Effective spacetime geometry of graviton condensates in 
f(R)
 gravity
Working in static spherically symmetric setup, recent study has demonstrated that the effective spacetime geometry of a Bose-Einstein condensate of weakly interacting gravitons is analogous to a gravastar, hence providing a bridge between these two attempts in describing the black hole interior. In this paper we make three generalizations: introducing a composite of two graviton condensates so that the exterior spacetime is not necessarily asymptotically flat, working in $f({\mathcal R})$ gravity, and extending the calculations to higher dimensions. We find that the effective spacetime geometry is again analogous to a gravastar, but the interior can be de Sitter or Anti-de Sitter and the exterior can be Schwarzschild, Schwarzschild-de Sitter, or Schwarzschild-Anti de Sitter, where the cosmological constant for the exterior must be smaller than the one for the interior. These geometries are determined by the modified gravity function $f({\mathcal R})$, in contrast to previous works where they were selected by hand. We also presented a new possible value for the size of the interior condensate provided a certain restriction is satisfied, which would not be met if we are still working in ordinary gravity with four spacetime dimensions. This restriction can be seen manifested in the behavior of the interior graviton wavelength as a function of spacetime dimension.