118524940
n-DBI gravity
n-DBI gravity is a gravitational theory introduced in [1], motivated by Dirac-BornInfeld type conformal scalar theory and designed to yield non-eternal inflation spontaneously. It contains a foliation structure provided by an everywhere time-like vector field n, which couples to the gravitational sector of the theory, but decouples in the small curvature limit. We show that any solution of Einstein gravity with a particular curvature property is a solution of n-DBI gravity. Amongst them is a class of geometries isometric to Reissner-Nordström-(Anti) de Sitter black hole, which is obtained within the spherically symmetric solutions of n-DBI gravity minimally coupled to the Maxwell field. These solutions have, however, two distinct features from their Einstein gravity counterparts: 1) the cosmological constant appears as an integration constant and can be positive, negative or vanishing, making it a variable quantity of the theory; 2) there is a non-uniqueness of solutions with the same total mass, charge and effective cosmological constant. Such inequivalent solutions cannot be mapped to each other by a foliation preserving diffeomorphism. Physically they are distinguished by the expansion and shear of the congruence tangent to n, which define scalar invariants on each leave of the foliation. herdeiro@ua.pt hirano@eken.phys.nagoya-u.ac.jp ysato@th.phys.nagoya-u.ac.jp 1
119200792
n-DBI Gravity in a nutshell
We present a new model of gravity which explicitly breaks Lorentz-invariance by the introduction of a unit time-like vector field, thereby giving rise to an extra (scalar) degree of freedom. We discuss its cosmology, exact solutions and the dynamics of the scalar mode. We show that it predicts inflation without an inflaton and admits the black hole solutions of General Relativity (GR). We argue that the scalar mode is well behaved and contains none of the pathologies previously found in similar models.
118614910
On the scalar graviton in n-DBI gravity
n-DBI gravity is a gravitational theory which yields near de Sitter inflation spontaneously at the cost of breaking Lorentz invariance by a preferred choice of foliation. We show that this breakdown endows n-DBI gravity with one extra physical gravitational degree of freedom: a scalar graviton. Its existence is established by Dirac’s theory of constrained systems. Firstly, studying scalar perturbations around Minkowski spacetime, we show that there exists one scalar degree of freedom and identify it in terms of the metric perturbations. Then, a general analysis is made in the canonical formalism, using ADM variables. It is useful to introduce an auxiliary scalar field, which allows recasting n-DBI gravity in an Einstein-Hilbert form but in a Jordan frame. Identifying the constraints and their classes we confirm the existence of an extra degree of freedom in the full theory, besides the two usual tensorial modes of the graviton. We then argue that, unlike the case of (the original proposal for) Hoyrava-Lifschitz gravity, there is no evidence that the extra degree of freedom originates pathologies, such as vanishing lapse, instabilities and strong self-coupling at low energy scales.
119121074
Causal dynamical triangulation with extended interactions in 1+1 dimensions
Abstract We study the Causal Dynamical Triangulation (CDT) with extended interactions in 1 + 1 dimensions applying the method in the non-critical String Field Theory (SFT) constructed by Ishibashi and Kawai. For this model, we solve Schwinger–Dysonʼs Equation (SDE) for disk amplitude perturbatively, and find a matrix model in the continuum limit reproducing the SDE in the non-critical SFT approach as the loop equation.
118614939
Physical states in the canonical tensor model from the perspective of random tensor networks
A bstractTensor models, generalization of matrix models, are studied aiming for quantum gravity in dimensions larger than two. Among them, the canonical tensor model is formulated as a totally constrained system with first-class constraints, the algebra of which resembles the Dirac algebra of general relativity. When quantized, the physical states are defined to be vanished by the quantized constraints. In explicit representations, the constraint equations are a set of partial differential equations for the physical wave-functions, which do not seem straightforward to be solved due to their non-linear character. In this paper, after providing some explicit solutions for N = 2, 3, we show that certain scale-free integration of partition functions of statistical systems on random networks (or random tensor networks more generally) provides a series of solutions for general N. Then, by generalizing this form, we also obtain various solutions for general N. Moreover, we show that the solutions for the cases with a cosmological constant can be obtained from those with no cosmological constant for increased N. This would imply the interesting possibility that a cosmological constant can always be absorbed into the dynamics and is not an input parameter in the canonical tensor model. We also observe the possibility of symmetry enhancement in N = 3, and comment on an extension of Airy function related to the solutions.
