Monopole planets and galaxies

Abstract

Spherical clusters of SU(2) BPS monopoles are investigated here. A large class of monopole solutions is found using an abelian approximation, where the clusters are spherically symmetric, although exact solutions cannot have this symmetry precisely. Monopole clusters generalise the Bolognesi magnetic bag solution of the same charge, but they are always larger. Selected density profiles give structures analogous to planets of uniform density, and galaxies with a density decaying as the inverse square of the distance from the centre. The Bolognesi bag itself has features analogous to a black hole, and this analogy between monopole clusters and astrophysical objects with or without black holes in their central region is developed further. It is also shown that certain exact, platonic monopoles of small charge have sizes and other features consistent with what is expected for magnetic bags.