Anisotropic Spacetime Research Articles

Unified first law in inhomogeneous and anisotropic spacetime models: Some general prescription

In this paper we investigate the interrelationship between the unified first law of thermodynamics and the Friedmann equations for three anisotropic and inhomogeneous spacetime metrics namely Lemaître–Tolman–Bondi and Friedmann–Lemaître–Robertson–Walker type inhomogeneous spacetime model and Kantowski–Sachs type model. In all these models we find that Einstein field equations are obtained by projecting the unified first law along and perpendicular to the Kodama vector and also along two linearly independent directions.

Butterfly velocity in quadratic gravity

We present a systematic procedure of finding the shock wave equation in anisotropic spacetime of quadratic gravity with Lagrangian . The general formula of the butterfly velocity is derived. We show that the shock wave equation in the planar, spherical or hyperbolic black hole spacetime of Einstein–Gauss–Bonnet gravity is the same as that in Einstein gravity if the space is isotropic. We consider the modified AdS spacetime deformed by the leading correction of the quadratic curvatures and find that the fourth order derivative shock wave equation leads to two butterfly velocities if . We also show that the butterfly velocity in the D = 4 planar black hole is not corrected by the quadratic gravity if , which includes the R2 gravity. In general, the correction of butterfly velocity by the quadratic gravity may be positive or negative, depending on the values of α, β, γ and temperature. We also investigate the butterfly velocity in the Gauss–Bonnet massive gravity.

Renormalization Group in Field Theories with Quantum Quenched Disorder.

We study the renormalization group flow in general quantum field theories with quenched disorder, focusing on random quantum critical points. We show that in disorder-averaged correlation functions the flow mixes local and nonlocal operators. This leads to a new critical exponent related to the disorder (as in classical disorder). We show that the time coordinate is rescaled at each renormalization group step, leading to anisotropic spacetime scaling at critical points. We write a universal formula for the dynamical scaling exponent z for weak disorder.

Expanding, shearing and accelerating isotropic plane symmetric universe with conformal Kasner geometry

We construct a model of a universe filled with a perfect fluid with isotropic particle pressure. The anisotropic plane symmetric Kasner spacetime is used as a seed metric and through a conformal mapping a perfect fluid is generated. The model is inhomogeneous, irrotational, shearing and accelerating. For negative time, the universe is expanding, while for positive time, it is collapsing. With the aid of graphical three-dimensional plots, it is established that the density and pressure hypersurfaces are smooth and singularity free. Additionally, the sound-speed index is computed and the fluid obeys the causality criterion. The non-static exact solution found may prove useful in the study of gravitational waves.

Holographic Dark Energy Model With Time Varying Deceleration Parameter

Two minimally interacting fluids; dark matter and holographic dark energy components has been studied in a spatially homogeneous and anisotropic Bianchi type-I space-time. The solutions of the Einstein’s field equations are obtained under the assumption of time varying deceleration parameter (Abdussattar and S. Prajapati, Astrophys. Space Sci. 331, 65, 2011) which represents transition of the universe from the early decelerating phase to the recent accelerating phase. It is shown that for large expansion the model reduces to model while for suitable choice of interaction between dark matter and holographic dark energy the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increase in time. Allowing for time dependent deceleration parameter the solutions of the field equations and some physical and geometric properties of the model along with physical acceptability of the solutions have also been discussed in details.Â

Dynamics of axially symmetric anisotropic modified holographic Ricci dark energy model in Brans-Dicke theory of gravitation

In this paper, we have derived field equations of Brans-Dicke (Phys. Rev. 124, 925 (1961)) theory of gravitation with the help of an axially symmetric anisotropic Bianchi-type space-time in the presence of dark matter and anisotropic modified holographic Ricci dark energy. We have presented a cosmological model solving the field equations. We have used i) the hybrid expansion law, ii) a relation between metric potentials and iii) the modified holographic Ricci dark energy defined by Chen and Jing (Phys. Lett. B 679, 144 (2009)) to solve the field equations. We have determined the cosmological parameters, namely, EoS parameter, matter energy density, anisotropic dark energy density, Skewness parameter, deceleration and jerk parameters. A detailed physical discussion of these dynamical parameters is presented through a graphical representation. We observe that we have a quintessence model which exhibits a smooth transition from decelerated phase to an accelerated phase of the universe. This situation is quite in agreement with the scenario of modern cosmology.

