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We study inflation in scalar–tensor perturbative quantum gravity driven by a one-loop effective potential. We consider effective potentials generated by three models. The first model describes a single scalar field with a nonvanishing mass. The second model describes a massless scalar field with nonminimal coupling to the Einstein tensor. The third model generalizes the Coleman–Weinberg model for the gravitational case. The first model can be consistent with the observational data for N∼70 e-foldings. The second model can be consistent with the observational data for N∼40 e-foldings. We did not find parameters that make the generalized Coleman–Weinberg model consistent with the observational data. We discuss the implications of these results and ways to improve them with other terms of effective action. [ABSTRACT FROM AUTHOR]

The quantum superposition principle states that quantum-mechanical systems such as atoms can be placed in a superposition of mass-energy eigenstates. Inspired by this idea and the seminal conjecture of Bekenstein, who proposed that black holes in quantum gravity must possess a discrete mass eigenspectrum, here we analyze the effects produced by a black hole in a superposition of masses. Analogous to using the electromagnetic field to probe atoms, we consider a quantum scalar field on the spacetime background sourced by the black hole mass superposition. From the resulting spectra, as measured by a hypothetical two-level system interacting with the field, we infer signatures of discretization of the black hole mass in support of Bekenstein's conjecture. [ABSTRACT FROM AUTHOR]

In statistical theory, the Tsallis entropy is an extended form of the Boltzmann–Gibbs entropy. The dimensionless parameter δ is employed to state the quantitative difference from the standard scenario. The concepts of Tsallis entropy and the future event horizon are employed in formulating the present new Tsallis holographic dark energy (NTHDE) model. The model attempts to explain the properties of dark energy using the foundation of quantum gravity. The differential equation characterizing the evolution of the NTHDE density parameter is obtained. Expressions stating the dynamic behavior such as equation of state (EoS), deceleration and jerk parameters are obtained in terms of the NTHDE density parameter. For δ < 1 , the quintessence nature of scalar field could completely characterize the NTHDE. A reconstruction of the scalar field's dynamics and quintessence potential is attempted. We demonstrate that the diagnosis made by statefinder is adaptive enough to distinguish between quintessence and cosmological constant-based dark energy models. Additionally, observational data obtained from CC + SNIa + 5 8 0 union 2.1 sources are used to evaluate the model's effectiveness. [ABSTRACT FROM AUTHOR]

In this paper, the geometry of quantum gravity is quantized in the sense of being noncommutative (first quantization) but it is also quantized in the sense of being emergent (second quantization). A new mechanism for quantum geometry is proposed in which noncommutative geometry can emerge from "one-matrix multitrace scalar matrix models" by probing the statistical physics of commutative phases of matter. This is in contrast to the usual mechanism in which noncommutative geometry emerges from "many-matrix singletrace Yang–Mills matrix models" by probing the statistical physics of noncommutative phases of gauge theory. In this novel scenario, quantized geometry emerges in the form of a transition between the two phase diagrams of the real quartic matrix model and the noncommutative scalar phi-four field theory. More precisely, emergence of the geometry is identified here with the emergence of the uniform-ordered phase and the corresponding commutative (Ising) and noncommutative (stripe) coexistence lines. The critical exponents and Wigner's semicircle law are used to determine the dimension and the metric, respectively. Arguments from the saddle point equation, from Monte Carlo simulation and from the matrix renormalization group equation are provided in support of this scenario. [ABSTRACT FROM AUTHOR]

In recent times, it has been perceived that incorporation of branes in higher dimensional theories of quantum gravity is very effective to differentiate those theories which successfully satisfy the swampland criteria. Moreover, warm inflation is considered to be most appropriate single scalar field inflationary model in the framework of swampland criteria. In this regard, we study two well-known Chaplygin gas (CG) models embedded in Randall–Sundrum braneworld. It is known that the de Sitter constraint is derived by taking into account Bousso's covariant entropy bound. We discuss the application of this result in the context of Chaplygin-brane models and find the condition on the slope of the potential. Furthermore, it is demonstrated that in view of warm inflation dynamics this condition remains essentially unchanged indicating the compatibility of the models with swampland criteria. In addition, we investigate the constraint on the warm inflation parameter imposed by the de Sitter conjecture which shows that the warm inflation is in accordance with de Sitter bound in the strong as well as in weak dissipative regime for both models of Chaplygin gas. [ABSTRACT FROM AUTHOR]

