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Kerr-Schild Geometry from Cosmology to Microworld and Space-Time Structure

The Kerr-Schild (KS) geometry is linked tightly with the auxiliary \emph{flat} Minkowski background. Nevertheless, it describes many curved space-times and the related physical models, starting from cosmology and black holes to the microworld of the spinning elementary particles and the pre-quantum structure of vacuum fluctuations. We consider here a KS model of the Bubble Universe -- a semi-closed Universe with a rotating de Sitter (or anti-de Sitter) space embedded in an external flat space-time. When the solution has two horizons, it may also be interpreted as an Universe inside a black hole. In micro-world the KS geometry yields a model of the spinning particle consistent with gravity and describes a pre-quantum twistorial structure of space-time with the beam-like fluctuations of metric consistent with the beamlike fluctuations of electromagnetic vacuum. These light-like twistor-beam excitations are consistent with gravity and generalize the known pp-wave solutions. Following Wheeler's estimations of the density of vacuum fluctuations we arrive at the general conclusion that Universe should be flat and have a zero cosmological constant. It enforces us to return to an `effective flat geometry' filled by the electromagnetic background radiation.