The primordial magnetic field in our cosmic backyard
The Big Bang is still shrouded in mystery in many respects. Cosmologists use many different ways to try and get information about the first moments of our universe. One possibility are cosmic magnetic fields, which were created by the Big Bang and should have survived to this day. A number of highly speculative mechanisms have been proposed for this so-called magnetogenesis, but in addition there is a simple plasma-physical effect, the Harrison effect, which should have produced magnetic fields at the birth of the cosmos. This effect describes how vortex movements in the plasma of the early universe produce electric currents due to friction with the very strong radiative field, thus inducing a magnetic field. Knowing the plasma vortices in that early time, one could calculate in detail how these magnetic fields were generated. If one also knew the plasma motions since then, one could calculate what these magnetic fields should look like today.
The necessary information is contained in the distribution of the galaxies around us, as this is the result of the motion of matter since the early universe. As we know the laws leading to the formation of galaxies, from today's galaxy distribution it is possible – with some uncertainty – to trace the evolution of the matter distribution from the early universe to the present day. This means that the information necessary is available to predict the magnetic fields generated by the Harrison effect in today's universe (Highlight 12/2009: Mapping of the Universe beyond the Known). An international team of scientists led by the MPA used this logic to calculate today's remnants of the primordial magnetic fields in our cosmic neighbourhood, i.e. in the surrounding 300 million light years.
These magnetic fields are extremely weak, twenty-seven orders of magnitude smaller than the Earth's magnetic field (see Figures). In spite of the weakness, the team was able to precisely predict the magnetic field structure as viewed from Earth (Figures 1 und 2) and at known places in the Universe (Figure 3) – unfortunately these fields are far smaller than the current measuring threshold. Nevertheless, these calculations show that we can understand our cosmos with high precision and calculate subtle effects within. And who knows how precisely we will be able to measure magnetic fields in 100 years – Einstein also thought that the gravitational waves he predicted would be too weak to detect...