Massless Monopoles and Multipronged Strings

We investigate the role of massless magnetic monopoles in the N=4 supersymmetric Yang-Mills Higgs theories. They can appear naturally in the 1/4-BPS dyonic configurations associated with multi-pronged string configurations. Massless magnetic monopoles can carry nonabelian electric charge when their associated gauge symmetry is unbroken. Surprisingly, massless monopoles can also appear even when the gauge symmetry is broken to abelian subgroups.Comment: 10 pages, LaTex file, more comments added. (To appear in Phys. Lett. B

Entropy Production during Asymptotically Safe Inflation

The Asymptotic Safety scenario predicts that the deep ultraviolet of Quantum Einstein Gravity is governed by a nontrivial renormalization group fixed point. Analyzing its implications for cosmology using renormalization group improved Einstein equations we find that it can give rise to a phase of inflationary expansion in the early Universe. Inflation is a pure quantum effect here and requires no inflaton field. It is driven by the cosmological constant and ends automatically when the renormalization group evolution has reduced the vacuum energy to the level of the matter energy density. The quantum gravity effects also provide a natural mechanism for the generation of entropy. It could easily account for the entire entropy of the present Universe in the massless sector.Comment: 17 pages, 4 figures, Invited contribution to the special issue of Entropy on "Entropy in Quantum Gravity

Nucleon-Nucleon Scattering and Effective Field Theory: Including Pions Non-perturbatively

Next to leading order effective field theory calculations are performed for ${}^1S_0$ NN scattering using subtractive renormalization procedure. One pion exchange and contact interaction potentials are iterated using Lippman-Schwinger equation. Satisfactory fit to the Nijmegen data is obtained for the momenta up to 300 MeV in the centre of mass frame. Phase shifts are also compared with the results of KSW approach where pions are included perturbatively.Comment: 7 pages, 3 figures, references added, to appear in Phys. Lett.

Six-dimensional Methods for Four-dimensional Conformal Field Theories

The calculation of both spinor and tensor Green's functions in four-dimensional conformally invariant field theories can be greatly simplified by six-dimensional methods. For this purpose, four-dimensional fields are constructed as projections of fields on the hypercone in six-dimensional projective space, satisfying certain transversality conditions. In this way some Green's functions in conformal field theories are shown to have structures more general than those commonly found by use of the inversion operator. These methods fit in well with the assumption of AdS/CFT duality. In particular, it is transparent that if fields on AdS$_5$ approach finite limits on the boundary of AdS$_5$, then in the conformal field theory on this boundary these limits transform with conformal dimensionality zero if they are tensors (of any rank), but with conformal dimension 1/2 if they are spinors or spinor-tensors.Comment: Version accepted for publication in Physical Review D. References to earlier work added in footnote 2. Minor errors corrected. 24 page

Dark Energy and the Nature of the Graviton

Does the existence of dark energy suggest that there is more to the graviton than we think we know?Comment: 4 page