Inclusion Of Gravitational Effects Research Articles

Field dynamics and tunneling in a flux landscape

We investigate field dynamics and tunneling between metastable minima in a landscape of type IIB flux compactifications, utilizing monodromies of the complex structure moduli space to continuously connect flux vacua. After describing the generic features of a flux-induced potential for the complex structure and type IIB axiodilaton, we specialize to the mirror quintic Calabi-Yau to obtain an example landscape. Studying the cosmological dynamics of the complex structure moduli, we find that the potential generically does not support slow-roll inflation and that in general the landscape separates neatly into basins of attraction of the various minima. We then discuss tunneling, with the inclusion of gravitational effects, in many-dimensional field spaces. A set of constraints on the form of the Euclidean paths through field space are presented, and then applied to construct approximate instantons mediating the transition between de Sitter vacua in the flux landscape. We find that these instantons are generically thick wall and that the tunneling rate is suppressed in the large-volume limit. We also consider examples where supersymmetry is not broken by fluxes, in which case near--Bogomolnyi-Prasad-Sommerfeld thin-wall bubbles can be constructed. We calculate the bubble-wall tension, finding that it scales like a D- or NS-brane bubble, and comment on the implications of this correspondence. Finally, we present a brief discussion of eternal inflation in the flux landscape.

Effect of gravity on false-vacuum decay rates for O(4)-symmetric bubble nucleation.

The self-gravity of quantum fields is often considered to be a negligible perturbation upon a background spacetime and not of much physical interest. Its importance is determined by the ratio of the mass of the field to the Planck mass, this ratio being very small for those fields that we are most familiar in dealing with. However, it is conceivable that either in the very early Universe or even today a false-vacuum decay could occur associated with a field of appreciable mass. The effect of self-gravity upon false-vacuum decay was initially studied within the ``thin-wall'' approximation by Coleman and De Luccia. Their analysis involved the approximate solution of the coupled Euclideanized field and Einstein equations with the assumption of O(4)-symmetric bubble nucleation. In this paper we consider the range of validity of the ``thin-wall'' approximation by comparing the Coleman--De Luccia results with exact numerical results for a quartic polynomial potential. We also extend the analysis into regimes for which the ``thin-wall'' approximation is inapplicable. In the case of an initially de Sitter space decaying into Minkowski space, we find a smooth transition between the Coleman--De Luccia mode of bubble formation and the Hawking-Moss transition, wherein the entire spacetime tunnels ``at once'' to the maximum of the potential. In the case of the decay of an initially Minkowski space to an anti--de Sitter space, we find that there is a ``forbidden region'' of vacuum potential parameters for which decay is not possible. At energies far below the Planck scale, the boundary of this region is accurately described by the thin-wall prediction obtained by Coleman and De Luccia. At energies near the Planck scale, however, the actual ``forbidden region'' is significantly smaller than predicted by the thin-wall approximation; thus, vacuum decays are possible which appear to be forbidden by thin-wall calculations. In all cases, the inclusion of gravitational effects tends to thicken the wall of the bubble.

On the properties of magnetoacoustic surface waves

The nature of magnetoacoustic surface waves at a single magnetic interface, one side of which is field-free, is explored for the case of parallel propagation. The interface may support a slow surface wave or both slow and fast surface waves, depending upon the ordering of the sound speeds in the two media. Phase-speeds and penetration depths of the waves and the associated pressure perturbations and motions are investigated for a variety of field strengths and sound speeds. The fast wave disturbs the interface more than the slow wave. In the magnetic field region the slow wave is mainly longitudinal in nature whilst the fast surface wave is transverse for strong fields, longitudinal for weaker fields. In the field-free region both waves are longitudinal in character. The running penumbral wave phenomenon may provide an example of a magnetoacoustic surface mode, though any direct comparison requires the inclusion of gravitational effects.

Hot spots on neutron stars. II - The observer's sky

The gravitational lensing properties of neutron stars are explored, and the results are applied to x-ray emission from radio and x-ray binary pulsars. A simple method is described and illustrated for calculating the apparent brightness and appearance of the observer's sky for any model of emission from the stellar surface. The case where the emission arises solely from two antipodal hot spots is examined. It is found that for typical neutron star parameters the inclusion of gravitational effects makes it 3-10 times less likely, on average, that a given observer will see only one hot spot of the star, assuming no bias in the angle between the magnetic and spin axes. It is also shown that when relativistic effects are included, the modulation of the light curve arising from two antipodal hot spots is virtually independent of the size of the spots. Thus, unlike the flat space result, it is impossible to obtain arbitrary degrees of modulation by simply considering smaller emitting regions. 29 references.