3D Spacetimes Construction from $$(1+1)$$ ( 1 + 1 ) D Kitaev Superconductor Model

This is a proceedings based on the talk that was made in the workshop “Black Holes, Gravitational Waves and Spacetime Singularities” at Vatican Observatory on 9–12 May 2017. Since then we have constructed some corrections, hence this paper includes them. Here we employ the $$(1 + 1)$$ D Kitaev superconductor model and perform the Wilsonian renormalization group transformation in a real space. We regard the running energy scale or the repetition number of RG transformation as a new emergent direction of the spacetime, thus constructing a one dimensional higher spacetime. The solution for the beta function is plugged into the effective action and then the RG information is encoded in the curvature of the emergent spacetime. We found that there are two fixed points in which one corresponds to the Lifshitz spacetime with the dynamical critical exponent $$z=2$$ and the other fixed point corresponds to the AdS $$_{3}$$ spacetime. The topological superconducting phase induces a naked singularity in the spacetime and the trivial superconducting phase produces regular spacetime. Except the fixed points, each parameter phase shows spatial anisotropic spacetime description as it goes to IR limit.

Reconstruction method of f(R) gravity for isotropic and anisotropic spacetimes

We present the reconstruction method of $f(R)$ gravity for the homogeneous and anisotropic Bianchi-I spacetime, which was previously formulated only for homogeneous and isotropic FLRW spacetime. We argue in this paper that for anisotropic spacetimes, the total anisotropy behaves as an independent metric degree of freedom on top of the average scale factor in $f(R)$ gravity. This is not like $GR$, where specifying the form of the average scale factor as a function of time also specify the total anisotropy as a function of time uniqely. We link this peculiar fact to an interesting intertwining between the definition of Ricci scalar for anisotropic metric and anisotropy evolution equation in $f(R)$ gravity. Consequently, specifying an anisotropic solution of $f(R)$ gravity implies specifying both the average scale factor and the total anisotropy as functions of time. The reconstruction method hence formulated is applied to two scenarios where anisotropy suppression is important, namely, a quasi de-Sitter expansion as required in inflation, and a power law contraction as required in ekpyrotic bounce models.

The rest momentum as an additional property of a massive particle in Finsler space-time

It is shown that a flat relativistically-invariant Finslerian space-time with partially broken 3D isotropy, i.e. an axially-symmetric space of events is a natural generalization of the Minkowski space of special relativity. The respective Finsler metric depends on two constant parameters, one of which determines the magnitude of spatial anisotropy or, in other words, the degree of deviation of the Finsler metric from the Minkowski metric, while the second parameter represents the physically preferred direction in 3D space. The replacement of the Minkowski line element in the action integral for a free massive particle by the Finsler line element leads to the Lagrange function which describes the relativistic particle in partially anisotropic 3D space. The respective function allows us to conclude that, like the situation in Minkowski space, if particle’s velocity tends to zero, energy of the particle tends to its absolute minimum, i.e. to ordinary rest energy. As for momentum of the particle, unlike the situation in Minkowski space, it does not tend to zero, if particle’s velocity tends to zero. Thus, in the anisotropic space, in addition to the rest energy, any massive particle obtains one more observable parameter - the rest momentum directed along the locally preferred direction in 3D space. Due to the directly proportional dependence of the rest momentum on the magnitude of local space-time anisotropy and also on mass of a particle and speed of light, the fact that in our time the magnitude of the local space-time anisotropy is extremely small does not at all mean the impossibility of detecting anisotropy in astrophysical processes with a giant energy release and, accordingly, with a giant mass defect. Obviously, in the processes with giant mass defects, one can also expect noticeable momentum defects. With allowance for the law of conservation of momentum, this implies a possible self-acceleration of matter and local isotropization of the Universe.

Holographic butterfly velocities in brane geometry and Einstein-Gauss-Bonnet gravity with matters

In the first part of the paper we generalize the butterfly velocity formula to anisotropic spacetime. We apply the formula to evaluate the butterfly velocities in M-branes, D-branes and strings backgrounds. We show that the butterfly velocities in M2-branes, M5-branes and the intersection M2$\bot$M5 equal to those in fundamental strings, D4-branes and the intersection F1$\bot$D4 backgrounds, respectively. These observations lead us to conjecture that the butterfly velocity is generally invariant under a double-dimensional reduction. In the second part of the paper, we study the butterfly velocity for Einstein-Gauss-Bonnet gravity with arbitrary matter fields. A general formula is obtained. We use this formula to compute the butterfly velocities in different backgrounds and discuss the associated properties.

