Tanmay Vachaspati
The creation of spacetimes with horizons is discussed, focussing on baby universes and black holes as examples. There is a complex interplay of quantum theory and General Relativity in both cases, leading to consequences for the future of the universe and the information loss paradox, and to a deeper understanding of quantum gravity.
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We show that the dynamical realization of the acceleration-enlarged Galilean symmetry leads to nonrelativistic massless particles whose energy may be negative. We present a fluid mechanical generalisation of this observation and use it to contruct a nonrelativistic two-dimensional fluid model which possesses solutions with a negative energy density. Considering this model as describing dark energy in a baby universe (two space dimensions) we show that its negative energy density leads to a repulsive gravitational interaction of the fluid with any test body.
Under certain circumstances, the collision of magnetic monopoles, topologically locked-in regions of false vacuum, leads to topological inflation and the creation of baby universes. The future evolution of initial data represented by the two incoming monopoles may contain a timelike singularity but this need not be the case. We discuss the global structure of the spacetime associated with monopole collisions and also that of topological inflation. We suggest that topological inflation within magnetic monopoles leads to an eternally reproducing universe.
Late time acceleration of our Universe caused by the absorption of baby universes
本文提出宇宙吸收婴儿宇宙的机制，通过修正弗里德曼方程解释晚期宇宙加速，无需宇宙学常数，并支持局域测量的哈勃常数值。
We reconsider the Coleman's mechanism that solves the Cosmological Constant Problem by the baby universes. We use the Lorentzian path integral and allow each universe has a different effective field theory and a vacuum. By using the probability distribution of coupling constants, it is shown that the cosmological constant of our universe does not necessary become small due to the effects of other universes.
Scaling Functions for Baby Universes in Two-Dimensional Quantum Gravity
我们最近提出的转移矩阵形式主义耦合到$(2, 2k-1)$极小模型的二维量子引力。我们发现，在测地（欧几里德）时间的父宇宙的传播是伴随着不断发射的婴儿宇宙，并推导出一个分布函数描述其大小。$k\to \infty~ (c\to -\infty)$极限通常被认为对应于经典几何，我们确实在宇宙分布函数中发现了一个经典峰值。然而，我们也观察到戏剧性的量子效应与婴儿宇宙在有限的长度尺度。
We study a partially fine-grained description of an evaporating black hole by introducing an open baby universe with a boundary. Since the Page's calculation of the entropy of Hawking radiation involves an ensemble average over a class of states, one can formally obtain a fine-grained state by purifying this setup. For AdS black holes with a holographic dual, this purification amounts to introducing an additional boundary (i.e., baby universe) and then connecting it to the original black hole through an Einstein-Rosen bridge. We uncover several details of this setup. As applications, we briefl…
We study quantum entanglements of baby universes which appear in non-perturbative corrections to the OSV formula for the entropy of extremal black holes in Type IIA string theory compactified on the local Calabi-Yau manifold defined as a rank 2 vector bundle over an arbitrary genus G Riemann surface. This generalizes the result for G=1 in hep-th/0504221. Non-perturbative terms can be organized into a sum over contributions from baby universes, and the total wave-function is their coherent superposition in the third quantized Hilbert space. We find that half of the universes preserve one set of…
The behaviour of baby universes has been an important ingredient in understanding and quantifying non-critical string theory or, equivalently, models of two-dimensional Euclidean quantum gravity coupled to matter. Within a regularized description based on dynamical triangulations, we amend an earlier conjecture by Jain and Mathur on the scaling behaviour of genus-$g$ surfaces containing particular baby universe `necks', and perform a nontrivial numerical check on our improved conjecture.
