We reanalyze brane inflation with brane-brane interactions at an angle, which include the special case of brane-anti-brane interaction. If nature is described by a stringy realization of the brane world scenario today (with arbitrary compactification), and if some additional branes were present in the early universe, we find that an inflationary epoch is generically quite natural, ending with a big bang when the last branes collide. In an interesting brane inflationary scenario suggested by generic string model-building, we use the density perturbation observed in the cosmic microwave background and the coupling unification to find that the string scale is comparable to the GU T scale.

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Brane inflation in superstring theory predicts that cosmic strings (but not domain walls or monopoles) are produced towards the end of the inflationary epoch. Here, we discuss the production, the spectrum and the evolution of such cosmic strings, properties that differentiate them from those coming from an abelian Higgs model. As D-branes in extra dimensions, some type of cosmic strings will dissolve rapidly in spacetime, while the stable ones appear with a spectrum of cosmic string tensions. Moreover, the presence of the extra dimensions reduces the interaction rate of the cosmic strings in some scenarios, resulting in an order of magnitude enhancement of the number/energy density of the cosmic string network when compared to the field theory case.

We find successful models of D-brane/anti-brane inflation within a string context. We work within the GKP-IKLT class of type IIB string vacua for which many moduli are stabilized through fluxes, as recently modified to include 'realistic' orbifold sectors containing standard-model type particles. We allow all moduli to roll when searching for inflationary solutions and find that inflation is not generic inasmuch as special choices must be made for the parameters describing the vacuum.But given these choices inflation can occur for a reasonably wide range of initial conditions for the brane and antibrane. We find that D-terms associated with the orbifold blowing-up modes play an important role in the inflationary dynamics. Since the models contain a standard-model-like sector after inflation, they open up the possibility of addressing reheating issues. We calculate predictions for the CMB temperature fluctuations and find that these can be consistent with observations, but are generically not deep within the scale-invariant regime and so can allow appreciable values for dn s /d ln k as well as predicting a potentially observable gravity-wave signal.It is also possible to generate some admixture of isocurvature fluctuations.

We investigate the problem of fine tuning of the potential in the IKLIMT warped flux compactification scenario for brane-antibrane inflation in Type IIB string theory. We argue for the importance of an additional parameter ψ0 (approximated as zero by IKLIMT), namely the position of the antibrane, relative to the equilibrium position of the brane in the absence of the antibrane. We show that for a range of values of a particular combination of the Kähler modulus, warp factor, and ψ0, the inflaton potential can be sufficiently flat. We point out a novel mechanism for dynamically achieving flatness within this part of parameter space: the presence of multiple mobile branes can lead to a potential which initially has a metastable local minimum, but gradually becomes flat as some of the branes tunnel out. Eventually the local minimum disappears and the remaining branes slowly roll together, with assisted inflation further enhancing the effective flatness of the potential. With the addition of Kähler and superpotential corrections, this mechanism can completely remove the fine tuning problem of brane inflation, within large regions of parameter space. The model can be falsified if future cosmic microwave background observations confirm the hint of a large running spectral index.