Large N Quantum Gravity ∗ Alessandro Codello † 2 January 12, 2012 1 0 2 n a J SISSA 1 via Bonomea 265, I-34136 Trieste, Italy 1 ] c q - r Abstract g [ We obtain the effective action of four dimensional quantum gravity, induced by N 2 v massless matter fields, by integrating the RG flow of the relative effective average ac- 8 0 tion. By considering the leading approximation in the large N limit, where one neglects 9 the gravitational contributions with respect to the matter contributions, we show how 1 . different aspects of quantum gravity, as asymptotic safety, quantum corrections to the 8 0 Newtonian potential and the conformal anomaly induced effective action, are all repre- 1 1 sented by different terms of the effective action when this is expanded in powers of the : v curvature. i X r a 1 Introduction Even if we are still lacking a quantum theory of gravitational phenomena we are starting to accumulate interesting partial results which will probably be important bits of the final theory. These are, among others, the conformal anomaly induced effective action, which was first written down in [1]; the low energy corrections to the Newtonian gravitational potential analyzed in [2]; the possible ultraviolet (UV) finite completion of the theory described in the asymptotic safety scenario [3]. In this paper we want to show how these different aspects can ∗Contribution to appear in "New Journal of Physics Focus Issue on Quantum Einstein Gravity". †[email protected] 1 all be seen as arising from different terms in the gravitational effective action when this is expandedinpowersofthecurvature. Inparticular,theUVpropertiesofthetheoryarerelated to the zero and first order terms, i.e. to the renormalization of Newton’s and cosmological constants, and to the renormalization of the coupling constants of all higher order local invariants; quantum gravitational corrections to the Newtonian potential are encoded in the finite part of the curvature square terms, while the conformal anomaly induced effective action is just one of the possible four curvature structures. We will use the effective average action formalism to obtain the gravitational effective action induced by N0 massless scalar fields, N1 massless Dirac spinors and N1 abelian gauge 2 fields, interacting solely with the background geometry, as the result of the integration of the 1 RG flow. In the large N expansion one assumes that the number of matter fields N grows large. In the leading approximation one simply neglects the gravitational contributions with respect to the matter contributions: this eliminates computational and conceptual issues related to the treatment of gravitational fluctuations. Within the effective average action formalism this point of view has been analyzed in [6], where the focus was on the local part of the effective average action and on the related renormalizability issues. Here we will extend the large N analysis to a non-local truncation of effective average action where this is expanded in powers of the curvature. 