Within the framework of quantum gravity and modified entropy-area formalism, the Tsallis holographic dark energy is an effort to peep into a mysterious content of the Universe, the dark energy, to analyze its nature. The Tsallis parameter δ provides the main characteristic of the Tsallis holographic dark energy. Opting the value of Tsallis parameter δ ≤ 2 , a quintessence scalar field description of the Tsallis holographic dark energy model can be obtained. In this work, we present this quintessential explanation of the Tsallis holographic dark energy with δ ≤ 2 and reconstruct the dynamics of the scalar field and the potential of quintessence. [ABSTRACT FROM AUTHOR]

In this paper, by the use of entangled and nonentangled coherent and squeezed state formalism of two-mode nonclassical states, we studied the chaotic inflationary model of a massive scalar field with quadratic potential in the semiclassical gravity, derived from canonical quantum gravity. It was found that the semiclassical quantum gravity leads to the same power-law expansion of the universe as that of the matter-dominated era τ 2 3 in an oscillatory phase of the scalar field in all the nonclassical quantum states considered. The coherently oscillating scalar field in the expanding universe suffers from the phenomenon of particle creation which restricts the duration of stable coherent oscillations of the scalar field dependent on the parameters of the states considered and affect in a certain way the abundant particle production owing to the parametric resonance of bosonic fields coupled to this coherently oscillating scalar field. [ABSTRACT FROM AUTHOR]

A class of effective field theory called delta-theory, which improves ultraviolet divergences in quantum field theory, is considered. We focus on a scalar model with a quartic self-interaction term and construct the delta theory by applying the so-called delta prescription. We quantize the theory using field variables that diagonalize the Lagrangian, which include a standard scalar field and a ghost or negative norm state. As well known, the indefinite metric may lead to the loss of unitary of the S -matrix. We study the optical theorem and check the validity of the cutting equations for three processes at one-loop order, and found suppressed violations of unitarity in the delta coupling parameter of the order of ξ 4 . [ABSTRACT FROM AUTHOR]

In this paper, the linearized field equations related to the quadratic curvature gravity theory have been obtained in the four-dimensional de Sitter (dS) space–time. The massless spin-2 field equations have been written in terms of the Casimir operators of dS group making use of the ambient space notations. By imposing some simple constraints, arisen from group theoretical interpretation of the field equations, a new four-dimensional Gauss–Bonnet (GB-)like action has been introduced with the related field equations transforming according to the unitary irreducible representations (UIRs) of dS group. Since, the field equations transform according to the UIRs of dS group, the GB-like action, we just obtained, is expected to be a successful model of modified gravity. For more clarity, the gauge invariant field equations have been solved in terms of a gauge-fixing parameter 𝒞. It has been shown that the solution can be written as the multiplication of a symmetric rank-2 polarization tensor and a massless minimally coupled scalar field on dS space. The Krein–Gupta–Bleuler quantization method has been utilized and the covariant two-point function has been calculated in terms of the massless minimally coupled scalar two-point function, using the ambient space notations. It has been written in terms of dS intrinsic coordinates from the ambient space counterpart. The two-point functions are dS invariant and free of any theoretical problems. It means that the proposed model is a successful model of modified gravity and it can produce significant results in the contexts of classical theory of gravity and quantum gravity toy models. [ABSTRACT FROM AUTHOR]

We review some recent works by Carone, Erlich and Vaman on composite gravitons in metric-independent quantum field theories, with the aim of clarifying a number of basic issues. Focusing on a theory of scalar fields presented previously in the literature, we clarify the meaning of the tunings required to obtain a massless graviton. We argue that this formulation can be interpreted as the massless limit of a theory of massive composite gravitons in which the graviton mass term is not of Pauli–Fierz form. We then suggest closely related theories that can be defined without such a limiting procedure (and hence without worry about possible ghosts). Finally, we comment on the importance of finding a compelling ultraviolet completion for models of this type, and discuss some possibilities. [ABSTRACT FROM AUTHOR]

