Fluid/gravity correspondence and the CFM black brane solutions R. Casadio,1,2,∗ R. T. Cavalcanti,1,3,† and Rold˜ao da Rocha4,‡ 1Dipartimento di Fisica e Astronomia, Universit`a di Bologna via Irnerio 46, I-40126 Bologna, Italy 2I.N.F.N., Sezione di Bologna, via B. Pichat 6/2, I-40127 Bologna, Italy 6 3Centro de Ciˆencias Naturais e Humanas, Universidade Federal do ABC - UFABC, 1 0 09210-580, Santo Andr´e, Brazil 2 4Centro de Matem´atica, Computa¸c˜ao e Cogni¸c˜ao, Universidade Federal do ABC - UFABC, p e 09210-580, Santo Andr´e, Brazil. S 0 We consider the lower bound for the shear viscosity-to-entropy density ratio, obtained 3 from the fluid/gravity correspondence, in order to constrain the post-Newtonian parameter ] h of brane-world metrics. In particular, we analyse the Casadio-Fabbri-Mazzacurati (CFM) t - effectivesolutionsforthegravitysideofthecorrespondenceandarguethatincludinghigher p e order terms in the hydrodynamic expansion can lead to a full agreement with the experi- h [ mental bounds, for the Eddington-Robertson-Schiff post-Newtonian parameter in the CFM 2 metrics.Thislendsfurthersupporttothephysicalrelevanceoftheviscosity-to-entropyratio v 2 lower bound and fluid/gravity correspondence. Hence we show that CFM black branes are, 2 2 effectively, Schwarzschild black branes. 3 0 PACS numbers: 11.25.Tq, 11.25.-w, 04.50.Gh . 1 0 6 1 : v i X r a ∗ [email protected] † [email protected] ‡ [email protected] 2 I. INTRODUCTION Black hole solutions of the Einstein equations in general relativity (GR) are useful tools to investigate the space-time structure and quantum effects in any possible theory of gravity. In particular,modelswithextraspatialdimensions areprominentcandidatesasextensionsofEinstein GR[1],leadingtoimportantconsequences,notonlyforblackholephysics[2–4],butalsoforparticle physics,cosmology,aswellasfortheastrophysicsofsupermassiveobjects[5].Inthiscontext,black strings play a notable role in the quest for physically viable and realistic models [6–8]. Fluid dynamics is an effective description of interacting quantum field theories (QFTs) at long wavelengths [9]. As a low energy effective field theory, fluid dynamics is determined by a derivative expansion of the local fluid variables, and is used to describe near equilibrium systems. The deriva- tive expansion is determined by the symmetries of the system and its thermodynamical features. Transport coefficients, such as viscosities and conductivities, assess how perturbations propagate underrelaxationand,besidesbeingtheoreticallypredictable,theycanbemeasuredexperimentally. The AdS/CFT correspondence, which relates the coupled dynamics of SU(N) gauge theories to gravityinanti-deSitter(AdS)spaces[10–13],hasbeenusedtoconnectthosetworealmsofmodern physics, namely gravity and QFT. It holographically relates a strongly interacting QFT at large N with the dynamics provided by gravity in asymptotically AdS space-time. In the long wavelength limit, the fluid/gravity correspondence has been recently proposed as a framework related to the AdS/CFT correspondence [14–16], mapping black holes in asymptotically AdS space-times to the fluiddynamicsof astronglycoupledboundaryfieldtheory.Oneof themostremarkablepredictions ofAdS/CFTandfluid/gravitycorrespondenceistheratiooftheshearviscosity-to-entropydensity, which is universal for a large class of strongly coupled (gauge theory) isotropic plasmas [9]. The universality of this ratio plays a prominent role for gauge theories that are dual to gravitational backgrounds [14–20]. Moreover, the same results were obtained by employing the Green-Kubo formula in Ref. [21]. On the other hand, black strings present hydrodynamic features such as viscosity, diffusion