R. Kolevatov, S. Mironov
We study whether it is possible to design a "classical" spatially flat bouncing cosmology or a static, spherically symmetric asymptotically flat Lorentzian wormhole in cubic Galileon theories interacting with an extra scalar field. We show that bouncing models are always plagued with gradient instabilities, while there are always ghosts in wormhole backgrounds.
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We revisit the evolutions of scalar perturbations in a non-singular Galileon bounce. It is known that the second order differential equation governing the perturbations is numerically unstable at a point called $γ$-crossing. This instability is usually circumvented using certain gauge choices. We show that the perturbations can be evolved across this point by solving the first order differential equations governing suitable gauge invariant quantities without any instabilities. We demonstrate this method in a matter bounce scenario described by the Galileon action.
We investigate the bounce and cyclicity realization in the framework of weakly broken galileon theories. We study bouncing and cyclic solutions at the background level, reconstructing the potential and the galileon functions that can give rise to a given scale factor, and presenting analytical expressions for the bounce requirements. We proceed to a detailed investigation of the perturbations, which after crossing the bouncing point give rise to various observables, such as the scalar and tensor spectral indices and the tensor-to-scalar ratio. Although the scenario at hand shares the disadvant…
We show that a nonsingular bounce, free of ghosts and gradient instabilities, can be realized in the framework of Horndeski or generalized Galileon cosmology. In particular, we first review that the theoretical no-go theorem, which states that the above is impossible, is based on two very strong assumptions, namely that a particular quantity cannot be discontinuous during the bounce, and that there is only one bounce. However, as we show in the present work, the first assumption not only can be violated in a general Horndeski/Galileon scenario, but also it is necessarily violated at the bounce…
Cosmological Perturbations Through a Non-Singular Ghost-Condensate/Galileon Bounce
研究了超视界宇宙学扰动在非奇异反弹时空中的传播。我们考虑的模型结合了鬼凝聚与伽利略项，以诱导一个鬼自由反弹。我们的计算是在调和规范下进行的，这确保了线性化的运动方程始终保持良好的定义和非奇异性。我们发现，尽管事实上，反弹附近的声速成为虚构的，超视界曲率扰动保持基本恒定的反弹。事实上，我们表明，有一个时间接近反弹，所有波长的曲率扰动需要暂时完全恒定。我们将我们的计算与其他量规中进行的计算相关联，并评论与文献中先前结果的关系。
Galileon bounce after ekpyrotic contraction
我们考虑一个简单的宇宙学模型，它包括一个长的Ekpyrotic收缩阶段和光滑的反弹后，Ekpyrotic行为是由于一个标量场与负指数势，而Galileon场产生反弹。我们给出了一个分析图片反弹如何发生在弱引力制度，然后进行数值分析，将我们的结果扩展到一个非微扰制度。
We study a nonsingular bounce inflation model, which can drive the early universe from a contracting phase, bounce into an ordinary inflationary phase, followed by the reheating process. Besides the bounce that avoided the Big-Bang singularity which appears in the standard cosmological scenario, we make use of the Horndesky theory and design the kinetic and potential forms of the lagrangian, so that neither of the two big problems in bouncing cosmology, namely the ghost and the anisotropy problems, will appear. The cosmological perturbations can be generated either in the contracting phase or …
Galileon Bouncing Inflation after BICEP2
我们提出了一个非奇异的情况下，一个通货膨胀的时代后，从早期宇宙的收缩情况反弹。宇宙的收缩应该是缓慢的，这样初始的各向异性就不会增长得太快而成为主导并破坏反弹。在反弹之后，宇宙进入暴胀区域和再加热阶段，在那里产生原始扰动。预计扰动的张量标量比与新发布的数据$r=0.2^{+0.07}_{-0.05}$一致。增加反弹过程的目的是为了摆脱恼人的大爆炸奇点，这通常存在于纯暴胀模型中。
We discuss the possibility of constructing stable, static, spherically symmetric, asymptotically flat Lorentzian wormhole solutions in General Relativity coupled to a generalized Galileon field $π$. Assuming that Minkowski space-time is obtained at $\partial π=0$, we find that there is tension between the properties of the energy-momentum tensor required to support a wormhole (violation of average null energy conditions) and stability of the Galileon perturbations about the putative solution (absence of ghosts and gradient instabilities). In 3-dimensional space-time, this tension is strong eno…
Towards Anisotropy-Free and Non-Singular Bounce Cosmology with Scale-invariant Perturbations
我们研究了无幽灵广义Galileon宇宙学框架下的非奇异反弹实现，它可以进一步摆脱各向异性问题。考虑类Ekpyrotic势，我们可以得到一个总的状态方程（EoS）大于一个在收缩阶段，这是必要的演化是稳定的对小的各向异性波动。由于这样一个大的EoS禁止伽利略场产生所需形式的扰动，我们另外引入曲率场，它可以在一般情况下产生所观察到的近尺度不变的光谱。特别是，我们提供了近似的解析和精确的半解析表达式下，弹跳的情况是与观测一致的。最后，组合的Galileon-curvaton系统在反弹后没有大撕裂。
We present a new class of nonsingular bounce cosmology free from instabilities, using a single scalar field coupled to gravity within the framework of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. In this type of scenarios, the gradient instability that widely exists in nonsingular bounce cosmologies in the framework of scalar-tensor and Horndeski/Galileon theories is removed by the effects of new operators introduced by the DHOST, due to the modification that they later bring about to the dispersion relation of perturbations. Hence, our results demonstrate that there is indeed a…
We recently showed how it is possible to use a cubic Galileon action to construct classical cosmological solutions that enter a contracting null energy condition (NEC) violating phase, bounce at finite values of the scale factor and exit into an expanding NEC-satisfying phase without encountering any singularities or pathologies. A drawback of these examples is that singular behavior is encountered at some time either just before or just after the NEC-violating phase. In this Letter, we show that it is possible to circumvent this problem by extending our method to actions that include the next…
We consider a class of generalized Galileon theories within General Relativity in space-times of more than two spatial dimensions. We show that these theories do not admit stable, static, spherically symmetric, asymptotically flat and traversable Lorentzian wormholes.
