Tanmay Vachaspati

Magnetic fields from cosmological phase transitions

Abstract Cosmological phase transitions in the early universe lead to gradients in the vacuum expectation value of the Higgs field. The variation of the Higgs field results in the presence of electromagnetic fields after the completion of the phase transition. This mechanism would also produce magnetic fields of other unbroken gauge symmetries such as SU(3)c. We estimate the strength of the electromagnetic fields produced due to the electroweak phase transition.

Semilocal and electroweak strings

Abstract We review a class of non-topological defects in the standard electroweak model, and their implications. Starting with the semilocal string, which provides a counterexample to many well-known properties of topological vortices, we discuss electroweak strings and their stability with and without external influences such as magnetic fields. Other known properties of electroweak strings and monopoles are described in some detail and their potential relevance to future particle accelerator experiments and to baryon number violating processes is considered. We also review recent progress on the cosmology of electroweak defects and the connection with superfluid helium, where some of the effects discussed here could possibly be tested.

Causality and cosmic inflation

In the context of inflationary models with a pre-inflationary stage, in which the Einstein equations are obeyed, the null energy condition is satisfied, and spacetime topology is trivial, we argue that homogeneity on super-Hubble scales must be assumed as an initial condition. Models in which inflation arises from field dynamics in a Friedmann-Robertson-Walker background fall into this class but models in which inflation originates at the Planck epoch may evade this conclusion. Our arguments rest on causality and general relativistic constraints on the structure of spacetime. We discuss modifications to existing scenarios that may avoid the need for initial large-scale homogeneity. (c) 1999 The American Physical Society.

Estimate of the primordial magnetic field helicity

Electroweak baryogenesis proceeds via changes in the non-Abelian Chern-Simons number. It is argued that these changes generate a primordial magnetic field with left-handed helicity. The helicity density of the primordial magnetic field today is then estimated to be given by approximately 10(2)n(b), where n(b) approximately 10(-6)/cm(3) is the present cosmological baryon number density. With certain assumptions about the inverse cascade we find that the field strength at recombination is approximately 10(-13) G on a comoving coherence scale approximately 0.1 pc.

Detailed stability analysis of electroweak strings

We give a detailed stability analysis of the Z-string in the standard electroweak model. We identify the mode that determines the stability of the string and numerically map the region of parameter space where the string is stable. For sin2θW = 0.23, we find that the strings are unstable for a Higgs mass larger than 23 GeV. Given the latest constraints of the Higgs mass from LEP, this shows that, if the standard electroweak model is realized in nature, the existing vortex solutions are unstable.

Cosmic microwave background anisotropy from wiggly strings

We investigate the effect of wiggly cosmic strings on the cosmic microwave background radiation anisotropy and matter power spectrum by modifying the string network model used by Albrecht {ital et al.} We employ the wiggly equation of state for strings and the one-scale model for the cosmological evolution of certain network characteristics. For the same choice of simulation parameters we compare the results with and without including wiggliness in the model and find that wiggliness together with the accompanying low string velocities leads to a significant peak in the microwave background anisotropy and to an enhancement in the matter power spectrum. For the cosmologies we have investigated (standard CDM and CDM plus a cosmological constant), and within the limitations of our modeling of the string network, the anisotropy is in reasonable agreement with current observations but the COBE normalized amplitude of density perturbations is lower than what the data suggest. In the case of a cosmological constant and CDM model, a bias factor of about 2 is required. {copyright} {ital 1999} {ital The American Physical Society}

Quantum radiation from quantum gravitational collapse

We study quantum radiation emitted during the collapse of a quantized, gravitating, spherical domain wall. The amount of radiation emitted during collapse now depends on the wavefunction of the collapsing wall and the background spacetime. If the wavefunction is initially in the form of a sharp wavepacket, the expectation value of the particle occupation number is determined as a function of time and frequency. The results are in good agreement with our earlier semiclassical analysis and show that the quantum radiation is non-thermal and evaporation accompanies gravitational collapse.

Cosmic Sparks from Superconducting Strings

We investigate cosmic sparks from cusps on superconducting cosmic strings in light of the recently discovered millisecond radio burst by Lorimer et al.. We find that the observed duration, fluence, spectrum, and event rate can be reasonably explained by grand unification scale superconducting cosmic strings that carry currents approximately 10{5} GeV. The superconducting string model predicts an event rate that falls off only as S{-1/2}, where S is the energy flux, and hence predicts a population of very bright bursts. Other surveys, with different observational parameters, are shown to impose tight constraints on the superconducting string model.

Chiral effects and cosmic magnetic fields

In the presence of cosmic chiral asymmetry, chiral-vorticity and chiral-magnetic effects can play an important role in the generation and evolution of magnetic fields in the early universe. We include these chiral effects in the magnetic field equations and find solutions under simplifying assumptions. Our numerical and analytical results show the presence of an attractor solution in which chiral effects produce a strong, narrow, Gaussian peak in the magnetic spectrum and the magnetic field becomes maximally helical. The peak in the spectrum shifts to longer length scales and becomes sharper with evolution. We also find that the dynamics may become non-linear for certain parameters, pointing to the necessity of a more complete analysis.

Seeking string theory in the cosmos

We review the existence, formation and properties of cosmic strings in string theory, the wide variety of observational techniques that are being employed to detect them, and the constraints that current observations impose on string theory models.