arXiv subjects

Brandon Carter

Publications and source records attributed to Brandon Carter.

At least 73 records · Page 4Linked to original sources

arXiv2000

Centrifugal buoyancy as a mechanism for neutron star glitches

The frequent glitches (sudden increases of the apparent angular velocity) observed in certain pulsars are generally believed to be attributable to discontinuous angular momentum transfer to the outer neutron star crust from a differentially rotating superfluid layer, but the precise mechanism is not quite elucidated. Most explanations invoke vortex pinning as the essential mechanism responsible for the build up of strain in the crust that is relaxed, either by fracture of the solid structure or by discontinous unpinning, during the glitch. It is shown here that there is another mechanism that could give rise to strain, and subsequent fracture, of the solid crust, even if vortex pinning is ineffective: this is the effective force arising from the deficit of centrifugal buoyancy that will be present whenever there is differential rotation. This centrifugal buoyancy deficit force will be comparable in order of magnitude, but opposite in direction, to the force that would arise from vortex pinning if it were effective.

astro-ph↗

arXiv2000

Chiral Vortons and Cosmological Constraints on Particle Physics

We investigate the cosmological consequences of particle physics theories that admit stable loops of current-carrying string - vortons. In particular, we consider chiral theories where a single fermion zero mode is excited in the string core, such as those arising in supersymmetric theories with a D-term. The resulting vortons formed in such theories are expected to be more stable than their non-chiral cousins. General symmetry breaking schemes are considered in which strings formed at one symmetry breaking scale become current-carrying at a subsequent phase transition. The vorton abundance is estimated and constraints placed on the underlying particle physics theories from cosmological observations. Our constraints on the chiral theory are considerably more stringent than the previous estimates for more general theories.

hep-ph↗

arXiv2000

Dilatonic formulation for conducting cosmic string models

It is shown how the the introduction of a suitably defined dilatonic auxiliary field, $Φ$ say, makes it possible for the non-linear Lagrangian for a generic elastic string model, of the kind appropriate for representing superconducting cosmic strings, to be converted into a standardised form as the sum of a kinetic term that is just homogeneously quadratic in the relevant scalar phase gradient (as in a simple linear model) together with a potential energy term, $V$ say, that is specified as a generically non-linear function of $Φ$. The explicit form of this function is derived for various noteworthy examples, of which the most memorable is that of the transonic string model, as characterised by a given mass scale, $m$ say, for which this potential energy density will be expressible in terms of the zero current limit value $Φ_{_0}$ of $Φ$ by $ V= {1\over 2} m \big(Φ_{_0}^{-2} Φ^2+ Φ_{_0}^{2} Φ^{-2} \big)$.

hep-th↗

arXiv2000

Cancellation of linearised axion-dilaton self interaction divergence in strings

The force densities exerted on a localised material system by linearised interaction with fields of axionic and dilatonic type are shown to be describable very generally by relatively simple expressions that are well behaved for fields of purely external origin, but that will be subject to ultraviolet divergences requiring regularisation for fields arising from self interaction in submanifold supported ``brane'' type systems. In the particular case of 2-dimensionally supported, i.e. string type, system in an ordinary 4-dimensional background it is shown how the result of this regularisation is expressible in terms of the worldsheet curvature vector $K^μ$, and more particularly that (contrary to what was suggested by early work on this subject) for a string of Nambu Goto type the divergent contribution from the dilatonic self action will always be directed oppositely to its axionic counterpart. This makes it possible for the dilatonic and axionic divergences to entirely cancel each other out (so that there is no need of a renormalisation to get rid of ``infinities'') when the relevant coupling coefficents are related by the appropriate proportionality condition provided by the low energy limit of superstring theory.

hep-th↗

arXiv1999

Bogomol'nyi Limit For Magnetic Vortices In Rotating Superconductor

This work is the sequel of a previous investigation of stationary and cylindrically symmetric vortex configurations for simple models representing an incompressible non-relativistic superconductor in a rigidly rotating background. In the present paper, we carry out our analysis with a generalized Ginzburg-Landau description of the superconductor, which provides a prescription for the radial profile of the normal density within the vortex. Within this framework, it is shown that the Bogomol'nyi limit condition marking the boundary between type I and type II behavior is unaffected by the rotation of the background.