rates, diffusion constants and other transport coefficients [22], besides temperature and entropy. Fromtheperspectiveoftheholographicprinciple[10],ablackstringcorrespondstoacertainfinite- temperature QFT in lower dimensions. In this context, the hydrodynamic features of the horizon of a black string can be identified with the hydrodynamic behaviour of a dual theory. For such 3 QFTs, the ratio of the shear viscosity η to entropy density s is bounded by the universal value [17] η (cid:126) (cid:38) (cid:39) 6.08×10−13 Ks , (1) s 4πk B for an extended range of thermal QFTs. The above inequality holds for all known substances, including, for example, liquid helium, water, and the quark-gluon plasma, produced at the Rel- ativistic Heavy Ion Collider (RHIC). Therefore, it has been conjectured to represent a universal lower bound for all materials 1, η 1 (cid:38) , (2) s 4π which is called the Kovtun-Starinets-Son (KSS) bound [16, 17]. In the case of Einstein gravity, the ratio η/s equals 1/4π. This bound is obtained in bulk supergravity from a recent calculation in N = 4 supersymmetric SU(N ) Yang-Mills theories, in the regime of infinite N and large ’t Hooft c c coupling g2N [18], yielding c (cid:20) (cid:21) η 1 135ζ(3) = 1+ +··· , (3) s 4π 8(2g2N )3/2 c where ζ(3) is the Ap´ery constant, and the next-to-leading term represents the first string theory correction to GR. The KSS bound is a powerful tool for studying strongly interacting systems, like the quark-gluon plasma, and is further employed to analyse trapped atomic gases. Strongly interacting Fermi gases of atoms have been noticed to be ruled by hydrodynamics [23], presenting finite shear viscosity at finite temperature. The bound (2) holds in relativistic QFTs at finite temperature and chemical potential [16, 17], for at least a single component non-relativistic gas of particles with either spin zero or spin 1/2 [24]. The entropy of the dual QFT equals the entropy of a black string, which is proportional to the area A (volume) of its event horizon in Planck units, S = A/4G , where G is the five-dimensional gravitational constant. 5 5 According to the AdS/CFT duality, any given asymptotically AdS bulk geometry should be equivalent to specific states of the gauge theory on the boundary. In fact, the AdS bulk geometry can be mapped into vacuum states of the gauge theory, whereas AdS-Schwarzschild black holes are related to thermal states of the corresponding gauge theory on the boundary. Hence, a fluid can be associated with the bulk black hole solution. Moreover, the bulk dynamics is governed by 1 We shall mostly use units with (cid:126) = c = k = 1, four-dimensional indices will be represented by Greek letters, B µ=0,...,3 and five-dimensional indices by capital latin letters, A=0,...,4. 4 Einstein’sequations,withtheAdS-Schwarzschildblackholeasatypicalsolution[25].Eachdifferent solution is equivalent to a thermal state, corresponding to black strings in AdS [26]. The dynamical framework can be realised by a system in local thermodynamical equilibrium, corresponding to a non-uniform black string in AdS, which evolves according to effective hydrodynamical equations corresponding to the Einstein’s equations [25]. Such a system will subsequently relax towards a global equilibrium state, corresponding to a uniform black string in AdS [26]. Motivated by the above picture, we shall here explore the fluid/gravity duality for the five- dimensional black string [5, 27, 28] associated with the effective four-dimensional Casadio-Fabbri- Mazzacurati (CFM) solutions [29]. CFM metrics generalise the Schwarzschild solution and have a parametrised post-Newtonian (PPN) form with parameter β, which is the usual