The Universe at Extreme Scales

粒子物理学和宇宙学的研究将最大的尺度，即整个宇宙的尺度，与最小的尺度，即基本粒子的尺度联系起来。通过了解宇宙在大爆炸后是如何演化的，我们可以了解哪些粒子还没有被发现，例如在大型强子对撞机（LHC）上，反之亦然。关于早期宇宙，通常认为它经历了一段快速膨胀的时期，称为暴胀。然而，许多悬而未决的问题仍然存在。例如，宇宙暴胀的机制是什么？我们能否将暴胀与量子引力联系起来？有趣的是，最近对引力波的观测可能在这方面提供了指导。暴胀预言了一种引力波背景，其性质取决于暴胀模型的细节。因此，如果这一背景被观测到，它可能有助于我们进一步揭示大爆炸后暴胀时代的细节。引力波也可能揭示其他的谜题，即那些与暗能量和暗物质有关的谜题。同样，爱因斯坦引力理论的替代理论，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，大型强子对撞机通过质子和原子核在非常高的能量下碰撞来探测最小的长度尺度。为了检验标准模型（SM），我们目前非常成功的基本粒子理论，到极端，有必要计算SM过程的高精度，并作出超出标准模型（BSM）的物理预测。前者可以使用超越通常的费曼图的先进技术来完成。对于后者，人们可以采取这样的观点，即SM是有效场论（EFT），仅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的高精度相冲突，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，它们都产生了电弱对称性破缺和轻希格斯玻色子。这些理论与夸克和胶子的量子色动力学（QCD）理论有一个共同点，即它们是强相互作用的。这意味着他们不能很容易地解决解析，但服从高性能计算设施的数值模拟。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，当宇宙在大爆炸后冷却下来时，它经历了一系列的相变。在其中一次，夸克和胶子结合成强子，即我们今天观察到的粒子。QCD相变目前正在大型强子对撞机上探索，通过碰撞重离子，激发QCD光谱如何随温度变化的定量预测。事实上，即使是对真空中QCD谱的理解仍然有一部分是未解决的，可能会指导BSM物理学。量子场论（QFT）描述了跨越巨大能量尺度的物理过程，从上面提到的基本相互作用到低维和凝聚态系统。许多新的现象和QFT的详细结构预计将超出传统的微扰方法或数值模拟的范围。对偶性提供了迄今为止不相关的理论之间的联系，使得以前被认为是遥不可及的易处理的问题。随着新对偶的发现，QFT的丰富性比天真的预期要大。类似地，与黑洞动力学相关的热平衡之外的动力学、热化过程或量子信息的演化，都受益于新的方法，其中一些方法是由量子信息驱动的。

Research in particle physics and cosmology connects the largest scales, those of the Universe as a whole, with then 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 linkinflation to quantum gravity, a theory that still eludes us? Interestingly, the recent observations of gravitational waves mayprovide a guide here. Inflation predicts a gravitational-wave background with properties depending on the details of theinflationary model. Hence if this background is observed, it may help us to further uncover details of the inflationary epochafter the Big Bang. Gravitational waves may also shed light on other puzzles, namely those related to dark energy and darkmatter. 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-precisionfrom the LHC, is an outstanding challenge. Two main classes of candidate theories are so-called near-conformalgauge 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 earlyUniverse and elementary particles. Namely, as the Universe cooled down after the Big Bang, it underwent a series ofphase transitions. During one of those, quarks and gluons combined into hadrons, i.e. the particles we observe today. TheQCD phase transition is currently being explored at the LHC, by colliding heavy ions, motivating quantitative predictions onhow 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 numericalsimulations. Dualities provide links between hitherto unrelated theories, making tractable questions previously consideredto 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 forblack hole dynamics, benefits from new approaches, some of which are motivated by quantum information.