S.-H. Henry Tye

Construction of Fermionic String Models in Four-Dimensions

The construction of four-dimensional closed fermionic string models is discussed. The approach is based on a fermionic formulation of all internal (i.e. toroidally compactified) coordinates. Modular invariance, world sheet supersymmetry, (super)conformal invariance and proper space-time spin-statistics impose stringent constraints on the model building. Using these constraints on the boundary conditions (spin structure) of the world sheet fermions, we obtain a simple set of rules for constructing ultraviolet-finite closed fermionic string models. For a large subclass of these models, this “spin structure” construction can be related to bosonic constructions via the fermionic charge lattice. These charge lattices are odd lorentzian self-dual lattices shifted by a fixed. vector and form a nontrivial generalization of the lorentzian self-dual even-integer lattices considered by Narain. In particular, four-dimensional models with N = 4, N = 2, and N = 1 supersymmetry as well as non-supersymmetric tachyon-free chiral models can easily be constructed. Some models may be interpreted as charge lattices moded by discrete symmetries – in particular Z 2 type orbifolds. String interactions and other related issues are also discussed.

Cosmic string production towards the end of brane inflation

Abstract Towards the end of the brane inflationary epoch in the brane world, cosmic strings (but not texture, domain walls or monopoles) are copiously produced during brane collision. These cosmic strings are Dp-branes with (p−1) dimensions compactified. We elaborate on this prediction of the superstring theory description of the brane world. Using the data on the temperature anisotropy in the cosmic microwave background, we estimate the cosmic string tension μ to be around Gμ≃10−7. This in turn implies that the anisotropy in the cosmic microwave background comes mostly from inflation, but with a component (of order 10%) from cosmic strings. This cosmic string effect should also be observable in gravitational wave detectors and maybe even pulsar timing measurements.

TeV scale superstring and extra dimensions

Utilizing the idea of extra large dimensions, it has been suggested that the gauge and gravity couplings unification can happen at a scale as low as 1 TeV. In this paper, we explore this phenomenological possibility within string theory. In particular, we discuss how the proton decay bound can be satisfied in Type I string theory. The string picture also suggests different scenarios of gauge and gravitational couplings unification. The various scenarios are explicitly illustrated with a specific 4-dimensional N=1 supersymmetric chiral Type I string model with Pati-Salam-like gauge symmetry. We point out certain features that should be generic in other Type I strings.

Brane Interaction as the Origin of Inflation

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 GUT scale.

Closer towards inflation in string theory

In brane inflation, the relative brane position in the bulk of a brane world is the inflaton. For branes moving in a compact manifold, the approximate translational (or shift) symmetry is necessary to suppress the inflaton mass, which then allows a slow-roll phase for enough inflation. Following recent works, we discuss how inflation may be achieved in superstring theory. Imposing the shift symmetry, we obtain the condition on the superpotential needed for inflation and suggest how this condition may be naturally satisfied.

The production, spectrum and evolution of cosmic strings in brane inflation

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.

Cosmological expansion in the Randall-Sundrum brane world scenario

The cosmology of the Randall-Sundrum scenario for a positive tension brane in a 5D universe with localized gravity has been studied previously. In the radiation-dominated universe, it was suggested that there are two solutions for the cosmic scale factor

Some aspects of brane inflation

a(t):

Dual identities inside the gluon and the graviton scattering amplitudes

the standard solution

Type IIB Orientifolds with NS-NS Antisymmetric Tensor Backgrounds

a\ensuremath{\sim}{t}^{1/2},