Observing astrophysical magnetic fields is difficult. Nonetheless, fields of surprisingly consistent amplitudes on the order of microgauss have been discovered in many galaxies and clusters of galaxies. So far, the generation of these fields has, remained a mystery. For a long time, scientists tried to conceive of a mechanism by which tiny primordial fields would be created in the early universe. Later, during gravitational collapse, such fields could be amplified—for example, by means of a dynamo mechanism—and thereby lead to the observed fields in galaxies and clusters. Even if the seed fields needed for dynamo amplification were as small as 10~(-25) G or smaller, these primordial seed fields have been shown to be severely constrained by the gravity wave background that they induce. As Ichiki et al. report on page 827 of this issue, there is another possibility. They show that second-order cosmological perturbations necessarily generate magnetic fields that are of the right order to be amplified by the dynamo mechanism into the currently observed fields in galaxies and clusters.
展开▼