In this paper, we look for AdS solutions to generalized gravity theories in the bulk in various spacetime dimensions. The bulk gravity action includes the action of a non-minimally coupled scalar field with gravity, and a higher-derivative action of gravity. The usual Einstein–Hilbert gravity is induced when the scalar acquires a nonzero vacuum expectation value. The equation of motion in the bulk shows scenarios where AdS geometry emerges on-shell. We further obtain the action of the fluctuation fields on the background at quadratic and cubic orders. [ABSTRACT FROM AUTHOR]

According to the holographic bound, there is only a finite density of degrees of freedom in space when gravity is taken into account. Conventional quantum field theory does not conform to this bound, since in this framework, infinitely many degrees of freedom may be localized to any given region of space. In this paper, we explore the viewpoint that quantum field theory may emerge from an underlying theory that is locally finite-dimensional, and we construct a locally finite-dimensional version of a Klein–Gordon scalar field using generalized Clifford algebras. Demanding that the finite-dimensional field operators obey a suitable version of the canonical commutation relations makes this construction essentially unique. We then find that enforcing local finite dimensionality in a holographically consistent way leads to a huge suppression of the quantum contribution to vacuum energy, to the point that the theoretical prediction becomes plausibly consistent with observations. [ABSTRACT FROM AUTHOR]

In this work, we intend to address the matter–antimatter asymmetry via the gravitational baryogenesis mechanism in the background of a quantum theory of gravity. We investigate this mechanism under the framework of Hořava–Lifshitz gravity. We will compute the baryon-to-entropy ratio in the chosen framework and investigate its physical viability against the observational bounds. We also conduct the above study for various sources of matter like scalar field and Chaplygin gas as specific examples. We speculate that quantum corrections from the background geometry will lead to interesting results. [ABSTRACT FROM AUTHOR]

We compare two different approaches to the treatment of the Wheeler–DeWitt (WDW) equation and the introduction of time in quantum cosmology. One approach is based on the gauge-fixing procedure in theories with first-class constraints and the construction of the corresponding Hilbert space of quantum states. The other approach uses the Born–Oppenheimer (BO) method, based on the existence of two energy scales in the model under consideration. We apply both to a very simple cosmological model, including a massless scalar field filling a flat Friedmann universe, and observe that they give similar predictions. We also discuss the problem of time in nonrelativistic quantum mechanics and some questions concerning the correspondence between classical and quantum theories. [ABSTRACT FROM AUTHOR]

We examine the influence of exact Lorentz-violating symmetry mechanism on the radiative quantum corrections to the critical exponents for massless q -deformed O(N) λ ϕ 4 scalar field theories. For that, we employ three different and independent field-theoretic renormalization group methods for computing analytically the q -deformed critical exponents up to next-to-leading order. Then, we generalize the former finite loop level results for any loop order. We show that the Lorentz-violating q -deformed critical exponents, obtained through the three methods, turn out to be identical and furthermore the same as their Lorentz-invariant q -deformed ones. We argue that this result is in accordance with the universality hypothesis. [ABSTRACT FROM AUTHOR]

We examine the recently introduced idea of Spin-Field Correspondence (SFC) focusing on the example of the spin system described by the XXZ Heisenberg model with external magnetic field. The Hamiltonian of the resulting nonlinear scalar field theory is derived for arbitrary value of the anisotropy parameter Δ. We show that the linear scalar field theory is reconstructed in the large spin limit. For Δ = 1 , a nonrelativistic scalar field theory satisfying the Born reciprocity principle is recovered. As expected, for the vanishing anisotropy parameter Δ → 0 , the standard relativistic Klein–Gordon field is obtained. Various aspects of the obtained class of the scalar fields are studied, including the fate of the relativistic symmetries and the properties of the emerging interaction terms. We show that, in a certain limit, the so-called polymer quantization of the field variables is recovered. This and other discussed properties suggest a possible relevance of the considered framework in the context of quantum gravity. [ABSTRACT FROM AUTHOR]