Spinor Field Nonlinearity and Space-Time Geometry

Within the scope of Bianchi type VI,VI0,V, III, I, LRSBI and FRW cosmological models we have studied the role of nonlinear spinor field on the evolution of the Universe and the spinor field itself. It was found that due to the presence of non-trivial non-diagonal components of the energy-momentum tensor of the spinor field in the anisotropic space-time, there occur some severe restrictions both on the metric functions and on the components of the spinor field. In this report we have considered a polynomial nonlinearity which is a function of invariants constructed from the bilinear spinor forms. It is found that in case of a Bianchi type-VI space-time, depending of the sign of self-coupling constants, the model allows either late time acceleration or oscillatory mode of evolution. In case of a Bianchi VI0 type space-time due to the specific behavior of the spinor field we have two different scenarios. In one case the invariants constructed from bilinear spinor forms become trivial, thus giving rise to a massless and linear spinor field Lagrangian. This case is equivalent to the vacuum solution of the Bianchi VI0 type space-time. The second case allows non-vanishing massive and nonlinear terms and depending on the sign of coupling constants gives rise to accelerating mode of expansion or the one that after obtaining some maximum value contracts and ends in big crunch, consequently generating space-time singularity. In case of a Bianchi type-V model there occur two possibilities. In one case we found that the metric functions are similar to each other. In this case the Universe expands with acceleration if the self-coupling constant is taken to be a positive one, whereas a negative coupling constant gives rise to a cyclic or periodic solution. In the second case the spinor mass and the spinor field nonlinearity vanish and the Universe expands linearly in time. In case of a Bianchi type-III model the space-time remains locally rotationally symmetric all the time, though the isotropy of space-time can be attained for a large proportionality constant. As far as evolution is concerned, depending on the sign of coupling constant the model allows both accelerated and oscillatory mode of expansion. A negative coupling constant leads to an oscillatory mode of expansion, whereas a positive coupling constant generates expanding Universe with late time acceleration. Both deceleration parameter and EoS parameter in this case vary with time and are in agreement with modern concept of space-time evolution. In case of a Bianchi type-I space-time the non-diagonal components lead to three different possibilities. In case of a full BI space-time we find that the spinor field nonlinearity and the massive term vanish, hence the spinor field Lagrangian becomes massless and linear. In two other cases the space-time evolves into either LRSBI or FRW Universe. If we consider a locally rotationally symmetric BI(LRSBI) model, neither the mass term nor the spinor field nonlinearity vanishes. In this case depending on the sign of coupling constant we have either late time accelerated mode of expansion or oscillatory mode of evolution. In this case for an expanding Universe we have asymptotical isotropization. Finally, in case of a FRW model neither the mass term nor the spinor field nonlinearity vanishes. Like in LRSBI case we have either late time acceleration or cyclic mode of evolution. These findings allow us to conclude that the spinor field is very sensitive to the gravitational one.

Anisotropic power spectrum and the observed low-l power in PLANCK CMB data

In this work, we study a direction dependent power spectrum in anisotropic Finsler space-time. We use this direction dependent power spectrum to address the low-l power observed in WMAP and PLANCK data. The angular power spectrum of the temperature fluctuations has a lower amplitude in comparison to the ΛCDM model in the multipole range l = 2 − 40. Our theoretical model gives a correction to the isotropic angular power spectrum due to the breaking of rotational invariance of the primordial power spectrum. We estimate best-fit model parameters along with the six ΛCDM cosmological parameters using the PLANCK likelihood code in CosmoMC software. We find that this modified angular power spectrum fits the CMB temperature data in the multipole range l = 2 − 10 to a good extent but fails for the whole multipole range l = 2 − 40.

Condensate of massive graviton and dark matter

We study coherently oscillating massive gravitons in the ghost-free bigravity theory. This coherent field can be interpreted as a condensate of the massive gravitons. We first define the effective energy-momentum tensor of the coherent massive gravitons in a curved spacetime. We then study the background dynamics of the Universe and the cosmic structure formation including the effects of the coherent massive gravitons. We find that the condensate of the massive graviton behaves as a dark matter component of the Universe. From the geometrical point of view the condensate is regarded as a spacetime anisotropy. Hence, in our scenario, dark matter is originated from the tiny deformation of the spacetime. We also discuss a production of the spacetime anisotropy and find that the extragalactic magnetic field of a primordial origin can yield a sufficient amount for dark matter.

A dynamical system approach to Bianchi III cosmology for Hu–Sawicki type f(R) gravity

The cosmological dynamics of spatially homogeneous but anisotropic Bianchi type-III space-time is investigated in presence of a perfect fluid within the framework of Hu–Sawicki model. We use the dynamical system approach to perform a detailed analysis of the cosmological behaviour of this model for the model parameters \(n=1, c_1=1\), determining all the fixed points, their stability and corresponding cosmological evolution. We have found stable fixed points with de Sitter solution along with unstable radiation like fixed points. We have identified a matter like point which act like an unstable spiral and when the initial conditions of a trajectory are very close to this point, it stabilizes at a stable accelerating point. Thus, in this model, the universe can naturally approach to a phase of accelerated expansion following a radiation or a matter dominated phase. It is also found that the isotropisation of this model is affected by the spatial curvature and that all the isotropic fixed points are found to be spatially flat.