cond-mat.supr-con↗

arXiv1999

Energy of Magnetic Vortices in Rotating Superconductor

We carry out a systematic analytic investigation of stationary and cylindrically symmetric vortex configurations for simple models representing an incompressible non-relativistic superconductor in a rigidly rotating background. It is shown that although the magnetic and kinetic contributions to the energy per unit length of such a vortex are separately modified by the background angular velocity, its effect on the total energy per unit length cancels out. For a type II superconductor threaded by a parallel array of such vortices, this result implies that the relevant macroscopic magnetic field strength H will not be equal to the large scale average of the local magnetic induction B (as has previously been suggested) but instead that H will simply be equal to the external London field that characterizes the value of B outside the vortices.

cond-mat.supr-con↗

arXiv1999

Old and New Processes of Vorton Formation

After a brief explanation of the concept of a vorton, quantitative estimates of the vorton population that would be produced in various cosmic string scenarios are reviewed. Attention is drawn to previously unconsidered mechanisms that might give rise to much more prolific vorton formation that has been envisaged hitherto.

hep-ph↗

arXiv1999

Vortex Dynamics in Superfluids

Superfluid condensates are known to occur in contexts ranging from laboratory liquid helium to neutron stars, and are also likely to occur in cosmological phenomena such as axion fields. In the zero temperature limit, such condensates are describable at a mesoscopic level by irrotational configurations of simple relativistic perfect fluid models. The general mechanical properties of such models are presented here in an introductory review giving special attention to the dynamics of vorticity flux 2-surfaces and the action principles governing both individual flow trajetories and the evolution of the system as a whole. Macroscopic rotation of such a condensate requires the presence of a lattice of quantised vortex defects, whose averaged tension violates perfect fluid isotropy. It is shown that for any equation of state (relating the mass density $ρ$ to the pressure $P$) the mesoscopic perfect fluid model can be extended in a uniquely simple and natural manner to a corresponding macroscopic model (in a conformally covariant category) that represents the effects of the vortex fibration anisotropy. The limiting case of an individual vortex defect is shown to be describable by a (``global'') string type model with a variable tension ${\cal T}$ (obtained as a function of the background fluid density) whose ``vorton'' (i.e. closed loop equilibrium) states are derived as an exercise.

gr-qc↗

arXiv1999

Dynamics and integrability property of the chiral string model

The effect of fermionic string conductivity by purely right (or purely left) moving ``zero modes'' is shown to be governed by a simple Lagrangian characterising a certain ``chiral'' (null current carrying) string model whose dynamical equations of motion turn out to be explicitly integrable in a flat spacetime background.

hep-th↗

arXiv1999

Renormalisation of gravitational self interaction for wiggly strings

It is shown that for any elastic string model with energy density $U$ and tension $T$, the divergent contribution from gravitational self interaction can be allowed for by an action renormalisation proportional to $(U-T)^2$. This formula is applied to the important special case of a bare model of the transonic type (characterised by a constant value of the product $UT$) that represents the macroscopically averaged effect of shortwavelength wiggles on an underlying microscopic model of the Nambu-Goto type (characterised by $U=T$).

hep-th↗

arXiv1998

Formally renormalisable gravitationally self interacting string models

It has recently been shown how the effect of the divergent part of the gravitational self interaction for a classical string model in 4 dimensions can be allowed for by a renormalisation of its stress energy tensor and in the elastic case a corresponding renormalisation of the off shell action. It is shown here that that it is possible to construct a new category of elastic string models for which this effect is describable as a renormalisation in the stricter ``formal'' sense, meaning that it only requires a rescaling of one of the fixed parameters characterising the model.

hep-th↗

arXiv1998

Relativistic models for Superconducting-Superfluid Mixtures

The material below the crust of a neutron star is understood to be describable in terms of three principal independently moving constituents, identifiable as neutrons, protons, and electrons, of which the first two are believed to form mutually coupled bosonic condensates. The large scale comportment of such a system will be that of a positively charged superconducting superfluid in a negatively charged ``normal'' fluid background. As a contribution to the development of the theory of such a system, the present work shows how, subject to neglect of dissipative effects, it is possible to set up an elegant category of simplified but fully relativistic three-constituent superconducting superfluid models whose purpose is to provide realistic approximations for cases in which a strictly conservative treatment is sufficient. A "mesoscopic" model, describing the fluid between the vortices, is constructed, as well as a "macroscopic" model taking into account the average effect of quantised vortices.

