Posts Tagged scalar field

Recent Postings from scalar field

Fast-roll solutions from two scalar field inflation

The cosmological equations of motion of scalar fields are commonly not easy to be analytically solved, which makes necessary to use approximation methods, as the {\it slow-roll} regime. In such an approximation one considers the scalar field potentials to be nearly flat. On the other hand, the so called {\it fast-roll} regime considers exactly flat potentials. Our purpose in this work is to obtain solutions for a two scalar field quintessence model in the fast-roll regime. Cosmological interpretations for such solutions are also presented.

Cosmological simulations with disformally coupled symmetron fields

We use N-body simulations to study the matter distribution in disformal gravity. The disformal model studied here is a conformally coupled symmetron field with an additional exponential disformal term. We conduct cosmological simulations with the aim to find the impact of the new disformal terms in the matter power spectrum, halo mass function and radial profile of the scalar field. This is done by calculating the disformal geodesic equation and the equation of motion for the scalar field, then implementing them into the N-body code ISIS, which is a modified gravity version of the code RAMSES. The presence of a conformal symmetron field increases both the power spectrum and mass function compared to standard gravity on small scales. Our main result is that the newly added disformal terms tend to counteract this effects and can make the evolution slightly closer to standard gravity. We finally show that the disformal terms give rise to oscillations of the scalar field in the centre of the dark matter haloes.

Cosmological simulations with disformally coupled symmetron fields [Cross-Listing]

We use N-body simulations to study the matter distribution in disformal gravity. The disformal model studied here is a conformally coupled symmetron field with an additional exponential disformal term. We conduct cosmological simulations with the aim to find the impact of the new disformal terms in the matter power spectrum, halo mass function and radial profile of the scalar field. This is done by calculating the disformal geodesic equation and the equation of motion for the scalar field, then implementing them into the N-body code ISIS, which is a modified gravity version of the code RAMSES. The presence of a conformal symmetron field increases both the power spectrum and mass function compared to standard gravity on small scales. Our main result is that the newly added disformal terms tend to counteract this effects and can make the evolution slightly closer to standard gravity. We finally show that the disformal terms give rise to oscillations of the scalar field in the centre of the dark matter haloes.

Cosmological simulations with disformally coupled symmetron fields [Cross-Listing]

We use N-body simulations to study the matter distribution in disformal gravity. The disformal model studied here is a conformally coupled symmetron field with an additional exponential disformal term. We conduct cosmological simulations with the aim to find the impact of the new disformal terms in the matter power spectrum, halo mass function and radial profile of the scalar field. This is done by calculating the disformal geodesic equation and the equation of motion for the scalar field, then implementing them into the N-body code ISIS, which is a modified gravity version of the code RAMSES. The presence of a conformal symmetron field increases both the power spectrum and mass function compared to standard gravity on small scales. Our main result is that the newly added disformal terms tend to counteract this effects and can make the evolution slightly closer to standard gravity. We finally show that the disformal terms give rise to oscillations of the scalar field in the centre of the dark matter haloes.

Cosmological simulations with disformally coupled symmetron fields [Cross-Listing]

We use N-body simulations to study the matter distribution in disformal gravity. The disformal model studied here is a conformally coupled symmetron field with an additional exponential disformal term. We conduct cosmological simulations with the aim to find the impact of the new disformal terms in the matter power spectrum, halo mass function and radial profile of the scalar field. This is done by calculating the disformal geodesic equation and the equation of motion for the scalar field, then implementing them into the N-body code ISIS, which is a modified gravity version of the code RAMSES. The presence of a conformal symmetron field increases both the power spectrum and mass function compared to standard gravity on small scales. Our main result is that the newly added disformal terms tend to counteract this effects and can make the evolution slightly closer to standard gravity. We finally show that the disformal terms give rise to oscillations of the scalar field in the centre of the dark matter haloes.

