Abstract
Magnetic fields are observed on a large range of scales in the universe. Up until recently, the evidence always pointed to magnetic fields associated with some kind of structure, from planets to clusters of galaxies. Blazar observations have been used to posit the first evidence of truly cosmological magnetic fields or void magnetic fields. A cosmological magnetic field generated in the very early universe before recombination has implications for the cosmic microwave background (CMB), large scale structure as well as the 21 cm line signal. In particular, the Lorentz term causes a change in the linear matter power spectrum. Its implication on the 21 cm line signal was the focus of our recent simulations which will be summarised here. Modelling the cosmological magnetic field as a gaussian random field numerical solutions were found for magnetic fields with present day amplitudes of 5 nG and negative spectral indices which are within the range of observational constraints imposed by the cosmic microwave background (CMB).
Highlights
There are undisputed observations of magnetic fields associated with upto the largest structures such as clusters of galaxies
Depending on the models values are of the order of 10−25 –10−15 nG (e.g., [8,9,10,11]) which are significantly below magnetic field strengths found in galaxies which are of the order of 10−6 G (e.g., [12])
Not associated with any virialized structure, are very interesting since they could be evidence for the existence of truly cosmological magnetic fields. Such fields could serve as seeds for galactic magnetic fields if generated in the early universe
Summary
There are undisputed observations of magnetic fields associated with upto the largest structures such as clusters of galaxies (cf., e.g., [1,2,3,4,5,6,7]). When CMB photons pass through a region of neutral hydrogen they might be absorbed and/or cause stimulated emission which changes the ratio of number densities in the singlet and triplet states, respectively, and the spin temperature. Earlier on, at higher redshifts gas density is still high enough so that collisions between hydrogen atoms, electrons and protons are still frequent enough to induce significant collisional excitation and de-excitation of the triplet ground state This collisional coupling leads to a spin temperature different from the CMB temperature. As can be appreciated from Equation (1) once the background cosmology is chosen the 21 cm line signal is determined by the difference of the spin temperature as well as the number density of neutral hydrogen The latter will follow the matter density field. The effects of this on the 21 cm signal have been considered in [24]
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