Beyond general relativity

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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 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.