We propose a novel construction of a third quantised baby universe Hilbert space $\mathcal{H}_{BU}$ for the quantum gravity path integral. In contrast to the original description of $α$-parameters, both the bulk and boundary microscopic parameters are fixed in our proposal. Wormholes and baby universes appear due to refined observables, of the boundary dual quantum field theories, that crucially involve the space of representations of the gauge group. Irreducible representations, on which the path integral factorises, give rise to field theoretic superselection sectors and replace the $α$ stat…
Observing 4D Baby Universes in Quantum Gravity
我们通过识别所谓的婴儿宇宙来测量四维单纯量子引力的分形结构。这使得一个简单的决定临界指数$\g$连接到四维流形的熵。
Can the Interaction between Baby Universes Generate a Big Universe?
我们探索一个简单的玩具模型的相互作用的宇宙，建立一个小的婴儿宇宙可以变得大（$\gg$普朗克长度），如果第三个量子化机制被考虑在内。
We argue that the holographic description of four-dimensional BPS black holes naturally includes multi-center solutions. This suggests that the holographic dual to the gauge theory is not a single AdS_2 times S^2 but a coherent ensemble of them. We verify this in a particular class of examples, where the two-dimensional Yang-Mills theory gives a holographic description of the black holes obtained by branes wrapping Calabi-Yau cycles. Using the free fermionic formulation, we show that O(e^{-N}) non-perturbative effects entangle the two Fermi surfaces. In an Euclidean description, the wave-funct…
Baby Universes in AdS$_3$
研究AdS$_3$中欧几里得几何的洛伦兹切片，产生闭合婴儿宇宙，其空间几何由亏格g≥2的曲面给出。通过二维共形场论提供替代壳态解释，显示婴儿宇宙几何始终次主导，且通过混合CFT态验证其可靠性。
Can a late-time cosmological model based on baby universe absorption explain the z-variation of w?
该研究提出一个基于婴儿宇宙吸收的晚期宇宙模型，无需宇宙常数即可解释宇宙指数膨胀，并导致w(z)参数随红移变化，且在足够大的红移下w(z)小于-1。
See hep-ph/0304045
We formulate the baby universe construction rigorously by giving a primordial role to the algebra of observables of quantum gravity rather than the Hilbert space. Utilizing diffeomorphism invariance, we study baby universe creation and annihilation via change in topology. We then construct the algebra of boundary observables for holographic theories and show that it enhances to contain an 'extra' Abelian tensor factor to describe the bulk in the quantum regime; via the gravitational path integral we realize this extra tensor factor, at the level of the Hilbert space, in the context of the GNS …
We reformulate recent insights into black hole information in a manner emphasizing operationally-defined notions of entropy, Lorentz-signature descriptions, and asymptotically flat spacetimes. With the help of replica wormholes, we find that experiments of asymptotic observers are consistent with black holes as unitary quantum systems, with density of states given by the Bekenstein-Hawking formula. However, this comes at the cost of superselection sectors associated with the state of baby universes. Spacetimes studied by Polchinski and Strominger in 1994 provide a simple illustration of the as…
We study $\widehat{\text{CGHS}}$ gravity, a variant of the matterless Callan-Giddings-Harvey-Strominger model. We show that it describes a universal sector of the near horizon perturbations of non-extremal black holes in higher dimensions. In many respects this theory can be viewed as a flat space analog of Jackiw-Teitelboim gravity. The result for the Euclidean path integral implies that $\widehat{\text{CGHS}}$ is dual to a Gaussian ensemble that we describe in detail. The simplicity of this theory allows us to compute exact quantities such as the quenched free energy and provides a useful pl…
On rips and cosmological singularities in a universe merging with baby universes
本文在宇宙与其他“婴儿”宇宙合并以驱动晚期膨胀的多元宇宙场景下，首次探测了撕裂场景与未来宇宙学奇点，并通过检验两个合并模型证明此类模型中不会出现任何撕裂或未来奇点。
We discuss the essential features of baby-universe production, starting from a description of black holes and wormholes, in terms of the causal structure of spacetime, and following a qualitative review of the connection between vacuum decay and inflation in early universe cosmology. Related open questions are also addressed, especially in connection with the possibility that baby universes could be formed within our present universe - either in a future particle accelerator, or spontaneously.
