5.2 Compact alternatives to black holes

As a localized scalar field configuration, a boson star can be constructed as a non-interacting compact object, as long as one does not include any explicit coupling to any electromagnetic or other fields. In that respect, it resembles a BH, although it lacks a horizon. Can observations of purported BHs be fully explained by massive boson stars? See Ref. [183] for a review of such observations.

Neutron stars also lack horizons, but, in contrast to a boson star, have a hard surface. A hard surface is important because one would expect accretion onto such a surface to have observable consequences. Can a boson star avoid such consequences? Yuan, Narayan and Rees consider the the viability of 10M āŠ™ boson stars as BH candidates in X-ray binaries [223]. They find that accreting gas collects not at the surface (which the star lacks), but instead at the center, which ultimately should lead to Type I X-ray bursts. Because these bursts are not observed, the case against boson stars as black hole mimickers is weakened (at least for BH candidates in X-ray binaries).

Ref. [105] considers a simplified model of accretion and searches for boson-star configurations that would mimic an accreting black hole. Although they find matches, they find that light deflection about a boson star will differ from the BH they mimic because of the lack of a photon sphere. Further work, studies the scalar field tails about boson stars and compares them to those of BHs [153]. If indeed a boson star collapses to a BH, then one could hope to observe the QNM of the massive scalar field, as described in [114].

Some of the strongest evidence for the existence of BHs is found at the center of most galaxies. The evidence suggests supermassive objects (of the order of millions of solar mass) occupying a small region (of order an astronomical unit) [32]. While definitive evidence for a BH horizon from conventional electromagnetic telescopes is perhaps just on the “horizon” [42], there are those who argue the viability of supermassive boson stars at galactic centers [214]. There could potentially be differences in the (electromagnetic) spectrum between a black hole and a boson star, but there is considerable freedom in adjusting the boson star potential to tweak the expected spectrum [103]. However, there are stringent constraints on BH alternatives to Sgr A* by the low luminosity in the near infrared [43]. In particular, the low luminosity implies a bound on the accretion rate assuming a hard surface radiating thermally and, therefore, the observational evidence favors a black hole because it lacks such a surface.

However, the observation of gravitational waves from such objects may be able to distinguish BHs from BSs [28]. Such a test would occur in the bandwidth for a space-based observatory such as the beleaguered LISA mission. Because BSs allow for orbits within what would otherwise be a black-hole event horizon, geodesics will exhibit extreme pericenter precession resulting in potentially distinguishable gravitational radiation [128]. In any case, observations of supermassive objects at the centers of galaxies can be used to constrain the scalar field parameters of possible mimickers [17].

There are other possible BH mimickers, and a popular recent one is the gravastar [158]. Common among all these alternatives, and most significantly, is the lack of an event horizon. Both gravastars and BSs undergo an ergoregion instability for high spin (Jāˆ•(GM 2) > 0.4[44]. As mentioned above for BSs, gravitational waves may similarly be able to distinguish gravastars from BHs [175].

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