2 Effective average action for matter fields on curved space Following [6] we consider massless matter fields on a curved four dimensional manifold equipped with a metric g . The bare action we consider describes N massless scalar fields, µν 0 N1 massless Dirac spinors and N1 abelian gauge fields interacting only with the background 2 geometry: N1 N0 1 χ 2 S[φ,ψ,A ,c¯,c;g] = d4x√g ∂ φ ∂µφ + φ2R + ψ¯ /ψ µ ˆ  2 µ i i 12 i i∇ i Xi=1(cid:20) (cid:21) Xi=1 +N1 1F Fµν + 1 (∂ Aµ)2 +∂ c¯ ∂µc . (1) 4 µν,i i 2α µ i µ i i  Xi=1(cid:20) (cid:21) 1N is any of N0,N1,N1  2 2 Here χ is a parameter, only when χ = 1 the scalar action is conformally invariant if the scalar has conformal weight one. The Dirac operator is defined using the covariant Dirac matrices γµ = eµγa. Note also that on an arbitrary curved manifold the abelian ghosts do a not decouple and cannot be discarded; we will choose the gauge α = 1 from now on. The effective average action Γ [φ,ψ,A ,c¯,c;g] is a scale depended generalization of the k µ standard effective action (depending on the infrared (IR) cutoff scale k) that interpolates smoothly between the bare action for k and the full quantum effective action for k 0 → ∞ → [4]. It satisfies an exact RG flow equation describing its flow as the IR scale k is shifted. The flow equation relevant to the field content we are considering reads as follows: 1 ∂ R [g] t k ∂ Γ [ϕ;g] = Tr , (2) t k 2 Γ(2,0)[ϕ;g]+R [g] k k (2,0) where Γ [ϕ,g] is the Hessian of the effective average action taken with respect to the k field multiplet ϕ = (φ,ψ,A ,c¯,c) and the trace is a "super-trace" on this space. Equation µ (2) is exact and is both UV and IR finite. The flow equation (2) can be seen as a RG improvement of the one-loop effective action which is derived from the modified bare action S[ϕ,g] S[ϕ,g]+∆S [ϕ,g], where ∆S [ϕ,g] is a cutoff action quadratic in the fields. This k k → cutoffactionisconstructedinsuchawaytosuppressthepropagationofallfieldmodessmaller than the RG scale k. The effective average action and the exact RG flow it satisfies (2) offer a different approach to quantization: in theory space, the space of "all" action functionals, the bare action represents the UV starting point of a RG trajectory which reaches the quantum effective action in the IR. The integration of successive modes is done step by step as the cutoff scale k is lowered. More details on the construction of the effective average action on curved backgrounds or in presence of quantized gravity can be found in [5]. As explained in [8], since we are considering matter fields interacting only with the back- ground geometry, we can replace the the Hessian of the effective average action in the rhs of (2) with the Hessian of the bare action (1). After performing the field multiplet trace we 2 find : N N1 ∂tΓk[φ,ψ,Aµ,c¯,c;g] = 0Tr0hk(∆0) 2Tr1 hk(∆1) 2 − 2 2 2 1 +N Tr h (∆ ) Tr h (∆ ) , (3) 1 1 k 1 0 k gh 2 − (cid:20) (cid:21) where we defined the function h (z) = ∂tRk(z) and Tr indicates a trace over spin-s fields. k z+Rk(z) s 2We are using a Type II cutoff operator in the nomenclature of [5]. 3 The differential operators introduced in (3) are the following: χ 1 ∆0 = ∆+ R ∆1 = ∆+ R (∆1)µν = ∆gµν +Rµν ∆gh = ∆, (4) 6 2 4 where ∆ = gµν is the covariant Laplacian. In the next section we will determine the µ ν − ∇ ∇ RG flow of the effective average action by calculating the functional traces on the rhs of (3). 3 Flow equations and beta functions The traces on the rhs of the flow equation (3) can be expanded in powers of the curvature by employing the the non-local heat kernel expansion [7] as was done in [8, 9]. To second order 3 we find the following result : N0 4N1 +2N1 (4π)2∂ Γ [g] = − 2 Q [h ] d4x√g t k 2 2 k ˆ (1 χ)N0 +2N1 4N1 + − 2 − Q [h ] d4x√gR 12 1 k ˆ ∞ + d4x√gR dsh˜ (s)sF (s∆) R ˆ ˆ k R " 0 # ∞ + d4x√gC ds˜h (s)sF (s∆) Cµναβ ˆ µναβ ˆ k C " 0 # N0 +11N1 +62N1 2 Q [h ] ddx√gE − 720 0 k ˆ N0 +N1 3N1 + 2 − Q [h ] ddx√g∆R 60 0 k ˆ +O( 3), (5) R where