# 28th Texas Symposium on Relativistic Astrophysics

13-18 December 2015
International Conference Centre Geneva
Europe/Zurich timezone

## Lower limits on the magnetic field strength in the early universe

15 Dec 2015, 15:05
20m
Level 2, Room 13 (International Conference Centre Geneva)

### Level 2, Room 13

#### International Conference Centre Geneva

17 Rue de Varembé, 1211 Geneva

### Speaker

Prof. Reinhard Schlickeiser (Ruhr University Bochum, Germany)

### Description

Two recent estimates of lower limits for the stochastic primordial magnetic fields are reviewed. The first estimate pioneered by Neronov and Vovk (2010) is based on GeV-TeV $\gamma$-ray observations of distant blazars by air-Cherenkov telecopes and the FERMI satellite. The generated $e^{\pm }$ pair beams from double photon collisions with the extragalactic background light have been expected to initiate a full electromagnetic cascade as in vacuum. However, as the cascaded GeV inverse Compton scattered gamma-rays have not been detected, the existence of small irregular intergalactic magnetic fields, scattering the produced pairs, has been predicted. However, the generated initial pair beams are subject to rapid electrostatic and electromagnetic kinetic plasma instabilities (Broderick et al. 2012, Schlickeiser et al. 2012) in the unmagnetized fully-ionized intergalactic medium, so that less kinetic initial pair energy for the cascade emission is available, explaining the non-detected GeV $\gamma$-rays. The second estimate calculates the magnetic (and electric) equilibrium wavenumber spectrum of aperiodic collective fluctuations in the thermal isotro\-pic electron-proton intergalactic plasma using the generalized Kirchhoff laws, accounting self-consistently for the simultaneous competition of spontaneous emission and absorption processes. By integrating the wavenumber spectrum over all wavenumber values provides for the total magnetic field strength in the IGM $|\delta B|=\sqrt{(\delta B)^2}\simeq 10^{-17}$ G with maximum length scales $\le 10^{15}$ cm. This guaranteed magnetic field in the form of randomly distributed aperiodic fluctuations, produced by the spontaneous emission of the isotropic thermal IGM plasma, sets a robust lower limit on stochastic primordial magnetic fields, and serves as seed field for amplification by later possible plasma instabilities from anisotropic plasma particle distribution functions, MHD instabilities and/or the MHD dynamo process.

### Primary author

Prof. Reinhard Schlickeiser (Ruhr University Bochum, Germany)