Stationary cylindrically symmetric spacetimes with a massless scalar field and a non-positive cosmological constant

The general stationary cylindrically symmetric solution of Einstein-massless scalar field system with a non-positive cosmological constant is presented. It is shown that the general solution is characterized by four integration constants. Two of these essential parameters have a local meaning and characterize the gravitational field strength. The other two have a topological origin, as they define an improper coordinate transformation that provides the stationary solution from the static one. The Petrov scheme is considered to explore the effects of the scalar field on the algebraic classification of the solutions. In general, these spacetimes are of type I. However, the presence of the scalar field allows us to find a non-vacuum type O solution and a wider family of type D spacetimes, in comparison with the vacuum case. The mass and angular momentum of the solution are computed using the Regge-Teitelboim method in the case of a negative cosmological constant. In absence of a cosmological constant, the curvature singularities in the vacuum solutions can be removed by including a phantom scalar field, yielding non-trivial locally homogeneous spacetimes. These spacetimes are of particular interest, as they have all their curvature invariants constant.

Curvature Singularity in f(R) Theories of Gravity [Cross-Listing]

Although f(R) modifications of late time cosmology is successful in explaining present cosmic acceleration, it is very difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point (close to its minima) in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of f(R) gravity that incorporates several viable f(R) gravity models in the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

Curvature Singularity in f(R) Theories of Gravity [Cross-Listing]

Although f(R) modifications of late time cosmology is successful in explaining present cosmic acceleration, it is very difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point (close to its minima) in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of f(R) gravity that incorporates several viable f(R) gravity models in the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

Curvature Singularity in f(R) Theories of Gravity

Although f(R) modifications of late time cosmology is successful in explaining present cosmic acceleration, it is very difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point (close to its minima) in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of f(R) gravity that incorporates several viable f(R) gravity models in the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

Curvature Singularity in f(R) Theories of Gravity [Cross-Listing]

Although f(R) modifications of late time cosmology is successful in explaining present cosmic acceleration, it is very difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point (close to its minima) in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of f(R) gravity that incorporates several viable f(R) gravity models in the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

Curvature Singularity in f(R) Theories of Gravity [Cross-Listing]

Although f(R) modifications of late time cosmology is successful in explaining present cosmic acceleration, it is very difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point (close to its minima) in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of f(R) gravity that incorporates several viable f(R) gravity models in the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

Horndeski's Stars

We consider the sector of Horndeski’s gravity characterized by a coupling between the kinetic scalar field term and the Einstein tensor. Our goal is to find realistic neutron star configurations in this framework. We show that, in a certain limit, there exist solutions that are identical to the Schwarzschild metric outside the star but change considerably inside, where the scalar field is not trivial. We study numerically the equations and find the region of the parameter space where neutron stars exist. We determine their internal pressure and mass-radius relation, and we compare them with standard general relativity models.

Horndeski's Stars [Cross-Listing]

We consider the sector of Horndeski’s gravity characterized by a coupling between the kinetic scalar field term and the Einstein tensor. Our goal is to find realistic neutron star configurations in this framework. We show that, in a certain limit, there exist solutions that are identical to the Schwarzschild metric outside the star but change considerably inside, where the scalar field is not trivial. We study numerically the equations and find the region of the parameter space where neutron stars exist. We determine their internal pressure and mass-radius relation, and we compare them with standard general relativity models.

Horndeski's Stars [Cross-Listing]

We consider the sector of Horndeski’s gravity characterized by a coupling between the kinetic scalar field term and the Einstein tensor. Our goal is to find realistic neutron star configurations in this framework. We show that, in a certain limit, there exist solutions that are identical to the Schwarzschild metric outside the star but change considerably inside, where the scalar field is not trivial. We study numerically the equations and find the region of the parameter space where neutron stars exist. We determine their internal pressure and mass-radius relation, and we compare them with standard general relativity models.

Strictly finite range forces from the signum-Gordon field: exact results in two spatial dimensions

Exact formula for the force between two identical static point charges coupled to the scalar field of two-dimensional signum-Gordon model is obtained. Pertinent solution of the field equation is found in the form of one dimensional integral. The force exactly vanishes when the distance between the charges exceeds certain critical value.

