arXiv subjects

P. M. Saffin

Publications and source records attributed to P. M. Saffin.

At least 19 recordsLinked to original sources

arXiv2025

Oscillons and bubbles in $Q$-ball dynamics

We show that, in the thin-wall regime, $Q$-ball--anti-$Q$-ball collisions reveal chaotic behaviour. This is explained by the resonant energy transfer mechanism triggered by the internal modes hosted by the $Q$-balls and by the existence of {\it ephemeral} states, that is unstable, sometimes even short-lived, field configurations that appear as intermediate states. The most important examples of such states are the {\it bubble} of the false broken vacuum, which as intermediate states govern the $QQ^*$ annihilation, and the {\it charged oscillons}. The usually short-lived bubble can be dynamically temporarily stabilized, which explains their importance in the dynamics of $Q$-balls. This happens due to the excitation of massless Goldstone modes, which, exerting pressure on the bubble boundaries or being trapped as bound modes, prevent the bubble from collapsing.

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arXiv2007

Cosmology with Twisted Tori

We consider the cosmological role of the scalar fields generated by the compactification of 11-dimensional Einstein gravity on a 7D elliptic twisted torus, which has the attractive features of giving rise to a positive semi-definite potential, and partially fixing the moduli. This compactification is therefore relevant for low energy M-theory, 11D supergravity. We find that slow-roll inflation with the moduli is not possible, but that there is a novel scaling solution in Friedmann cosmologies in which the massive moduli oscillate but maintain a constant energy density relative to the background barotropic fluid.

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arXiv2006

Scaling in a SU(2)/Z_3 model of cosmic superstring networks

Motivated by recent developments in superstring theory in the cosmological context, we examine a field theory which contains string networks with 3-way junctions. We perform numerical simulations of this model, identify the length scales of the network that forms, and provide evidence that the length scales tend towards a scaling regime, growing in proportion to time. We infer that the presence of junctions does not in itself cause a superstring network to dominate the energy density of the early Universe.

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arXiv2006

Scaling solutions and geodesics in moduli space

In this paper we consider cosmological scaling solutions in general relativity coupled to scalar fields with a non-trivial moduli space metric. We discover that the scaling property of the cosmology is synonymous with the scalar fields tracing out a particular class of geodesics in moduli space - those which are constructed as integral curves of the gradient of the log of the potential. Given a generic scalar potential we explicitly construct a moduli metric that allows scaling solutions, and we show the converse - how one can construct a potential that allows scaling once the moduli metric is known.

hep-th↗

arXiv2006

The dynamics of coset dimensional reduction

The evolution of multiple scalar fields in cosmology has been much studied, particularly when the potential is formed from a series of exponentials. For a certain subclass of such systems it is possible to get `assisted` behaviour, where the presence of multiple terms in the potential effectively makes it shallower than the individual terms indicate. It is also known that when compactifying on coset spaces one can achieve a consistent truncation to an effective theory which contains many exponential terms, however, if there are too many exponentials then exact scaling solutions do not exist. In this paper we study the potentials arising from such compactifications of eleven dimensional supergravity and analyse the regions of parameter space which could lead to scaling behaviour.

hep-th↗

arXiv2006

M-theory on seven-dimensional manifolds with SU(3) structure

In this paper we study M-theory compactifications on seven-dimensional manifolds with SU(3) structure. As such manifolds naturally pick out a specific direction, the resulting effective theory can be cast into a form which is similar to type IIA compactifications to four dimensions. We derive the gravitino mass matrix in four dimensions and show that for different internal manifolds (torsion classes) the vacuum preserves either no supersymmetry, or N=2 supersymmetry or, through spontaneous partial supersymmetry breaking, N=1 supersymmetry. For the latter case we derive the effective N=1 theory and give explicit examples where all the moduli are stabilised without the need of non-perturbative effects.

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arXiv2005

A practical model for cosmic (p,q) superstrings

We propose a model with a U(1)_A x U(1)_B gauge symmetry which contains topological strings carrying magnetic flux under each of the U(1)s. By calculating the tension of the first few low energy strings we show that bound states, containing flux under each of the U(1)s, are stable against decay to their constituents of lower winding number. As the model contains only Abelian gauge symmetries this model provides a pragmatic solution to the numerical modelling of cosmic-superstrings. Whilst these defects do not satisfy a BPS bound we argue that they bear sufficient similarities to warrant such a study.

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arXiv2005

On the evolution of cosmic-superstring networks

We model the behaviour of a network of interacting (p,q) strings from IIB string theory by considering a field theory containing multiple species of string, allowing us to study the effect of non-intercommuting events due to two different species crossing each other. This then has the potential for a string dominated Universe with the network becoming so tangled that it freezes. We give numerical evidence, explained by a one-scale model, that such freezing does not take place, with the network reaching a scaling limit where its density relative to the background increases with N, the number of string types.

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arXiv2005

Type IIB Conifold Transitions in Cosmology

We study four-dimensional low energy effective actions for conifold transitions of Calabi-Yau spaces in the context of IIB supergravity. The actions are constucted by examining the mass of D3-branes wrapped on collapsing/expanding three-cycles. We then study the cosmology of the conifold transition, including consequences for moduli stabilization, taking into account the effect of the additional states which become light at the transition. We find, the degree to which the additional states are excited is essential for whether the transition is dynamically realized.

