Original publication

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Sebastian Hutschenreuter, Sebastian Dorn, Jens Jasche, Franco Vazza, Daniela Paoletti, Guilhem Lavaux, Torsten A. Enßlin
The primordial magnetic field in our cosmic backyard

Highlight: April 2018

The primordial magnetic field in our cosmic backyard

April 01, 2018

At the very beginning of the Universe, not only elementary particles and radiation were generated but also magnetic fields. A team of researchers led by the Max Planck Institute for Astrophysics now calculated what these magnetic fields should look like today in the local universe – in great detail and in 3D. To achieve this, first they traced back the current distribution of matter to the time of the Big Bang; this distribution of matter was then used to calculate the generation of the magnetic field; and finally the resulting fields were translated back to the present. Thus, the researchers were able to predict the structure and morphology of the primordial magnetic field in our cosmic neighbourhood for the first time. This field is incredibly weak; nevertheless, the prediction could help to address the challenge of measuring it.

<p>Fig. 1: Sky view of the Harrison magnetic field strength averaged within a sphere with 300 million light years radius around the Earth. The region with stronger fields on the left side of the image is the Perseus Pisces galaxy cluster, the one in the upper part is the Virgo cluster.</p> Zoom Image

Fig. 1: Sky view of the Harrison magnetic field strength averaged within a sphere with 300 million light years radius around the Earth. The region with stronger fields on the left side of the image is the Perseus Pisces galaxy cluster, the one in the upper part is the Virgo cluster.

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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.

<p>Fig. 2: Sky view of the magnetic field strength and orientation of the magnet field components perpendicular to the line of sight, again averaged a sphere with 300 million light years radius around the Earth. The texture indicates the direction of the field lines.</p> Zoom Image

Fig. 2: Sky view of the magnetic field strength and orientation of the magnet field components perpendicular to the line of sight, again averaged a sphere with 300 million light years radius around the Earth. The texture indicates the direction of the field lines.

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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.

<p>Fig. 3: A slice through the Perseus Pisces galaxy cluster in the present Universe with the matter distribution depicted in grey and the blue arrows highlighting the Harrison magnetic field.</p> Zoom Image

Fig. 3: A slice through the Perseus Pisces galaxy cluster in the present Universe with the matter distribution depicted in grey and the blue arrows highlighting the Harrison magnetic field.

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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...

 

 
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