stands for any curvature. The non-local heat kernel structure functions F (x) and C R F (x) in (5) are linear combinations of the basic non-local heat kernel structure function R f(x) = 1dξe−xξ(1−ξ) and read as follows: 0 ´ 5N0 20N1 +10N1f(x) 1 (3 2χ)N0 2N1 2N1f(x) F (x) = − 2 − + − − 2 − R 48 x2 48 x (13−12χ)N0 +8N12 −22N1 1 + (3−2χ)2N0 −2N1f(x) − 288 x 576 3Here we define Γk[g] Γk[0,0,0,0,0;g] ≡ 4 N0 4N1 +2N1f(x) 1 N1 2N1f(x) F (x) = − 2 − 2 − C 4 x2 − 4 x +N0 +2N12 −10N1 1 + N1f(x). (6) 24 x 8 ˜ In (5) h (s) is the (inverse) Laplace transform of the function h (x) and the Q-functionals k k are defined as Q [f] = 1 ∞dzzn−1f(z) when n > 0 and as Q [f] = ( 1)f(n)(0) when n Γ(n) 0 n − ´ n 0. ≤ Since the RG flow has generated all possible terms compatible with diffeomorphism in- variance on the rhs of (5), we need now to consider the most general truncation for the gravitational effective average action Γ [g] to insert in the lhs of (5). As proposed in [8, 9], k we consider an ansatz where the effective average action is expanded in powers of the cur- vature and where the scale dependence is carried both by running couplings and by running (possibly non-local) structure functions. To second order in the curvatures the expansion of 4 the gravitational effective average reads as follows : 1 Γ [g] = d4x√g (2Λ R)+Rf (∆)R+ k ˆ 16πG k − R,k (cid:20) k 1 1 +C f (∆)Cµναβ + E + ∆R +O( 3). (7) µναβ C,k ρk τk # R In (7) the couplings Λ and G are the running cosmological and Newton’s constants, f k k C,k and f are the two independent curvature square running structure functions (which at this R,k point are arbitrary functions of the covariant Laplacian ∆), while ρ and τ are the running k k couplings related to the Euler and totalderivative invariants. The other two curvature square running couplings λ and ξ are related to the running structure functions by the following k k relations: 1 1 f (0) = f (0) = . (8) C,k R,k 2λ ξ k k We can extract the beta functions for the couplings Λ ,G ,ρ ,τ and the flow equations k k k k for the running structure functions f , f by comparing (5) with (7). In particular, by C,k R,k matching the coefficients of the operators √g and √gR we find the following relations: ´ ´ (4π)2∂ Λk = N0 −4N12 +2N1Q [h ] t 2 k 8πG 2 (cid:18) k(cid:19) 1 (1 χ)N0 +2N1 4N1 (4π)2∂ = − 2 − Q [h ], (9) t 1 k −16πG 12 (cid:18) k(cid:19) 4This implyes also that we are adding to the action (1) the non-dynamical term ΓΛ[g]. 5 while by matching the non-local curvature square terms we find the following flow equation for both running structure functions: ∞ 1 ˜ ∂ f (x) = dsh (s)sF (sx), (10) t i,k (4π)2 ˆ k i 0 where i = C,R and x stands for ∆. If we insert the explicit form of the functions F (x) from i (6) we can rewrite equation (10) explicitly in terms of Q-functionals: (3 2χ)2N 2N 1 (4π)2∂ f (x) = − 0 − 1 dξQ [h (z +xξ(1 ξ))]+ t R,k 576 ˆ 0 k − 0 (3 2χ)N0 2N1 2N1 1 + − − 2 − dξQ [h (z +xξ(1 ξ))] 48 ˆ 1 k − 0 (13 12χ)N0 +8N1 22N1 − 2 − Q [h ] 1 k − 288 5N0 20N1 +10N1 1 + − 2 dξQ [h (z +xξ(1 ξ))] Q [h ] (11) 48x2 (ˆ0 2 k − − 2 k ) N 1 (4π)2∂ f (x) = 1 dξQ [h (z +xξ(1 ξ))]+ t C,k 8 ˆ 0 k − 0 N1 2N1 1 2 − dξQ [h (z +xξ(1 ξ))]+ − 4x ˆ 1 k − 0 N0 +2N1 10N1 + 2 − Q [h ]+ 1 k 24x N0 4N1 +2N1 1 + − 2 dξQ [h (z +xξ(1 ξ))] Q [h ] . (12) 4x2 (ˆ0 2 k − − 2 k ) Finally, matching the remaining local curvature square terms gives: 1 N0 +11N1 +62N1 (4π)2∂ = 2 Q [h ] t 0 k ρk! − 360 1 N0 +N1 3N1 (4π)2∂ = 2 − Q [h ]. (13) t 0 k τ 30 (cid:18) k(cid:19) The flow equations (9), (11), (12) and (13) completely describe the RG flow of the effective average action (7). The Q-functionals in the flow equations just derived can be evaluated once a particular cutoff shape function has been chosen; it is possible to evaluate them analytically if we 6 employ the so called "optimized" cutoff shape function R (z) = (k2 z)θ(k2 z). We k − − easily find the cutoff shape independent value Q [h ] = 2 and the results Q [h ] = 2k2n, 0 k n k n! while the evaluation of the parametric integrals of Q-functional present in (11) and (12) is more involved and we refer to the Appendix of [9] for further details. We can now write down the beta functions for the dimensionless cosmological constant, Λ = k2Λ˜ , and for the k k dimensionless Newton’s constant, G = k2−dG˜ , which follow from (9): k k ∂ Λ˜ = 2Λ˜ + N0 −4N12 +2N1G˜ + (1−χ)N0 +2N21 −4N1Λ˜ G˜ t k k k k k − 4π 6π ∂ G˜ = 2G˜ + (1−χ)N0 +2N21 −4N1G˜2. (14) t k k 6π k The first terms on the rhs of (14) represent the canonical scaling of the cosmological and Newton’s constants, while the other terms represent the running induced by the interaction with matter fields. Note also that conformal scalars, i.e. χ = 1, do not contribute to the running of Newton’s constant. Equations (11) and (12) can be compactly rewritten as: 1 x ∂ f (x) = g , (15) t i,k (4π)2 i k2 (cid:18) (cid:19) where we defined the following cutoff shape dependent functions: ∞ g (u) ds˜h (s)sF (sk2u). (16) i ≡ ˆ k i 0 In (16) u = x/k2 stands for the covariant Laplacian in units of k; note that the UV regime corresponds to small values of u while the IR regime corresponds to large values of u. The integrals in (16) can be evaluated analytically [9], we find the following forms: N0 +6N1 +12N1 N0 +6N1 +12N1 8N0 +8N1 64N1 g (u) = 2 2 2 − C 120 −" 120 − 120u 16N0 64N1 +32N1 4 + − 2 1 θ(u 4) 120u2 #s − u − g (u) = (1−χ)2N0 (1−χ)2N0 + (1−χ)N0 +N21 −2N1 R 72 −" 72 18u N0 4N1 +2N1 4 + − 2 1 θ(u 4). (17) 18u2 #s − u − Note that the form of (17) implies that conformally invariant matter, contributes to g (u) R 7 and thus to the flow of f (x), showing that for k = 0 the flow generates non-conformally R,k 6 invariant interactions. We will see later that the conformal invariance of the curvature square terms will bepartially restored in the IR atk = 0. The constant terms in(17), when matched with(8), give thebeta functions forthecouplings λ andξ . These, together with theexplicit k k forms of the beta functions (13), are: 1 N0 +6N1 +12N1 ∂ λ = 2 λ2 t k −(4π)2 60 k 1 (1 χ)2N ∂ ξ = − 0ξ2 t k −(4π)2 72 k 1 N0 +11N1 +62N1 ∂ ρ = 2 ρ2 t k (4π)2 180 k 1 N0 +N1 3N1 ∂ τ = 2 − τ2 (18) t k −(4π)2 15 k The numerical coefficients in (18) are scheme independent, i.e. they don’t depend on R (z) k and on the other details of the cutoff choice. Note that the beta function of the coupling ξ k vanishes when one is considering conformally invariant matter. The beta functions (14) and (18) agree with those obtained in [6]. 4 Asymptotic safety We proceed now to integrate the RG flow of the couplings of the local curvature invariants present in the truncation we are considering, i.e. the cosmological, Newton’s, and the cou- pling constants of the curvature square terms. We will see that precisely the flow of these couplings is related to the UV renormalization of the theory, that in the effective average action formalism is encoded into the initial conditions of RG flow. These relations will tell us how to choose the bare couplings in order to achieve a finite continuum limit when the UV