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。它提供了一个很好的描述太阳系中天体的运动，双星通过引力辐射失去能量的方式，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，要提出一个引力的量子理论是出了名的困难;也就是说，一个理论是我们从亚原子世界理解的物理学和爱因斯坦的引力图像作为宇宙几何的表现形式的和谐融合。为了解决这些缺点，人们提出了许多激进的想法，例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缺陷或不完整。不经过实验检验，就不可能知道这些替代模型中哪一个是正确的。我研究的主要目标是找出这些模型对实际实验或观察的影响。 例如，我研究的模型中，我们的量子力学的标准图片，理论，统治亚原子世界，得到了深刻的方式修改。这种变化改变了宇宙在非常早期的行为，当时星系的种子是由原始量子波动创造的。这影响了我们今天观测到的大爆炸的残余辐射，因此提供了一种检验理论的方法。我也对一些模型感兴趣，这些模型假设在最小尺度上，时空的结构不是连续的，就像液态水一样，而是颗粒状的，就像沙子一样。 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。 如果我们追踪这些波到越来越远的过去，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、粒状结构。 在今天的宇宙中，这些波的形状--以及它们所产生的结构--直接受到它们过去行为的影响。 因此，在今天的天体位置上，我们可能会看到我们宇宙颗粒性质的回声。 通过推导其他理论对观测的精确影响，我希望获得新的和新颖的方法来约束或排除这些模型，从而使我们更接近于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.  **