Exact solutions of the hard universe and a magnetic primordial field in an anisotropic cosmological space-time of the Petrov D type

Exact solutions of the hard universe and a magnetic primordial field in an anisotropic cosmological space-time of the Petrov D type

A Cylindrically Symmetric and Static Anisotropic Fluid Spacetime and the Naked Singularity

A cylindrically symmetric and static solution of Einstein’s field equations was presented. The spacetime is conformally flat and regular everywhere except on the symmetry axis where it possesses a naked curvature singularity. The matter-energy source anisotropic fluids violate the weak energy condition (WEC) and diverge on the symmetry axis. We discuss geodesics motion of free test-particles near to the singularity, geodesic expansion in the metric to understand the nature of singularity which is naked or covered, and finally the C-energy of the spacetime.

Exact cosmological solution of a scalar field of type +cosh in anisotropic space-time of Petrov type D

Exact cosmological solution of a scalar field of type +cosh in anisotropic space-time of Petrov type D

A regime of linear stability for the Einstein-scalar field system with applications to nonlinear Big Bang formation

We linearize the Einstein-scalar field equations, expressed relative to constant mean curvature (CMC)-transported spatial coordinates gauge, around members of the well-known family of Kasner solutions on $(0,\infty) \times \mathbb{T}^3$. The Kasner solutions model a spatially uniform scalar field evolving in a (typically) spatially anisotropic spacetime that expands towards the future and that has a "Big Bang" singularity at $\lbrace t = 0 \rbrace$. We place initial data for the linearized system along $\lbrace t = 1 \rbrace \simeq \mathbb{T}^3$ and study the linear solution's behavior in the collapsing direction $t \downarrow 0$. Our first main result is the proof of an approximate $L^2$ monotonicity identity for the linear solutions. Using it, we prove a linear stability result that holds when the background Kasner solution is sufficiently close to the Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) solution. In particular, we show that as $t \downarrow 0$, various time-rescaled components of the linear solution converge to regular functions defined along $\lbrace t = 0 \rbrace$. In addition, we motivate the preferred direction of the approximate monotonicity by showing that the CMC-transported spatial coordinates gauge can be viewed as a limiting version of a family of parabolic gauges for the lapse variable; an approximate monotonicity identity and corresponding linear stability results also hold in the parabolic gauges, but the corresponding parabolic PDEs are locally well-posed only in the direction $t \downarrow 0$. Finally, based on the linear stability results, we outline a proof of the following result, whose complete proof will appear elsewhere: the FLRW solution is globally nonlinearly stable in the collapsing direction $t \downarrow 0$ under small perturbations of its data at $\lbrace t = 1 \rbrace$.

Dark energy cosmological models with general forms of scale factor

In this paper, we have constructed dark energy models in an anisotropic Bianchi-V space-time and studied the role of anisotropy in the evolution of dark energy. We have considered anisotropic dark energy fluid with different pressure gradients along different spatial directions. In order to obtain a deterministic solution, we have considered three general forms of scale factor. The different forms of scale factors considered here produce time varying deceleration parameters in all the cases that simulates the cosmic transition. The variable equation of state (EoS) parameter, skewness parameters for all the models are obtained and analyzed. The physical properties of the models are also discussed.

Cosmological perturbations and stability of nonsingular cosmologies with limiting curvature

We revisit nonsingular cosmologies in which the limiting curvature hypothesis is realized. We study the cosmological perturbations of the theory and determine the general criteria for stability. For the simplest model, we find generic Ostrogradski instabilities unless the action contains the Weyl tensor squared with the appropriate coefficient. When considering two specific nonsingular cosmological scenarios(one inflationary and one genesis model), we find ghost and gradient instabilities throughout most of the cosmic evolution. Furthermore, we show that the theory is equivalent to a theory of gravity where the action is a general function of the Ricci and Gauss-Bonnet scalars, and this type of theory is known to suffer from instabilities in anisotropic backgrounds. This leads us to construct a new type of curvature invariant scalar function. We show that it does not have Ostrogradski instabilities, and it avoids ghost and gradient instabilities for most of the interesting background inflationary and genesis trajectories. We further show that it does not possess additional new degrees of freedom in an anisotropic spacetime. This opens the door for studying stable alternative nonsingular very early universe cosmologies.