Eddington- Robertson-Schiff parameter used to describe the classical tests of GR 2. This parameter allows for a direct comparison with experimental data in the weak-field limit, which is accurate enough to include most solar system tests [30]. In general, one can construct a dual fluid stress tensor by solvingthebulkEinsteinequationsinasymptoticallyAdSspacetimeandtakingthelongwavelength limit, but there are only a few black hole solutions that admit such a hydrodynamic dual on the boundary, even asymptotically. The prototype is the Schwarzschild metric, however the CFM solutions can also be obtained as four-dimensional vacuum brane solutions of Einstein gravity with a cosmological constant. In a second-order derivative theory of five-dimensional gravity interacting with other fields in AdS, for instance type II B supergravity in AdS ×S5, the CFM solutions can 5 be realised as universal sub-sectors, regarding pure gravity with negative cosmological constant, in the long-wavelength limit. Onthegravityside,weshallthusconsiderafive-dimensionalCFMblackstring,associatedwith afour-dimensionalCFMblackhole,andthefour-dimensionaldualfluiddescriptionwillnecessarily carryanimprintoftheparameterβ.Thefluid/gravitycorrespondencewillthenbeshowntoprovide aboundforβ,correspondingtotheuniversalKSSbound(2).Inparticular,boththeshearviscosity and black string entropy will depend on β, and Eq. (2) will read η(β) 1 (cid:38) , (4) s(β) 4π which is precisely what gives rise to the bound on β. We shall also argue that this bound can be 2 As usual [1], the four-dimensional brane-world black hole is viewed as a transverse section of the five-dimensional black string, producing real world post-Newtonian corrections. 5 made to agree with observations, provided higher order terms are included in the hydrodynamic expansion of the stress tensor. This paper is organised as follows: in Section II the CFM solutions are reviewed; in Section III, the Green-Kubo formula is used to bound the PPN parameter through the shear viscosity-to- entropy density ratio in the linear regime, and we show how the estimates improve by including higher-order hydrodynamical terms in Section IV; finally we discuss our results in Section V. II. CASADIO-FABBRI-MAZZACURATI BRANE-WORLD SOLUTIONS SolutionsoftheEinsteinfieldequationsonthebranearenotuniquelydeterminedbythematter energy density and pressure, since gravity can propagate into the bulk and generates a Weyl term onto the brane itself. Taking that into account, the CFM metrics [27] are vacuum brane solutions, like the tidally charge metric in Ref. [31], and contain a PPN parameter β, which can be measured on the brane. The case β = 1 corresponds to the exact Schwarzschild solution on the brane, and extends into a homogeneous black string in the bulk. Furthermore, it was observed in Ref. [29] that β ≈ 1, in solar system measurements. More precisely, the deflection of light in the classical tests of GR provides the bound |β−1| (cid:46) 0.003 [30]. The parameter β also measures the difference between the inertial mass and the gravitational mass of a test body, besides affecting the perihelion shift and describing the Nordtvedt effect [29]. Finally, measuring β provides information regarding the vacuum energy of the brane-world or, equivalently, the cosmological constant [1, 29]. We recall the effective four-dimensional Einstein equations on the brane can be expressed as Λ κ4 (cid:104)g (cid:16) (cid:17) (cid:105) G = 8πG T − 4 g + 5 µν T2−T Tαβ +T T −T Tα −E , (5) µν N µν 2 µν 4 2 αβ µν µα ν µν where κ2 = 8πG and G = κ2σ/48π (σ being the brane tension), T denotes the energy- 5 5 N 5 µν momentum tensor of brane matter, T ≡ Tµ, and E is the Weyl tensor term. For brane vacuum, µ µν T = 0, and absorbing the bulk cosmological constant Λ into the bulk warp factor, the above µν 4 field equations reduce to R = −E . (6) µν µν We are in particular interested in static spherically symmetric systems on the brane, whose general metric can be written as g dxµdxν = −N(r)dt2+B(r)dr2+r2dΩ2 . (7) µν 6 TheCFMmetricsareobtainedbyrelaxingtheconditionN(r) = B−1(r),valid,forexample,forthe SchwarzschildandReissner-Nordstr¨ommetrics.Infact,forblackstrings,suchaconditionwillresult in a central singularity extending all along the extra dimension and a singular bulk horizon [29], a configuration which moreover suffers of the well-known Gregory-Laflamme instability [32]. The CFM solution I is obtained, by fixing N(r) equal to the Schwarzschild form and then determining B(r) from the field equations (6), whereas the CFM solution II follows from the same procedure but starting from a metric coefficient N(r) of the Reissner-Nordstr¨om form [31]. A. CFM solution I For the first case, the metric coefficients in Eq. (7) are given by 2G M 1− 3GNM N (r) = 1− N and B (r) = 2r ≡ B(r) . (8) I I (cid:16) (cid:17)(cid:104) (cid:105) r 1− 2GNM 1− GNM(4β−1) r 2r The solution (8) depends upon just one parameter β and the Minkowski vacuum is recovered for M → 0. The horizon radius r = R on the brane is then determined by the algebraic equation 1/B(R) = 0, and this black hole is either hotter or colder than the Schwarzschild black hole of equal mass M depending upon the sign of (β −1) [29]. For example, assuming β = 5/4, one finds two solutions equal to the Schwarzschild radius R = 2G M. Note also that the four-dimensional S N Kretschmann scalar K(I) = R Rµνρσ diverges for r = 0 and r = 3G M/2 < R . µνρσ N S B. CFM solution II The second solution for the metric coefficients reads 2G M 2G2 M2 N (r) = 1− N + N (β−1) and B (r) = B(r) . (9) II r r2 II In this case the classical radius R for the black hole horizon is given by R = R and R = R (β− S S 1/4). Moreover the Kretschmann scalar diverges at each black hole horizon [5], hence indicating a physical singularity. A comprehensive analysis of the causal structure and further features on both CFM solutions can be found in Refs. [5, 27, 29]. 7 III. BOUNDING THE PPN PARAMETER IN THE LINEAR REGIME As previously mentioned, hydrodynamics can be viewed as a low-energy effective description for the low-momentum regime of correlation functions. We are particularly interested in the Green- Kuboformulainthelinearresponsetheory,whereanimportantfluidtransportcoefficient,theshear viscosity, arises in response to a perturbation in the fluid stress-energy tensor. The Kubo formula can further relate the shear viscosity to the absorption cross section of low-energy gravitons [19] 3. Let us consider an action S, and introduce a set of sources Ja coupled to a set of operators Oa, namely, S (cid:55)→ S +(cid:82) d4xJ (x)Oa(x), where xµ = (x0,xi). If the space-time is flat and the vacuum a expectation values (VEV) of all the Oa vanish in the absence of the sources J , one has a (cid:90) (cid:104)Oa(x)(cid:105) = − dyGa|b(x;y)J (y) , (10) R b where G (x;y) = G (x−y) is the retarded Green function of Oa in position space given by R R iGa|b(x;y) = θ(x0−y0)(cid:104)[Oa(x),Ob(y)](cid:105) . (11) R Using the interaction picture in QFT [13], one can immediately see that the variation δ(cid:104)Oa(q)(cid:105) = −Ga|b(q)J (q) , (12) R b where G (q) denotes the retarded Green function in momentum space with qµ = (ω,k). This R simple linear formalism can be generalised to curved space-times and non-vanishing VEV in order to describe a fluid in the conditions of interest here. A common way to derive the Kubo formula is, in fact, to couple four-dimensional