The authors study the dynamics of flat Friedmann-Robertson-Walker cosmologies (FRW) in the presence of a triplet of massive vector fields with SO(3) global symmetry. They find an E³-symmetric ansatz for the vector fields that is compatible with the E³-invariant FRW metric and propose a method to make an invariant ansatz for more general cosmological models. They use the techniques of dynamical systems to study qualitatively the behaviour of the model and find, in particular, that the effective equation of state of the system changes gradually from a radiation-dominated to a matter-dominated form and that the scale of the transition depends on the mass of the gauge fields.

A unified description of inflation, dark energy, and dark matter is presented within a two-scalar-field cosmological model. Inflation, assumed to be of the warm type, is driven by one of the scalar fields, which, shortly after the end of the inflationary period, decouples from radiation and begins to oscillate rapidly around the minimum of its potential, thus behaving like cold dark matter; the second scalar field emerges, at recent times, as the dominant component of the Universe, giving rise to a second era of accelerated expansion. For certain values of the parameters of the model, the cosmological solutions arising in this triple unification of inflation, dark energy, and dark matter are viable, reproducing the main features of the evolution of the Universe.

A general dilaton gravity theory in 1+1 spacetime dimensions, with a cosmological constant $\lambda$ and a new dimensionless parameter $\omega$, contains as special cases the constant curvature theory of Teitelboim and Jackiw, the theory equivalent to vacuum planar general relativity, the first-order string theory, and a two-dimensional purely geometrical theory. The equations of this general two-dimensional theory admit several different black holes with various types of singularities. The singularities can be spacelike, timelike, or null, and there are even cases without singularities. Evaluation of the ADM mass, as a charge density integral, is possible in some situations by carefully subtracting the black hole solution from the corresponding linear dilation at infinity.

Three dimensional black holes in a generalized dilaton gravity action theory are analysed. The theory is specified by two fields, the dilaton and the graviton, and two parameters, the cosmological constant and the Brans-Dicke parameter. It contains seven different cases, of which one distinguishes as special cases, string theory, general relativity and a theory equivalent to four dimensional general relativity with one Killing vector. We study the causal structure and geodesic motion of null and timelike particles in the black hole geometries and find the ADM masses of the different solutions.

We investigate the late-time evolution of the Universe within a cosmological model in which dark matter and dark energy are identified with two interacting scalar fields. Using methods of qualitative analysis of dynamical systems, we identify all cosmological solutions of this model. We show that viable solutions—in the sense that they correspond to a cosmic evolution in which a long enough matter-dominated era is followed by a current era of accelerated expansion—can be found in several regions of the parameter space. These solutions can be divided into two categories, namely, solutions that evolve to a state of everlasting accelerated expansion, in which the energy density of the dark-matter field rapidly approaches zero and the evolution becomes entirely dominated by the dark-energy field, and solutions in which the stage of accelerated expansion is temporary and the ratio between the energy densities of dark energy and dark matter tends, asymptotically, to a constant nonzero value.