energy” methods for obtaining local a priori estimates associated with an elliptic partial differential equation. This entails multiplying the Euler–Lagrange equations 11/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. by appropriate functions and integrating by parts. For example, consider the following equation arising from the harmonic map equations of Eq. (2.4) and the fact that log ρ is a harmonic function: ij (log f − log ρ ) =− ∇ω ·∇ω . (6.21) i j 2 2 Let χ be a smooth cut-off function with supp χ ⊂ B \B ; multiply Eq. (6.21)by χ (log f −log ρ ) 2r r/4 and integrate to ﬁnd ij 2 2 2 2 2 χ |∇(log f − log ρ )| dx = χ (log f − log ρ ) ∇ω ·∇ω dx i j B \B B \B 2r r/4 2r r/4 2 2 − 2 χ(log f − log ρ )∇χ ·∇(log f − log ρ )dx. B \B 2r r/4 (6.22) It follows that 2 2 2 2 2 χ |∇(log f − log ρ )| dx ≤ sup | log f − log ρ | |∇χ | dx B \B B \B B \B 2r r/4 2r r/4 2r r/4 ij 2 2 + 2 sup | log f − log ρ | χ ∇ω ·∇ω dx. (6.23) i j B \B B \B 2r r/4 2r r/4 2 −1 ij 0 Similarly, multiplying the harmonic map equation for ω by χ f f (ω − ω ) and integrating i j produces ij 0 0 ij f (ω − ω )(ω − ω ) f i j i j 2 2 χ ∇ω ·∇ω dx ≤ c sup |∇χ | dx. (6.24) i j f f B \B B \B B \B 2r r/4 2r r/4 2r r/4 Therefore, 2 2 2 χ |∇(log f − log ρ )| dx B \B 2r r/4 ij 0 0 f (ω − ω )(ω − ω ) i j i j 2 2 ≤ c sup | log f − log ρ | 1 + sup |∇χ | dx. (6.25) B \B B \B B \B 2r r/4 2r r/4 2r r/4 Since d(, ) → 0as r →∞, the term in brackets on the right-hand side of Eq. (6.25) decays at inﬁnity. Thus, by choosing a cut-off function χ ≡ 1on B \ B which vanishes outside B \ B , r r/2 2r r/4 so that |∇χ|∼ 1/r,wehave 2 2 |∇(log f − log ρ )| dx = o(r). (6.26) B \B r r/2 Similar considerations may be used to estimate all the remaining terms in the reduced energy, as well as the two other terms on the right-hand side of Eq. (6.20). It follows that cos θ α dx = o(r). (6.27) B \B r r/2 Moreover, an analogous procedure yields sin θ α dx = o(r). (6.28) B \B r r/2 12/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018 PTEP 2018, 053E01 M. Khuri et al. This, together with Eq. (6.9), gives the desired conclusion of Eq. (6.6). Acknowledgements M. Khuri acknowledges the support of National Science Foundation (NSF) Grant DMS-1708798. S. Yamada acknowledges the support of Japan Society for the Promotion of Science (JSPS) grants KAKENHI 24340009 and 17H01091. Funding Open Access funding: SCOAP . References [1] R. Emparan and H. S. Reall, Living Rev. Rel. 11, 6 (2008) [arXiv:0801.3471 [hep-th]] [Search INSPIRE]. [2] Y. Chen and E. Teo, Nucl. Phys. B 838, 207 (2010) [arXiv:1004.2750 [gr-qc]] [Search INSPIRE]. [3] S. Hollands and A. Ishibashi, Class. Quant. Grav. 29, 163001 (2012) [arXiv:1206.1164 [gr-qc]] [Search INSPIRE]. [4] G. Chen and X. Chen, arXiv:1505.01790 [math.DG]. [5] H. Lü, J. Mei, and C. N. Pope, Nucl. Phys. B 806, 436 (2009) [arXiv:0804.1152 [hep-th]] [Search INSPIRE]. [6] J. F. Plebañski, Ann. Phys. 90, 196 (1975). [7] J. F. Plebanski and M. Demianski, Ann. Phys. 98, 98 (1976). [8] M. Khuri, G. Weinstein, and S. Yamada, arXiv:1711.05229 [gr-qc] [Search INSPIRE]. [9] S. Hollands and S. Yazadjiev, Commun. Math. Phys. 302, 631 (2011) [arXiv:0812.3036 [gr-qc]] [Search INSPIRE]. [10] D. Ida, A. Ishibashi, and T. Shiromizu, Prog. Theor. Phys. Suppl. 189, 52 (2011) [arXiv:1105.3491 [hep-th]] [Search INSPIRE]. [11] D. Maison, Gen. Rel. Grav. 10, 717 (1979). [12] S. Hollands and S. Yazadjiev, Commun. Math. Phys. 283, 749 (2008) [arXiv:0707.2775 [gr-qc]] [Search INSPIRE]. [13] S. Tomizawa and H. Ishihara, Prog. Theor. Phys. Suppl. 189, 7 (2011) [arXiv:1104.1468 [hep-th]] [Search INSPIRE]. [14] R. C. Myers and M. J. Perry, Ann. Phys. 172, 304 (1986). [15] A. Chodos and S. Detweiler, Gen. Rel. Grav. 14, 879 (1982). [16] G. Weinstein, Math. Res. Lett. 3, 835 (1996). [17] D. Rasheed, Nucl. Phys. B 454, 379 (1995) [arXiv:hep-th/9505038][Search INSPIRE]. [18] T. Harmark, Phys. Rev. D 70, 124002 (2004) [arXiv:hep-th/0408141][Search INSPIRE]. [19] A. Alaee, M. Khuri, and H. Kunduri, Adv. Theor. Math. Phys. 20, 1397 (2016) [arXiv:1510.06974 [gr-qc]] [Search INSPIRE]. [20] A. Alaee, M. Khuri, and H. Kunduri, Ann. Henri Poincaré 18, 1703 (2017) [arXiv:1608.06589 [hep-th]] [Search INSPIRE]. [21] A. Alaee, M. Khuri, and H. Kunduri, Phys. Rev. Lett. 119, 071101 (2017) [arXiv:1705.08799 [hep-th]] [Search INSPIRE]. [22] M. Khuri and G. Weinstein, Calc. Var. Partial Differ. Equ. 55, 1 (2016) [arXiv:1502.06290 [gr-qc]] [Search INSPIRE]. 13/13 Downloaded from https://academic.oup.com/ptep/article-abstract/2018/5/053E01/5021511 by Ed 'DeepDyve' Gillespie user on 17 June 2018

Progress of Theoretical and Experimental Physics – Oxford University Press

**Published: ** May 30, 2018

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