gr-qc↗

arXiv1998

Regularisation of classical self interaction in strings

A general method of regularisation of classical self interaction in strings is extended from the electromagnetic case (for which it was originally developed) to the gravitation case, for which the result can also be represented as a renormalisation.

hep-th↗

arXiv1997

Has the black hole equilibrium problem been solved?

When the term ``black hole'' was originally coined in 1968, it was immediately conjectured that the only pure vacuum equilibrium states were those of the Kerr family. Efforts to confirm this made rapid progress during the ``classical phase'' from 1968 to 1975, and some gaps in the argument have been closed during more recent years. However the presently available demonstration is still subject to undesirably restrictive assumptions such as non-degeneracy of the horizon, as well as analyticity and causality in the exterior.

gr-qc↗

arXiv1997

Recent developments in Vorton Theory

This article provides a concise overview of recent theoretical results concerning the theory of vortons, which are defined to be (centrifugally supported) equilibrium configurations of (current carrying) cosmic string loops. Following a presentation of the results of work on the dynamical evolution of small circular string loops, whose minimum energy states are the simplest examples of vortons, recent order of magnitude estimates of the cosmological density of vortons produced in various kinds of theoretical scenario are briefly summarised.

astro-ph↗

arXiv1997

Differential rotation of relativistic superfluid in neutron stars

It is shown how to set up a mathematically elegant and fully relativistic superfluid model that can provide a realistic approximation (neglecting small anisotropies due to crust solidity, magnetic fields, et cetera, but allowing for the regions with vortex pinning) of the global structure of a rotating neutron star, in terms of just two independently moving constituents, one of which represents the differentially rotating neutron superfluid, while the other part represents the combination of all the other ingredients, including the degenerate electrons, the superfluid protons in the core, and the ions in the crust, whose electromagnetic interactions will tend to keep them locked together in a state of approximately rigid rotation. Order of magnitude estimates are provided for relevant parameters such as the resistive drag coefficient and the maximum pinning force.

astro-ph↗

arXiv1997

Moving vortex in relativistic irrotational perfect fluid or superfluid

Irrotational relativistic vortex configurations in uniform subsonic motion with respect to a surrounding perfect fluid are analysed for the purpose of application to superfluid layers in neutron stars. Asymptotic solutions are found by asymptotically expanding the flow equation at large distances from the vortex core and then by solving it order by order. The asymptotic effective tension and energy density that are needed for an averaged macroscopic description are thus obtained as functions of the vortex velocity, the vortex circulation, the asymptotic chemical potential and of parameters depending on the equation of state.

astro-ph↗

arXiv1997

Brane dynamics for treatment of cosmic strings and vortons

This course provides a self contained introduction to the general theory of relativistic brane models, of the category that includes point particle, string, and membrane representations for phenomena that can be considered as being confined to a worldsheet of the corresponding dimension (respectively one, two, and three) in a thin limit approximation. The first part of the course is concerned with purely kinematic aspects: it is shown how, to second differential order, the geometry (and in particular the inner and outer curvature) of a brane worldsheet of arbitrary dimension is describable in terms of the first, second, and third fundamental tensor; the extension to a foliation by a congruence of such worldsheets is also briefly discussed. In the next part, it is shown how -- to lowest order in the thin limit -- the evolution of such a brane worldsheet will always be governed by a simple tensorial equation of motion whose left hand side is the contraction of the second fundamental tensor with the relevant surface stress tensor, while the right hand side will simply vanish in the case of free motion and will otherwise be just the orthogonal projection of any external force density that may happen to act on the brane. (Allowance for first order deviations from such a thin limit treatment would require evolution equations of a more complicated kind of which a prototype example is presented.) The last part of the course concentrates on the case of a string, and particularly on the stationary (centrifugally supported) configurations known as vortons, which, if they are sufficiently stable, may be of considerable cosmological significance.

hep-th↗