Bouncing scenario of general relativistic hydrodynamics in extended gravity

In this paper,we have framed bouncing cosmological model of the Universe in the presence of general relativistic hydrodynamics in an extended theory of gravity.The metric assumed here is the flat Friedmann–Robertson–Walker space–time and the stress energy tensor is of perfect fluid.Since general relativity(GR)has certain issues with late time cosmic speed up phenomena,here we have introduced an additional matter geometry coupling that described the extended gravity to GR.The dynamical parameters are derived and analyzed.The dynamical behavior of the equation of state parameter has been analyzed.We have observed that the bouncing behavior is mostly controlled by the coupling parameter.     

Einstein–Cartan and general relativity cosmologies with a nonminimally coupled ghost scalar field

Exact general solutions for spatially flat isotropic and homogeneous cosmologies with a nonminimally coupled ghost scalar field that has polynomial potentials of the fourth degree are obtained in the framework of the Einstein–Cartan theory (ECT) and general relativity (GR). The special values of coupling constant $\xi $ and restrictions on $\xi $ are found for the above solutions. Some effects of torsion and scalar field potential are elucidated. It is shown that solutions can describe the bouncing models with the late-time accelerated expansion. It is demonstrated that some models admit the unified scenario for dark matter and dark energy: (i) both singular models in ECT with a de Sitter-like asymptotic and with the power-law $(t^{4/3})$ asymptotic at late times, (ii) singular and nonsingular models in GR with a de Sitter-like asymptotic at late times.     

Primordial magnetic fields from gravitationally coupled electrodynamics in nonsingular bounce cosmology

We in this paper study a class of mechanism of the production of the primordial magnetic field(PMF) in the non-singular bouncing cosmology, through the coupling of the electromagnetic field to gravity. We adopt an electrodynamic model with a coupling coefficient as a function of the scale factor a, i.e., f = 1 +(a/a?)~(-n), with a? and n 0 being constants. With analytical calculations, we find that this model can yield a blue tilted power spectrum of PMF on large scales from 1 Mpc to the Hubble length if the bounce scenario has experienced a contracting phase with an equation-of-state parameter larger than-1/3. Furthermore, in order to satisfy the constraints of observational data, the present mechanism favors the so-called ekpyrotic-bounce paradigm. The back-reaction of the energy density of PMF at the bouncing point can lead to additional theoretical constraints on the underlying bouncing paradigm.     

Multi-Component Einstein–Cartan Cosmologies

Flat Friedmann universes filled by radiation, stiff fluid and a nonminimally coupled ghost scalar field with polynomial potentials of the fourth degree V(Φ) are investigated in the framework of the Einstein–Cartan theory. Exact solutions are obtained and analyzed for an arbitrary coupling constant ξ. It is shown that both singular and bouncing models with the late-time accelerated expansion are possible. A comparative analysis of the cosmological models with and without stiff fluid is carried out. The role of sources in the evolution of models is elucidated. Some conclusions from comparison of the presented results with other approaches in literature for accelerated expansion are made.     

Causal horizons in a bouncing universe

As our understanding of the past in a bouncing universe is limited, it becomes difficult to propose a cosmological model which can give some understanding of the causal structure of the bouncing universe. In this article we address the issue related to the particle horizon problem in the bouncing universe models. It is shown that in many models the particle horizon does not exist, and consequently the horizon problem is trivially solved. In some cases a bouncing universe can have a particle horizon and we specify the conditions for its existence. In the absence of a particle horizon the Hubble surface specifies the causal structure of a bouncing universe. We specify the complex relationship between the Hubble surface and the particle horizon when the particle horizon exists. The article also address the issue related to the event horizon in a bouncing universe. A toy example of a bouncing universe is first presented where we specify the conditions which dictate the presence of a particle horizon. Next we specify the causal structures of three widely used bouncing models. The first case is related to quintom matter bounce model, the second one is loop quantum cosmology based bounce model and lastly f(R) gravity induced bounce model. We present a brief discussion on the horizon problem in bouncing cosmologies. We point out that the causal structure of the various bounce models fit our general theoretical predictions.     

Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity

Flame shape is an important observed characteristic of flames that can be used to scale flame properties such as heat release rates and radiation. Flame shape is affected by fuel type, oxygen levels in the oxidiser, inverse burning and gravity. The objective of this study is to understand the effect of high oxygen concentrations, inverse burning, and gravity on the predictions of flame shapes. Flame shapes are obtained from recent analytical models and compared with experimental data for a number of inverse and normal ethane flame configurations with varying oxygen concentrations in the oxidiser and under earth gravity and microgravity conditions. The Roper flame shape model was extended to predict the complete flame shapes of laminar gas jet normal and inverse diffusion flames on round burners. The Spalding model was extended to inverse diffusion flames. The results show that the extended Roper model results in reasonable predictions for all microgravity and earth gravity flames except for enhanced oxygen normal diffusion flames under earth gravity conditions. The results also show trends towards cooler flames in microgravity that are in line with past experimental observations. Some key characteristics of the predicted flame shapes and parameters needed to describe the flame shape using the extended Roper model are discussed.     