Static Self-Forces in a Five-Dimensional Black Hole Spacetime

We obtain the electric field and scalar field for a static point charge in closed form in the 5D Schwarzschild-Tangherlini black hole spacetime. We then compute the static self-force in each of these cases by assuming that the appropriate singular field is a 4D Hadamard Green’s function on the constant time Riemannian slice. It is well known that the Hadamard Green’s function involves an arbitrary regular biscalar $W_{0}(x,x’)$, whose coincidence limit $w(x)$ appears in the expression for the self-force. We develop an axiomatic approach to reduce this arbitrary function to a single arbitrary dimensionless coefficient. We show that in the context of this approach to regularization, the self-force does not depend on any undetermined length-scale and need not depend on the internal structure of the charge.

Static Self-Forces in a Five-Dimensional Black Hole Spacetime [Cross-Listing]

We obtain the electric field and scalar field for a static point charge in closed form in the 5D Schwarzschild-Tangherlini black hole spacetime. We then compute the static self-force in each of these cases by assuming that the appropriate singular field is a 4D Hadamard Green’s function on the constant time Riemannian slice. It is well known that the Hadamard Green’s function involves an arbitrary regular biscalar $W_{0}(x,x’)$, whose coincidence limit $w(x)$ appears in the expression for the self-force. We develop an axiomatic approach to reduce this arbitrary function to a single arbitrary dimensionless coefficient. We show that in the context of this approach to regularization, the self-force does not depend on any undetermined length-scale and need not depend on the internal structure of the charge.

Interaction between Maxwell field and charged scalar field in de Sitter universe

We study the theory of interaction between charged scalar field and Maxwell field in de Sitter background. Solving the equation of interacting fields we define the in-out fields as asymptotic free fields and construct the reduction formalism for scalar field. Then we derive the perturbation expansion of the scattering operator. The first order transition amplitudes corresponding to particle production from de Sitter vacuum and pair production in an external field are analysed. We show that all these effects are important only in strong gravitational fields and vanish in the flat limit.

Interaction between Maxwell field and charged scalar field in de Sitter universe [Replacement]

We study the theory of interaction between charged scalar field and Maxwell field in de Sitter background. Solving the equation of interacting fields we define the in-out fields as asymptotic free fields and construct the reduction formalism for scalar field. Then we derive the perturbation expansion of the scattering operator. The first order transition amplitudes corresponding to particle production from de Sitter vacuum and pair production in an external field are analysed. We show that all these effects are important only in strong gravitational fields and vanish in the flat limit.

Cosmological Constraints on Scalar Field Dark Matter [Cross-Listing]

In this paper we study a real scalar field as a possible candidate to explain the dark matter in the universe. In the context of a free scalar field with quadratic potential, we have used observational $H(z)$ data to constrain the dark matter mass to $m=\left(3.46^{+0.38+0.75+1.1}_{-0.43-0.92-1.5}\right)\times10^{-33}$ eV. This value is much below some previous estimates of $m\sim 10^{-22}$ eV found in some models, which we explain as being due to a slightly different formulation, but in complete agreement with a recent model based on a cosmological scalar field harmonic oscillator, for which $m\sim 10^{-32}$ eV. Although scalar field dark matter (SFDM) is much disfavored, as it gives rise to ultra hot dark matter and could halt structure formation, different scalar field potentials could alleviate this issue.

Cosmological Constraints on Scalar Field Dark Matter

In this paper we study a real scalar field as a possible candidate to explain the dark matter in the universe. In the context of a free scalar field with quadratic potential, we have used observational $H(z)$ data to constrain the dark matter mass to $m=\left(3.46^{+0.38+0.75+1.1}_{-0.43-0.92-1.5}\right)\times10^{-33}$ eV. This value is much below some previous estimates of $m\sim 10^{-22}$ eV found in some models, which we explain as being due to a slightly different formulation, but in complete agreement with a recent model based on a cosmological scalar field harmonic oscillator, for which $m\sim 10^{-32}$ eV. Although scalar field dark matter (SFDM) is much disfavored, as it gives rise to ultra hot dark matter and could halt structure formation, different scalar field potentials could alleviate this issue.

