The distance between BPS branes in string theory corresponds to a flat direction in the effective potential. Small deviations from supersymmetry may lead to a small uplifting of this flat direction and to brane inflation. However, this scenario can work only if the BPS properties of the branes and the corresponding flatness of the inflaton potential are preserved in the stable volume compactification. We present an "inflaton trench" mechanism that keeps the inflaton potential flat due to shift symmetry, which is related to near BPS symmetry in our model.

Among the inflationary models based on string theory, the D3/D7 model has the advantage that the flatness of the inflaton potential can be protected even with moduli stabilization. However, the Abrikosov-Nielsen-Olesen BPS cosmic strings produced at the end of original D3/D7 inflation lead to an additional contribution to the CMB anisotropy. To make this contribution consistent with the WMAP results one needs an extremely small gauge coupling in the effective D-term inflation model. Such couplings may be difficult to justify in string theory. Here we develop a generalized version of the D3/D7 brane model, which leads to semilocal strings, instead of the topologically stable ANO cosmic strings. We show that the semilocal strings have unbroken supersymmetry when embedded into supergravity with FI terms. The energy of these strings is independent of their thickness. We confirm the existing arguments that strings of such type disappear soon after their formation and do not pose any cosmological problems, for any value of the gauge coupling. This should simplify the task of constructing fully realistic models of D3/D7 inflation.

One of the main problems of inflation in string theory is finding models with a flat potential while simultaneously stabilizing the volume of the compactified space. This can be achieved in theories where the potential has (an approximate) shift symmetry in the inflaton direction. We will identify a class of models where the shift symmetry uniquely follows from the underlying mathematical structure of the theory. It is related to the symmetry properties of the corresponding coset space and the period matrix of special geometry, which shows how the gauge coupling depends on the volume and the position of the branes. In particular, for type IIB string theory on K3 × T 2 /Z 2 with D3 or D7 moduli belonging to vector multiplets, the shift symmetry is a part of SO(2, 2+n) symmetry of the coset space SU (1,1)) ×SO(2+n) . The absence of a prepotential, specific for the stringy version of supergravity, plays a prominent role in this construction, which may provide a viable mechanism for the accelerated expansion and inflation in the early universe.

We construct black hole attractor solutions for a wide class of N = 2 compactifications. The analysis is carried out in ten dimensions and makes crucial use of pure spinor techniques. This formalism can accommodate non-Kähler manifolds as well as compactifications with flux, in addition to the usual Calabi-Yau case. At the attractor point, the charges fix the moduli according to f k = Im (CΦ), where Φ is a pure spinor of odd (even) chirality in IIB (A). For IIB on a Calabi-Yau, Φ = Ω and the equation reduces to the usual one. Methods in generalized complex geometry can be used to study solutions to the attractor equation.

Abstract:We compute the masses of all moduli in the unstable deSitter vacua arising in the toy model of cosmological M-theory flux compactifications on the G 2 holonomy manifolds of [1]. The slow-roll parameters in the tachyonic directions are shown to be too large to be useful for conventional models of inflation. However, it appears that we can find fast roll regimes which could, under certain conditions, account for the current dark energy driven accelerated expansion of the universe. *