Magnetic monopoles with no strings

Previous treatments of the charge-monopole system have used either a lagrangian singular along the Dira string or a multiple valued action integral. Here we formulate an action principle for this system which involves no strings and is single. This is achieved by writing the lagrangian directly in terms of the variables of a suitable fibre bundle. Canonical quantization is carried out in a simple way and known results are recovered.     

Massless monopoles and multi-pronged strings

We investigate the role of massless magnetic monopoles in the N=4 supersymmetric Yang-Mills Higgs theories. They can appear naturally in the 1/4-BPS dyonic configurations associated with multi-pronged string configurations. Massless magnetic monopoles can carry nonabelian electric charge when their associated gauge symmetry is unbroken. Surprisingly, massless monopoles can also appear even when the gauge symmetry is broken to abelian subgroups.     

Fundamental cosmic strings

Cosmic strings are linear concentrations of energy that may be formed at phase transitions in the very early universe. At one time they were thought to provide a possible origin for the density inhomogeneities from which galaxies eventually develop, though this idea has been ruled out, primarily by observations of the cosmic microwave background (CMB). Fundamental strings are the supposed building blocks of all matter in superstring theory or its modern version, M-theory. These two concepts were originally very far apart, but recent developments have brought them closer. The ‘brane-world’ scenario in particular suggests the existence of macroscopic fundamental strings that could well play a role very similar to that of cosmic strings.

In this paper, we outline these new developments, and also analyse recent observational evidence, and prospects for the future.     

Self-gravitating general-relativistic cosmic strings

We examine the coupled Einstem-Euler-Lagrange equations for nonstationary cosmic strings. Self-consistent solutions to all the equations are found under the assumption that the energy-momentum tensor is of the formT _t ^t =T _z ^z while all other components vanish. It is shown that the strings are necessarily static in this case and that the scalar field potential must be of the usual quartic form with the coupling constants satisfying e²=8.     

Stable charged cosmic strings

We study the quantum stabilization of a cosmic string by a heavy fermion doublet in a reduced version of the standard model. We show that charged strings, obtained by populating fermionic bound state levels, become stable if the electroweak bosons are coupled to a fermion that is less than twice as heavy as the top quark. This result suggests that extraordinarily large fermion masses or unrealistic couplings are not required to bind a cosmic string in the standard model. Numerically we find the most favorable string profile to be a simple trough in the Higgs vacuum expectation value of radius ≈10(-18) m. The vacuum remains stable in our model, because neutral strings are not energetically favored.     

An experimental search for cosmic monopoles

A simple search for a flux of massive cosmic monopoles predicted by the grand unified theories of strong, electromagnetic and weak interactions, was performed using an existing cosmic ray telescope of scintillation counters. The experiment was sensitive to monopoles, coming from above and underneath the earth surface, with a velocity 7 × 10^?3 < β < 0.6. An upper limit of 6 × 10^?5 monopoles per cm² sr yr (90% c.l.) su obtained.     

Cosmological evolution of monopoles connected by strings

Phase transitions in the early universe can give rise to a system of vacuum strings. Assuming that the strings are topologically stable, it has been recently shown that they can produce density fluctuations which can subsequently lead to galaxy formation. The present paper examines the cosmological evolution of topologically unstable strings. In this case the system consists of strings having monopoles and antimonopoles at the ends, as well as of closed loops. Various physical processes in the system are discussed in detail. It is shown that monopoles and strings disappear long before any appreciable density fluctuations on a galactic scale can be produced.     

Cosmic axions from cosmic strings

The possibility of a new constraint on the Peccei-Quinn symmetry breaking scale, arising from the decay of cosmic axion strings, is discussed.