Astrophysical Wormholes

Abstract

Wormholes are hypothetical spacetime structures with non-trivial topology capable of connecting either two distant regions of the same universe or two different universes, as illustrated in Fig. 1. The entrances of a wormhole are called the “mouths” of the wormhole and the spacetime region connecting the mouths is called the “throat” of the wormhole. The simplest wormhole configuration has two mouths connected by a throat, but more complex structures are also possible [1]. Strictly speaking, wormholes are not a prediction of general relativity or of other theories of gravity. They are spacetime structures that can potentially exist in curved spacetimes, so in a very wide class of gravity models. Formation mechanisms and stability of these spacetime structures depend on the specific gravity theory and often present problems, so the existence of wormholes in the Universe is challenging. However, they cannot be ruled out. The first wormhole solution was proposed by Einstein and Rosen in 1935 and is known as the EinsteinRosen bridge [2]. Misner and Wheeler coined the term “wormhole” in Ref. [3]. Among the wormhole solutions, “traversable wormholes” are of particular interest: they can be crossed in both directions and they could thus be potential shortcuts for interstellar or intergalactic trips to move from one region to another region of the Universe that would be unfeasible to reach within human timescales without exceeding the speed of light. Traversable wormholes were discovered in 1973 by Bronnikov [4] and, independently, Ellis [5], but they became popular after the papers by Morris and Thorne [6, 7] and Visser [8]. In general relativity, traversable wormholes require exotic matter to keep the mouths open (see, however, Ref. [9]). In the absence of exotic matter, the wormhole throat collapses and the mouths close [6–8]. The initial wormhole may become a normal black hole [10]. However, in other theories of gravity wormholes may exist even without exotic matter or without matter at all; see, for instance, Ref. [11]. Since we do not know if general relativity is the correct theory of gravity, nor if exotic matter can exist in the