Accelerated expansion as predicted by an ether theory of gravitation

Cosmology is investigated within a new, scalar theory of gravitation, which is a preferred-frame bimetric theory with flat background metric. Before coming to cosmology, the motivation for an "ether theory" is exposed at length; the investigated concept of ether is presented in some detail: it is an elastically compressible fluid, and gravity is seen as Archimedes's thrust due to the macroscopic pressure gradient in that fluid; the construction of the theory, based on this concept, is explained; and the current status of the experimental confrontation is analyzed, also in some detail. An analytical cosmological solution is obtained for a general form of the energy-momentum tensor. According to that theory, expansion is necessarily accelerated by the vacuum and even by matter. In one case, the theory predicts expansion, the density increasing without limit as time goes back to infinity, High density is thus obtained in the past, without a big-bang singularity. in the other case, the universe follows a sequence of (nonidentical) contraction-expansion cycles, each with finite maximum energy density; the current expansion phase will end by infinite dilution in some six billion years. The density ratio of the present cycle (ratio of the maximum to current densities) is not determined by the current density and the current Hubble parameter H-0 unless a special assumption is made ("no cosmological time-dilation"). Since cosmological redshifts approaching z = 4 are observed, the density ratio should be at least 100. From this and the estimate of H-0, the time since the maximum density is constrained to be larger than several hundred billion years. Yet if a high density ratio, compatible with the standard explanation for the light elements and the 2.7 K radiation, is assumed, then the age of the universe is much larger still.