The geometrical-optics expansion reduces the problem of solving wave equations to one of the solving transport equations along rays. Here, we consider scalar, electromagnetic and gravitational waves propagating on a curved spacetime in general relativity. We show that each is governed by a wave equation with the same principal part. It follows that: each wave propagates at the speed of light along rays (null generators of hypersurfaces of constant phase); the square of the wave amplitude varies in inverse proportion to the cross-section of the beam; and the polarization is parallel-propagated along the ray (the Skrotskii/Rytov effect). We show that the optical scalars for a beam, and various Newman–Penrose scalars describing a parallel-propagated null tetrad, can be found by solving transport equations in a second-order formulation. Unlike the Sachs equations, this formulation makes it straightforward to find such scalars beyond the first conjugate point of a congruence, where neighboring rays cross, and the scalars diverge. We discuss differential precession across the beam which leads to a modified phase in the geometrical-optics expansion. [ABSTRACT FROM AUTHOR]

We obtain spinning boson star solutions and hairy black holes with synchronized hair in the Einstein–Klein–Gordon model, wherein the scalar field is massive, complex and with a nonminimal coupling to the Ricci scalar. The existence of these hairy black holes in this model provides yet another manifestation of the universality of the synchronization mechanism to endow spinning black holes with hair. We study the variation of the physical properties of the boson stars and hairy black holes with the coupling parameter between the scalar field and the curvature, showing that they are, qualitatively, identical to those in the minimally coupled case. By discussing the conformal transformation to the Einstein frame, we argue that the solutions herein provide new rotating boson star and hairy black hole solutions in the minimally coupled theory, with a particular potential, and that no spherically symmetric hairy black hole solutions exist in the nonminimally coupled theory, under a condition of conformal regularity. [ABSTRACT FROM AUTHOR]

The external field approximation (EFA) provides a mechanism whereby classical gravity and inertia emerge from a quantum background. The two-point vector, that in EFA represents gravity, is the byproduct of symmetry breaking and is accompanied by classical, vortical structures. Its interaction range is, in general, that of the metric tensor, but, in the context of a simple symmetry-breaking model, the range can be made finite by the presence of massive scalar particles. Vortices that conceal matter can then be produced, making it effectively 'dark'. In EFA, fermion relativistic vortices can be induced, in particular, by rotation. [ABSTRACT FROM AUTHOR]

Tunneling of scalar particles across the event horizon of rotating BTZ black hole is investigated using the Generalized Uncertainty Principle to study the corrected Hawking temperature and entropy in the presence of quantum gravity effects. We have determined explicitly the various correction terms in the entropy of rotating BTZ black hole including the logarithmic term of the Bekenstein-Hawking entropy , the inverse term of and terms with inverse powers of , in terms of properties of the black hole and the emitted particles - mass, energy and angular momentum. In the presence of quantum gravity effects, for the emission of scalar particles, the Hawking radiation and thermodynamics of rotating BTZ black hole are observed to be related to the metric element, hence to the curvature of space-time. [ABSTRACT FROM AUTHOR]

Partially massless theory in three dimensions is revisited and its relationship with the self-dual massive gravity is considered. The only mode of the partially massless theory is shown explicitly through an action for a scalar field on (A)dS background. This fact can be generalized to higher dimensions. This degree of freedom is altered when a triadic Chern-Simons is introduced, giving rise to the self-dual massive gravity on (A)dS background. We present another physical system with partially massless symmetry and its connection with topologically massive gravity is discussed. [ABSTRACT FROM AUTHOR]

In this short review we describe some aspects of -deformation. After discussing the algebraic and geometric approaches to -Poincaré algebra we construct the free scalar field theory, both on noncommutative -Minkowski space and on curved momentum space. Finally, we make a few remarks concerning the interacting scalar field. [ABSTRACT FROM AUTHOR]

