From the Planck Scale to the Hubble Scale - Theoretical Physics At KCL

Our research focuses on theoretical physics and the quest for the ultimate laws governing the Universe we inhabit and everything that we see around us. There are two sides to the kind of theoretical physics we research - quantum physics and gravity.

Quantum physics, particularly quantum field theory, describes particles and the forces like those that make up regular matter as well as ones that can only be detected at particle experiments, like the Large Hadron Collider at CERN. The development of quantum field theory in conjunction with experiments have led to the creation of the Standard Model of particle physics over the past century. This hugely successful theory received its final confirmation in 2012 with the discovery of the Higgs boson.

Gravity, as first formulated by Newton and then in more detail by Einstein's General Relativity describes the motion of celestial bodies and the expansion of the Universe as being due to the curvature of space-time. There have been multiple observations over the years in agreement with Einstein's theory. One of the most exciting was the discovery in 2015 of gravitational waves, a key prediction of the theory. Our ability to detect these ripples of space-time has fundamentally changed what we can learn about the Universe.

While there are some tantalising indications about new physics at the LHC, so far the Standard Model and General Relativity have passed every experimental test we can throw at them, often to amazing accuracy. Our research aims at gaining deeper insights into those theories and resolving outstanding questions in the relation to observations.

Some of the puzzles we are studying are as follows. There is plenty of evidence for dark matter in the Universe - an additional particle beyond those in with the standard model. There is also evidence for dark energy, a mysterious energy field which accelerates the Universe's expansion and doesn't behave like a particle at all. And does the Universe contain so much more matter than anti-matter. We know that some new physics beyond just the Standard model with gravity had to take place at the very start of the Universe to set up the initial conditions of the Universe we inhabit. More fundamentally, quantum theory and general relativity don't play well with each other and lead to inconsistencies in extreme situations like at the beginning of the Universe and in black holes.

It is precisely these questions that we hope to research with this grant. In order to find out what lies beyond the standard model and what the dark matter is we use observations at particle colliders, underground detectors and in space. We can examine what can and cannot be observed at the Large Hadron collider, what dark matter is doing inside galaxies in space and increasingly we are using gravitational waves to peer deeper into the Early Universe than we have been able to in the past.

To reconcile quantum physics and gravity, we study string theory and its many avatars. At a basic level, string theory replaces particles and quantum fields with extended objects - the strings. The resulting theory accommodates both quantum physics and gravity and reveals many new features. One particular example are "holographic" theories where gravity provides an alternative description to quantum field theory, rather than a contradiction that needs to be reconciled. We explore a multitude of theories in varying dimensions of space-time and the objects within them, from particles to black holes. The more learn of all possible physical theories, the better we will understand the laws governing our Universe.

Roughly speaking, the Theory Group in the Maths Department works more closely on string theory, black holes, and abstract properties of quantum field theory and gravity while the Theory Group in the Physics Department works on particle physics phenomenology, dark matter, the early Universe and the relation of gravity to particle physics.