We study the cosmological evolution of a type-0 string theory by employing noncriticality, which may be induced by fluctuations of the D3 brane worlds. We check the consistency of the approach to O(α ′ ) in the corresponding σ-model. The ten-dimensional theory is reduced to an effective four-dimensional model, with only time dependent fields.We show that the four-dimensional universe has an inflationary phase and graceful exit from it, while the other extra dimensions are stabilized to a constant value, with the fifth dimension much larger than the others. We pay particular attention to demonstrating the rôle of tachyonic matter in inducing these features. The Universe asymptotes, for large times, to a non-accelerating linearly-expanding Universe with a time-dependent dilaton and a relaxing to zero vacuum energy a lá quintessence.

We consider a three-dimensional brane-universe moving in a Type 0 String background. The motion induces on the brane a cosmological evolution which, for some range of the parameters, exhibits an inflationary phase. a

An analysis of a spherically symmetric braneworld configuration is performed when the intrinsic curvature scalar is included in the bulk action; the vanishing of the electric part of the Weyl tensor is used as the boundary condition for the embedding of the brane in the bulk. All the solutions outside a static localized matter distribution are found; some of them are of the Schwarzschild-(A)dS 4 form. Two modified Oppenheimer-Volkoff interior solutions are also found; one is matched to a Schwarzschild-(A)dS 4 exterior, while the other is not. A nonuniversal gravitational constant arises, depending on the density of the considered object; however, the conventional limits of Newton's constant are recovered. An upper bound of the order of TeV for the energy string scale is extracted from the known solar system measurements ͑experiments͒. On the contrary, in the usual brane dynamics, this string scale is calculated to be larger than TeV.

We study the cosmological evolution of a D3-brane universe in a type 0 string background. We follow the brane universe along the radial coordinate of the background and we calculate the energy density which is induced on the brane because of its motion in the bulk. We find that, for some typical values of the parameters and for a particular range of values of the scale factor of the brane universe, the effective energy density is dominated by a term proportional to 1/(log a) 4 indicating a slowly varying inflationary phase. For larger values of the scale factor the effective energy density takes a constant value and the brane universe enters its usual inflationary period.

We study the geometry of M5-branes wrapping a 2-cycle which is Special Lagrangian with respect to a specific complex structure in a CalabiYau two-fold. Using methods recently applied to the three-fold case, we are again able find a characterization of the geometry, in terms of a nonintegrable almost complex structure and a (2,0) form. This time, however, due to the hyper-Kähler nature of the underlying 2-fold we also have the freedom of choosing a different almost complex structure with respect to which the wrapped 2-cycle is holomorphic. We show that this latter almost complex structure is integrable. We then relate our geometry to previously found geometries of M5-branes wrapping holomophic cycles and go further to prove some previously unknown results for M5-branes on holomorphic cycles. HUTP-05/A0040BCCUNY-HEP /05-04 hep-th/xxxxxxx *