Valerio Faraoni, Andrea Giusti, Jeremy Côté
Following an existing procedure in general relativity, the turnaround radius of a spherical structure is studied in scalar-tensor gravity using a new prescription for the analog of the Hawking-Hayward quasilocal mass in this class of theories.
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The Brown-York quasilocal energy is applied to three cosmological problems which have previously been studied with the Hawking-Hayward quasilocal energy (Newtonian simulations of large scale structure formation, turnaround radius in the present accelerating universe, and lensing by the cosmological constant). It is found that, in an appropriate gauge and to first order in the amplitude of the cosmological perturbations describing local structures, the Hawking-Hayward and the Brown-York quasilocal masses predict the same results.
In an accelerating universe in General Relativity there is a maximum radius above which a shell of test particles cannot collapse, but is dispersed by the cosmic expansion. This radius could be used in conjunction with observations of large structures to constrain the equation of state of the universe. We extend the concept of turnaround radius to modified theories of gravity for which the gravitational slip is non-vanishing.
Turnaround Radius for charged particles in the Reissner-Nordström deSitter spacetime
本研究分析了Reissner-Nordström de Sitter时空中带电测试粒子的转向半径，同时结合共形耦合引力的MTZ解考察标量测试电荷的情形，发现两类相互作用下转向半径均与粒子能量相关。
We revisit the concept of turnaround radius in cosmology, in the context of modified gravity. While preliminary analyses were limited to scalar-tensor/$F(R)$ gravity, we extend the definition and the study of this quantity to a much broader class of theories including also quantum $R^2$ gravity. The turnaround radius is computed in terms of the parameters of the theory and it is shown that a deviation not larger than 10\% of this quantity from its value in Einstein's theory could constrain the model parameters and even rule out some current theories.
We apply the Hawking-Hayward quasi-local energy construct to obtain in a rigorous way the turnaround radius of cosmic structures in General Relativity. A splitting of this quasi-local mass into local and cosmological parts describes the interplay between local attraction and cosmological expansion.
Our intuitive understanding of cosmic structure formation works best in scales small enough so that bound, relaxed gravitating systems are no longer adjusting their radius; and large enough so that space and matter follow the average expansion of the Universe. Yet one of the most robust predictions of $Λ$CDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from the center of a bound structure. The maximum possible value of the turnaround radius within the framework of the $Λ$CDM model is, for a given mass $M$, equal…
Constraints of extended gravity theory in the regime of Universe accelerated extension: turnaround radius
该研究利用转向半径定义与含宇宙学常数的广义相对论的观测吻合性，分析两类扩展引力模型的球对称解行为，结合星系团数据为扩展引力理论参数提供了额外约束。
The turnaround radius of a large structure in an accelerating universe has been studied only for spherical structures, while real astronomical systems deviate from spherical symmetry. We show that, for small deviations from spherical symmetry, the gauge-invariant characterization of the turnaround size using the Hawking-Hayward quasi-local mass and spherical symmetry still applies, to first order in the cosmological perturbation potentials and in the deviations from sphericity. This is the first step to include non-spherical systems in the physics of turnaround.