Quantum black holes are described by a large number of macroscopically indistinguishable microstates. Correlation functions of fields outside the horizon at long time separation probe this indistinguishability. The simplest of these, the thermal two-point function, oscillates erratically around a nonperturbatively small average "ramp" and "plateau" after an initial period of decay; these non-decaying averaged features are signatures of the discreteness of the black hole spectrum. For a theory described by an ensemble of Hamiltonians, the two-point function follows this averaged behavior. In th…
String tension fundamentally determines the properties of strings; yet its value is often assigned arbitrarily, creating a fine-tuning problem. We describe a mechanism for dynamically generating string tension in a flat or almost flat spacetime, using the modified measures formalism, which in turn naturally generates a new type of stringy brane-world scenario. Such a scenario allows strings to achieve near infinite tension confining the strings to two very close expanding surfaces, but the infinite tensions also threatens to distort the near-flat embedding spacetime through large back reaction…
Can we live in a baby universe formed by a delayed first-order phase transition?
该研究探讨了婴儿宇宙起源的可能性，提出在扩展的标准模型中，延迟一级相变可能使我们所处的宇宙具有高概率，并预测了可被实验检测的重中性规范玻色子存在。
A baby-universe model of cosmic inflation is analyzed using quantum Darwinism. In this model cosmic inflation can be approximated as Brownian motion of a quantum field, and quantum Darwinism implies that decoherence is the result of quantum Brownian motion of the wave function. The quantum Darwinism approach to decoherence in the baby-universe cosmic-inflation model yields the decoherence times of the baby-universes. The result is the equation relating the baby-universe's decoherence time with the Hubble parameter. A brief discussion of the relation between Darwinism and determinism is provide…
The validity of the Coleman mechanism, which automatically tunes the fundamental constants, is examined in two-dimensional and four-dimensional quantum gravity theories. First, we consider two-dimensional Euclidean quantum gravity on orientable closed manifolds coupled to conformal matter of central charge $c \leq1$. The proper time Hamiltonian of this system is known to be written as a field theory of noncritical strings, which can also be viewed as a third quantization in two dimensions. By directly counting the number of random surfaces with various topologies, we find that the contribution…
General relativity predicts that the inner horizon of an astronomically realistic rotating black hole is subject to the mass inflation instability. The inflationary instability acts like a gravity-powered particle accelerator of extraordinary power, accelerating accreted streams of particles along the principal outgoing and ingoing null directions at the inner horizon to collision energies that would, if nothing intervened, typically exceed exponentially the Planck energy. The inflationary instability is fueled by ongoing accretion, and is occurring inevitably in essentially every black hole i…
Can Baby Universe Absorption Explain Dark Energy?
本研究检验婴儿宇宙并合解释暗能量的模型，发现纯模型拟合效果差，扩展含宇宙学常数的版本有两个允许参数区，可优于ΛCDM模型并缓解哈勃张力。
Black hole thermodynamics suggests that, in order to describe the physics of distant observers, one may model a black hole as a standard quantum system with density of states set by the Bekenstein-Hawking entropy $S_\mathrm{BH}$. This idea has long been considered to be in strong tension with Hawking's prediction that radiation from black holes is nearly thermal, and with low-energy gravity more generally. But the past two years have shown that low-energy gravity does offer a self-consistent description of black hole evaporation consistent with the above idea, and which in particular reproduce…
The quantum gravity path integral involves a sum over topologies that invites comparisons to worldsheet string theory and to Feynman diagrams of quantum field theory. However, the latter are naturally associated with the non-abelian algebra of quantum fields, while the former has been argued to define an abelian algebra of superselected observables associated with partition-function-like quantities at an asymptotic boundary. We resolve this apparent tension by pointing out a variety of discrete choices that must be made in constructing a Hilbert space from such path integrals, and arguing that…
Group-averaged baby universe field theory
本文从群平均振幅构建婴宇宙场论，阐明其场对易性与线性场方程，并推广至一般拓扑，在特定模型中给出全拓扑的精确福克空间表示。
We investigate the decay of metastable de Sitter, Minkowski and anti-de Sitter vacua catalyzed by a black hole and a cloud of strings. We apply the method to the creation of the four dimensional bubble universe in the five dimensional anti-de Sitter spacetime recently proposed by Banerjee, Danielsson, Dibitetto, Giri and Schillo. We study the bounce action for the creation and find that the bubble with very small cosmological constant, of order $Λ^{(4)}/M^2_4 \sim 10^{-120}$, is favored by the catalysis by assuming appropriate mass scales of the black hole and the cloud of strings to reproduce…
