Emergent Gravity from Hidden Sectors and TT Deformations
Total Page:16
File Type:pdf, Size:1020Kb
Preprint typeset in JHEP style - HYPER VERSION CCTP-2020-11 ITCP-2020/11 DCPT-20/15 Emergent gravity from hidden sectors and TT deformations ♮ ♭,♮ ♭, P. Betzios , E. Kiritsis , V. Niarchos † ♮ APC, AstroParticule et Cosmologie, Universit´eParis Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cit´e, 10, rue Alice Domon et L´eonie Duquet, 75205 Paris Cedex 13, France. ♭ Crete Center for Theoretical Physics, Institute for Theoretical and Computational Physics, Department of Physics University of Crete, Heraklion, Greece † Department of Mathematical Sciences and Centre for Particle Theory, Durham University, Durham DH1 3LE, UK Abstract: We investigate emergent gravity extending the paradigm of the AdS/CFT correspondence. The emergent graviton is associated to the (dynamical) expectation value of the energy-momentum tensor. We derive the general effective description of such dynamics, and apply it to the case where a hidden theory gen- erates gravity that is coupled to the Standard Model. In the linearized description, generically, such gravity is massive with the presence of an extra scalar degree of arXiv:2010.04729v2 [hep-th] 3 Nov 2020 freedom. The propagators of both the spin-two and spin-zero modes are positive and well defined. The associated emergent gravitational theory is a bi-gravity theory, as is (secretly) the case in holography. The background metric on which the QFTs are defined, plays the role of dark energy and the emergent theory has always as a solution the original background metric. In the case where the hidden theory is holographic, the overall description yields a higher-dimensional bulk theory coupled to a brane. The effective graviton on the brane has four-dimensional characteristics both in the UV and IR and is always massive. Keywords: Emergent gravity, quantum gravity, cosmological constant, holography, dark energy. Contents 1. Introduction and results 2 1.1 Results 8 1.2 Outlook 14 2. A general setup for emergent gravity 17 3. The effective action for the emergent graviton: a simple example 19 3.1 Gapped theories 23 3.2 The linearized induced interaction 27 4. Linearized emergent gravity from a hidden sector 30 4.1 Integrating-in a metric in perturbation theory 31 4.2 The induced gravitational interaction 41 4.3 IR massive gravity actions 45 4.4 Generalization to arbitrary interactions 47 4.5 Features of the emergent linearised effective theory 48 5. The non-linear theory and diffeomorphism invariance 49 6. The low-energy emergent gravitational dynamics 54 6.1 A simpler example 57 7. The holographic hidden QFT 59 8. Discussion 66 Acknowledgements 75 Appendix 76 A. Defining the stress energy correlators 76 A.1 The T Tˆ interactiononanarbitrarybackground 78 B. Ward identities and the energy-momentum two-point function 81 C. Translational invariance constraints on the Schwinger functional 84 D. Conservation of energy momentum and the emergent metric 87 E. The calculation of the effective action 89 –1– F. Linearised analysis of the effective action and equations 92 F.1 The linearized analysis in the case of a single theory 92 F.2 The linearized analysis of the T Tˆ deformation 98 F.2.1 Matching the fluctuation equations 101 F.3 The Schwinger functional and the effective action at the linearized level102 G. Solutions to the effective Einstein equation of a single theory 108 G.1 Einstein manifolds 108 G.2 General spherically symmetric ansatz 108 G.2.1 Perturbation theory 110 G.2.2 Secondorderperturbationtheory 113 G.3 Cosmological solutions 115 G.4 Conformally flat Cosmology 117 H. Inverting a general stress energy two-point function 118 I. The spectral representation of the stress-tensor two-point function122 I.1 Renormalization in momentum space 124 I.2 The static potential 129 I.3 Improvements 129 J. The Energy-momentum two-point function of free fields 130 J.1 Free bosons 130 J.2 Free fermions 134 J.3 The correlator in real space 135 J.4 The static potential 139 References 140 1. Introduction and results There is a widespread belief among experts that gravity remains todate the weak link in our attempts to understand the universe. On one hand, in many contexts (cosmology, black holes), gravity and quantum mechanics seem to give results that violently clash against each other. On the other hand, in all the cases in which only one, or the other of these frameworks seems relevant (as in gravitational physics in the solar system, or particle physics at terrestrial experiments) both theories pass tests with flying colors. Both gravity and the gauge theories that describe all the known interactions, are theories that rely on local symmetries. It is not understood why this is so. It has –2– been advocated that this fact may not be an accident, [1]. However, the similarity stops at that point: gauge theories can be UV complete