Scheme: University Research Fellowship
Organisation: University of Nottingham
Dates: Oct 2013-Sep 2016
Summary: Lay summary:
Why does the Universe extend as far as the moon? This seemingly absurd question encapsulates the greatest mystery in theoretical Physics -- the cosmological constant problem (CCP). The absurdity is not in the question, but in the currently preferred models of Particle Physics and Gravitation. Taken to hold true, they imply that the electron endows the vacuum with enough energy to curve the Universe back onto itself over a distance of no more than 100000 km. Less than the distance travelling by the astronauts of Apollo XI.
We have recently made a breakthrough in tackling the CCP. For the first time ever, there now exists a simple and consistent mechanism that can protect a gravitational Universe from the vacuum energy of the electron, and all other particles in the Standard Model. Consistent solutions to the CCP have been so elusive, that real progress like this is very likely to tell us something about how Nature really works.
The idea was to modify Einstein's theory of gravity on a global scale. That means you would have to look at the entire Universe over its entire lifetime to establish that our theory differed from Einstein's. But this subtle difference is enough to force a cancellation that eats up the electron vacuum energy, and protects the Universe from a catastrophic level of curvature.
Dates: Oct 2008-Sep 2013
Summary: What is gravity? To Newton it was the force that caused the apple to fall to earth and held the planets in orbit around the Sun. To Einstein is was the curvature of space and time. To a particle physicist it is the exchange of a "virtual" called the graviton. To a modern day string theorist, it is the exchange of tiny loops of vibrating strings, within a sea of extra dimensions.
Although we know a lot about gravity, it remains the most mysterious of the fundamental forces, largely because it is so much weaker than the rest. But despite this feebleness, its long range and attractive nature guarantee that it dominates the Universe on the largest scales. My research is devoted to understanding this mysterious force at both the largest and the smallest scales.
Consider the smallest scales. This is the realm of quantum gravity, where Einstein's equations break down. If we can make sense of this then we essentially have the key to understanding our own existence. This is because the physics of the very very early Universe was dominated by quantum gravity, so if we can understand that, perhaps we can understand how the Universe began.
Now consider the largest scales. This is the dark realm, where dark energy and dark matter reign supreme. Dark energy, in particular, is a mysterious substance that causes the Universe to expand at an accelerated rate. Particle physics has failed miserably to explain what dark energy might be. Since its properties are inferred through gravity, perhaps we need to better understand how gravity behaves on the largest scales before we can make any real sense of dark energy. Is the acceleration of the Universe really just a signal that Einstein's theory of gravity breaks down on large scales? Was Einstein wrong?