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损失能量的方式，以及距离小到一毫米的质量之间的吸引力。但这个模型并不完美：最近的观测表明，宇宙的膨胀正在加速，而不是像你从普通相对论中所期望的那样减速。此外，提出量子引力理论是出了名的困难；也就是说，这个理论是我们从亚原子世界理解的物理学和爱因斯坦的万有引力理论的和谐融合，引力是宇宙几何形状的一种表现。为了解决这些缺点，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在某种程度上是有缺陷或不完整的。如果不进行实验测试，就不可能知道这些备选模型中哪一个是正确的。我研究的主要目标是弄清楚这些模型对实际实验或观察有什么影响。例如，在我研究的模型中，我们对量子力学的标准理解，即支配亚原子世界的理论，得到了深刻的修改。当星系的种子是由原始量子涨落产生的时候，这样的修正改变了宇宙在非常早期的行为。这影响了我们今天观察到的大爆炸遗留辐射，因此提供了一种检验理论的方法。我还对一些模型感兴趣，这些模型假设在最小的尺度上，时空的结构不是连续的，像液态水一样，而是粒状的，像沙子一样。这个想法很重要，因为我们今天在宇宙中看到的一切都是由空间和时间的波动形成的。如果我们在过去越来越远的地方追踪这些波，它们会变得越来越小，直到它们最终小到足以“看到”时空的离散颗粒结构。在今天的宇宙中，这些波的形状——以及它们所产生的结构——直接受到它们在过去的行为的影响。因此，我们有可能在今天的天体位置上看到宇宙颗粒性质的回声。通过推导替代理论对观测的精确影响，我希望获得新的和新颖的方法来限制或排除这些模型，从而使我们更接近理解宇宙引力的真实本质。

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.