We study the vacuum Cauchy problem for K-essence, i.e. cosmologically relevant scalar-tensor theories that involve first-order derivative self-interactions, and which pass all existing gravitational wave bounds. We restrict to spherical symmetry and show that there exists a large class of theories for which no breakdown of the Cauchy problem occurs outside apparent black hole horizons, even in the presence of scalar shocks/caustics, except for a small set of initial data sufficiently close to critical black hole collapse. We characterise these problematic initial data, and show that they lead to large or even diverging (coordinate) speeds for the characteristic curves. We discuss the physical relevance of this problem and propose ways to overcome it.
In this paper, we study the dynamics of k-essence in loop quantum cosmology (LQC). The study indicates that the loop quantum gravity (LQG) effect plays a key role only in the early epoch of the universe and is diluted at the later stage. The fixed points in LQC are basically consistent with that in standard Friedmann-Robertson-Walker (FRW) cosmology. For most of the attractor solutions, the stability conditions in LQC are in agreement with that for the standard FRW universe. But for some special fixed point, more tighter constraints are imposed thanks to the LQG effect.
We reexamine $k$-essence dark energy models with a scalar field $ϕ$ and a factorized Lagrangian, $\mathcal L = V(ϕ)F(X)$, with $X = \frac{1}{2} \nabla_μϕ\nabla^μϕ.$ A value of the equation of state parameter, $w$, near $-1$ requires either $X \approx 0$ or $dF/dX \approx 0$. Previous work showed that thawing models with $X \approx 0$ evolve along a set of unique trajectories for $w(a)$, while those with $dF/dX \approx 0$ can result in a variety of different forms for $w(a)$. We show that if $dV/dϕ$ is small and $(1/V)(dV/dϕ)$ is roughly constant, then the latter models also converge toward a s…
We consider modifications of general relativity characterized by a special noncovariant constraint on metric coefficients, which effectively generates a perfect-fluid type of matter stress tensor in Einstein equations. Such class of modified gravity models includes recently suggested generalized unimodular gravity (GUMG) theory and its simplest version -- unimodular gravity (UMG). We make these gravity models covariant by introducing four Stueckelberg fields and show that in the case of generalized unimodular gravity three out of these fields dynamically decouple. This means that the covariant…
From Dark Radiation to Dark Energy: Unified Cosmological Evolution in K-essence Models
Universidad de Guanajuato（瓜纳华托大学）；Centro de Investigación y de Estudios Avanzados del I.P.N.（墨西哥国立自治研究所高级研究中心）；Secretaría de Ciencia, Humanidades, Tecnología e Innovación（科学、人文、技术与创新秘书处）
We calculate the cosmological complexity under the framework of scalar curvature perturbations for a K-essence model with constant potential. In particular, the squeezed quantum states are defined by acting a two-mode squeezed operator which is characterized by squeezing parameters $r_k$ and $ϕ_k$ on vacuum state. The evolution of these squeezing parameters are governed by the $Schr\ddot{o}dinger$ equation, in which the Hamiltonian operator is derived from the cosmological perturbative action. With aid of the solutions of $r_k$ and $ϕ_k$, one can calculate the quantum circuit complexity betwee…
Scalar-tensor theories with first-derivative self interactions, known as $k$-essence, may provide interesting phenomenology on cosmological scales. On smaller scales, however, initial value evolutions (which are crucial for predicting the behavior of astrophysical systems, such as binaries of compact objects) may run into instabilities related to the Cauchy problem becoming potentially ill-posed. Moreover, on local scales the dynamics may enter in the nonlinear regime, which may lie beyond the range of validity of the infrared theory. Completions of $k$-essence in the ultraviolet, when they ar…
We investigate the cosmological evolution of the power law K-essence dark energy (DE) model $F(X)= -\sqrt{X} + X$ with a new interaction $Q = αρ_mρ_{ϕ}H^{-1}$ in FRWL spacetime. The evolution behavior of dark energy under this interaction is analyzed by using dynamical systems method, and ten critical points are obtained. Among those critical points, a new stable point, which we called Scaling-like dark energy(DE) solution, is very important and interesting. The cosmological meaning of this attractor is different from the Scaling solution and dark energy dominated solution. For some value of m…
