5.3 As origin of dark matter

Studies of stellar orbits within various galaxies produce rotation curves, which indicate galactic mass within the radius of the particular orbit. The discovery that these curves remain flat at large radius suggests the existence of a large halo of massive, yet dark, matter that holds the galaxy together despite its large rotation. However, what precise form of matter could fulfill the observational constraints is still very much unclear. Scalar fields are an often used tool in the cosmologist’s toolkit, but one cannot have a regular, static configuration of scalar field to serve as the halo [178] (see [69Jump To The Next Citation Point] as discussed in Section 6.3 for a discussion of rotating boson stars with embedded, rotating BH solutions). Instead, galactic scale boson stars are one possible candidate.

Boson stars can be matched onto the observational constraints for galactic dark matter halos [145, 199]. However, multi-state boson stars that superpose various boson-star solutions (e.g., an unexcited solution with an excited solution) can perhaps find better fits to the constraints [215]. Boson stars at the galactic scale may not exhibit general relativistic effects and can be effectively considered as Bose–Einstein condensates (BEC) with angular momentum [185].

The solitonic nature of boson stars (see Figure 1View Image) lends itself naturally to the wonderful observation of dark matter in the Bullet Cluster [146]. Ref. [144] attempts to determine a minimum galactic mass from such a match.

Interestingly, Ref. [19] foregoes boson stars and instead looks for quasi-stationary scalar solutions about a Schwarzschild black hole that could conceivably survive for cosmological times. Another approach is to use scalar fields for both the dark matter halo and the supermassive, central object. Ref. [8] looks for such a match, but finds no suitable solutions. Quite a number of more exotic models viably fit within current constraints, including those using Q-balls [71].

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