arXiv subjects

E. J. Copeland

Publications and source records attributed to E. J. Copeland.

60 records · Page 4Linked to original sources

arXiv1995

Oscillons: Resonant Configurations During Bubble Collapse

Oscillons are localized, non-singular, time-dependent, spherically-symmetric solutions of nonlinear scalar field theories which, although unstable, are extremely long-lived. We show that they naturally appear during the collapse of subcritical bubbles in models with symmetric and asymmetric double-well potentials. By a combination of analytical and numerical work we explain several of their properties, including the conditions for their existence, their longevity, and their final demise. We discuss several contexts in which we expect oscillons to be relevant. In particular, their nucleation during cosmological phase transitions may have wide-ranging consequences.

hep-ph↗

arXiv1995

Quantum Dynamics Beyond the Gaussian Approximation

The time dependent quantum variational principle is emerging as an important means of studying quantum dynamics, particularly in early universe scenarios. To date all investigations have worked within a Gaussian framework. Here we present an improved method which is demonstrated to be superior to the Gaussian approach and may be naturally extended to the field--theoretic case.

gr-qc↗

arXiv1994

Characteristics of Cosmic String Scaling Configurations

Using the formalism developed in a previous paper we analyze the cosmological implications of our conclusions concerning the scaling behaviour of a network of cosmic strings, in particular the idea that once gravitational back-reaction becomes important the transient scaling regime so far explored by numerical simulations will be replaced by a new `full scaling' regime, with slightly different parameters. This has consequences for the normalization of the string tension on all scales. We show how the new scaling solutions affect the gravitational wave bound from nucleosynthesis and the milli-second pulsar, and also describe the consequences from large scale structure up to COBE scales.

hep-ph↗

arXiv1994

String Cosmology with a Time-Dependent Antisymmetric Tensor Potential

We present a class of exact solutions for homogeneous, anisotropic cosmologies in four dimensions derived from the low-energy string effective action including a homogeneous dilaton $ϕ$ and antisymmetric tensor potential $B_{μν}$. Making this potential time-dependent produces an anisotropic energy-momentum tensor, and leads us to consider a Bianchi I cosmology. The solution for the axion field must then only be a linear function of one spatial coordinate. This in turn places an upper bound on the product of the two scale factors evolving perpendicular to the gradient of the axion field. The only late-time isotropic solution is then a {\em contracting} universe.

hep-th↗

arXiv1994

Low energy effective string cosmology

We give the general analytic solutions derived from the low energy string effective action for four dimensional Friedmann-Robertson-Walker models with dilaton and antisymmetric tensor field, considering both long and short wavelength modes of the $H$-field. The presence of a homogeneous $H$-field significantly modifies the evolution of the scale factor and dilaton. In particular it places a lower bound on the allowed value of the dilaton. The scale factor also has a lower bound but our solutions remain singular as they all contain regions where the spacetime curvature diverges signalling a breakdown in the validity of the effective action. We extend our results to the simplest Bianchi I metric in higher dimensions with only two scale factors. We again give the general analytic solutions for long and short wavelength modes for the $H$ field restricted to the three dimensional space, which produces an anisotropic expansion. In the case of $H$ field radiation (wavelengths within the Hubble length) we obtain the usual four dimensional radiation dominated FRW model as the unique late time attractor.

hep-th↗

arXiv1993

Evolution of cosmic string configurations

We extend and develop our previous work on the evolution of a network of cosmic strings. The new treatment is based on an analysis of the probability distribution of the end-to-end distance of a randomly chosen segment of left-moving string of given length. The description involves three distinct length scales: $ξ$, related to the overall string density, $\barξ$, the persistence length along the string, and $ζ$, describing the small-scale structure, which is an important feature of the numerical simulations that have been done of this problem. An evolution equation is derived describing how the distribution develops in time due to the combined effects of the universal expansion, of intercommuting and loop formation, and of gravitational radiation. With plausible assumptions about the unknown parameters in the model, we confirm the conclusions of our previous study, that if gravitational radiation and small-scale structure effects are neglected, the two dominant length scales both scale in proportion to the horizon size. When the extra effects are included, we find that while $ξ$ and $\barξ$ grow, $ζ$ initially does not. Eventually, however, it does appear to scale, at a much lower level, due to the effects of gravitational back-reaction.

hep-ph↗