Observational signatures of quantum gravity

我们目前最好的引力理论是爱因斯坦的广义相对论。它很好地描述了太阳系中天体的运动，双星脉冲星通过引力辐射失去能量的方式，以及相距小至一毫米的质量之间的引力。但该模型并不尽如人意：最近的观测表明，宇宙的膨胀正在加速，而不是像你可能从普通相对论中预期的那样减速。此外，提出量子引力理论也是出了名的困难；也就是说，这个理论是我们从亚原子世界中理解的物理学和爱因斯坦关于引力作为宇宙几何表现的图景的和谐融合。为了解决这些缺陷，人们提出了许多激进的想法，比如宇宙有额外维度的可能性，或者我们对量子力学的理解存在某种缺陷或不完整。如果不对这些替代模型进行实验测试，就不可能知道它们中的哪一个是正确的。我研究的主要目标是找出这些模型对实际实验或观察有什么影响。例如，我研究了一些模型，在这些模型中，我们对量子力学的标准图景--支配亚原子世界的理论--进行了深刻的修改。这些变化在很早的时候就改变了宇宙的行为，当时星系的种子是由原始量子涨落创造出来的。这影响了我们今天观察到的大爆炸的残骸辐射，因此提供了一种测试该理论的方法。我还对一些模型感兴趣，这些模型假定时空的结构在最小的尺度上不是连续的，像液态水一样，而是像沙子一样的颗粒状。这个想法很重要，因为我们今天在宇宙中看到的一切都是从空间和时间的波动中形成的。如果我们将这些波追踪到越来越远的过去，它们会变得越来越小，直到它们最终变得足够小，可以“看到”离散的、颗粒状的时空结构。在今天的宇宙中，这些波的形状-以及它们产生的结构-直接受到它们过去的行为的影响。因此，今天在天体的位置上可能会看到我们宇宙颗粒性的回声。通过推导出另类理论对观测的精确影响，我希望获得约束或排除这些模型的新的、新颖的方法，从而使我们更接近理解宇宙中引力的真实性质。

Our current best theory of gravity is Einstein's general relativity. It provides an excellent description of the motion of bodies in the solar system, the way binary pulsars lose energy via gravitational radiation, and the force of attraction between masses separated by distances as small as a millimetre. But all is not well with the model: Recent observations have shown that the expansion of the universe is accelerating, not decelerating as you might expect from ordinary relativity. Also, it is notoriously difficult to come up with a quantum theory of gravity; that is, a theory that is a harmonious blend of the physics we understand from the subatomic world and Einstein's picture of gravitation as a manifestation of the geometry of the universe. To address these shortcomings, many radical ideas have been proposed, such as the possibility that the universe has extra dimensions or that our understanding of quantum mechanics is somehow flawed or incomplete. It is impossible to know which of these alternative models is correct without subjecting them to experimental tests. The primary goal of my research is to work out what effects these models have on actual experiments or observations. For example, I study models where our standard picture of quantum mechanics, the theory that governs the subatomic world, gets modified in a profound way. Such modifications alter the behaviour of the universe at very early times, when the seeds of galaxies were created by primordial quantum fluctuations. This affects the relic radiation from the big bang we observe today, and hence provides a means of testing the theory. I am also interested in models that assume that on the smallest scales the structure of spacetime is not continuous, like a liquid water, but granular, like sand.  This idea is important because everything we see in the universe today formed out of waves in space and time.  If we track these waves further and further into the past, they get smaller and smaller until they are eventually tiny enough to "see" the discrete, grainy structure of spacetime.  The shape of these waves---and the structures they spawned---in today's universe is directly influenced by their behaviour in the past.  So it may be possible to see the echoes of the granular nature of our universe in the positions of celestial bodies today.   By deriving the precise effects alternative theories have on observations, I hope to obtain new and novel ways of constraining or ruling out these models, thus bringing us closer to understanding the true nature of gravity in the universe.