From a spacetime perspective, the dynamics of magnetic field lines of force-free electromagnetic fields can be rewritten into a quite similar form for the dynamics of strings, i.e., dynamics of "field sheets". Using this formalism, we explicitly show that the field sheets of stationary and axisymmetric force-free electromagnetic fields have identical intrinsic properties to the world sheets of rigidly rotating Nambu-Goto strings. Thus, we conclude that the Blandford-Znajek process is kinematically identical to an energy-extraction mechanism by the Nambu-Goto string with an effective magnetic t…
In this paper, we have used the reconstructed DBI-essence dark energy density to modify the mass accretions of black holes and wormholes. In general, the black hole mass accretion does not depend on the metric or local Einstein geometry. That is why we have used a generalized mechanism by reconstructing the DBI-essence dark energy reconstruction with f(R) gravity. We have used some particular forms of the scale factor to analyze the accretion phenomena. We have shown the effect of cosmic evolution in the proper time variation of black hole mass accretion. Finally, we have studied the validity …
We construct new black brane solutions in the context of Horndeski gravity, in particular in its K-essence sector. These models are supported by axion scalar fields that depend only on the horizon coordinates. The dynamics of these fields is determined by a K-essence term that includes the standard kinetic term $X$ and a correction of the form $X^{k}$. We find both neutral and charged exact and analytic solutions in $D$-dimensions, which are asymptotically anti de Sitter. Then, we describe in detail the thermodynamical properties of the four-dimensional solutions and we compute the dual hologr…
Early dark energy (EDE) that becomes subdominant around the epoch of matter-radiation equality can be used to ease the Hubble tension. However, there is a theoretical problem that why the energy scale of EDE is in coincidence with that of matter-radiation equality when their physics are completely unrelated. Sakstein and Trodden [Phys. Rev. Lett. 124, 161301 (2020)] proposed a mechanism to solve this coincidence problem with $\mathcal{O}({\rm eV})$-mass neutrino. In this paper, in order to solve the coincidence problem, we propose a new scenario for EDE, in which the onset and ending of EDE ar…
Universidad de La Frontera（拉弗龙特拉大学）；Universidad Católica del Norte（北天主教学会大学）；Durban University of Technology（德班理工大学）；North-West University（西北大学）；Universidad Católica de Temuco（特木科天主教大学）；National Institute for Theoretical and Computational Sciences (NITheCS)（南非国家理论与计算科学研究所）
Boltzmann Dynamics in K-essence Cosmology: Photon Propagation in an Emergent Spacetime
University of Calcutta（加尔各答大学）；Sister Nivedita University（西塔·尼维迪塔大学）；Inter-University Center for Astronomy and Astrophysics（跨大学天文与天体物理中心）；Prabhat Kumar College（普拉巴特·库马尔学院）；Institute of Astronomy, Space and Earth Science（天文、空间与地球科学研究院）
We investigated the scenario of time-dependent diffusive interaction between dark matter and dark energy and showed that such a model can be accommodated within the observations of luminosity distance - redshift data in Supernova Ia (SNe Ia) observations. We obtain constrains on different relevant parameters of this model from the observational data. We consider a homogeneous scalar field $ϕ(t)$ driven by a $k-$essence Lagrangian of the from $L = V(ϕ)F(X)$ with constant potential $V(ϕ) = V$, to describe the dynamics of dark energy in this model. Using the temporal behaviour of the FRW scale fa…
In this work we study a modified version of $f(R)$ gravity in which higher order kinetic terms of a scalar field are added in the action of vacuum $f(R)$ gravity. This type of theory is a type of $k$-essence $f(R)$ gravity, and it belongs to the general class of $f(R,ϕ,X)$ theories of gravity, where $ϕ$ is a scalar field and $X=\frac{1}{2}\partial^μϕ\partial_μϕ$. We focus on the inflationary phenomenology of the model, in the slow-roll approximation, and we investigate whether viable inflationary evolutions can be realized in the context of this theory. We use two approaches, firstly by imposi…
Unified Dark Energy and Dust Dark Matter Dual to Quadratic Purely Kinetic K-Essence