G-Bounce
我们提出了一个广泛的类模型，实现了反弹在一个空间平坦的弗里德曼宇宙在标准广义相对论。我们考虑的理论的关键成分是一个非正则的，最小耦合的标量场属于一类的理论与动力学重力编织/Galileon的自耦合。在这些模型中，宇宙从收缩到膨胀平滑地演化，在转折点附近既没有鬼也没有梯度不稳定性。演化的终点可以是一个标准的辐射主导时代或暴胀阶段。我们制定必要的限制拉格朗日需要获得一个健康的反弹，并说明我们的结果与简单的系统，包括最近提出的伽利略成因的情况下的相图。
Stability of nonsingular cosmologies in Galileon models with torsion. A no-go theorem for eternal subluminality
该研究将伽利略模型非奇异宇宙学的不可行定理推广至含挠率时空，证明在额外假设引力子亚光速下仍成立，但短暂超光速阶段（$c_g\geq\sqrt{2}c$）可构造全时间稳定且非奇异的反弹宇宙模型。
One of the fundamental questions of theoretical cosmology is whether the universe can undergo a non-singular bounce, i.e., smoothly transit from a period of contraction to a period of expansion through violation of the null energy condition (NEC) at energies well below the Planck scale and at finite values of the scale factor such that the entire evolution remains classical. A common claim has been that a non-singular bounce either leads to ghost or gradient instabilities or a cosmological singularity. In this letter, we examine cubic Galileon theories and present a procedure for explicitly co…
Bouncing Galileon Cosmologies
我们提出了非奇异的，均匀的和各向同性的共形Galileon模型的弹跳解。我们表明，这样的解决方案必然开始与辐射为主的收缩阶段。其次是一个五分之一的情况下，背景状态方程跨越宇宙学常数边界允许一个非奇异的反弹，这反过来又是其次是伽利略成因。我们分析了在这种背景下的宇宙学扰动谱。我们的结果表明，起伏的演变顺利，没有任何病理，但绝热模式形成一个蓝色的倾斜光谱。为了获得当前观测所要求的标度不变的原始功率谱，我们引入了一个光标量场与Galileon的动力学耦合。我们发现两个耦合产生一个标度不变的频谱，其中之一需要微调的初始条件。这个模型还预测了一个蓝色的倾斜光谱的引力波源于量子真空波动在收缩阶段。
We study a model for a non-singular cosmic bounce in N=1 supergravity, based on supergravity versions of the ghost condensate and cubic Galileon scalar field theories. The bounce is preceded by an ekpyrotic contracting phase which prevents the growth of anisotropies in the approach to the bounce, and allows for the generation of scale-invariant density perturbations that carry over into the expanding phase of the universe. We present the conditions required for the bounce to be free of ghost excitations, as well as the tunings that are necessary in order for the model to be in agreement with c…
From configuration to dynamics -- Emergence of Lorentz signature in classical field theory
在任何场论中使用的洛伦兹度规结构允许人们实现因果关系的相对论概念，并定义时间维度的概念。本文研究了在微观层次上度规是黎曼的可能性，即局部欧几里得的可能性，以及我们通常认为是基本的洛伦兹结构实际上是在具有正定度规的四维空间的某些区域中出现的有效性质。在这样的模型中，在某些超曲面上没有动力学或签名翻转;相反，所有的场在这些区域中发展出洛伦兹动力学，因为它们以有效的度规传播。它表明，人们可以构建一个体面的经典场论标量，矢量和（狄拉克）旋量在平坦时空。然后表明，重力可以包括在内，但有效的洛伦兹度规的理论不是广义相对论，而是协变伽利略型。从稳定性，等效原理和基本常数的恒定性所产生的限制进行了详细的描述，并给出了洛伦兹度规出现的唯象图像。这一结构虽然仅限于本文中的经典领域，但却提供了一种关于时间概念的新观点。
The worldvolume actions of 3+1 dimensional bosonic branes embedded in a five-dimensional bulk space can lead to important effective field theories, such as the DBI conformal Galileons, and may, when the Null Energy Condition is violated, play an essential role in cosmological theories of the early universe. These include Galileon Genesis and "bouncing" cosmology, where a pre-Big Bang contracting phase bounces smoothly to the presently observed expanding universe. Perhaps the most natural arena for such branes to arise is within the context of superstring and $M$-theory vacua. Here, not only ar…
We present a systematic study of the cosmological dynamics resulting from an effective Hamiltonian, recently derived in loop quantum gravity using Thiemann's regularization and earlier obtained in loop quantum cosmology (LQC) by keeping the Lorentzian term explicit in the Hamiltonian constraint. We show that quantum geometric effects result in higher than quadratic corrections in energy density in comparison to LQC causing a non-singular bounce. Dynamics can be described by the Hamilton's or the Friedmann-Raychaudhuri equations, but the map between the two descriptions is not one-to-one. A car…