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arXiv2004

Dynamics of Q-Balls in an expanding universe

We analyse the evolution of light Q-balls in a cosmological background, and find a number of interesting features. For Q-balls formed with a size comparable to the Hubble radius, we demonstrate that there is no charge radiation, and that the Q-ball maintains a constant physical radius. Large expansion rates cause charge migration to the surface of the Q-ball, corresponding to a non-homogeneous internal rotation frequency. We argue that this is an important phenomenon as it leads to a large surface charge and possible fragmentation of the Q-ball. We also explore the deviation of the Q-ball profile function from the static case. By introducing a parameter $ε$, which is the ratio of the Hubble parameter to the frequency of oscillation of the Q-ball field, and using solutions to an analytically approximated equation for the profile function, we determine the dependence of the new features on the expansion rate. This allows us to gain an understanding of when they should be considered and when they can be neglected, thereby placing restrictions on the existence of homogeneous Q-balls in expanding backgrounds.

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arXiv2004

Type IIA Killing spinors and calibrations

We consider the dimensional reduction of eleven dimensional supergravity to type IIA in ten dimensions, and study the conditions for supersymmetry in terms of p-form spinor bi-linears of the supersymmetry parameter. For a bosonic solution to be supersymmetric these p-forms must satisfy a set of differential relations, which we derive in full; the supersymmetry variations of the dilatino give a set of algebraic relations which are also derived. These results are then used to provide the generalized calibration conditions for some of the basic brane solutions, we also follow up a suggestion of Hackett-Jones and Smith and present a calibration condition for IIA supertubes. We find that a probe supertube satisfies this bound but does not saturate it, with the bound successfully accounting for the D0 charge of the supertube but not the string charge; we speculate that there should be a stronger calibration inequality than the one given.

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arXiv2004

M-theory compactification, fluxes and AdS_4

We analyze supersymmetric solutions of M-theory based an a seven-dimensional internal space with SU(3) structure and a four-dimensional maximally symmetric space. The most general supersymmetry conditions are derived and we show that a non-vanishing cosmological constant requires the norms of the two internal spinors to differ. We find explicit local solutions with singlet flux and the space being a warped product of a circle, a nearly-Kahler manifold and AdS_4. The embedding of solutions into heterotic M-theory is also discussed.

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arXiv2003

Singular tachyon kinks from regular profiles

We demonstrate how Sen's singular kink solution of the Born-Infeld tachyon action can be constructed by taking the appropriate limit of initially regular profiles. It is shown that the order in which different limits are taken plays an important role in determining whether or not such a solution is obtained for a wide class of potentials. Indeed, by introducing a small parameter into the action, we are able circumvent the results of a recent paper which derived two conditions on the asymptotic tachyon potential such that the singular kink could be recovered in the large amplitude limit of periodic solutions. We show that this is explained by the non-commuting nature of two limits, and that Sen's solution is recovered if the order of the limits is chosen appropriately.

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arXiv2003

Decay Modes of Intersecting Fluxbranes

Just as the single fluxbrane is quantum mechanically unstable to the nucleation of a locally charged spherical brane, so intersecting fluxbranes are unstable to various decay modes. Each individual element of the intersection can decay via the nucleation of a spherical brane, but uncharged spheres can also be nucleated in the region of intersection. For special values of the fluxes, however, intersecting fluxbranes are supersymmetric, and so are expected to be stable. We explicitly consider the instanton describing the decay modes of the two--element intersection (an F5-brane in the string theory context), and show that in dimensions greater than four the action for the decay mode of the supersymmetric intersection diverges. This observation allows us to show that stable intersecting fluxbranes should also exist in type 0A string theory.

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arXiv2002

String theory and the Classical Stability of Plane Waves

The presence of fields with negative mass-squared typically leads to some form of instability in standard field theories. The observation that, at least in the light-cone gauge, strings propagating in plane wave spacetimes can have worldsheet scalars with such tachyon-like masses suggests that the supergravity background may itself be unstable. To address this issue, we perform a perturbative analysis around the type IIB vacuum plane wave, the solution which most obviously generates worldsheet scalars with negative mass-squared. We argue that this background is perturbatively stable.

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arXiv2001

A note on the Supergravity Description of Dielectric Branes

We comment on the recent papers by Costa et al and Emparan, which show how one might generate supergravity solutions describing certain dielectric branes in ten dimensions. The ``basic'' such solutions describe either N fundamental strings or N D4-branes expanding into a D6-brane, with topology M^2 x S^5 or M^5 x S^2 respectively. Treating these solutions in a unified way, we note that they allow for precisely two values of the radius of the relevant sphere, and that the solution with the smaller value of the radius has the lower energy. Moreover, the possible radii in both cases agree up to numerical factors with the corresponding solutions of the D6-brane worldvolume theory. We thus argue that these supergravity solutions are the correct gravitational description of the dielectric branes of Emparan and Myers.

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arXiv2001

Electroweak preheating on a lattice

In many inflationary models, a large amount of energy is transferred rapidly to the long-wavelength matter fields during a period of preheating after inflation. We study how this changes the dynamics of the electroweak phase transition if inflation ends at the electroweak scale. We simulate a classical SU(2)xU(1)+Higgs model with initial conditions in which the energy is concentrated in the long-wavelength Higgs modes. With a suitable initial energy density, the electroweak symmetry is restored non-thermally but broken again when the fields thermalize. During this symmetry restoration, baryon number is violated, and we measure its time evolution, pointing out that it is highly non-Brownian. This makes it difficult to estimate the generated baryon asymmetry.

hep-ph↗

arXiv2001

Fluxbranes from p-branes

It is shown how magnetic fluxbrane, Fp-brane, solutions are related to electric black p-branes by analytic continuation. Viewing the transverse space of branes as a warped cone, one finds that the cone base of the p-brane becomes the world-volume of the F(D-p-3)-brane and the world volume of the p-brane becomes the cone base of the F(D-p-3)-brane. An explicit example of the correspondence is given for a 2-brane and F6-brane of 11D supergravity.

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