From a quantum field perspective, General Relativity is a low-energy quantum effective field theory (EFT) capturing the leading-order dynamics of a quantum massless spin-2 field: the graviton. While this means there are no issues in treating General Relativity as a quantum field theory, understanding what is the high-energy "UV" completion of the theory is one of the deepest open problems in physics. This is a bottom-up approach to quantum gravity, using the tools of EFT to systematically explore possible extensions of General Relativity, complementary to the top-down perspective provided for instance by string theory.
Gravity as a Quantum Effective Field Theory
Since the foundation of the theory group at Imperial close to seven decades ago, the Salam Centre has led extensive studies towards understanding gravity as an EFT, including seminal work by Abdus Salam.
Foundational work at the Centre also established the sigma-model approach to string theory EFT, providing the first systematic framework for deriving the low-energy operators of gravity directly from string theory, explicitly formulating all dimension-8 operators. Members of the Centre have also proved the asymptotic freedom and UV finiteness of quadratic curvature gravity (Stelle Gravity).
Research at the Centre also investigates how basic axioms of physics, related to unitarity, analyticity, and causality, impose powerful positivity bounds on the EFT coefficients of any gravitational theory. Crucially, causality in gravitational EFTs is subtler than it looks. New approaches to causality have been pioneered, showing how scattering time-delays can be used to derive new bounds even in situations where standard S-matrix methods fail. This opens the door to constraining theories of inflation and dark energy using the same fundamental principles.
The deep connection between these bounds has also been explored at the horizon of near-extremal black holes, suggesting that the consistency of quantum gravity leaves observable imprints even in the classical physics of black holes.
A Graviton with Mass?
While the graviton is expected to be massless, or at least lighter than light itself, in theory it may not be entirely massless. But what if it has a tiny, non-zero mass?
This simple question has profound consequences. A massive graviton would alter how gravity propagates across cosmic distances, potentially explaining the accelerated expansion of the universe without invoking additional amounts of dark energy and tackle the cosmological constant (quantum vacuum energy) problem.
Members of the Centre have constructed the full non-linear Lorentz-invariant theory of a massive spin-2 particle. Understanding its dynamics in general setups has however remained a challenging question. Among various probes, the Centre has derived the first manifestly well-posed dynamical formulation of massive gravity, enabling for the first time numerical simulations of gravitational collapse in this theory. This work has since been extended to develop formulations amenable to full 4-dimensional simulations, opening the door to realistic predictions for gravitational wave observatories like LIGO, Virgo, and the future space-based detector LISA.
Rethinking the Foundations of Gravity
The Centre has also pioneered novel approaches to gravity from radically different angles, including by asking whether the very constants of nature might vary across cosmic time, offering alternatives to inflation and new directions in theoretical cosmology.
More recently, a programme exploring unimodular gravity and thermodynamic gravity was explored at the Salam Centre in which the laws of gravity emerge from more fundamental thermodynamic principles. In this framework, time itself can be understood as the canonical dual of the constants of nature, leading to a well-defined inner product and enforcing unitarity in quantum cosmology.