Gravitational-wave background in bouncing models from semi-classical, quantum and string gravity
本文研究了三种以反弹取代大爆炸的引力模型的原初谱和引力波背景，发现pre-big-bang模型的GWB可被LISA和Einstein Telescope探测，而另两种模型则不可探测或被排除。
We study the cosmological sector of the Lorentzian Barrett-Crane (BC) model coupled to a free massless scalar field in its Group Field Theory (GFT) formulation, corresponding to the mean-field hydrodynamics obtained from coherent condensate states. The relational evolution of the condensate with respect to the scalar field yields effective dynamics of homogeneous and isotropic cosmologies, similar to those previously obtained in $\text{SU}(2)$-based EPRL-like models. Also in this manifestly Lorentzian setting, in which only continuous $\text{SL}(2,\mathbb{C})$-representations are used, we obta…
We study the dynamics of perturbations representing deviations from perfect isotropy in the context of the emergent cosmology obtained from the group field theory formalism for quantum gravity. Working in the mean field approximation of the group field theory formulation of the Lorentzian EPRL model, we derive the equations of motion for such perturbations to first order. We then study these equations around a specific simple isotropic background, characterised by the fundamental representation of $\mbox{SU(2)}$, and in the regime of the effective cosmological dynamics corresponding to the bou…
Closed strings in Misner space: a toy model for a Big Bounce ?
米斯纳空间，也被称为洛伦兹轨道折叠$R^{1,1}/boost$，是弦论中宇宙学奇点最简单的例子之一。在这堂课中，我们回顾了在这种背景下闭弦的半经典传播，特别强调了轨道的扭曲部分。还讨论了三能级散射振幅和单圈真空振幅。
Arrows of Time in the Bouncing Universes of the No-boundary Quantum State
我们在一个小超空间模型中，从宇宙的量子态（NBWF）的无边界理论出发，导出了宇宙的时间箭头。时间的箭头被四维地看作是宇宙的四维洛伦兹历史的性质。这些历史的概率由NBWF预测。对于在最小半径处具有规则“反弹”的历史，我们发现波动在反弹处很小，并且在两侧的膨胀方向上增长。对于具有大爆炸和大挤压奇点的经典历史，我们发现涨落在一个奇点附近很小，并通过膨胀和收缩增长到另一个奇点。因此，由涨落增长定义的时间箭头在整个跳跃时空中指向一个方向，但在跳跃时空中是双向的。我们认为，电磁，热力学和心理箭头的时间是对齐的波动箭头。双向箭头的因果关系的影响进行了讨论。
Remote Hawking-Moss instanton and the Lorentzian path integral
该研究通过洛伦兹路径积分和Picard-Lefschetz理论，证明远程霍金-莫斯跃迁的量子隧穿振幅小于传统振幅，且随场距离增大而显著抑制。
(2+1) Lorentzian quantum cosmology from spin-foams: opportunities and obstacles for semi-classicality
该研究构建(2+1)维洛伦兹量子宇宙学的有效自旋泡沫模型，通过驻相近似和耦合有质量标量场，分析半经典性的机遇与挑战，为量子反弹等场景研究奠定基础。
We study the instantons (or bounces) in the Brown-Teitelboim (BT) mechanism of relaxation of cosmological constant which is a cosmological version of the Schwinger mechanism. The BT mechanism is a false vacuum decay of (A)dS$_{d+1}$ (and $R^{1, d}$) spaces via spontaneous nucleations of spherical $(d-1)$-branes and thus ostensibly has bearings on (A)dS$_{d+1}$/CFT$_d$ holography. In this paper we focus on the four-dimensional case, although the higher or lower-dimensional generalization is straightforward. As is the case with pair productions near black hole and de Sitter horizons, we show tha…
We investigate Euclidean wormholes in Einstein gravity with a massless scalar field in de Sitter space. Euclidean wormholes are possible due to the analytic continuation of the time as well as complexification of fields, where we need to impose the classicality after the Wick-rotation to the Lorentzian signatures. For some parameters, wormholes are preferred than Hawking-Moss instantons, and hence wormholes can be more fundamental than Hawking-Moss type instantons. Euclidean wormholes can be interpreted in three ways: (1) classical big bounce, (2) either tunneling from a small to a large unive…
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