Cosmological Constraints on Scalar Field Dark Matter [Cross-Listing]

In this paper we study a real scalar field as a possible candidate to explain the dark matter in the universe. In the context of a free scalar field with quadratic potential, we have used observational $H(z)$ data to constrain the dark matter mass to $m=\left(3.46^{+0.38+0.75+1.1}_{-0.43-0.92-1.5}\right)\times10^{-33}$ eV. This value is much below some previous estimates of $m\sim 10^{-22}$ eV found in some models, which we explain as being due to a slightly different formulation, but in complete agreement with a recent model based on a cosmological scalar field harmonic oscillator, for which $m\sim 10^{-32}$ eV. Although scalar field dark matter (SFDM) is much disfavored, as it gives rise to ultra hot dark matter and could halt structure formation, different scalar field potentials could alleviate this issue.

Nonminimal Macroscopic Models of a Scalar Field Based on Microscopic Dynamics. II. Transport Equations

The article proposes generalizations of the macroscopic model of plasma of scalar charged particles to the cases of inter-particle interaction with multiple scalar fields and negative effective masses of these particles. The model is based on the microscopic dynamics of a particle at presence of scalar fields. The theory is managed to be generalized naturally having strictly reviewed a series of its key positions depending on a sign of particle masses. Thereby, it is possible to remove the artificial restriction contradicting the more fundamental principle of action functional additivity. Additionally, as a condition of internal consistency of the theory, particle effective mass function is found.

Quasi normal modes and the area spectrum of a near extremal de Sitter black hole with conformally coupled scalar field

In this paper we have studied a black hole in de Sitter space which has a conformally coupled scalar field in the background. This black hole is also known as the $MTZ$ black hole. We have obtained exact values for the quasi normal mode frequencies under massless scalar field perturbations. We have demonstrated that when the black hole is near-extremal, that the wave equation for the massless scalar field simplifies to a Schr$\ddot{o}$dinger type equation with the well known P$\ddot{o}$shler-Teller potential. We have also used 6th order WKB approximation to compute quasinormal mode frequencies to compare with exact values obtained via the P$\ddot{o}$shler-Tell method for comparison. As an application, we have obtained the area spectrum using modified Hods approach and show that it is equally spaced.

Hyperbolic Inflation in the Light of Planck 2015 [Cross-Listing]

Rubano and Barrow have discussed the emergence of a dark energy, with late-time cosmic acceleration arising from a self-interacting homogeneous scalar field with a potential of hyperbolic power type. Here, we study the evolution of this scalar field potential back in the inflationary era. Using the hyperbolic power potential in the framework of inflation, we find that the main slow-roll parameters, like the scalar spectral index, the running of the spectral index and the tensor-to-scalar fluctuation ratio can be computed analytically. Finally, in order to test the viability of this hyperbolic scalar field model at the early stages of the Universe, we compare the predictions of that model against the latest observational data, namely Planck 2015.

Hyperbolic Inflation in the Light of Planck 2015

Rubano and Barrow have discussed the emergence of a dark energy, with late-time cosmic acceleration arising from a self-interacting homogeneous scalar field with a potential of hyperbolic power type. Here, we study the evolution of this scalar field potential back in the inflationary era. Using the hyperbolic power potential in the framework of inflation, we find that the main slow-roll parameters, like the scalar spectral index, the running of the spectral index and the tensor-to-scalar fluctuation ratio can be computed analytically. Finally, in order to test the viability of this hyperbolic scalar field model at the early stages of the Universe, we compare the predictions of that model against the latest observational data, namely Planck 2015.

