The Cosmological Bootstrap: a New Approach to the Primordial Universe

在过去几十年对宇宙的观察中出现的最引人注目的事实之一是，宇宙中物质在星际距离上的分布显示出令人震惊的高度规律性。我们所观察到的一切都是如此：我们在夜空中看到的星系及其周围的暗物质；宇宙微波背景中的光子，即宇宙炎热过去的微弱余辉，以及难以捉摸的中微子。由于我们的宇宙在其所有可观察到的历史中一直在膨胀，我们可以肯定地知道，这种非凡的规律性肯定是在大爆炸的前几分之一秒内孕育的。我们描述宇宙历史上那个时期的主要范式叫暴胀，它假设空间以指数级的速度膨胀，微小的量子涨落被拉伸到宇宙的大小，最终决定了一切事物的空间分布。这张非常早期的宇宙图片有着深刻的含义。首先，它告诉我们，量子力学，即支配原子和亚原子尺度并使数字时代成为可能的一套物理定律，也是理解可观测宇宙的巨大宇宙学距离的关键。其次，这告诉我们，我们可以利用宇宙学测量在亚原子尺度上研究物理定律。第三，由于宇宙的膨胀是一种引力现象，量子力学必须发挥关键作用，我们有独特的机会了解理论物理的圣杯：量子引力。在过去的30年里，人们建立了越来越复杂的暴胀和非常早期的宇宙模型，并将它们与观测进行比较。不幸的是，尽管我们掌握着庞大的宇宙学数据集，但越来越明显的是，模型空间中存在着巨大的退化，这不能通过更好的观测来解决。关键的障碍是我们试图模拟随着时间的推移宇宙的演化，但这不是我们今天可以观察到的。我们所能看到的是这种演变的最终结果。另一个障碍是，对特定模型的依赖使得在通胀预测中找到一般性质和模式变得越来越困难。这一提议的目标是设计一种不涉及时间的对我们原始宇宙的描述。换句话说，我们希望能够预测通胀的可能结果和预测，而不必写下并解决所有可能的模型。相反，我们想要完全依赖于我们对基础物理的理解的支柱。这些一般原理，例如对称性、一元性和局域性，是我们描述亚原子粒子的核心，因此应该作为我们理解早期宇宙的起点。为了达到描述“没有时间的时间”的目标，我们将引入在过去20年里彻底改变了我们对粒子物理的理解的方法和技术。这一进展尚未转移到宇宙学领域，在那里它实际上具有最高的潜力。将这种方法应用于宇宙学，将使我能够预测一般的、与模型无关的膨胀结果，更具体地说，可以预测在符合我们所知的物理定律的情况下可以产生的详细统计分布。这项研究的成果将是对量子力学如何工作的新理解，当结合非平凡的引力背景时。它将打开利用宇宙学观测发现新粒子的大门，这些粒子超出了我们目前标准模型中已知的那些粒子。我们将能够发现自然界中正在发挥作用的新力量，甚至可能了解一些关于量子引力微扰制度的知识。

One of the most remarkable facts that has emerged from observations of the cosmos in the past few decades is that the distribution of stuff in the universe at intergalactic distances displays a shockingly high degree of regularity. This is true for everything we have observed: the galaxies that we see in the night sky and the Dark Matter that surrounds them; the photons in the cosmic microwave background, namely the faint afterglow of the universe' hot past, and the elusive neutrinos. Because our universe has been expanding for all of its observable history, we know for certain that this remarkable regularity must have been seeded during the first fraction of a second of the Big Bang. Our leading paradigm to describe that period in the history of the universe is called inflation and it posits that space expanded exponentially fast and small quantum fluctuations were stretched to cosmological size and eventually determined the spatial distribution of everything. This picture of the very early universe has profound implications. First, it tells us that quantum mechanics, namely the set of physical laws that rule the atomic and subatomic scales and that enabled the digital age, is also the key to understanding the large cosmological distances of the observable universe. Second, this tells us that we can study the laws of physics at subatomic scale using cosmological surveys. Third, because the expansion of the universe is a gravitational phenomenon and since quantum mechanics must play a crucial role, we have the unique opportunity to learn about the holy grail of theoretical physics: quantum gravity.In the past 30 years, people have made more and more elaborate models of inflation and the very early universe and they have compared them to observations. Unfortunately, despite the huge cosmological dataset at our disposal, it has become increasingly clear that there is a vast degeneracy in the space of models, which cannot be resolved by better observations. The key obstacle is that we have tried to model the evolution of the universe as time progresses, but that's not something we can observe today. All we can see is the end result of that evolution. A further obstacle is that the reliance on specific models has made it harder and harder to find general properties and patterns in the predictions of inflation.The goal of this proposal is to devise a description of our primordial universe that does not involve time. In other words, we want to be able to predict what the possible outcomes and predictions of inflation are, without having to write down and solve all possible models. Rather, we want to rely exclusively on the pillars of our understanding of fundamental physics. These general principles, such as for example symmetries, unitarity and locality, sit at the core of our description of subatomic particles and should therefore be taken as a starting point for our understanding of the early universe.To reach our goal of describing "time without time", we will import the approach and technology that has revolutionised our understanding of particle physics in the past two decades. This progress has not yet been transferred to the realm of cosmology, where it actually has the highest potential. Applying this approach to cosmology will allow me to predict the general, model-independent outcomes of inflation and more specifically the detailed statistical distributions that can be generated while being compatible with the laws of physics as we know them. The output of this research will be a new understanding of how quantum mechanics works when combined with non-trivial gravitation backgrounds. It will open the door to use cosmological observations to discovering new particles beyond those that we know in the current standard model. We will be able to discover new forces at play in nature and perhaps even learn something about the perturbative regime of quantum gravity.