The Universe at Extreme Scales

粒子物理学和宇宙学的研究将宇宙作为一个整体的最大尺度与最小的基本粒子尺度联系起来。通过试图了解宇宙在大爆炸后是如何演化的，我们可能会洞察到哪些粒子尚未被发现，例如在大型强子对撞机(LHC)上，反之亦然。就早期宇宙而言，人们通常认为它经历了一段快速膨胀的时期，称为暴胀。然而，仍有许多悬而未决的问题。例如，宇宙膨胀的机制是什么，我们能否将膨胀与量子引力联系起来，这是一个我们仍然无法理解的理论？有趣的是，最近对引力波的观测可能会在这里提供指导。通货膨胀预测的是引力波背景，其性质取决于通货膨胀模型的细节。因此，如果观察到这一背景，它可能有助于我们进一步揭示大爆炸后通货膨胀时代的细节。引力波也可能解开其他谜题，即那些与暗能量和暗物质有关的谜题。同样，爱因斯坦一般引力理论的可能替代理论，旨在解决暗能量/物质之谜，可能会在引力波中留下印记。与此相反，大型强子对撞机探测的是最小的长度尺度，通过在非常高的能量下碰撞质子和原子核。为了将我们目前非常成功的基本粒子理论标准模型(SM)检验到极致，有必要对SM过程进行高精度的计算，并做出超出标准模型(BSM)的物理预测。前者可以使用超越通常费曼图的先进技术来实现。对于后者，人们可以认为SM是一种有效场论(EFT)，只在一定的能量尺度下有效。要了解哪些新的BSM相互作用可以在低能量下产生SM，而不与LHC的高精度测试相冲突，这是一个突出的挑战。两类主要的候选理论是所谓的近共形规范理论和复合希格斯模型，它们都会导致电弱对称破缺和轻希格斯玻色子。它们甚至可能提供暗物质候选者。这些理论与夸克和胶子理论、量子色动力学(QCD)有一个共同点，即它们之间存在着强烈的相互作用。这意味着它们不能很容易地通过解析来解决，但可以在高性能计算设施上进行数值模拟。QCD的研究提供了早期宇宙物理和基本粒子之间的联系。也就是说，随着宇宙在大爆炸后冷却，它经历了一系列的相变。在其中一个过程中，夸克和胶子结合成强子，也就是我们今天观察到的粒子。大型强子对撞机目前正在探索QCD相变，通过碰撞重离子，激发对QCD光谱如何随温度变化的定量预测。事实上，即使对真空中的QCD光谱的理解也在一定程度上仍未解决，并可能引导我们走向BSM物理。量子场论(QFT)描述了从上面提到的基本相互作用到低维和凝聚物质系统的广泛能量范围的物理过程。许多新的现象和QFT的详细结构预计将超出传统微扰方法或数值模拟的范围。二元性提供了迄今不相关的理论之间的联系，使得以前被认为遥不可及的容易处理的问题变得遥不可及。随着新的二元性的发现，QFT的丰富性超出了天真的预期。同样，与黑洞动力学相关的脱离热平衡的动力学，即热正态化过程，或量子信息的演化，也受益于新的方法，其中一些方法是由量子信息理论推动的。

Research in particle physics and cosmology connects the largest scales, those of the Universe as a whole, with the smallest, namely those of fundamental particles. By trying to understand how the Universe evolved after the Big Bang, we may gain insight into which particles are yet to be discovered, e.g. at the Large Hadron Collider (LHC), and vice versa.Concerning the early Universe, it is commonly understood that it underwent a period of rapid expansion, called inflation. However, many open questions remain. For instance, what is the mechanism of cosmological inflation, and, can we link inflation to quantum gravity, a theory that still eludes us? Interestingly, the recent observations of gravitational waves may provide a guide here. Inflation predicts a gravitational-wave background with properties depending on the details of the inflationary model. Hence if this background is observed, it may help us to further uncover details of the inflationary epoch after the Big Bang. Gravitational waves may also shed light on other puzzles, namely those related to dark energy and dark matter. Again, possible alternative theories to Einstein's general theory of gravity, which are designed to solve the dark energy/matter puzzles, may leave their imprint in gravitational waves.In contrast to this, the LHC probes the smallest length scales, by colliding protons and nuclei at very high energies. In order to test the Standard Model (SM), our current highly successful theory of elementary particles, to the extreme, it is necessary to compute SM processes to high precision, and make predictions of physics beyond the Standard Model (BSM). The former can be done using advanced techniques which go beyond the usual Feynman diagrams. For the latter, one may take the viewpoint that the SM is an effective field theory (EFT), valid up to a certain energy scale only. To understand which novel BSM interactions can give rise to the SM at low energies, without conflicting with high-precision tests from the LHC, is an outstanding challenge. Two main classes of candidate theories are so-called near-conformal gauge theories and Composite Higgs models, which both give rise to electroweak symmetry breaking and a light Higgs boson. They may even provide dark matter candidates.These theories have a commonality with the theory of quarks and gluons, Quantum Chromodynamics (QCD), namely that they are strongly interacting. This implies that they cannot be solved easily analytically, but are amenable to numerical simulations on high-performance computing facilities. The study of QCD provides a link between the physics of the early Universe and elementary particles. Namely, as the Universe cooled down after the Big Bang, it underwent a series of phase transitions. During one of those, quarks and gluons combined into hadrons, i.e. the particles we observe today. The QCD phase transition is currently being explored at the LHC, by colliding heavy ions, motivating quantitative predictions on how the QCD spectrum changes with temperature. In fact, even understanding the QCD spectrum in vacuum is still partly unsolved and may guide toward BSM physics.Quantum field theories (QFTs) describes physical processes across a vast range of energy scales, from fundamental interactions, as mentioned above, to low-dimensional and condensed matter systems. Many new phenomena and the detailed structure of QFTs are anticipated to lie beyond the confines of traditional perturbative methods or numerical simulations. Dualities provide links between hitherto unrelated theories, making tractable questions previously considered to be out of reach. With new dualities being discovered, the richness of QFT is larger than naively expected. Similarly, dynamics out of thermal equilibrium, the process of thermalisation, or the evolution of quantum information, relevant for black hole dynamics, benefits from new approaches, some of which are motivated by quantum information theory.