Hyperbolic Inflation in the Light of Planck 2015 [Cross-Listing]

Rubano and Barrow have discussed the emergence of a dark energy, with late-time cosmic acceleration arising from a self-interacting homogeneous scalar field with a potential of hyperbolic power type. Here, we study the evolution of this scalar field potential back in the inflationary era. Using the hyperbolic power potential in the framework of inflation, we find that the main slow-roll parameters, like the scalar spectral index, the running of the spectral index and the tensor-to-scalar fluctuation ratio can be computed analytically. Finally, in order to test the viability of this hyperbolic scalar field model at the early stages of the Universe, we compare the predictions of that model against the latest observational data, namely Planck 2015.

Hyperbolic Inflation in the Light of Planck 2015 [Cross-Listing]

Rubano and Barrow have discussed the emergence of a dark energy, with late-time cosmic acceleration arising from a self-interacting homogeneous scalar field with a potential of hyperbolic power type. Here, we study the evolution of this scalar field potential back in the inflationary era. Using the hyperbolic power potential in the framework of inflation, we find that the main slow-roll parameters, like the scalar spectral index, the running of the spectral index and the tensor-to-scalar fluctuation ratio can be computed analytically. Finally, in order to test the viability of this hyperbolic scalar field model at the early stages of the Universe, we compare the predictions of that model against the latest observational data, namely Planck 2015.

Exact solutions for gravitational collapse with a dilaton field in arbitrary dimensions

We present time-dependent analytic solutions to the Einstein equations coupled with a dilaton (scalar) field. The background geometry for the solutions is a product of an N-dimensional spherically symmetric space and a d-dimensional flat space. We discuss the global properties of the spacetime.

Stochastic quantization and holographic Wilsonian renormalization group of scalar theories with arbitrary mass

We have studied a mathematical relationship between holographic Wilsonian renormalization group(HWRG) and stochastic quantization(SQ) of scalar field with arbitrary mass in AdS spacetime. In the stochastic theory, the field is described by an equation with a form of harmonic oscillator with time dependent frequency and its Euclidean action also shows explicit time dependent kernel in it. We have obtained the stochastic 2-point correlation function and demonstrate that it reproduces the radial evolution of the double trace operator correctly via the suggested relation given in arXiv:1209.2242. Moreover, we justify our stochastic procedure with time dependent kernel by showing that it can map to a new stochastic theory with a standard kernel without time dependence.

Solution of the Hyperon Puzzle within a Relativistic Mean-Field Model

The equation of state of cold baryonic matter is studied within a relativistic mean-field model with hadron masses and coupling constants depending on the scalar field. All hadron masses undergo a universal scaling, whereas the coupling constants are scaled differently. The appearance of hyperons in dense neutron star interiors is accounted for, however the equation of state remains sufficiently stiff if a reduction of the $\phi$ meson mass is included. Our equation of state matches well the constraints known from analyses of the astrophysical data and the particle production in heavy-ion collisions.

Construction of the Energy-Momentum Tensor for Wilson Actions

Given an arbitrary Wilson action of a real scalar field, we discuss how to construct the energy-momentum tensor of the theory. Using the exact renormalization group, we can determine the energy-momentum tensor implicitly, but we are short of obtaining an explicit formula in terms of the Wilson action.

Nonminimal Macroscopic Models of a Scalar Field Based on Microscopic Dynamics. I. Extension of the Theory for Negative Masses

The article proposes generalizations of the macroscopic model of plasma of scalar charged particles to the cases of inter-particle interaction with multiple scalar fields and negative effective masses of these particles. The model is based on the microscopic dynamics of a particle at presence of scalar fields. The theory is managed to be generalized naturally having strictly reviewed a series of its key positions depending on the sign of particle masses. Thereby, it is possible to remove the artiicial restriction contradicting the more fundamental principle of action functional additivity.