116895105
n-DBI Gravity: A Short Overview
We present a model of gravity motivated by the Dirac–Born–Infeld type conformal scalar theory it yields when the Universe is conformally flat. We show that, if the Universe is permeated by a perfect fluid of radiation, the theory naturally predicts two eras of accelerated expansion mediated by a radiation dominated epoch, with a large hierarchy between the two effective cosmological constants, thus providing an alternative inflation scenario. This theory, dubbed n-DBI gravity, contains a preferred unit vector field, everywhere time-like, which breaks diffeomorphism invariance and gives rise to an extra scalar degree of freedom. We analyze the dynamics of this mode and conclude that it is free from some of the pathologies found in similar models, namely the issues of vanishing lapse short distance instability and strong coupling. We also show that the standard black holes of General Relativity are solutions of this theory.
118547294
Potential analysis in holographic Schwinger effect
A bstractWe analyze electrostatic potentials in the holographic Schwinger effect. The potential barrier for the pair production is estimated by a static potential consisting of static mass energies, an electric potential from an external electric-field, and the Coulomb potential between a particle and an antiparticle. Given that the Coulomb potential is supposed to be evaluated by the minimal surface attaching on the conformal boundary as usual, the critical field, where the potential barrier vanishes, exhibits a deviation of 30% from the one obtained from the Dirac-Born-Infeld (DBI) action. We reconsider this issue by reexamining the Coulomb-potential part, which is evaluated by the classical action of a string solution attaching on a probe D3-brane sitting at an intermediate position in the bulk AdS. Then the resulting critical-field completely agrees with the DBI result. This agreement gives rise to a strong support for the holographic scenario. We also discuss the finite-temperature case and the temperature-dependent critical-field also agrees with the DBI result.
119213971
Holographic Schwinger effect in confining phase
A bstractWe consider the Schwinger effect in confining phase by using a holographic setup. The potential analysis is performed for the confining D3-brane and D4-brane back-grounds. We find the critical electric field above which there is no potential barrier and the system becomes unstable catastrophically. An intriguing point is that no Schwinger effect occurs when the electric field is smaller than the confining string tension.
119114995
Holographic description of the Schwinger effect in electric and magnetic field
A bstractWe consider a generalization of the holographic Schwinger effect proposed by Semenoff and Zarembo to the case with constant electric and magnetic fields. There are two ways to turn on magnetic fields, i) the probe D3-brane picture and ii) the string world-sheet picture. In the former picture, magnetic fields both perpendicular and parallel to the electric field are activated by a Lorentz transformation and a spatial rotation. In the latter one, the classical solutions of the string world-sheet corresponding to circular Wilson loops is generalized to contain two additional parameters encoding the presence of magnetic fields.
118500260
Universal aspects of holographic Schwinger effect in general backgrounds
A bstractWe consider universal aspects of a holographic Schwinger effect in general backgrounds with an external homogeneous electric field. The argument is based on the potential analysis developed in our previous work. Under some conditions, there always exists a critical electric field, above which the potential barrier vanishes and the system becomes unstable catastrophically. The critical value agrees with the one obtained from the Dirac-Born-Infeld action. For general confining backgrounds, we show that the Schwinger effect does not occur when the electric field is weaker than the confining string tension.
17956151
Schwinger pair production rate in confining theories via holography
We study the Schwinger pair production in confining theories. The production rate in an external electric field E is numerically evaluated by using the holographic description. There exist two kinds of critical values of the electric field, i) E=E_c, above which there is no potential barrier and particles are freely generated, ii) E=E_s, below which the confining string tension dominates the electric field and the pair production does not occur. We argue the universal exponents associated with the critical behaviors.