  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: it is a compressible fluid, and
gravity is seen as Archimedes' thrust due to the pressure gradient in that
fluid. The construction of the theory is explained and the current status of
the experimental confrontation is analysed, both 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, both by
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 (non-identical) contraction-expansion
cycles, each with finite maximum energy density; the current expansion phase
will end by infinite dilution in some six billions of 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 constant H0, unless a
special assumption is made. Since cosmological redshifts approaching z = 4 are
observed, the density ratio should be at least 100. From this and the estimate
of H0, the time spent since the maximum density is constrained to be larger
than several hundreds of billions of 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.