Hamiltonian operator for loop quantum gravity coupled to a scalar field

We present the construction of a physical Hamiltonian operator in the deparametrized model of loop quantum gravity coupled to a free scalar field. This construction is based on the use of the recently introduced curvature operator, and on the idea of so-called "special loops". We discuss in detail the regularization procedure and the assignment of the loops, along with the properties of the resulting operator. We compute the action of the squared Hamiltonian operator on spin network states, and close with some comments and outlooks.

Hamiltonian operator for loop quantum gravity coupled to a scalar field [Cross-Listing]

We present the construction of a physical Hamiltonian operator in the deparametrized model of loop quantum gravity coupled to a free scalar field. This construction is based on the use of the recently introduced curvature operator, and on the idea of so-called "special loops". We discuss in detail the regularization procedure and the assignment of the loops, along with the properties of the resulting operator. We compute the action of the squared Hamiltonian operator on spin network states, and close with some comments and outlooks.

Singular deformations of nearly $R^2$ inflation potentials [Cross-Listing]

We investigate in which cases a singular evolution with a singularity of Type IV, can be consistently incorporated in deformations of the $R^2$ inflationary potential. After demonstrating the difficulties that the single scalar field description is confronted with, we use a general two scalar fields model without other matter fluids, to describe the Type IV singular evolution, with one of the two scalar fields being canonical. By appropriately choosing the non-canonical scalar field, we show that the canonical scalar field corresponds to a potential that is nearly the $R^2$ inflation potential. If the Type IV singularity occurs at the end of inflation, the Universe’s dynamical evolution near inflation is determined effectively by the canonical scalar field and at late-time the evolution is effectively determined by the non-canonical scalar. We also discuss the evolution of the Universe in terms of the effective equation of state and we show that the Type IV singularity, that occurs at the end of inflation, drives late-time acceleration. If however the singularity occurs at late-time, this might affect the inflationary era. We also investigate which Jordan frame pure $F(R)$ gravity corresponds to the nearly $R^2$ inflation scalar potentials we found. The stability of the solutions in the two scalar fields case is also studied and also we investigate how Type IV singularities can be incorporated in certain limiting cases of $R+R^p$ gravity in the Einstein frame. Finally, we briefly discuss a physical appealing scenario triggered by instabilities in the dynamical system that describes the evolution of the scalar fields.

Singular deformations of nearly $R^2$ inflation potentials

We investigate in which cases a singular evolution with a singularity of Type IV, can be consistently incorporated in deformations of the $R^2$ inflationary potential. After demonstrating the difficulties that the single scalar field description is confronted with, we use a general two scalar fields model without other matter fluids, to describe the Type IV singular evolution, with one of the two scalar fields being canonical. By appropriately choosing the non-canonical scalar field, we show that the canonical scalar field corresponds to a potential that is nearly the $R^2$ inflation potential. If the Type IV singularity occurs at the end of inflation, the Universe’s dynamical evolution near inflation is determined effectively by the canonical scalar field and at late-time the evolution is effectively determined by the non-canonical scalar. We also discuss the evolution of the Universe in terms of the effective equation of state and we show that the Type IV singularity, that occurs at the end of inflation, drives late-time acceleration. If however the singularity occurs at late-time, this might affect the inflationary era. We also investigate which Jordan frame pure $F(R)$ gravity corresponds to the nearly $R^2$ inflation scalar potentials we found. The stability of the solutions in the two scalar fields case is also studied and also we investigate how Type IV singularities can be incorporated in certain limiting cases of $R+R^p$ gravity in the Einstein frame. Finally, we briefly discuss a physical appealing scenario triggered by instabilities in the dynamical system that describes the evolution of the scalar fields.

