  Gravitation is described in the context of a dilatonic theory that is
conformally related to general relativity. All dimensionless ratios of
fundamental dimensional quantities, e.g. particle masses and the Planck mass,
as well as the relative strengths of the fundamental interactions, are fixed
constants. An interplay between the positive energy density associated with
relativistic matter (and possibly with negative spatial curvature) and the
negative energy associated with dynamical dilaton phase results in a
non-singular, flat cosmological model with no horizon, and -- as a direct
consequence of absence of phase transitions in the early universe -- with no
production of topological defects. The (logarithmic) time-derivative of the
field modulus is degenerate with the Hubble function, and all cosmological
epochs of the standard model are unchanged except at the very early universe.
We demonstrate that both linear order perturbation theory and the spherical
collapse model are equivalent to those in the standard model, up to
modifications caused by the phase of the (complex) scalar field and its
perturbations. Consequently, our alternative theory automatically passes the
main classical cosmological tests. Quantum excitations of the phase of the
scalar field generate a slightly red-tilted spectrum of adiabatic and gaussian
scalar perturbations on the largest scales. However, this framework does not
provide a similar mechanism for producing primordial gravitational waves on
these scales. A spherically symmetric vacuum solution that approximately
describes the exterior of gravitationally bound systems (e.g., stars and
galaxies) by a modified Schwarzschild-de Sitter metric, augmented with an
additional linear potential term, could possibly explain galactic rotation
curves and strong gravitational lensing with no recourse to dark matter.
