Dark Matter with Genuine Spin-2 Fields

Abstract

Gravity is the only force which is telling us about the existence of Dark Matter. I will review the idea that this must be the case because Dark Matter is nothing else than a manifestation of Gravity itself, in the guise of an additional, massive, spin-2 particle.Keywords: dark matter theory; bigravity theory; general relativity alternatives; cosmology of dark matter Where is Dark Matter?Roughly 85% of all the matter of the Universe appears to be in the form of Dark Matter (DM).As of yet, we do not know what the origin and properties of DM are. That there is DM in our Universe we know from its gravitational effects [1]. Typically, DM is modeled as a cold relic density of some yet unknown particle, which was produced early in the evolution of the Universe, which possesses very weak interactions with Standard Model (SM) particles. Despite many efforts to debunk this field, DM remains very elusive. Here, I review the idea that DM is automatically built into the only known consistent extension of General Relativity (GR) to an additional interacting massive spin-2 field; since this field is engineered to interact only gravitationally, it escapes all non-gravitational detection methods [2-6]. Bimetric TheoryMultiple spin-2 particles in four dimensions have only recently been shown to be a consistent, ghost-free possibility, thanks to the construction of ghost-free bimetric theory (see [7] for a review). As the name says, this model describes, on top of the usual graviton with zero mass, a second, massive, propagating spin-2 particle [8]. We can begin with the (fully non-linear) action, which describes two dynamical tensor fields g µν and f µν [8]:here, m g and α m g are the mass scales that determine the strength of (self-)interaction for the two fields, whereas m tells the energy scale for the massive spin-2 field (see below). For the model to be consistent, the form of the potential V(g −1 f ) is constrained to be [9,10]:β n e n g −1 f ,