Singular deformations of nearly $R^2$ inflation potentials [Cross-Listing]

We investigate in which cases a singular evolution with a singularity of Type IV, can be consistently incorporated in deformations of the $R^2$ inflationary potential. After demonstrating the difficulties that the single scalar field description is confronted with, we use a general two scalar fields model without other matter fluids, to describe the Type IV singular evolution, with one of the two scalar fields being canonical. By appropriately choosing the non-canonical scalar field, we show that the canonical scalar field corresponds to a potential that is nearly the $R^2$ inflation potential. If the Type IV singularity occurs at the end of inflation, the Universe’s dynamical evolution near inflation is determined effectively by the canonical scalar field and at late-time the evolution is effectively determined by the non-canonical scalar. We also discuss the evolution of the Universe in terms of the effective equation of state and we show that the Type IV singularity, that occurs at the end of inflation, drives late-time acceleration. If however the singularity occurs at late-time, this might affect the inflationary era. We also investigate which Jordan frame pure $F(R)$ gravity corresponds to the nearly $R^2$ inflation scalar potentials we found. The stability of the solutions in the two scalar fields case is also studied and also we investigate how Type IV singularities can be incorporated in certain limiting cases of $R+R^p$ gravity in the Einstein frame. Finally, we briefly discuss a physical appealing scenario triggered by instabilities in the dynamical system that describes the evolution of the scalar fields.

Resonant Primordial Gravitational Waves Amplification [Cross-Listing]

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Resonant Primordial Gravitational Waves Amplification [Replacement]

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Resonant Primordial Gravitational Waves Amplification [Replacement]

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Resonant Primordial Gravitational Waves Amplification [Cross-Listing]

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Resonant Primordial Gravitational Waves Amplification

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Resonant Primordial Gravitational Waves Amplification [Replacement]

We propose a mechanism to evade the Lyth bound in models of inflation. We minimally extend the conventional single-field inflation model in general relativity (GR) to a theory with non-vanishing graviton mass in the very early universe. The modification primarily affects the tensor perturbation, while the scalar and vector perturbations are the same as the ones in GR with a single scalar field at least at the level of linear perturbation theory. During the reheating stage, the graviton mass oscillates coherently and leads to resonant amplification of the primordial tensor perturbation. After reheating the graviton mass vanishes and we recover GR.

Nonminimal coupling and the cosmological constant problem [Cross-Listing]

We consider a universe with a positive effective cosmological constant and a nonminimally coupled scalar field. When the coupling constant is negative, the scalar field exhibits linear growth at asymptotically late times, resulting in a decaying effective cosmological constant. The Hubble rate in the Jordan frame reaches a self-similar solution, $H=1/(\epsilon t)$, where the principal slow roll parameter $\epsilon$ depends on $\xi$, reaching maximally $\epsilon=2$ (radiation era scaling) in the limit when $\xi\rightarrow -\infty$. Similar results are found in the Einstein frame (E), with $H_E=1/(\epsilon_E t)$, but now $\epsilon_E \rightarrow 4/3$ as $\xi\rightarrow -\infty$. Therefore in the presence of a nonminimally coupled scalar de Sitter is not any more an attractor, but instead (when $\xi<-1/2$) the Universe settles in a decelerating phase. Next we show that, when the scalar field $\phi$ decays to matter with $\epsilon_m>4/3$ at a rate $\Gamma\gg H$, the scaling changes to that of matter, $\epsilon\rightarrow \epsilon_m$, and the energy density in the effective cosmological becomes a fixed fraction of the matter energy density, $M_{\rm P}^2\Lambda_{E\rm eff}/\rho_m={\rm constant}$, exhibiting thus an attractor behavior. While this may solve the (old) cosmological constant problem, it does not explain dark energy. Provided one accepts tuning at the $1\%$ level, the vacuum energy of neutrinos can explain the observed dark energy.