regularization is removed. Since (14) is a closed system for Λ˜ and G˜ , we can start to look for non-Gaussian fixed k k points of the RG flow, that we need to find in order to construct a continuum limit, by first solving: ∂ Λ˜ = 0 ∂ G˜ = 0. (19) t k t k It’s easy to see that the system (19) admits both a Gaussian fixed point Λ˜ = G˜ = 0 and a k k non-Gaussian one [6]: 8 3.0 2.5 2.0 1.5 1.0 0.5 -2 -1 0 1 2 Figure 1: Renormalization group flow in the (Λ˜ ,G˜ ) plane form equations (24) and (28). k k Λ˜∗ = 3 N0 −4N21 +2N1 G˜∗ = 12π . (20) −4(1 χ)N0 +2N1 4N1 −(1 χ)N0 +2N1 4N1 − 2 − − 2 − Note that to have a fixed point with positive Newton’s constant we need to satisfy the inequality (1 χ)N0 +2N1 4N1 > 0. The stability matrix around the non-Gaussian fixed − 2 − 5 point (20) is readily calculated and shows that it is attractive in the UV . This shows that both the cosmological and Newton’s constants are asymptotically safe couplings. This fact shows that, within the the truncation we are considering and in the large N limit, four 6 dimensional quantum gravity is asymptotically safe . The beta function system (14) can be easily integrated analytically. Integrating the second equation in (14) from the UV scale Λ to the IR scale k gives the following relation connecting the k-dependent, fixed point and 7 bare Newton’s constants : 1 1 1 1 Λ 2 k 2 0 = + . (21) G˜k G˜∗ G˜Λ − G˜∗!(cid:18)k0(cid:19) k ! In (21) we introduced the reference scale k , which will play a role similar to the renormal- 0 ization scale µ of standard perturbation theory. To make (21) finite in the continuum limit, 5It has eigenvalues 4, 2. 6See [5] for a discus−sion−of asymptotic safety when also gravitationalfluctuations are considered. 7If we define the couplings Ak = 8πΛGkk and Bk = 16π1Gk the system (14) becomes ∂tA˜k = 4(A˜∗ −A˜k) and ∂tB˜k = 2(B˜∗ B˜k) where Ak = A˜kk4 and Bk = B˜kk2. These relations immediately give A˜k = A˜∗+(A˜Λ A˜∗) Λk −4 and B˜k =B˜∗+(B˜Λ B˜∗) Λk 2 from which we read off (21) and (26). − − (cid:0) (cid:1) (cid:0) (cid:1) 9 i.e. in the limit Λ , we need to renormalize Newton’s constant; this can be done by → ∞ imposing the following condition: 1 1 Λ 2 = C , (22) G˜Λ − G˜∗!(cid:18)k0(cid:19) G where C is a constant that we will determine later. Solving (22) with respect to bare G ˜ Newton’s constant G gives: Λ ˜ G˜ = G∗ , (23) Λ 1+CGG˜∗ kΛ0 2 (cid:16) (cid:17) which shows how we need to choose G˜Λ to approach G˜∗, as Λ , in order that the lhs of → ∞ (22)remainsconstant. Inserting now(22)in(21)givesthefunctionalformofthek-dependent Newton’s constant: ˜ G˜ = G∗ . (24) k 1+CGG˜∗ kk0 2 (cid:16) (cid:17) Equations (23) and (24) are formally the same at the level of the approximation we are considering but have different physical meanings: the first equation tells us how the bare coupling changes as we vary the UV scale Λ, while the second equation shows how the k- dependent coupling flows as we integrate more and more degrees of freedom by lowering the RG scale k. From (24) we see that the dimensionful Newton’s constant, G = k−2G˜ , reaches k k the following renormalized value at k = 0: 1 1 G = C = , (25) 0 C k2 ⇒ G G k2 G 0 0 0 which fixes the value of the constant introduced (22). Similarly, we obtain the following relation between the k-dependent, fixed point and bare cosmological constants: Λ˜k Λ˜∗ Λ˜Λ Λ˜∗ Λ 4 k0 4 = + . (26) G˜k G˜∗ G˜Λ − G˜∗!(cid:18)k0(cid:19) k ! After we impose the UV renormalization condition: Λ˜Λ Λ˜∗ Λ 4 = C , G˜Λ − G˜∗!(cid:18)k0(cid:19) Λ 10