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In quantum field theory, the decay of an extended metastable state into the real ground state is known as ``false vacuum decay'' and it takes place via the nucleation of spatially localized bubbles. Despite the large theoretical effort to estimate the nucleation rate, experimental observations were still missing. Here, we observe bubble nucleation in isolated and highly controllable superfluid atomic systems, and we find good agreement between our results, numerical simulations and instanton theory opening the way to the emulation of out-of-equilibrium quantum field phenomena in atomic systems., 12 pages, 9 figures

This note analyses on the long wavelength dynamics of a two horizon analogue black hole system in one spatial dimension. By introducing an effective scalar-dilaton model we show that closed form expressions can be obtained for the time-dependent Hawking flux and the energy density of the Hawking radiation. We show that, in the absence superluminal modes, there is a vacuum instability. This instability is recognisable to relativists as the analogue to the destabilisation of the Cauchy horizon of a black hole due to vacuum polarization., Comment: 12 pages, 6 figures

Particle production from the vacuum is a remarkable aspect of particle physics. Prime examples are the Schwinger process of particle production in strong electric fields and the Hawking process of particle production from black holes. These processes can be viewed as quantum tunnelling of particles from the vacuum. The tunnelling approach, and the closely related instanton or complex path approaches, are reviewed here with emphasis on paths in the complex coordinate plane. The method is applied to particle production from a black hole in a magnetic field, where ultra-high energy charged particles are produced., Comment: 18 pages, 12 figures. Conference talk for "Avenues of QFT in Curved Spacetime"

We perform an analysis of the supercooled state in an analogue of an early universe phase transition based on a one dimensional, two-component Bose gas with time-dependent interactions. We demonstrate that the system behaves in the same way as a thermal, relativistic Bose gas undergoing a first order phase transition. We propose a way to prepare the state of the system in the metastable phase as an analogue to supercooling in the early universe. While we show that parametric resonances in the system can be suppressed by thermal damping, we find that the theoretically estimated thermal damping in our model is too weak to suppress the resonances for realistic experimental parameters. However, we propose that experiments to investigate the effective damping rate in experiments would be worthwhile., Comment: 21 pages, 5 figures

The problem of finding a minimal-time path for an aeroplane travelling in a wind flow has a simple formulation in terms of analogue gravity. This paper gives an elementary explanation with equations and some numerical solutions., 6 pages, 6 figures, LaTeX

Casimir forces from fermions in an extra dimension can stabilise Randall-Sundrum models. Contrary to previous expectations, the stabilisation can produce large electroweak hierarchies without fine tuning. The Casimir stabilisation occurs in the same five-dimensional fermion models which have been found to give the observed fermion mass hierarchies. This is shown for masses generated by the Higgs mechanism and by a Higgsless symmetry breaking mechanism, where new light Kaluza-Klein states are presented, Comment: LaTeX 4 pages

This talk reviews our recent work showing how tiny black holes can act as nucleation sites for the decay of the metastable Higgs vacuum. We start by discussing the formation of thin wall bubbles of true vacuum inside a false vacuum, and show how adding a black hole lowers the action of the Euclidean tunneling solution, thus strongly enhancing the probability of vacuum decay. We then review numerical results for the Higgs vacuum showing that the decay rate is even higher for these "thick wall" bubbles. The results imply either tiny black holes are not a component of our universe, or BSM corrections to the Higgs potential must stabilise our vacuum., 6 pages, 2 figures. Talk presented at ICHEP2016, 38th International Conference on High Energy Physics, 3-10 August 2016, Chicago, USA. To appear in the conference proceedings

Stability of the Higgs vacuum during early universe inflation is dependent on how the Higgs couples to the spacetime curvature. Limits on the curvature coupling parameter at the electroweak scale $��_{EW}$ are shown to be consistent only when quantum gravity effects are included and a covariant quantisation procedure, such as Vilkovisky-DeWitt, is adopted. Stability requires $��_{EW}>-0.03$ for a top quark mass $M_t=173.34\,{\rm GeV}$., 5 pages, 3 figures, RevTex

The quantisation of scalar field theory and Einstein gravity is investigated using a fully covariant background field formalism, including Vilkovisky-DeWitt corrections. The one-loop divergences, which are relevant for the consistency of the low-energy effective theory, differ substantially from non-covariant calculations. The results are applied to the effective action of Higgs inflation, with a non-minimal gravity coupling parameter $\xi$, where the use of a formalism which is independent of field redefinitions is particularly important. Consistency of the one-loop effective action requires the cut-off $\Lambda, Comment: 11 pages, LaTeX. v2 corrects some coefficients

This short note considers the effects of quantum theory on the linear evolution of the magnetic fields during and after inflation. The analysis appears to show that the magnetic fields decay exponentially in the high-temperature radiation era due to a combination of ohmic dissipation and vacuum polarisation., Comment: This paper has been withdrawn by the author. The paper only considers the omega>k regime, but the undamped modes have omega

A special class of non-minimal operators which are relevant for quantum field theory is introduced. The general form of the heat kernel coefficients of these operators on manifolds without boundary is described. New results are presented for the traces of the first two heat kernel coefficients for vector, Yang-Mills and perturbative gravity. It is argued that non-minimal operators can be used to define gauge-fixing independent actions and solve the conformal mode problem in quantum gravity., Some minor typos corrected. Thanks to R. Percacci for telling us about (4.42)

