Research

My research focuses on modified theories of gravity. In particular, I am very interested in scalar-tensor theories of gravity with screening mechanisms such as the chameleon, symmetron and environmentally-dependent dilaton.

Dark Energy and Modified Gravity

We were all taught in school that the universe is expanding but in the last decade or so there have been several observations that the universe is actually accelerating, that is it is expanding faster and faster. Well so what you might say, if it's expanding then what does it matter whether it is accelerating or not? Well actually if you think about it for a second then this acceleration really doesn't make sense. We all know that gravity is an attractive force and that it pulls things together. Furthermore, despite being the weakest of the four fundamental forces, gravity is the force that dominates the large scale behaviour of the universe. So even if the universe is expanding we would expect that gravity would try to pull everything together in the universe not push it apart and so whatever is causing this expansion, it is very unlikely that it is gravity. This mysterious energy that is driving the acceleration of the universe has come to be known as dark energy.

So what could possibly be causing this strange behaviour? One possibility is that there is some strange new matter which doesn't attract, but another possibility is that gravity itself behaves differently on large, cosmological scales, to how it manifests on small, solar-system scales. It is this possibility which I am interested in and one goal when constructing new modified gravity models is to investigate whether or not they can drive the cosmic acceleration.

There are other reasons to study modified gravity too. For example, many theories of fundamental physics including string theory and supergravity include new light scalar particles that couple to matter gravitationally. This results in additional or fifth forces and so these theories are really theories of modified gravity. From this point of view, we are stuck with modified theories so it is a good idea to understand them better.

Screening Mechanisms

It's all good and well finding a modified theory of gravity that predicts the acceleration, but these theories are really modifications to gravity as described by General Relativity and Einstein's equations. Why would you want to do that, Einstein gravity works so well? You might ask. Well it does, in fact it works so well that modifying it ad hoc usually violates the most stringent experiments that we have performed in order to test how well it works. Either that or the constraints are so tight that the new theories are unobservable. Einstein's equations explain everything from the perihelion of Mercury to the structure and evolution of our sun and so whatever we do to General Relativity on large scales we better have some way of recovering what we already know about our own solar-system.

This can be accomplished if the theory includes a screening mechanism. These are mechanisms where the fifth-forces become negligible in our own galaxy and therefore do not affect the results of our local experiments. Scalar-Tensor theories, where a new scalar degree of freedom couples conformally to matter through the metric, include such a mechanism and it is these theories which I have spent the last few years working on.

The Chameleon Mechanism

The chameleon mechanism is one such screening mechanism present in scalar-tensor theories:

The conformal coupling of the new particle to matter results in the energy and mass of the particle becoming dependent on the local matter density. The chameleon mechanism exploits this by forcing the mass of the particle to increasingly higher values in dense environments. The range of the force mediated by the particle is inversely proportional to its mass and so this means the range of the force becomes incredibly short in dense environments - so short that it can have no observable consequences and it is therefore screened out. It is this blending in of the particle with its environment that has led to it being dubbed the chameleon particle and the associated gravity theory chameleon gravity. In high density-environments such as our galaxy's dark-matter halo or our solar-system the force is so short ranged that we cannot see it but in low-density regions such as the cosmological background the range can be significant and we can have interesting new effects such as changes to the large-scale structure of galaxy distributions and modifications to the Cosmic Microwave Background.

In addition to this, there are also the symmetron and environmentally-dependent Damour-Polyakov effect, which work in a similar manner by driving the coupling to matter to negligible values in high-density environments.

Stellar Structure

One aspect of chameleon gravity in which I am particularly interested is the difference in the structure and phenomenology of stars. Stars are gravitationally bound objects which support themselves against gravitational collapse due to their internal pressure. This pressure is provided by burning elements in its core - firstly Hydrogen and then Helium, Carbon, Oxygen and many more depending on how heavy the star is - releasing light at the same time. These internal processes can result in a rich phenomenology over the stars life-time such as pulsation, magnetism and internal waves. Altering the behaviour of gravity can dramatically change the way in which these stars behave and by calculating quantitative predictions we can use real data to place constraints on new models.

In chameleon gravity our own galaxy must be screened, however there are other galaxies - dwarf galaxies - which we expect to be at least partially or even fully unscreened. In these galaxies the stars themselves will also be unscreened and will hence feel a stronger gravitational force than they would have done had they been located in our galaxy. This force requires more pressure to balance it and must hence burn more nuclear fuel per unit time leading to higher surface temperatures and more light being released. Stars in unscreened galaxies are brighter than those in our own. Since stars only have a finite supply of fuel this also means that they die faster than stars in our own galaxy, in some cases three times as fast.

UV Completions

I am also very interested in finding UV completions of chameleon gravity within fundamental theories such as string theory and supergravity. Presently, these theories are phenomenological and it is important to see if we can make them fit in with the rest of particle physics. To this end, I have been working on embedding these theories into local supersymmetry and supergravity and have recently been developing a generalised bottom-up framework. It has been possible to find several models that can be realised within these theories and, more importantly, there are other models which are precluded. Using this framework we have been able to establish a no-go theorem for supersymmetric theories of screened modified gravity: only those which derive from no-scale Kähler potentials are viable theories, the others either have their mechanisms destroyed or screen so efficiently that there is no possibility of ever distinguishing them from General Relativity.