A $k$-essence scalar field model having (non canonical) Lagrangian of the form $L=-V(ϕ)F(X)$ where $X=1/2g^{μν}\nabla_μϕ\nabla_νϕ$ with constant $V(ϕ)$ is shown to be consistent with luminosity distance-redshift data observed for type Ia Supernova. For constant $V(ϕ)$, $F(X)$ satisfies a scaling relation which is used to set up a differential equation involving the Hubble parameter $H$, the scale factor $a$ and the $k$-essence field $ϕ$. $H$ and $a$ are extracted from SNe Ia data and using the differential equation the time dependence of the field $ϕ$ is found to be: $ϕ(t) \sim λ_0 + λ_1 t + λ_2 t^2$. The constants $λ_i$ have been determined. The time dependence is similar to that of the quintessence scalar field (having canonical kinetic energy) responsible for homogeneous inflation. Furthermore, the scaling relation and the obtained time dependence of the field $ϕ$ is used to determine the $X$-dependence of the function $F(X)$.
We consider scalar dark matter models where the theory has a shift symmetry only broken by the scalar mass term. We restrict ourselves to K-essence kinetic terms where the shift symmetric part of the Lagrangian is a function of the first derivatives of the scalar field only. In the low-amplitude and nonrelativistic regime, which applies on large galactic scales, scalar clouds form solitons with a finite core. Close to the center of galaxies, where a supermassive Black Hole (BH) resides, we analyze the scalar field distribution and the fate of the dark matter soliton when subject to the BH grav…
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…
Is Phantom Divide Crossing in General Relativity Completely Impossible? Shortcomings in canonical and minimally coupled scalar field and Possible Solutions in $k$-essence Models
High Energy Accelerator Research Organization (KEK)（高能加速器研究机构）；Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University（名古屋大学小林-益川粒子宇宙起源研究所）；ICREA（加泰罗尼亚高级研究中心）；Institute of Space Sciences (IEEC-CSIC)（空间科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）；Aristotle University of Thessaloniki（塞萨洛尼基亚里士多德大学）；Khazar University（哈扎尔大学）
K-essence is a minimally-coupled scalar field whose Lagrangian density $\mathcal{L}$ is a function of the field value $ϕ$ and the kinetic energy $X=\frac{1}{2}\partial_μϕ\partial^μϕ$. In the thawing scenario, the scalar field is frozen by the large Hubble friction in the early universe, and therefore initial conditions are specified. We construct thawing k-essence models by generating Taylor expansion coefficients of $\mathcal{L}(ϕ, X)$ from random matrices. From the ensemble of randomly generated thawing k-essence models, we select dark energy candidates by assuming negative pressure and non-…
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 …
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…
Dynamical System of Scalar Field from 2-Dimension to 3-D and its Cosmological Implication
我们对大多数流行的标量场暗能量模型，包括（幻影）quintessence，（幻影）tachyon，k-essence和一般的非正则标量场模型，给出了三维动力学自治系统，将动力学变量从变量$(x, y, λ)$变为可观测的相关变量$(w_ϕ, Ω_ϕ, λ)$，并展示了k-本质三维系统可还原为（幻影）快子的那些标量场之间的密切关系，一般的非正则标量场可以归约为（幻影）quintessence，k-本质在某些特殊情况下也可以归约为（幻影）quintessence。对于三维动力系统的应用，我们研究了几种特殊情况，并给出了详细的精确动力学解。本文最后指出，用$(w_ϕ, Ω_ϕ, λ)$代替变量$(x, y, λ)$来表示动力系统的微分方程，用三维来研究动力系统比用二维来研究动力系统更方便，也更有物理意义。我们还提出了一个问题，在空间平坦的单标量场FRW宇宙模型中的混沌行为的可能性在普通物质的存在。
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 study gravitational collapse in K-essence model with shift symmetry. For these models, we have the formation of two types of horizons, event and sonic. For the particular case $K(X)=X+βX^2$ we found three different regimes. In the weak field regime the scalar field disperses to infinity, in the very strong regime both horizons form at the same time and finally for the intermediate regime, the sonic horizon could form first or both horizons form at the same time. The threshold of formation of black hole is found in the regime where the sonic horizon forms first. We observe a universal behavi…
We consider k-essence, a scalar-tensor theory with first-order derivative self-interactions that can screen local scales from scalar fifth forces, while allowing for sizeable deviations from General Relativity on cosmological scales. We construct fully nonlinear static stellar solutions that show the presence of this screening mechanism, and we use them as initial data for simulations of stellar oscillations and gravitational collapse in spherical symmetry. We find that for k-essence theories of relevance for cosmology, the screening mechanism works in the case of stellar oscillation and suppr…
In this work we shall study the late-time behavior of $k$-Essence $f(R)$ gravity without scalar potential, in the presence of matter and radiation perfect fluids. We quantify the late-time study by using the statefinder function $Y_H(z)=\frac{ρ_{DE}}{ρ_m^{(0)}}$, which is a function of the redshift and of the Hubble rate. By appropriately rewriting the Friedmann equation in terms of the redshift and of the function $Y_H(z)$, we numerically solve it using appropriate initial conditions, and we critically examine the effects of the $k$-Essence higher order kinetic terms. As we demonstrate, the e…
Observational Insights on DBI K-essence Models Using Machine Learning and Bayesian Analysis
University of Calcutta（加尔各答大学）；Haldia Government College（霍尔迪亚政府学院）；K. J. Somaiya College of Science and Commerce（K.J. 索马亚科学与商业学院）；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（尼维迪塔姐妹大学）；Prabhat Kumar College（普拉巴特·库马尔学院）；Institute of Astronomy, Space and Earth Science（天文学、空间与地球科学研究院）
Non-standard interactions (NSI) of neutrinos with matter mediated by a scalar field would induce medium-dependent neutrino masses which can modify oscillation probabilities. Generating observable effects requires an ultra-light scalar mediator. We derive general expressions for the scalar NSI using techniques of quantum field theory at finite density and temperature, including the long-range force effects, and discuss various limiting cases applicable to the neutrino propagation in different media, such as the Earth, Sun, supernovae and early Universe. We also analyze various terrestrial and s…
A light CP-even Standard Model (SM) gauge-singlet scalar $S$ can be produced abundantly in the supernova core, via the nucleon bremsstrahlung process $N N \to N N S$, due to its mixing with the SM Higgs boson. Including the effective $S$ coupling to both nucleons and the pion mediators, we evaluate the production amplitude for the $S$ particle and point out a key difference with the well-known light CP-odd scalar (axion) and vector boson (dark photon) cases. Taking the subsequent decay and re-absorption of $S$ into account, we present a complete calculation of the energy loss rate for the $S$ …