In the 1980's, work by Coleman and by Giddings and Strominger linked the physics of spacetime wormholes to `baby universes' and an ensemble of theories. We revisit such ideas, using features associated with a negative cosmological constant and asymptotically AdS boundaries to strengthen the results, introduce a change in perspective, and connect with recent replica wormhole discussions of the Page curve. A key new feature is an emphasis on the role of null states. We explore this structure in detail in simple topological models of the bulk that allow us to compute the full spectrum of associat…
Birth of baby universes from gravitational collapse in a modified-gravity scenario
本文研究 Palatini $f(\mathcal{R})$ 引力中玻色星坍缩，发现坍缩内部形成通过喉部与母宇宙相连的指数膨胀婴儿宇宙，且该喉部始终被视界隐藏，与外部观测者因果隔离。
We present a dynamical toy model for an expanding universe inside a black hole. The model is built by matching a spherically symmetric collapsing matter cloud to an expanding Friedmann-Robertson-Walker universe through a phase transition that occurs in the quantum-gravity dominated region, here modeled with semi-classical corrections at high density. The matching is performed on a space-like hyper-surface identified by the co-moving time at which quantum-gravity induced effects halt collapse. The purpose of the model is to suggest a possible reconciliation between the observation that black ho…
AdS/CFT, Ultralimits and Baby universes
本文在AdS/CFT对偶框架下，利用超极限与自由超滤子概念提出CFTₚ边界理论，指出引力路径积分可产生婴儿宇宙与时空虫洞，并将该思路应用于类AS²态。
We propose a solution of the naturalness problem in the context of the multiverse wavefunction without the anthropic argument. If we include microscopic wormhole configurations in the path integral, the wave function becomes a superposition of universes with various values of the coupling constants such as the cosmological constant, the parameters in the Higgs potential, and so on. We analyze the quantum state of the multiverse, and evaluate the density matrix of one universe. We show that the coupling constants induced by the wormholes are fixed in such a way that the density matrix is maximi…
Condensed matter black hole analogues may provide guidance in grappling with difficult questions about the role of short distance physics in the Hawking effect. These questions bear on the very existence of Hawking radiation, the correlations it may or may not carry, the nature of black hole entropy, and the possible loss of information when a black hole evaporates. We describe a model of black hole formation and evaporation and the loss of information to a disconnected universe in a thin film of 3He-A, and we explain why the existence of Hawking radiation has not yet been demonstrated in this…
The Baby Universe is Fine and the CFT Knows It: On Holography for Closed Universes
本文探讨了AdS/CFT中闭宇宙的全息编码机制，澄清了其与观察者克隆提议的关系并反驳了半经典物理崩溃的论点，提出末态投影模型以解释CFT如何决定闭宇宙的物理性质及时间箭头涌现。
The dynamics of baby universes which are branched off in pairs from a large universe and whose respective members are entangled to each other has been studied in quantum gravity and string theory. It is shown that the probability measure for such pairs essentially keeps its Planckian form when one of their two members is trapped by a Lorentzian tunnel and travels through it into another large universe, so that the two baby universes of the pairs preserve their mutual entanglement even when they are going to be finally branched in distinct large universes. The conclusion is thus drawn that big …
In this work, we extend a 2d topological gravity model of Marolf and Maxfield to have as its bulk action any open/closed TQFT obeying Atiyah's axioms. The holographic duals of these topological gravity models are ensembles of 1d topological theories with random dimension. Specifically, we find that the TQFT Hilbert space splits into sectors, between which correlators of boundary observables factorize, and that the corresponding sectors of the boundary theory have dimensions independently chosen from different Poisson distributions. As a special case, we study in detail the gravity model built …
Baby universes (inflationary or non--inflationary) are regions of spacetime that disconnect from the original ambient spacetime, which we take to be asymptotically flat spacetime. A particular kind of baby universe solution, involving string--like matter, is studied to show that it can be formed by ``investing'' an arbitrarily small amount of energy, i.e. it can appear from an almost flat space at the classical level. Since this possibility has not yet been clearly recognized in the literature, we then discuss in detail its properties, relevance and possible generalizations.