theories1, while the theory of Einstein gravity is non-renormalizable. In the case of gauge theories, we can say more. If we decree that experiment commands vector fields to mediate interactions, and that vector fields are good dy- namical variables all the way to the UV (i.e.. weakly coupled), then we know that their interactions must be controlled by a gauge theory (possibly spontaneously bro- ken). Sometimes, a similar argument can also be made for composite low-energy vectors, as is the case near the lower end of the conformal window in N=1 sQCD, [28]. Many different proposals have been put forward in the past in an effort to over- come the UV problems of gravity and which are reviewed in [2, 3, 4]. One particular set of ideas involves the concept of “emergent gravity” which is quite vague and has many different incarnations, [5]-[17]. This is to be contrasted with the concept of induced gravity pioneered by Sakharov, [18] and reviewed in [19, 20]. In this second line of thought, the graviton is a priori a dynamical field that is coupled to “mat- ter”. It is however the matter that determines the gravitational action via Wilsonian renormalization and quantum effects. It is reasonable to have a definition of emergent gravity as the theory where the graviton is a composite of the “fundamental” fields of the theory. This definition can become confusing when dualities are at play. Indeed, dualities relate two theories written in terms of different “fundamental” degrees of freedom. However, in most cases this is a good working definition. Therefore from this point of view, and in contrast with induced gravity, the emergent graviton is not a fundamental field at high energy, but only an effective low-energy field. In many cases, it emerges in the context of standard relativistic quantum field theory (QFT). There are however non-relativistic approaches, typically inspired by condensed matter theories with or without an explicit cutoff, where a low-energy graviton is claimed to emerge, [21]-[24]. Perturbative string theory, is a case that shares some features of the emergence phenomenon. The standard NSR description of the theory has dynamical fields (the coordinates of the string) that do not involve directly the space-time metric. The theory however, has explicit invariance under space-time diffeomorphisms2. In retrospect, a dynamical space-time metric (and many other fields) emerge in the theory, making it a quantum theory of gravity. String theory, at the same time does something, other approaches to quantiz- ing gravity cannot do. It provides a perturbatively well-defined theory of quantum gravity that is semiclassical3. 1Modulo complications with interactions mediated by scalars. 2And world-sheet diffeomorphisms as well. 3In the limit of weak coupling, where we know how to handle the theory. –3– String theory4 provides a quantized theory of gravity at a cost: it introduces an infinite number of “new” degrees of freedom, the excited states of the string and 1 a scale, the string scale M ℓ− at which these new states become effective, and s ∼ s resolve some of the problems of the gravitational and other interactions. The theory of strings is UV-finite but not UV-complete. The structure of closed string perturbation theory suggests a theory with a smart cutoff at the string scale, containing therefore no UV divergences (but plenty of IR divergences), [25]. Pertur- bative string theories are well-defined for energies well below the Planck scale, but ill-defined at energy transfers at or above the Planck scale where string perturbation theory breaks down, [26, 27]5. The presence of a non-renormalizable interaction in an effective QFT is not new in the field. We have two well-known, and historically important cases, whose resolution taught us interesting lessons. The first is the Fermi interaction. Its non- renormalizability was resolved by introducing new degrees of freedom (the W and Z bosons) and a related mass scale. In a sense, for gravity, string theory comes somewhat close (but is not identical) to this paradigm. It introduces new degrees of freedom and “resolves” the gravitational interaction. The new scale, Ms, is related 8 to the Planck scale MP of the non-renormalizable gravitational interaction as Ms = g2M 8 . In the perturbative formulation therefore we always find M M . s P s ≪ P Another idea that is in a sense similar, is the approach of quantizing gravity starting with an R2 Lagrangian, [29, 30, 31]. In that case, renormalizability is ensured by a new degree of freedom that is a ghost. Recently, modifications of this formalism have been proposed, [32, 33], but it is not clear yet, what is their interpretation. The other example is QCD. Here, at low energy we have a non-renormalizable IR-free theory of pions. Its characteristic scale is the pion decay constant, fπ, and it is analogous to the Planck scale in the gravity example.