The potential is responsible for the inflation of the universe as scalar field oscillates quickly around some point where has a minimum. The end of this stage has an important role on the further evolution stages of the universe. The created particles are responsible for reheating the universe at the end of this stage. The behavior of the inflation and reheating stages are often known as power law expansion , , respectively. The reheating temperature () and give us valuable information about the reheating stage. Recently, people have studied about the behavior of based on slow-roll inflation and initial condition of quantum normalization. It is shown that there is some discrepancy on due to the amount of under the condition of slow-roll inflation and quantum normalization [M. Tong, Class. Quantum Grav. 30 (2013) 055013.]. Therefore, the author is believed in [M. Tong, Class. Quantum Grav. 30 (2013) 055013.] that the quantum normalization may not be a good initial condition but it seems that, we can remove this discrepancy by determining the appropriate parameter and hence the obtained temperatures based on the calculated are in favor of both mentioned conditions. Then from given , we can calculate , tensor-to-scalar ratio and parameters based on the Planck and WMAP-9 data. The observed results of and have consistency with their constrains. Also the results of are in agreement with its general range and special range based on the DECIGO and BBO detectors. [ABSTRACT FROM AUTHOR]

When quantum gravity effects, that are based on generalized uncertainty principle with a minimal measurable length, are incorporated into black hole physics the Klein-Gordon and Dirac equations get modified. Using these modified equations we investigate tunneling of scalar particles and fermions from event and acceleration horizons of accelerating and rotating black holes and obtain the modified Hawking temperature with quantum gravity effects. We see that Hawking temperature depends on black hole parameters as well as the quantum numbers of emitted fermions. The quantum corrections slow down black hole evaporation and leave a black hole remnant. This contradicts complete evaporation of a black hole which is presaged by the standard temperature formula for black holes. The modified Hawking temperatures presented here, in appropriate limits, are consistent with the previous results in the literature. [ABSTRACT FROM AUTHOR]

We present wormhole as a solution of Euclidean field equations as well as the solution of the Wheeler-deWitt (WD) equation satisfying Hawking-Page wormhole boundary conditions in (4 + 1)-dimensional Kaluza-Klein cosmology. The wormholes are considered in the cases of pure gravity, minimally coupled scalar (imaginary) field and with a positive cosmological constant assuming dynamical extra-dimensional space. In above cases, wormholes are allowed both from Euclidean field equations and WD equation. The dimensional reduction is possible. [ABSTRACT FROM AUTHOR]

We study Friedmann inflation in general Horava-Lifshitz (HL) gravity with detailed and nondetailed and also without the projectability conditions. Accordingly, we derive the modifications in the Friedmann equations due to single scalar field potentials describing power-law and minimal-supersymmetrically extended inflation. By implementing four types of the equations-of-state characterizing the cosmic background geometry, the dependence of the tensorial and spectral density fluctuations and their ratio on the inflation field is determined. The latter characterizes the time evolution of the inflation field relative to the Hubble parameter. Furthermore, the ratio of tensorial-to-spectral density fluctuations is calculated in dependence on the spectral index. The resulting slow-roll parameters apparently differ from the ones deduced from the standard General Relativity (Friedmann gravity). We also observe that the tensorial-to-spectral density fluctuations continuously decrease when moving from nondetailed HL gravity, to Friedmann gravity, to HL gravity without the projectability, and to detailed HL gravity. This regular pattern is valid for three types of cosmic equations-of-state and different inflation potential models. The results fit well with the recent Planck observations. [ABSTRACT FROM AUTHOR]

Causal Dynamical Triangulations (CDT) provide a non-perturbative formulation of Quantum Gravity assuming the existence of a global time foliation. In our earlier study we analyzed the effect of including d copies of a massless scalar field in the two-dimensional CDT model with imaginary time. For d > 1 we observed the formation of a "blob", somewhat similar to that observed in four-dimensional CDT without matter. In the two-dimensional case the "blob" has a Hausdorff dimension DH = 3. In this paper, we study the spectral dimension DS of the two-dimensional CDT-universe, both for d = 0 (pure gravity) and d = 4. We show that in both cases the spectral dimension is consistent with DS = 2. [ABSTRACT FROM AUTHOR]

