Abstract
We study coherently oscillating massive gravitons in the ghost-free bigravity theory. This coherent field can be interpreted as a condensate of the massive gravitons. We first define the effective energy-momentum tensor of the coherent massive gravitons in a curved spacetime. We then study the background dynamics of the Universe and the cosmic structure formation including the effects of the coherent massive gravitons. We find that the condensate of the massive graviton behaves as a dark matter component of the Universe. From the geometrical point of view the condensate is regarded as a spacetime anisotropy. Hence, in our scenario, dark matter is originated from the tiny deformation of the spacetime. We also discuss a production of the spacetime anisotropy and find that the extragalactic magnetic field of a primordial origin can yield a sufficient amount for dark matter.
Highlights
The existence of gravitational waves was confirmed by the direct detections [1,2], and their quantum counterpart is called gravitons
We will show that the coherent magnetic field can yield a sufficient amount of the massive graviton condensate in order to explain the present abundance of dark matter
We provide a scenario in which a tiny deformation of the spacetime is dark matter in the ghost-free bigravity theory
Summary
The existence of gravitational waves was confirmed by the direct detections [1,2], and their quantum counterpart is called gravitons. The effective energy-momentum tensors of both massless and massive gravitons are defined in the similar way to the case of GR [17]. We explore those questions and find that the dark matter component can be regarded as the “condensate” of the massive graviton and it can give local structures of the Universe. We can define the energy-momentum tensor of the zero momentum mode of the massive graviton as long as the graviton mass is larger than the curvature scale of the background spacetime. We will show that the coherent magnetic field can yield a sufficient amount of the massive graviton condensate in order to explain the present abundance of dark matter. In Appendix C, we detail the calculations about the inhomogeneous modes of the massive graviton condensate
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