Emergent spacetime physics from quantum gravity

我们在这个项目中要解决的关键问题是：“什么是基本量子引力模型的有效宏观物理？”量子引力首先是一个物理问题。现有的早期宇宙学和黑洞物理的半经典模型从根本上是不完整的。例如，当前的宇宙学假想(膨胀、弹跳宇宙论、宇宙浮现假想)对宇宙在大爆炸或高能标度下的初始状态做出假设，或者假设新的物理学解决了他们不能完全控制的大爆炸奇点。只有量子引力理论才能证明这些假设的合理性，修改这样的宇宙学假设，或者提出新的假设。类似地，量子黑洞模型会产生在半经典物理学中无法解决的洞见和谜题。量子引力也可能有助于解释半经典物理学在解释观测方面取得的无可争议的成功。这需要牢固地连接到GR和QFT，展示它们的连续体结构(流形、度规、物质场)是如何从基本理论中出现的。因此，这个项目的主要总体目标是从量子引力模型中提取几何和物质的近似描述，识别它们在早期宇宙和黑洞周围的观测特征。在这种情况下，量子引力方法面临的问题类似于凝聚态理论家经常面临的问题(从系统的原子描述中提取宏观动力学)。在这个类比的启发下，我们探索了时空作为量子引力(具体地说，群场论)积木的凝聚的想法；这项早期工作是本项目的基础。该项目由两个主要方向组成，对应于我们想要获得基本量子引力模型的观测结果的两个物理背景。方向一：QG和基本宇宙学本项目第一个研究方向的重点问题是：在非常早期的宇宙中发生了什么，超出了半经典物理学所描述的范围，其量子引力起源的物理特征是什么？这个方向的主要目标是从微观量子引力模型中提取详细的有效宇宙动力学，捕捉早期宇宙的物理，方向II：QG和量子黑洞物理这个项目第二个研究方向的焦点问题是：“黑洞的量子性质是什么，决定了它们的热力学和信息处理特性？这个方向的主要目标是使用基本的量子引力模型来描述黑洞的微观结构和动力学、它们如何存储和处理信息以及它们的热力学性质。

The key question we want to address in this project is: “What is the effective macroscopic physics of fundamental quantum gravity models?”Quantum gravity is first and foremost a physical problem. Existing semi-classical models of early cosmology and black holes physics are fundamentally incomplete. For example, current cosmological scenarios (inflation, bouncing cosmologies, emergent universe scenarios) make assumptions about the initial state of the universe at the big bang or at high energy scales, or postulate new physics resolving the big bang singularity, that they cannot fully control. Only a theory of quantum gravity can justify these assumptions, modify such cosmological scenarios, or suggest new ones. Similarly, models of quantum black holes produce insights as well as puzzles that are unsolvable within semi-classical physics. Quantum gravity may also help to explain the indisputable successes of semi-classical physics in accounting for observations. This requires to connect solidly to GR and QFT, showing how their continuum structures (manifold, metric, matter fields) emerge from a fundamental theory.Accordingly, the main general objective of this project is the extraction of an approximate description of geometry and matter from quantum gravity models, identifying their observational signature, in the early universe and around black holes.The issues faced by quantum gravity approaches in this context are analogous to those faced routinely by condensed matter theorists (the extraction of macroscopic dynamics from the atomic description of a system). Inspired by this analogy, we explored the idea of spacetime as a condensate of quantum gravity (specifically, group field theory) building blocks; this early work is the basis of the present project.The project is constituted by two main directions, corresponding to the two physical contexts where we want to obtain observational consequences of fundamental quantum gravity models.Direction I: QG and fundamental cosmology The focus question of the first research direction of this project is:“What happened in the very early universe, beyond the regime described by semi-classical physics, and what are the physical signatures of its quantum gravity origin?”The main goal of this direction is to extract a detailed effective cosmological dynamics from microscopic quantum gravity models, capturing the physics of the early universe, and putting them in contact with observationsDirection II: QG and quantum black hole physicsThe focus question of the second research direction of this project is:“What is the quantum nature of black holes, responsible for their thermodynamic and information-processing properties? Can it be probed by gravitational waves observations?”The main goal of this direction is to use fundamental quantum gravity models to describe the microstructure and dynamics of black holes, how they store and process information and their thermodynamic properties.