International Journal of Modern Physics A

In this work, we review recent work on string cosmology. The need for an inflationary era is well known. Problems of Standard Cosmology such as horizon, flatness, monopole and entropy find an elegant solution in the inflationary scenario. On the other hand no adequate inflationary model has been constructed so far. In this review we discuss the attempts that have been made in the field of string theory for obtaining an adequate Cosmological Inflationary Epoch. In particular, orbifold compactifications of string theory which are constrained by target-space duality symmetry offer as natural candidates for the role of inflatons the orbifold moduli. Orbifold moduli dynamics is very constrained by duality symmetry and offers a concrete framework for discussing Cosmological Inflation. We discuss the resulting cosmology assuming that nonperturbative dynamics generates a moduli potential which respects target-space modular invariance. Various modular forms for the nonperturbative superpotential and Kähler potential which include the absolute modular invariant j(T) besides the Dedekind eta function η(T) are discussed. We also review scale-factor duality and pre-Big-Bang scenarios in which inflation is driven by the kinetic terms of the dilaton modulus. In this context we discuss the problem of graceful exit and review recent attempts for solving the problem of exiting from inflation. The possibility of obtaining inflation through the D-terms in string models with anomalous U_A(1) and other Abelian factors is reviewed. In this context we discuss how the slow-roll problem in supergravity models with F-term inflation can be solved by D-term inflation. We also briefly review the consequences of duality for a generalized Heisenberg uncertainty principle and the structure of space–time at short scales. The problem of the Cosmological Constant is also briefly discussed.

Recently,^1–3 it is shown that, the quantum effects of matter are well described by the conformal degree of freedom of the space–time metric. On the other hand, it is a well-known fact that according to Einstein's gravity theory, gravity and geometry are interconnected. In the new quantum gravity theory,^1–3 matter quantum effects completely determine the conformal degree of freedom of the space–time metric, while the causal structure of the space–time is determined by the gravitational effects of the matter, as well as the quantum effects through back reaction effects. This idea, previously, is realized in the framework of scalar–tensor theories. In this work, it is shown that quantum gravity theory can also be realized as a purely metric theory. Such a theory is developed, its consequences and its properties are investigated. The theory is applied, then, to black holes and the radiation-dominated universe. It is shown that the initial singularity can be avoided.

The asymptotic high momentum behavior of quantum field theories with cubic interactions is investigated using renormalization group techniques in the asymmetric limit x≪1. Particular emphasis is paid to theories with interactions involving more than one field where it is found that a matrix renormalization is necessary. Asymptotic scaling forms, in agreement with Regge theory, are derived for the elastic two-particle scattering amplitude and verified in one-loop renormalization group improved perturbation theory, corresponding to the summation of leading logs to all orders. We give explicit forms for the Regge trajectories of different scalar theories in this approximation and determine the signatures.

The heavy quark effective field theory (HQEFT) containing both effective "quark fields" and "antiquark fields" is investigated in detail. By integrating out (but not neglecting) the effective antiquark fields, we present a new formulation of effective theory which differs from the usual heavy quark effective theory (HQET) and exhibits valuable features because of the inclusion of the contributions from the antiquark fields. Matrix elements of vector and axial vector heavy quark currents between pseudoscalar and vector mesons containing a heavy quark (b or c) are then evaluated systematically up to the order of and parametrized by a set of universal form factors. With a consistent normalization condition between the effective heavy hadron states, the form factors at zero recoil are related to the ground state meson masses, which enables us to estimate the values of form factors at zero recoil. In particular, Luke's theorem comes out automatically in the new formulation of HQEFT without the need of imposing the equation of motion . Consequently, the differential decay rates of both B → D*lν and B→Dlν do not receive 1/m_Q order corrections at zero recoil, which is not the case in the usual HQET. Thus we quote that the Cabibbo–Kobayashi–Maskawa matrix element |V_cb| can nicely be extracted from either of these two exclusive semileptonic decays at the order of . Our estimates for |V_cb| are presented.