In this paper, we review the effective field theory of modified gravity in which the Lagrangian involves three-dimensional geometric quantities appearing in the 3+1 decomposition of spacetime. On the flat isotropic cosmological background, we expand a general action up to second-order in the perturbations of geometric scalars, by taking into account spatial derivatives higher than two. Our analysis covers a wide range of gravitational theories - including Horndeski theory/its recent generalizations and the projectable/nonprojectable versions of Hořava-Lifshitz gravity. We derive the equations of motion for linear cosmological perturbations and apply them to the calculations of inflationary power spectra as well as the dark energy dynamics in Galileon theories. We also show that our general results conveniently recover stability conditions of Hořava-Lifshitz gravity already derived in the literature. [ABSTRACT FROM AUTHOR]

The significant matter for the construction of the so-called no-boundary proposal is the assumption of signature transition, which has been a way to deal with the problem of initial conditions of the universe. On the other hand, results of Loop Quantum Gravity indicate that the signature change is related to the discrete nature of space at the Planck scale. Motivated by possibility of non-Archimedean and/or noncommutative structure of space-time at the Planck scale, in this work we consider the classical, p-adic and (spatial) noncommutative form of a cosmological model with Friedmann-Robertson-Walker (FRW) metric coupled with a self-interacting scalar field. [ABSTRACT FROM AUTHOR]

We discuss the implications of a purely classical (instead of quantum) theory of gravity for the primordial gravitational wave spectrum generated during inflation. We argue that for a scalar field driven inflation in a classical gravity the amplitude of the gravitational wave will be too small, irrespective of its primordial seed, to be detected in any forthcoming experiments. Therefore, a positive detection of the B-mode polarizations in the Cosmic Microwave Background (CMB) spectrum will naturally confirm the quantum nature of gravity itself. Furthermore there will be no upper limit on the scale of inflation in the case of classical gravity, and a high-scale model of inflation can easily bypass the observational constraints. [ABSTRACT FROM AUTHOR]

The scalar-tensor theories (STTs) of gravity in spacetime dimensions (D+1)>2 are studied. By performing Hamiltonian analysis, we obtain the geometrical dynamics of the theories from their Lagrangian. The Hamiltonian formalism indicates that the theories are naturally divided into two sectors by the coupling parameter ω. The Hamiltonian structures in both sectors are similar to the corresponding structures of four-dimensional cases. It turns out that, similar to the case of general relativity (GR), there is also a symplectic reduction from the canonical structure of (D+1) Yang-Mills theories coupled to the scalar field to the canonical structure of the geometrical STTs. Therefore, the non-perturbative loop quantum (LQG) gravity techniques can also be applied to the STTs in D+1 dimensions based on their connection-dynamical formalism. [ABSTRACT FROM AUTHOR]

We propose the holographic principle as a dynamical cutoff for any quantum theory of gravity, incorporating ideas of effective field theory. This is done by viewing the holographic bound as a limit on the number of degrees of freedom that can be turned on before the geometrical picture of gravity loses applicability. We illustrate the proposal by revisiting the problem of defining a measure for homogeneous and isotropic space-times coupled to a scalar field and conclude by discussing the implications to the single scalar field inflationary model. [ABSTRACT FROM AUTHOR]

In this paper, we consider a semiclassical description of the spherically symmetric gravitational collapse with a massless scalar field. In particular, we employ an effective scenario provided by holonomy corrections from loop quantum gravity (LQG), to the homogeneous interior spacetime. The singularity that would arise at the final stage of the corresponding classical collapse, is resolved in this context and is replaced by a bounce. Our main purpose is to investigate the evolution of trapped surfaces during this semiclassical collapse. Within this setting, we obtain a threshold radius for the collapsing shells in order to have horizons formation. In addition, we study the final state of the collapse by employing a suitable matching at the boundary shell from which quantum gravity effects are carried to the exterior geometry. [ABSTRACT FROM AUTHOR]

We summarize and expand our investigations concerning the soft graviton effects on microscopic matter dynamics in de Sitter space. The physical couplings receive IR logarithmic corrections which are sensitive to the IR cutoff at the one-loop level. The scale invariant spectrum in the gravitational propagator at the super-horizon scale is the source of the de Sitter symmetry breaking. The quartic scalar, Yukawa and gauge couplings become time dependent and diminish with time. In contrast, the Newton's constant increases with time. We clarify the physical mechanism behind these effects in terms of the conformal mode dynamics in analogy with 2d quantum gravity. We show that they are the inevitable consequence of the general covariance and lead to gauge invariant predictions. We construct a simple model in which the cosmological constant is self-tuned to vanish due to UV-IR mixing effect. We also discuss phenomenological implications such as decaying Dark Energy and SUSY breaking at the inflation era. The quantum effect alters the classical slow roll picture in general, if the tensor-to-scalar ratio r is as small as 0.01. [ABSTRACT FROM AUTHOR]