Nonminimal coupling and the cosmological constant problem

We consider a universe with a positive effective cosmological constant and a nonminimally coupled scalar field. When the coupling constant is negative, the scalar field exhibits linear growth at asymptotically late times, resulting in a decaying effective cosmological constant. The Hubble rate in the Jordan frame reaches a self-similar solution, $H=1/(\epsilon t)$, where the principal slow roll parameter $\epsilon$ depends on $\xi$, reaching maximally $\epsilon=2$ (radiation era scaling) in the limit when $\xi\rightarrow -\infty$. Similar results are found in the Einstein frame (E), with $H_E=1/(\epsilon_E t)$, but now $\epsilon_E \rightarrow 4/3$ as $\xi\rightarrow -\infty$. Therefore in the presence of a nonminimally coupled scalar de Sitter is not any more an attractor, but instead (when $\xi<-1/2$) the Universe settles in a decelerating phase. Next we show that, when the scalar field $\phi$ decays to matter with $\epsilon_m>4/3$ at a rate $\Gamma\gg H$, the scaling changes to that of matter, $\epsilon\rightarrow \epsilon_m$, and the energy density in the effective cosmological becomes a fixed fraction of the matter energy density, $M_{\rm P}^2\Lambda_{E\rm eff}/\rho_m={\rm constant}$, exhibiting thus an attractor behavior. While this may solve the (old) cosmological constant problem, it does not explain dark energy. Provided one accepts tuning at the $1\%$ level, the vacuum energy of neutrinos can explain the observed dark energy.

Multi-disformal invariance of nonlinear primordial perturbations [Cross-Listing]

We study disformal transformations of the metric in the cosmological context. We first consider the disformal transformation generated by a scalar field $\phi$ and show that the curvature and tensor perturbations on the uniform $\phi$ slicing, on which the scalar field is homogeneous, are non-linearly invariant under the disformal transformation. Then we discuss the transformation properties of the evolution equations for the curvature and tensor perturbations at full non-linear order in the context of spatial gradient expansion as well as at linear order. In particular, we show that the transformation can be described in two typically different ways: one that clearly shows the physical invariance and the other that shows an apparent change of the causal structure. Finally we consider a new type of disformal transformation in which a multi-component scalar field comes into play, which we call a "multi-disformal transformation". We show that the curvature and tensor perturbations are invariant at linear order, and also at non-linear order provided that the system has reached the adiabatic limit.

Multi-disformal invariance of nonlinear primordial perturbations

We study disformal transformations of the metric in the cosmological context. We first consider the disformal transformation generated by a scalar field $\phi$ and show that the curvature and tensor perturbations on the uniform $\phi$ slicing, on which the scalar field is homogeneous, are non-linearly invariant under the disformal transformation. Then we discuss the transformation properties of the evolution equations for the curvature and tensor perturbations at full non-linear order in the context of spatial gradient expansion as well as at linear order. In particular, we show that the transformation can be described in two typically different ways: one that clearly shows the physical invariance and the other that shows an apparent change of the causal structure. Finally we consider a new type of disformal transformation in which a multi-component scalar field comes into play, which we call a "multi-disformal transformation". We show that the curvature and tensor perturbations are invariant at linear order, and also at non-linear order provided that the system has reached the adiabatic limit.

Multi-disformal invariance of nonlinear primordial perturbations [Cross-Listing]

We study disformal transformations of the metric in the cosmological context. We first consider the disformal transformation generated by a scalar field $\phi$ and show that the curvature and tensor perturbations on the uniform $\phi$ slicing, on which the scalar field is homogeneous, are non-linearly invariant under the disformal transformation. Then we discuss the transformation properties of the evolution equations for the curvature and tensor perturbations at full non-linear order in the context of spatial gradient expansion as well as at linear order. In particular, we show that the transformation can be described in two typically different ways: one that clearly shows the physical invariance and the other that shows an apparent change of the causal structure. Finally we consider a new type of disformal transformation in which a multi-component scalar field comes into play, which we call a "multi-disformal transformation". We show that the curvature and tensor perturbations are invariant at linear order, and also at non-linear order provided that the system has reached the adiabatic limit.

Robinson-Trautman solution with scalar hair

Explicit Robinson-Trautman solution with minimally coupled free scalar field is derived and analyzed. It is shown that this solution contains curvature singularity which is initially naked but later the horizon envelopes it. We use quasilocal horizon definition and prove its existence in later retarded times using sub- and supersolution method combined with growth estimates. We show that the solution is generally of algebraic type II but reduces to type D in spherical symmetry.

 

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