Firewalls from wormholes in higher genus
本文研究JT引力中高亏格情形下黑洞通过发射多个婴儿宇宙隧穿至白洞或防火墙的概率，并给出了与黑洞年龄、熵和能量相关的精确概率公式。
We present Euclidean wormhole solutions describing possible bridges within the multiverse. The study is carried out in the framework of the third quantization. The matter content is modelled through a scalar field which supports the existence of a whole collection of universes. The instanton solutions describe Euclidean solutions that connect baby universes with asymptotically de Sitter universes. We compute the tunnelling probability of these processes. Considering the current bounds on the energy scale of inflation and assuming that all the baby universes are nucleated with the same probabil…
Spacetime wormholes can lead to surprises in black hole physics. We show that a very old black hole can tunnel to a white hole/firewall by emitting a large baby universe. We study the process for a perturbed thermofield double black hole in Jackiw-Teitelboim (JT) gravity, using the lowest order (genus one) spacetime wormhole that corresponds to single baby-universe emission. The probability for tunneling to a white hole is proportional to $t^2 e^{-2S}$ where $t$ is the age of the black hole and $S$ is the entropy of one black hole.
Super-slow phase transition catalyzed by BHs and the birth of baby BHs
本文研究原初黑洞催化的一阶相变，提出当哈勃体积内黑洞数少于1时发生超慢相变，可产生增强引力波信号，并解释暗物质丰度。
We recently formulated a model of the universe based on an underlying W3-symmetry. It allows the creation of the universe from nothing and the creation of baby universes and wormholes for spacetimes of dimension 2, 3, 4, 6 and 10. Here we show that the classical large time and large space limit of these universes is one of exponential fast expansion without the need of a cosmological constant. Under a number of simplifying assumptions our model predicts that w=-1.2 in the case of four-dimensional spacetime. The possibility of obtaining a w-value less than -1 is linked to the ability of our mod…
Finite cutoff JT gravity: Baby universes, Matrix dual, and (Krylov) Complexity
本文基于'复杂性=体积'猜想，研究有限截断JT引力中黑洞内部在晚期时间的增长，发现由于可积无关变形，谱特性发生非平凡改变，ERB长度增长速度比纯JT引力快。同时计算了变形理论中婴儿宇宙的发射概率，并发现只有在洛伦兹演化的条件下，变形参数才会改变发射概率。此外，发现变形理论相对于未变形理论的饱和时间依赖于逆温度，并讨论了双射矩阵模型的细微差别及可能的一刀切普遍性。最后，评论了由有限边界截断引起的非微扰谱曲线修正对模空间体积的可能修正。
"It from Bit": The Hartle-Hawking state and quantum mechanics for de Sitter observers
本文探讨了德西特空间内观测者所经历的量子力学与封闭宇宙一态性质的关系，通过解析婴儿宇宙希尔伯特空间与本体希尔伯特空间的区别，阐明了德西特熵的物理意义。
We argue that an understanding of the Arrow of Time is urgently required if recent string-theoretic ideas about cosmology are to be made to function. In the string Landscape, the nucleation of baby universes is postulated to "populate" the multiverse. Here we argue that baby universes can only have an Arrow if they \emph{inherit} one; the problem of explaining the Arrow is thus reduced to explaining it in the case of the original universe. Motivated by the Ooguri-Vafa-Verlinde formulation of "creation from nothing" in the context of string theory, we propose that the original universe was crea…