Conformal Couplings in Induced Gravity

It is found that the induced gravity with conformal couplings requires the conformal invariance in both classical and quantum levels for consistency. This is also true for the induced gravity with an extended conformal coupling interacting with torsion.     

Background Dynamics of Pre-inflationary Scenario in Brans-Dicke Loop Quantum Cosmology

Recently the background independent nonperturbative quantization has been extended to various theories of gravity and the corresponding quantum effective cosmology has been derived, which provides us with necessary avenue to explore the pre-inflationary dynamics. Brans-Dicke (BD) loop quantum cosmology (LQC) is one of such theories whose effective background dynamics is considered in this article. Starting with a quantum bounce, we explore the pre-inflationary dynamics of a universe sourced by a scalar field with the Starobinsky potential in BD-LQC. Our study is based on the idea that though Einstein's and Jordan's frames are classically equivalent up to a conformal transformation in BD theory, this is no longer true after quantization. Taking the Jordan frame as the physical one we explore in detail the bouncing scenario which is followed by a phase of a slow roll inflation. The three phases of the evolution of the universe, namely, bouncing, transition from quantum bounce to classical universe, and the slow roll inflation, are noted for an initially kinetic energy dominated bounce. In addition, to be consistent with observations, we also identify the allowed phase space of initial conditions that would produce at least 60 e-folds of expansion during the slow roll inflation.     

Formation and evolution of structure in loop cosmology

Inhomogeneous cosmological perturbation equations are derived in loop quantum gravity, taking into account corrections, in particular, in gravitational parts. This provides a framework for calculating the evolution of modes in structure formation scenarios related to inflationary or bouncing models. Applications here are corrections to the Newton potential and to the evolution of large scale modes which imply nonconservation of curvature perturbations possibly noticeable in a running spectral index. These effects are sensitive to quantization procedures and test the characteristic behavior of correction terms derived from quantum gravity.     

Inflation with an antisymmetric tensor field

We investigate the possibility of inflation with models of antisymmetric tensor field having minimal and nonminimal couplings to gravity. Although the minimal model does not support inflation, the nonminimal models, through the introduction of a nonminimal coupling to gravity, can give rise to stable de-Sitter solutions with a bound on the coupling parameters. The values of field and coupling parameters are sub-planckian. Slow roll analysis is performed and slow-roll parameters are defined which can give the required number of e-folds for sufficient inflation. Stability analysis has been performed for perturbations to antisymmetric field while keeping the metric unperturbed, and it is found that only the sub-horizon modes are free of ghost instability for de-Sitter space.     

Anisotropic cosmological models in f (R, T) theory of gravitation

A class of non-singular bouncing cosmological models of a general class of Bianchi models filled with perfect fluid in the framework of f(R,T) gravity is presented. The model initially accelerates for a certain period of time and decelerates thereafter. The physical behaviour of the model is also studied.     

Observational Constraints on Hybrid Scale Factor in f(Q,T)$f(Q,T)$ Gravity with Anisotropic Space-Time

This paper presents an accelerating cosmological model by constraining the free parameters using the cosmological datasets in an extended symmetric teleparallel gravity for the flat and anisotropic space-time. It employs a time variable deceleration parameter that behaves early deceleration and late time acceleration in the form of a hybrid scale factor (HSF). It obtains the present values of deceleration parameter and analyzes the late time behavior of the Universe based on the best-fit values of the free parameters. It derives the dynamical parameters of the model and obtains the equation of state parameter at present in the quintessence region; however, at late time it approaches to ΛCDM. The energy conditions are also analyzed to validate the modified gravity and it finds that the strong energy condition is violating. It establishes the importance of a hybrid scale factor in the late time cosmic phenomena issue.     

Unitarity bounds on low scale quantum gravity

We study the unitarity of models with low scale quantum gravity both in four dimensions and in models with a large extra-dimensional volume. We find that models with low scale quantum gravity have problems with unitarity below the scale at which gravity becomes strong. An important consequence of our work is that their first signal at the Large Hadron Collider would not be of a gravitational nature such as graviton emission or small black holes, but rather would be linked to the mechanism which fixes the unitarity problem. We also study models with scalar fields with non-minimal couplings to the Ricci scalar. We consider the strength of gravity in these models and study the consequences for inflation models with non-minimally coupled scalar fields. We show that a single scalar field with a large non-minimal coupling can lower the Planck mass in the TeV region. In that model, it is possible to lower the scale at which gravity becomes strong down to 14 TeV without violating unitarity below that scale.     