Inflationary models are characterized by the form of a potential which is arbitrary and usually parametrized so as to match the observational data. However, making use of the renormalization group methods to a gravity-scalar field system it is possible to find physically nontrivial potentials that might be relevant for cosmology and particle physics as well. I will pinpoint the way this class of potentials may be found as well as discuss their utility for inflationary models. [ABSTRACT FROM AUTHOR]

We present a novel mechanism for generating fermion masses through global anomalies at the three-loop level. In a gauge theory, global anomalies are triggered by the possible existence of scalar or pseudoscalar states and heavy fermions, whose masses may not necessarily result from spontaneous symmetry breaking. The implications of this mass-generating mechanism for model building are discussed, including the possibility of creating low-scale fermion masses by quantum gravity effects. [ABSTRACT FROM AUTHOR]

We revisit the issue of continuous signature transition from Euclidean to Lorentzian metrics in a cosmological model described by Friedmann-Robertson-Walker (FRW) metric minimally coupled with a self-interacting massive scalar field. Then, using a noncommutative (NC) phase space of dynamical variables deformed by generalized uncertainty principle (GUP), we show that the signature transition occurs even for a model described by the FRW metric minimally coupled with a free massless scalar field accompanied by a cosmological constant. This indicates that the continuous signature transition might have been easily occurred at early universe just by a free massless scalar field, a cosmological constant and a NC phase space deformed by GUP, without resorting to a massive scalar field having an ad hoc complicate potential. We also study the quantum cosmology of the model and obtain a solution of Wheeler-DeWitt (WD) equation which shows a good correspondence with the classical path. [ABSTRACT FROM AUTHOR]

Using a new effective action predicted from quantum gravity where the sign of the Euclidean Einstein action is reversed, we discuss Wentzel-Kramers-Brillouin (WKB) solutions in quantum cosmology and predict that this sign change might explain the origin of phantom scalars. [ABSTRACT FROM AUTHOR]

It is offered that F(R) modified gravities can be considered as nonperturbative quantum effects arising from Einstein gravity. It is assumed that nonperturbative quantum effects gives rise to the fact that the connection becomes incompatible with the metric, the metric factors and the square of the connection in Einstein-Hilbert Lagrangian have nonperturbative additions. In the simplest approximation both additions can be considered as functions of one scalar field. The scalar field can be excluded from the Lagrangian obtaining F(R) gravity. The essence of quantum correction to the affine connection as a torsion is discussed. [ABSTRACT FROM AUTHOR]

Asymptotic behavior of the scattering amplitude for two scalar particles by scalar, vector and tensor exchanges at high energy and fixed momentum transfers is reconsidered in quantum field theory. In the framework of the quasipotential approach and the modified perturbation theory a systematic scheme of finding the leading eikonal scattering amplitudes and its corrections are developed and constructed. The connection between the solutions obtained by quasipotential and functional approaches is also discussed. The first correction to leading eikonal amplitude is found. [ABSTRACT FROM AUTHOR]

Born-Infeld black holes in the scalar-tensor theories of gravity with massless scalar field have been recently obtained [I. Stefanov, S. Yazadjiev and M. Todorov, Phys. Rev. D75 (2007) 084036; Mod. Phys. Lett. A.23(34) (2008) 2915; Class. Quantum Gravity26 (2009) 015006]. The aim of the current paper is to study the effect of the inclusion of a potential for the scalar field in the theory, through a combination of analytical techniques and numerical methods. The black holes coupled to a massive scalar field have richer causal structure in comparison to the massless scalar field case. In the former case, the black holes may have a second, inner horizon. The presence of potential for the scalar field allows the existence of extremal black holes for certain values of the mass of the scalar field and the magnetic (electric) charge of the black hole. The solutions are stable against spherically symmetric perturbations. Arguments in favor of the general stability of the solutions coming from the application of the "turning point" method are also presented. [ABSTRACT FROM AUTHOR]