gravity g to the fluid and determine how the fluid energy-momentum tensor responds to gravitational µν perturbations 4. In particular, for small metric perturbations which vary slowly in space and time, g = g¯ +h with(cid:107)h (cid:107) (cid:28) 1,thestresstensorcanbedoublyexpandedinthemetricfluctuation µν µν µν µν h and in gradients [33]. This procedure yields a generalisation of Eq. (10) to higher orders for µν the operator T , namely µν (cid:90) (cid:104)Tµν(x)(cid:105) = (cid:104)Tµν(cid:105) − 1 d4yGµν|ρσ(x;y)h (y) h=0 2 R ρσ (cid:90) (cid:90) +1 d4y d4zGµν|ρσ|τζ(x;y,z)h (y)h (z)+... 8 R ρσ τζ ≡ (cid:104)Tµν(cid:105)+(cid:104)Tµν(cid:105)+(cid:104)Tµν(cid:105)+··· , (13) (0) (1) (2) 3 Some equivalent approaches can be found, e.g., in Refs. [14, 16]. 4 Notethat,althoughthespace-timeisflatinourlaboratories,theeffectofspace-timecurvatureonhydrodynamics becomes extremely relevant in astrophysics. 8 µν|... where G are retarded n-point correlators, with the measurement point z having the largest R time, and we also assume the metric fluctuations h vanish in the far past. The fluid response ρσ is then obtained from the stress tensor conservation law ∇ Tµν = 0, together with the condition µ that the fluid describes a conformal theory, namely, Tµ = 0. We shall here analyse the first-order µ formalism, and illustrate how our results can be further refined by considering second-order terms in the next section. At zero order in derivatives, the energy momentum tensor reads Tµν = ((cid:15)+P)uµuν +P g¯µν , (14) (0) where uµ is the fluid four-velocity, (cid:15) denotes its energy density, P the pressure and g¯ is again the µν unperturbed metric on the four-dimensional boundary where the fluid lives 5. Since in the linear response theory the metric can be seen as the source of the stress-energy tensor, the response of the two-point function of the associated perturbed metric can then be (1) written as the first order term (cid:104)T (cid:105) in the expansion (13) [34], µν (cid:90) (cid:104)Tµν(x)(cid:105) ∼ dyGµν|αβ(x;y)h (y) , (15) R αβ where the retarded Green function is Gµν|αβ = (cid:104)Tµν(x)Tαβ(y)(cid:105). In the gauge/gravity duality, R the metric is dual to the stress-energy tensor [12]. Hence, from the gravity point of view of the correspondence,theaboveperturbationshouldarisefrommetricfluctuationsh oftheappropriate µν black brane metric. Thefirst-orderexpansion(inderivatives)ofthehydrodynamicalstress-energytensor,alsoknown as the constitutive equation, includes dissipative terms, as well as shear and bulk viscosities. In curved space-times, the first order contribution is given by [35] (cid:20) (cid:18) (cid:19) (cid:21) 2 Tµν = −PµαPνβ η ∇ u +∇ u − g¯ ∇ uλ +ζg¯ ∇ uλ , (16) (1) α β β α 3 αβ λ αβ λ where η is the shear viscosity, ζ the bulk viscosity, ∇ represents the covariant derivative in the µ (generally curved) space-time metric g¯ . In addition, Pµν = g¯µν +uµuν is the projection tensor µν alongspatialdirections,thatenablesustoexpresstheconstitutiveequationinacovariantmanner. We shall next restrict ourselves to a particular type of perturbations which is simpler to treat using the AdS/CFT correspondence. In fact, we suppose, as usual, that the contribution to the 5 Minkowski space-time is just a particular case corresponding to a Schwarzschild-type black brane in the bulk. 9 shearviscosityη comesonlyfromthecomponenth = h (t)ofh ,withallotherh = 0[13,34]. 12 12 µν µν Since we assumed such fluctuations around thermal equilibrium are small, we can say the fluid has uniform temperature T(xµ) = T and is at rest in the chosen frame, that is uµ = (1,ui = 0). 