Cosmological Phase Transitions

Together with Angela Olinto (U of C) and Karsten Jedamzik (Max-Planck-Institut für Astrophysik, Garching bei München) I am working on improved estimates for the strength and spectrum of primordial magnetic fields produced during cosmological first order phase transitions. Bubble wall instabilities and a finite charge density around the phase boundaries due to charge separation and screening effects lead to electric currents along the bubble walls. The instability growth rates are positive only in a finite range of wavenumbers and are modified by damping due to a finite viscosity and heat conductivity caused by the diffusion of radiation. The growing electric currents in turn cause magnetic seed fields which can subsequently be amplified by dynamo effects if the fluid flow modified by the instabilities turns turbulent.

The physics involved in this scenario reaches from particle physics over the thermodynamics of phase transitions, kinetic theory of particle transport, and turbulence to magnetohydrodynamics and the (still poorly understood) theory of magnetic dynamos. Although it can therefore only be quite qualitative, this new scenario is investigated in as general terms as possible for both the electroweak and the quark-hadron (QCD) transition and the assumptions made are discussed in some detail, including a comparison with other mechanisms of large scale magnetic field generation. At least for the QCD transition, magnetic field strengths up to about 10**(-19) G on a scale of 10Mpc at zero redshift seem to be possible. This appears to be sufficient to seed turbulent magnetic dynamo processes which could amplify galactic fields up to the observed levels of micro Gauss.

Observations of average flux levels and distributions of energies and arrival times as well as angular images of ultra-high energy cosmic rays could be used in the future to distinguish such type of scenarios from scenarios where galactic magnetic fields are produced by adiabatic compression starting from much stronger primordial extragalactic magnetic fields of the order of 10**(-12) G - 10**(-9) G. On extragalactic scales such strong primordial fields could also result from reprocessing of strong primordial fields on very small scales by magnetohydrodynamical effects.

For the future I plan to extend my studies on aspects of phase transitions which are relevant for baryogenesis. The baryon asymmetry created during the electroweak phase transition depends, for example, on the bubble wall propagation and the degree of decoherence of particle wave functions due to scattering near the wall. I plan to study this by using the non-abelian Boltzmann equation which I developed for neutrino oscillations.

Some of my collaborators on this branch of my interests are in the Early Universe Gang at the Max Planck Institut für Astrophysik, Garching bei München.


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