The effect of modified gravity on weak lensing

We study the effect of modified gravity on weak lensing in a class of scalar-tensor theory that includes f(R) gravity as a special case. These models are designed to satisfy local gravity constraints by having a large scalar-field mass in a region of high curvature. Matter density perturbations in these models are enhanced at small redshifts because of the presence of a coupling Q that characterizes the strength between dark energy and non-relativistic matter. We compute a convergence power spectrum of weak lensing numerically and show that the spectral index and the amplitude of the spectrum in the linear regime can be significantly modified compared to the ΛCDM model for large values of |Q| of the order of unity. Thus weak lensing provides a powerful tool to constrain such large coupling scalar-tensor models including f(R) gravity.     

Absence of gravitational contributions to the running Yang–Mills coupling

The question of a modification of the running gauge coupling of (non-)Abelian gauge theories by an incorporation of the quantum gravity contribution has recently attracted considerable interest. In this Letter we perform an involved diagrammatical calculation in the full Einstein–Yang–Mills system both in cut-off and dimensional regularization at one-loop order. It is found that all gravitational quadratic divergencies cancel in cut-off regularization and are trivially absent in dimensional regularization so that there is no alteration to asymptotic freedom at high energies. This settles the previously open question of a potential regularization scheme dependence of the one-loop β function traditionally computed in the background field approach. Furthermore we show that the remaining logarithmic divergencies give rise to an extended effective Einstein–Yang–Mills Lagrangian with a counterterm of dimension six.     

Post-low bouncing in Mandarin Chinese: acoustic analysis and computational modeling

Post-low bouncing is a phenomenon whereby after reaching a very low pitch in a low lexical tone, F(0) bounces up and then gradually drops back in the following syllables. This paper reports the results of an acoustic analysis of the phenomenon in two Mandarin Chinese corpora and presents a simple mechanical model that can effectively simulate this bouncing effect. The acoustic analysis shows that most of the F(0) dynamic features profiling the bouncing effect strongly correlate with the amount of F(0) lowering in the preceding low-tone syllable, and that the additional F(0) raising commences at the onset of the first post-low syllable. Using the quantitative Target Approximation model, this bouncing effect was simulated by adding an acceleration adjustment to the initial F(0) state of the first post-low syllable. A highly linear relation between F(0) lowering and estimated acceleration adjustment was found. This relation was then used to effectively simulate the bouncing effect in both the neutral tone and the full tones. The results of the analysis and simulation are consistent with the hypothesis that the bouncing effect is due to a temporary perturbation of the balance between antagonistic forces in the laryngeal control in producing a very low pitch.     

Consistency Conditions and Constraints on Generalized f(R) Gravity with Arbitrary Geometry-Matter Coupling

We study the consistency conditions of the generalized f(R) gravity by extending f(R) gravity with non-minimal coupling to the generalized f(R) with arbitrary geometry-matter coupling.Specifically,we discuss the two particular models of generalized f(R) by means of consistencyconditions.It is found that the second model is not physically viable so as to be ruled out.Moreover,we further constrain the first model using the DolgovKawasaki stability criterion,and give the value ranges of the parameters in the first model.It is worth stressing that our results include the ones in f(R) gravity with non-minimal coupling as the special case of Q(L_m) = L_m.     

Hamiltonian analysis of linearized extension of Hořava–Lifshitz gravity

We investigate the Hamiltonian structure of linearized extended Ho?ava–Lifshitz gravity in a flat cosmological background following the Faddeev–Jackiw's Hamiltonian reduction formalism. The Hamiltonian structure of extended Ho?ava–Lifshitz gravity is similar to that of the projectable version of original Ho?ava–Lifshitz gravity, in which there is one primary constraint and so there are two physical degrees of freedom. In the infrared (IR) limit, however, there is one propagating degree of freedom in the general cosmological background, and that is coupled to the scalar graviton mode. We find that extra scalar graviton mode in an inflationary background can be decoupled from the matter field in the IR limit. But it is necessary to go beyond linear order in order to draw any conclusion of the strong coupling problem.     

Poincaré gauge theory with coupled even and odd parity spin‐0 modes: Cosmological normal modes

We are investigating the dynamics of a new Poincaré gauge theory of gravity model, which has cross coupling between the spin‐0⁺ and spin‐0^‐ modes. To this end we are considering a very appropriate situation – homogeneous‐isotropic cosmologies – which is relatively simple, and yet all the modes have non‐trivial dynamics which reveals physically interesting and possibly observable results. More specifically we consider manifestly isotropic Bianchi class A cosmologies. Here the first order equations obtained from an effective Lagrangian are linearized and the normal modes are found. These turn out to control the asymptotic late time cosmological normal modes. Numerical evolution confirms the late time asymptotic approximation and shows the expected effects of the cross parity pseudoscalar coupling.