Using the known result that the nucleation of baby universes in correlated pairs is equivalent to spacetime squeezing, we show in this letter that there exists a T-duality symmetry between two-dimensional warp drives, which are physically expressible as localized de Sitter little universes, and two dimensional Tolman-Hawking and Gidding-Strominger baby universes respectively correlated in pairs, so that the creation of warp drives is also equivalent to spacetime squeezing. Perhaps more importantly, it has been also seen that the nucleation of warp drives entails a violation of the Bell's inequ…
A Possible Resolution of the Black Hole Information Puzzle
在假设黑洞蒸发过程终止于黑洞内部衰变为婴儿宇宙的前提下，考虑了信息损失问题。我们表明，这样的理论可以被分解成超选择部门标记的第三个量子化的婴儿宇宙场算子的本征值，散射是单一的每个超选择部门。这个结果主要依赖于衰变时间的量子力学可变性。进一步论证了在黑洞静止坐标系中的衰变率必然与$e^{-S_{tot}}$成正比，其中$S_{tot}$是蒸发过程中产生的总熵，这意味着一个非常长寿命的残余物。
A Zoo of Axionic Wormholes
本文扩展了轴子-膨胀子虫洞解，发现膨胀子有质量时解集大增，并存在导致膨胀婴儿宇宙的新解族，且欧几里得作用量随振荡增加而减小。
We investigate contributions of spacetime wormholes, describing baby universe emission and absorption, to calculations of entropies and correlation functions, for example those based on the replica method. We find that the rules of the "wormhole calculus," developed in the 1980s, together with standard quantum mechanical prescriptions for computing entropies and correlators, imply definite rules for $\textit{limited}$ patterns of connection between replica factors in simple calculations. These results stand in contrast with assumptions that all topologies connecting replicas should be summed o…
The aim of this paper is to construct a quantum effective action for gravitational fields by the effective potential method in quantum field theory. The minimum of the quantum effective action gives an equation of quantum fluctuations. We discuss the quantum fluctuation in the flat spacetime and in the Schwarzschild spacetime. It is shown that a baby spacetime may be created from a classical vacuum through a quantum fluctuation.
Operator Algebras and Third Quantization
本文提出Poissonization算子代数框架，将拓扑变化导致的宇宙创生与湮灭描述为泊松过程，并统一解释了多边界谱形因子平台、闭婴儿宇宙Fock空间相干态以及Jackiew-Teitelboim引力中的晚期关联子。
Towards a holographic description of closed universes
研究AdS中婴儿闭合宇宙的全息描述，利用AdS/CFT的代数框架解决半经典婴儿宇宙边界理解难题，并指出有限$G_N$下宏观闭合宇宙应存在半经典描述。
Toward Krylov-based holography in double-scaled SYK
本文基于Jackiw-Teitelboim和正弦-稀释引力中的Krylov复杂度与测地线长度对偶性，发展了双标SYK模型Krylov子空间的全息字典，探讨了Krylov复杂度增长速率与虫洞速度的关系，以及高阶Krylov复杂度与复制虫洞的关联。
Firewalls in the non-perturbative bulk Hilbert space of JT gravity
本文研究了JT引力中非微扰bulk希格斯空间中防火墙几何体的概率行为，发现其在晚期趋于常数。
Observing Spacetime
研究量子引力复杂态中隐藏信息能否被量子可观测量获取。利用引力路径积分，发现一般探针无状态信息，微调探针可给出信号。渐近观察者不易确定宇宙状态，但可检验提议，还能检测隐藏在婴儿宇宙及双边界黑洞中的信息。
We show that, in the first order gravity theory coupled to axions, the instanton number of the Giddings-Strominger wormhole can be interpreted as the Nieh-Yan topological index. The axion charge of the baby universes is quantized in terms of the Nieh-Yan integers. Tunneling between universes of different Nieh-Yan charges implies a nonperturbative vacuum state. The associated topological vacuum angle can be identified with the Barbero-Immirzi parameter.
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