Rotating Boson Stars

Abstract

Recently, experimental evidence has been accumulated that fundamental scalar fields, like the Higgs boson, exist in Nature. The gravitational collapse of such a boson cloud would lead to a boson star (BS) as a new type of a compact object. Similarly as for white dwarfs and neutron stars (NSs), there exist a limiting mass, the Kaup limit, below which a BS is stable against complete gravitational collapse to a black hole (BH). Depending the self-interaction of the basic scalars, one can distinguish mini-, axi-dilaton, soliton, charged, oscillating and rotating BSs. Their compactness normally prevents a Newtonian approximation, however, modifications of general relativity (GR), as in the case of Jordan-Brans-Dicke theory, would provide them with gravitational memory. Balance between the quantum pressure due to Heisenberg’s uncertainty principle and gravity permits the existence of a completely stable branch of spherically symmetric configurations. Moreover, as a coherent state, like the vortices of Bose-Einstein condensates, it allows for rotating solutions with quantized angular momentum. In this review, we concentrate on the fascinating possibility of weakening the BH uniqueness theorem for rotating configurations and soliton-type collisions of excited BSs. (Dedicated to Carl Brans’ 80th birthday, the author’s professor at Princeton in the fall of 1973, then lecturing on complex relativity).