An analogy between asymmetric rotor molecules and anisotropic cosmology can be used to calculate new centrifugal distortion terms in the effective potential of asymmetric rotor molecules which have no internal 3-fold symmetry. The torsional potential picks up extra $\cos\alpha$ and $\cos2\alpha$ contributions, which are comparable to corrections to the momentum terms in methanol and other rotor molecules with isotope replacements., Comment: 6 pages, 1 figure, ReVTeX

The AdS/CFT correspondence suggests that there is a point in the phase diagram of strongly interacting gauge-theory matter where the viscosity approaches zero. This paper analyses the possibility that this point represents a superfluid and that the the system near this point in the phase diagram can be described by a Landau fluid. Superfluid vortices are constructed and the AdS analogue of vorticity quantisation is described. The production of vortices in the quark-gluon plasma during heavy ion collisions is discussed., 6 pages, 5 figures

Dissipative effects can lead to a friction term in the equation of motion for an inflaton field during the inflationary era. The friction term may be linear and localised, in which case it is described by a dissipation coefficient. The dissipation coefficient is calculated here in a supersymmetric model with a two stage decay process in which the inflaton decays into a thermal gas of light particles through a heavy intermediate. At low temperatures, the dissipation coefficient $\propto T^3$ in a thermal approximation. Results are also given for a non-equilibrium anzatz. The dissipation coefficient is consistent with a warm inflationary regime for moderate ($\sim 0.1$) values of the coupling constants., Comment: 21 pages, 11 figures, ReVTeX

This paper considers eleven dimensional supergravity on a manifold with boundary and the theories related to heterotic $M$-theory, in which the matter is confined to the boundary. New low energy actions and boundary conditions on supergravity fields are derived. Previous problems with infinite constants in the action are overcome. The new boundary conditions are shown to be consistent with supersymmetry, and their role in the ten dimensional reduction and gaugino condensation is briefly discussed., Comment: 16 pages, ReVTeX, new section on 10D reduction

Based on some recent work of the authors, we focus on the relationship between the Casimir energy of a Majorana spinor field for a Euclidean Einstein universe $S^4\times R$ and for a Euclidean de Sitter brane ($S^4$) embedded in AdS(5). This is for a conformally coupled massless field. Interestingly, the one brane effective potential is zero and the results are equivalent, as for the scalar case, when evaluated on the conformally related cylinder. However, using the actual metric this equivalence no longer holds because a non-trivial contribution from the path integral measure (known as the cocycle function) is non-zero., Comment: Talk given at the 12th Workshop on General Relativity and Gravitation, University of Tokyo, Tokyo, Japan, 25-28 Nov 2002

The vacuum energy of two CP(1) solitons on a torus is computed numerically. A numerical technique for zeta-function regularisation is proposed to remove the divergences of the vacuum energy. After performing the numerical regularisation, we observe the effect of the vacuum energy on the two soliton interactions., 16 pages, 6 figures

A generalised canonical formulation of gravity is devised for foliations of spacetime with codimension $n\ge1$. The new formalism retains n-dimensional covariance and is especially suited to 2+2 decompositions of spacetime. It is also possible to use the generalised formalism to obtain boundary contributions to the 3+1 Hamiltonian., Comment: 18 pages, revtex, 3 postscript figures included

Black hole solutions can be used to shed light on general issues in General Relativity and Quantum Physics. Black--hole hair, entropy and naked singularities are considered here, along with some implications for the inflationary universe scenario., 12 pages, NCL94-TP2 (LaTeX)

It is shown that improved potentials and corrected mass terms can be introduced by using a quadratic source term in the path integral construction for the effective action. The advantage of doing things this way is that we avoid ever having to deal with complex propagators in the loop expansion. The resulting effective action for electroweak phase transitions is similar to the usual results., 16 pages, NCL93-TP16, (REVTEX)

The problem of calculating the effective potential for a real scalar field with a double--well potential is a possible preliminary to understanding symmetry breaking phase transitions in the early universe. It is argued here that it is necessary to include multiple saddle points in the path integral formalism and that quadratic source terms are important. The resulting effective action has a well--defined propagator expansion with a positive mass and gives a new way of understanding high temperature expansions, but the results are equally valid at low or zero temperatures., Comment: 13 pages, NCL93-TP8 (REVTEX)

Some indication of conditions that are necessary for the formation of black holes from the collision of bubbles during a supercooled phase transition in the the early universe are explored. Two colliding bubbles can never form a black hole. Three colliding bubbles can refocus the energy in their walls to the extent that it becomes infinite., Comment: 12 pages, NCL93-TP13 (RevTeX)

Warm inflation dynamics is fundamentally based on a system-reservoir configuration in which the dynamics is dictated by a fluctuation-dissipation relation. Recent work by Cerioni et. al. (arXiv:0804.0163) examined dissipative dynamics with no associated fluctuation component. Their results, which are heavily dependent on initial conditions, are at odds with results in the warm inflation literature, especially the spectral indices. The inconsistency of their formalism is outlined here and it is shown how this would lead to their erroneous conclusions. It is then shown how, following the correct dynamical equations of warm inflation, the results in the literature are correct., Comment: Comment on arXiv:0804.0163 (3 pages)