Stochastic gravity extends semiclassical gravity by allowing for a systematic and self-consistent description of the metric fluctuations produced by the quantum fluctuations of the stress tensor. The effect of minimally coupled scalar fields with arbitrary mass in de Sitter spacetime is discussed, assuming that these fields are in the Bunch-Davies de Sitter invariant vacuum. The matter field fluctuations are described by the noise kernel which is obtained from the symmetrized two-point correlation of the stress tensor operator. The noise kernel is computed in terms of de Sitter invariant bi-tensors. It turns out that in a de Sitter background the two-point function of the linearized Einstein tensor, which is gauge invariant, is directly related to the noise kernel. [ABSTRACT FROM AUTHOR]

In this paper, the λϕ4 scalar field effective action, in the one-loop approximation, is calculated by using the Krein space quantization. We show that the effective action is naturally finite and the singularity does not appear in the theory. The physical interaction mass, the running coupling constant and β-function are then calculated. The effective potential which is calculated in the Krein space quantization is different from the usual Hilbert space calculation, however we show that β-function is the same in the two different methods. [ABSTRACT FROM AUTHOR]

We discuss the conformal symmetry between Jordan and Einstein frames considering their relations with the metric and Palatini formalisms for modified gravity. Appropriate conformal transformations are taken into account leading to the evident connection between the gravitational actions in the two mentioned frames and the Hilbert-Einstein action with a cosmological constant. We show that the apparent differences between Palatini and metric formalisms strictly depend on the representation while the number of degrees of freedom is preserved. This means that the dynamical content of both formalism is identical. [ABSTRACT FROM AUTHOR]

After a brief review of the linearized gravity in de Sitter (dS) four-dimensional space and ambient flat five-dimensional notations, the linearized field equation is written in terms of the Casimir operators of dS group. It is shown that the field equation is gauge invariant under some special gauge transformations. Because of this gauge freedom, a gauge-fixing parameter c is inserted in the field equation. It is shown that the solution to the field equation can be written as the multiplication of a generalized symmetric rank-2 polarization tensor and a massless minimally coupled scalar field in ambient space notations. The graviton two-point function has been thoroughly calculated, which is dS-invariant and free of any divergences. This two-point function has been expressed in terms of the intrinsic dS coordinates, from its ambient space counterpart, which is clearly dS-invariant and free of any divergences again. [ABSTRACT FROM AUTHOR]

The cosmological dynamics of a minimally coupled scalar field that couples to the background matter with thermal interactions is investigated using statefinder diagnostics. The time evolution of the statefinder pairs {r, s} and {r, q} are obtained under the circumstance that different values of model parameters are chosen. We show that the thermal coupling term does not affect the location of the late-time attractor, but exerts an influence on the evolution of the statefinder parameters. The most notable feature of the r–s plane for the thermal coupling model, which is distinguished from the other dark energy models, is that some part of the curve with thermal coupling can form a closed loop in the second quadrant (r > 1, s < 0). [ABSTRACT FROM AUTHOR]

This paper studies the cosmological equations for a scalar field φ in the framework of a quantum gravity modified Einstein–Hilbert Lagrangian where G and Λ are dynamical variables. It is possible to show that there exists a Noether symmetry for the point Lagrangian describing this scheme in a FRW universe. Our main result is that the Noether Symmetry Approach fixes both Λ = Λ(G) and the potential V = V(φ) of the scalar field. The method does not lead, however, to easily solvable equations, by virtue of the higher dimensionality of the reduced configuration space involved, the additional variable being the running Newton coupling. [ABSTRACT FROM AUTHOR]

The cosmological implications of an evolutionary quantum gravity are analyzed in the context of a generic inhomogeneous model. The Schrödinger problem is formulated and solved in the presence of a scalar field, an ultrarelativistic matter and a perfect gas regarded as the dust-clock. Considering the actual phenomenology, it is shown how the evolutionary approach overlaps the Wheeler-DeWitt one. [ABSTRACT FROM AUTHOR]