0 Moreover, this perturbation has spin-2 with respect to the spatial SO(3) group and it cannot excite linear order fluctuations of the velocity (which is a vector) or of the temperature (a scalar). Therefore, ui = 0 and T = T remain valid up to linear order. 0 The shear viscosity of the dual theory can be computed from gravity in a number of equivalent approaches [13, 14, 16]. We shall employ the Kubo formula, which relates the viscosity to equi- librium correlation functions, in order to compute the T12 component of the stress-energy tensor, in a large distance and long time scale regime. The non-vanishing contribution in the covariant derivative arises from the Christoffel symbol 1 ∇ u = ∂ u −Γα u = −Γ0 u = − ∂ h . (17) 1 2 1 2 12 α 12 0 2 0 12 The component ∇ u is obtained analogously and the other components vanish. Hence, only the 2 1 first two terms inside the brackets in Eq. (16) contribute to Tµν, and we have (1) δ(cid:104)T (cid:105) ∼ −η(∇ u +∇ u ) = −η∂ h . (18) (1)12 1 2 2 1 0 12 By taking the Fourier transform of Eq. (18), one obtains [34] δ(cid:104)T (ω,k = 0)(cid:105) = iωηh . (19) (1)12 12 AperturbedfluidLagrangianiscorrespondinglygivenbyδL = h (x0)Tµν(xα) = h (x0)T12(xα), µν 12 for which Eq. (12) reads δ(cid:104)T12(cid:105) = −G12|12(q)h , (20) R 12 where G12|12(q) = −i(cid:82) d4xe−iqµxµθ(x0)(cid:104)T12(xµ)T12(0)(cid:105). Eq. (19) represents the same linear re- R sponse relation as Eq. (20), and the Green-Kubo formula can therefore be obtained as [13, 34] 12|12 (cid:61)G (ω,0) η = − lim R , (21) ω→0 ω where (cid:61) denotes the imaginary part. Our first concern here is to provide analytic expressions for asymptotically AdS black strings which solve the full five-dimensional Einstein equations. The CFM black strings are “tubular”, 10 codimension one, black branes in AdS [5], corresponding to a late time behaviour of their dual fluid. Two families of such solutions are obtained from the same assumption that leads to the CFM brane metrics reviewed in Section II, namely by relaxing the condition that the metric coefficient g is(minus)theinverseofthemetriccoefficientg .Thesetwofamiliesofblackstringsarelikewise tt rr expressed in terms of the PPN parameter β [29], and reduce to the five-dimensional Schwarzschild blackstringsolution,whenβ → 1.CFMblackbranesinAdScanbealsoobtainedasaperturbative approachofCFMblackholesonthebrane[5].Ourfive-dimensionalmetricsare,inparticular,given by g dxAdxB = −N(r)dt2+B(r)dr2+r2dΩ2 , (22) AB where dΩ2 = (cid:96)−2d(cid:126)x2 = (cid:96)−2(dx2+dx2+dx2), the length (cid:96) is related to the AdS curvature [26, 36] 1 2 3 and the metric coefficients N and B equal the CFM expressions (8) or (9). The entropy of these black strings can be computed by using the Hawking-Bekenstein formula [16], A R3V 3 S = = , (23) 4G 4G (cid:96)3 5 5 (cid:82) where R is the horizon radius determined by 1/B(R) = 0 and V = (cid:96) dΩ. Moreover, the volume 3 density of entropy s = S/V must be finite, which requires that β < 5/4 in Eqs. (8) and (9). It 3 is also convenient to trade the radial coordinate r for the dimensionless u = R/r, and define the length M¯ = G M, so that N 1− 3M¯ u B(u) = 2R (24) (cid:16) (cid:17)(cid:104) (cid:105) 1− 2M¯ u 1− M¯ u(4β−1) R 2R and the metric finally reads R2 R2 g dxAdxB = −N(u)dt2+B(u) du2+ dΩ2 ≡ g du2+g dxµdxν . (25) AB u4 u2 uu µν Our main aim is now to predict a value for β exclusively from the KSS bound (2), implied by the fluid/gravity correspondence. The background metric (25) can be perturbed as g (cid:55)→ AB g +h [14,17].Asmentionedbefore,thecontributiontotheshearviscositycomesfromasingle AB AB component of the metric fluctuations. Denoting this term by φ = φ(t,u,(cid:126)x), the corresponding wave equation reads [16, 34] √ √ ∂ (cid:0) −gguu∂ φ(cid:1)+ −ggµν∂ ∂ φ = 0 . (26) u u µ ν