The accelerating behavior of cosmic fluid opposes to the gravitational attraction, at present epoch, whereas standard gravity is dominant at small scales. As a consequence, there exists a \emph{point} where the effects are counterbalanced, dubbed \emph{turnaround radius}, $r_{\text{ta}}$. By construction, it provides a bound on maximum structure sizes of the observed universe. Once an upper bound on $r_{\text{ta}}$ is provided, i.e. $R_{\text{TA,max}}$, one can check whether cosmological models guarantee structure formation. Here, we focus on $f(R)$ gravity, without imposing \emph{a priori} th…
In this work we study the quasilocal energy as in [11] for a constant radius surface in Kerr spacetime in Boyer-Lindquist coordinates. We show that under suitable conditions for isometric embedding, for a stationary observer the quasilocal energy defined in [11] for constant radius in a Kerr like spacetime is exactly equal to the Brown-York quasilocal energy [2]. By some careful estimations, we show that for a constant radius surface in the Kerr spacetime which is outside the ergosphere the embedding conditions for the previous result are satisfied. We discuss extremal solutions as described i…
The gravitational instability, responsible for the formation of the structure of the Universe, occurs below energy thresholds and above spatial scales of a self-gravitating expanding region, when thermal energy can no longer counterbalance self-gravity. I argue that at sufficiently-large scales, dark energy may restore thermal stability. This stability re-entrance of an isothermal sphere defines a turnaround radius, which dictates the maximum allowed size of any structure generated by gravitational instability. On the opposite limit of high energies and small scales, I will show that an ideal,…
The maximum size of a cosmic structure is given by the maximum turnaround radius -- the scale where the attraction due to its mass is balanced by the repulsion due to dark energy. We derive generic formulae for the estimation of the maximum turnaround radius in any theory of gravity obeying the Einstein equivalence principle, in two situations: on a spherically symmetric spacetime and on a perturbed Friedman-Robertson-Walker spacetime. We show that the two formulae agree. As an application of our formula, we calculate the maximum turnaround radius in the case of the Brans-Dicke theory of gravi…
Finite-cutoff holography and quasilocal thermodynamics of BTZ black holes in a cavity
本文提出有限截断形式的BTZ黑洞热力学，将圆环腔视为真实反 holographic 屏幕，结合准局部引力系统和AdS_3中的有限半径holography，探讨静态和旋转BTZ几何的热力学。
Three simple idealised models are studied in order to develop some intuition about the leading order effect of non-sphericity on the maximum turnaround size $R_{\rm TA,max}$ of large scale bound cosmic structures. Two of them describe intrinsically axisymmetric static mass distributions whereas the other is the Kerr-de Sitter metric where the axisymmetry is generated due to the rotation of the structure. In all the cases the fractional change $δR_{\rm TA,max}(θ)/R^{(0)}_{\rm TA,max}$ of $R_{\rm TA,max}$ of a given structure, compared to a spherical one with the same mass $M$, depends on the po…
We explore the (non)-universality of Martinez's conjecture, originally proposed for Kerr black holes, within and beyond general relativity. The conjecture states that the Brown-York quasilocal energy at the outer horizon of such a black hole reduces to twice its irreducible mass, or equivalently, to \sqrt{A} /(2\sqrt{pi}), where `A' is its area. We first consider the charged Kerr black hole. For such a spacetime, we calculate the quasilocal energy within a two-surface of constant Boyer-Lindquist radius embedded in a constant stationary-time slice. Keeping with Martinez's conjecture, at the out…
Studying spherical collapse and its implications in the Eddington-inspired Born-Infeld gravity theory
本文研究EiBI引力理论中球面坍缩的演化，探讨了物质梯度修正对线性坍缩阈值、转折过密度等参数的影响，揭示了EiBI修正对非线性结构形成的重要性。
In many cosmological models, including the $Λ$CDM concordance model, there exist a theoretical upper bounds on the size of collapsing structures. The most common formulations in the literature refer to a turnaround radius in spherical symmetry or a turnaround surface, defined as the zero-expansion boundary separating the outer Hubble flow from the inner flow of a collapsing fluid. In order to access a generic scenario, we propose an improvement of this cosmological test in terms of the maximum volume of the cosmological structures, which is equivalent to a zero-averaged expansion -- instead of…
Inhomogeneous dust collapse in five-dimensional AdS-Chern-Simons gravity
研究五维AdS-Chern-Simons引力中非均匀尘埃坍缩，得到精确内部解，证明壳聚焦为真实奇点，并揭示视界形成依赖于准局域质量阈值。
Approximations of the quasi-local Bartnik mass in general relativity
本研究利用 eth 算子和自旋加权球谐函数表达 ADM 质量，并将其推广至准局部 Bartnik 质量，进而提出一种深度学习方法以数值构造具有指定 Bartnik 质量的静态度量。
Even though many scalar field models of dark energy have been considered in the literature, there is another interesting class of dark energy models involving a fluid known as a Chaplygin gas. In addition to describing the dark energy, both scalar-tensor model and the Chaplygin gas model are suitable candidates for explaining the spherical cosmological collapse. One of the most well-known scalar field models is the quintessence model, which was first introduced to explain an accelerating expanding universe. Using a special form of the quintessence model that is equivalent to Chaplygin gas, we …
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