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Centre for Particle Theory

Cosmology

The major achievement of the cosmology programme has been a series of papers by Ruth Gregory demonstrating that the Higgs vacuum decays in the presence of tiny black holes The implication of this work is that primordial black holes, i.e. black holes left over from the early universe, would be in conflict with the Standard Model of particle physics. This work extends previous analyses of the tunnelling process to allow for nucleation seeds or impurities, by including a black hole in the tunnelling process. The action of the instanton is significantly reduced for small black hole masses. Applying this to the Standard Model Higgs, we conclude that black holes trigger very rapid decay of the Higgs vacuum. Another implication considered in this work was the possibility that black holes produced at the LHC would trigger vacuum decay in models with large extra dimensions.


Whilst investigating the Higgs potential at high energies, we obtained some radical results on the renormalisation of Higgs-gravity couplings. These couplings play a crucial role in the cosmology of the Higgs boson and in Higgs inflation. In this rapidly moving area, there are contrasting physical descriptions depending on the choice of frame (Einstein or Jordan). Choosing a quantisation scheme which is covariant under changes of frame gives new results for beta functions of the effective field theory and new physical predictions. The techniques needed to do these calculations have only been developed recently, and they open up a whole new avenue of research.

Future Directions

It looks increasingly likely that the standard model Higgs vacuum is metastable. This has profound cosmological consequences which may point the way towards BSM physics. The evolution of the Higgs field during the inflationary era is partially understood through approaches based on stochastic fields and some very limited bubble nucleation calculations.The bubble nucleation approach holds out the best hope of tracking the evolution of collapsing false-vacuum decay regions, but there are fundamental gaps in our understanding of bubble nucleation in curved spacetime. We plan to provide a consistent theory of bubble nucleation and obtain reliable cosmological bounds on Higgs physics.

Effective theories emerging in the micro-to-macro transition have been well studied in particle and solid state physics through the use of field theory and the renormalization group. We
will apply these ideas to obtain new IR descriptions of CDM with both effective viscous and stochastic source terms. We will focus on two aspects: 1) We will attempt a first principles derivation of the effective theory through coarse graining the microscopic description based on the collisionless Boltzmann equation. 2) We will use the stochastic viscous fluid model of CDM on a phenomenological basis and compute correlation functions to one and two loops, comparing against the results of standard perturbation theory, other approaches that neglect stochastic sources, and N-body simulations.

We will continue our efforts to explore the observational and theoretical implications of dark energy or modified gravity interacting with black holes. We aim to refine the calculations of observational impact of screened scalars using more realistic accretion disc profiles and upgrade to the Kerr geometry. We will also explore other signatures of modified gravity in the near horizon regime. A related project is to explore the practical aspects of thermodynamic volume of black holes in the context of a dynamical dark energy. This explores the theoretical side of black hole thermodynamics, but a better understanding of thermodynamic volume would be important both for cosmological effects of dark energy on black holes as well as horizon effects of modified
gravity.