## 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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