我们考虑一个修正的引力加单标量场模型，其中标量拉格朗日量对称地耦合到由黎曼度量行列式的平方根给出的标准黎曼体积形式（时空积分测度密度），以及另一个以辅助最大秩反对称张量规范场表示的非黎曼体积形式.正如在以前的论文中所示，相关的标量场动力学提供了一个精确的统一描述的暗能量通过动态生成的宇宙学常数，和暗物质作为一个“尘埃”流体与测地线流作为一个隐藏的诺特对称性的结果。在这里，我们通过考虑纯引力作用量的一个非平凡的修正，即f（R）= R - αR^2广义引力来扩展讨论。在导出后一种修正的引力标量场理论的相应的“爱因斯坦框架”有效作用量时，我们发现了明确的对偶性（在弱耦合与强耦合的意义上）统一动力学暗能量和尘埃流体暗物质的原始模型之间，一方面，一个特定的二次纯动力学“k本质”引力物质模型，其耦合常数仅对两个独立参数有特殊的依赖性，另一方面，本文还简要讨论了对偶纯动力学“k-本质”引力-物质模型的正则哈密顿处理和Wheeler-DeWitt量子化。
We establish a formal connection between the {\bf K}-essence emergent gravity scenario and generalizations of Vaidya spacetime. Choosing the {\bf K}-essence action to be of the Dirac-Born-Infeld variety, the physical spacetime to be a general static spherically symmetric black hole and restricting the {\bf K}-essence scalar field to be a function solely of the advanced or the retarded time, we show that the emergent gravity metric resembles closely the generalized Vaidya metrics for null fluid collapse proposed by Husain. Imposing null energy conditions on the emergent energy-momentum tensor d…
Matching Scherrer's k essence argument with alterations of Di Quark scalar fields permitting an eventual comological constant dominated inflationary expansion
Gravitational Wave Propagation in K-essence Cosmology: Theory and Observational Constraints
University of Calcutta（加尔各答大学）；Ben-Gurion University of the Negev（内盖夫本-古里安大学）；Frankfurt Institute for Advanced Studies (FIAS)（法兰克福高等研究所）；Bahamas Advanced Study Institute and Conferences（巴哈马高级研究院与会议）；Sister Nivedita University（尼维蒂达修女大学）；Pontificia Universidad Católica de Valparaíso（瓦爾帕萊索天主教大學）；Prabhat Kumar College（普拉巴特庫瑪學院）；Institute of Astronomy, Space and Earth Science（天文、太空與地球科學研究所）
In this paper, we study the late time cosmology of a non-canonical scalar field ($k$-essence) coupled to a vector field in a Bianchi-I background. Specifically, we study three cases: canonical scalar field (quintessence) with exponential potential as a warm up, the dilatonic ghost condensate, and the Dirac Born Infeld field with exponentials throat and potential. By using a dynamical system approach, we show that anisotropic dark energy fixed points can be attractors for a suitable set of parameters of each model. We also numerically integrate the associated autonomous systems for particular i…
对具有非标准动力学项（k-本质型）的测试标量场在Schwarzschild黑洞上的吸积进行了数值模拟。我们找到了Dirac Born Infeld型标量场的球吸积的动力学完整解。模拟结果表明，吸积最终沉降到一个众所周知的稳定的解决方案。这种特定的分析稳态解保持两个独立的视野。标准视界是以极限光速传播的普通粒子的视界，而另一个声速视界是以声速传播的k-本质扰动围绕这个吸积背景。对于k-本质扰动传播superluminally的情况下，我们表明，可以发送信号从一个黑洞内的方法固定的解决方案。我们还发现在吸积过程中，一个幽灵凝聚模型会稳定下来。
Sources of matter for wormholes in a k-essence theory
University of Valladolid（巴利亚多利德大学）；Universidade Federal do Ceará（塞阿拉联邦大学）；Universidade Federal do Espírito Santo（圣埃斯皮里图联邦大学）；Universidade Federal do Pará（帕拉联邦大学）；National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)（莫斯科工程物理国立研究大学）
In the context of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, we study cosmological solutions and their stability properties. In particular, we explicitly illustrate the crucial role of degeneracy by showing how the higher order homogeneous equations in the physical frame (where matter is minimally coupled) can be recast in a system of equations that do not involve higher order derivatives. We study the fixed points of the dynamics, finding the conditions for having a de Sitter attractor at late times. Then we consider the coupling to matter field (described for convenience by a k-…
Fine Tuning Free Paradigm of Two Measures Theory: K-Essence, Absence of Initial Singularity of the Curvature and Inflation with Graceful Exit to Zero Cosmological Constant State
We investigate the behavior of the scalar field in $f (R, T )$ gravity (Harko et al., Phys. Rev. D 84, 024020, 2011) inside the structure of a flat FRW cosmological model, where $R$ and $T$ have their usual meaning. The deterministic solution of FEs has been settled by considering the scale factor $S(t) = t^{m}e^{kt}$, where $m$ and $k$ are positive constant. Here we utilize three ongoing imperatives ($H_{0}=73.8$ and $ q_{0}=-0.73$) from SNe Ia Union observation (Cunha, Phys. Rev. D 79, 047301, 2009); ($H_{0}=73.8$ and $ q_{0}=-0.55$) from Type Ia Supernova (SNIa) observation in joined with (…
A Starobinsky-like Potential in k-Essence Framework: Attractor Dynamics, Reheating, and Consistency with ACT DR6
Hunan Institute of Technology（湖南工学院）；SGT University（SGT大学）；Centre for Cosmology and Science Popularization (CCSP), SGT University（SGT大学宇宙学与科学普及中心）；Eurasian International Centre for Theoretical Physics（欧亚国际理论物理中心）
