On the Weyl - Eddington - Einstein affine gravity in the context of modern cosmology

Abstract

We propose new models of an `affine' theory of gravity in $D$-dimensional space-times with symmetric connections. They are based on ideas of Weyl, Eddington and Einstein and, in particular, on Einstein's proposal to specify the space - time geometry by use of the Hamilton principle. More specifically, the connection coefficients are derived by varying a `geometric' Lagrangian that is supposed to be an arbitrary function of the generalized (non-symmetric) Ricci curvature tensor (and, possibly, of other fundamental tensors) expressed in terms of the connection coefficients regarded as independent variables. In addition to the standard Einstein gravity, such a theory predicts dark energy (the cosmological constant, in the first approximation), a neutral massive (or, tachyonic) vector field, and massive (or, tachyonic) scalar fields. These fields couple only to gravity and may generate dark matter and/or inflation. The masses (real or imaginary) have geometric origin and one cannot avoid their appearance in any concrete model. Further details of the theory - such as the nature of the vector and scalar fields that can describe massive particles, tachyons, or even `phantoms' - depend on the concrete choice of the geometric Lagrangian. In `natural' geometric theories, which are discussed here, dark energy is also unavoidable. Main parameters - mass, cosmological constant, possible dimensionless constants - cannot be predicted, but, in the framework of modern `multiverse' ideology, this is rather a virtue than a drawback of the theory. To better understand possible applications of the theory we discuss some further extensions of the affine models and analyze in more detail approximate (`physical') Lagrangians that can be applied to cosmology of the early Universe.