Independent Research on Fundamental Astrophysics

# ASFYON

Asfyon was established in 1988 by a group of researchers and teachers.

Founder: Dr Reinoud Jan Slagter [ member of EPS Hyperspace and IOP ]

## The most prominent research domains are now:

## *** dark-energy and cosmic strings

## ** large extra dimensions and warped spacetimes

## ** causality issues

A self-gravitating cosmic string is one of the few interesting solutions of compact objects of general relativy (GR) theory, just like the famous Kerr black hole solution. This stringlike solution is a nice example of the interaction of a quantum field, i.e., the abelian Higgs field, with GR. This U(1) scalar-gauge field serves as source in the Einstein field equations.

Although cosmic strings are theoretically predicted in 4D general relativistic models, there is still no observational evidence of their existence. From recent observations of the cosmic microwave background it is concluded that these cosmic strings cannot provide a satisfactory explanation for the bulk of density perturbations. They even would not survive inflation.

It was conjectured that only in a 5D warped brane world model there will be observable imprint of these so-called cosmic superstrings on the induced effective 4D brane metric for values of the symmetry breaking scale larger than the GUT values. The warp factor makes these strings consistent with the empirical bounds. In a time-dependent setting, it seems that there is wave-like energy-momentum transfer to infinity on the brane, a high-energy brane world behavior. Evidence of this information from the bulk geometry could be found in the gravitational cosmic background radiation via gravitational wave energy momentum affecting the brane evolution. Fluctuations of the brane when there is a U(1) gauge field present, are comparable with the proposed brane tension fluctuations, or branons, whose relic abundance can be a dark matter candidate. The critical distinction between dispersion of the scalar waves and singular behavior fade away when a time dependent warp factor is present. The spacetime around a spinning cosmic string shows additionally un-physical behavior, i.e., it would possess closed time like curves (CTC). This controversy with Hawking's chronology protection conjecture persists till today, at least for some physicists. In the stationary case, it was recognized that these so-called Gott (2+1) dimensional spacetimes violate physical boundary conditions. On a time dependent non-diagonal spacetime, complications arise, because one no longer can consider the cosmic string as infinitely thin. In contrast whit the Kerr solution, where the CTC is hidden behind the horizon, it turns out that when one approaches the core of the string to experience the CTC, the proper time on the core stops flowing. The warped 5D counterpart model of the Gott (3+1)-dimensional spacetime possesses CTC's without super-luminal behavior in contrast with the 4D model, where the effective one-particle generator of the CTC has a tachyonic center of mass. So

once again, one encounters causal obstructions.