Warped product metrics with rank-1 ricci tensor

Locally 2 + 2 warped product metrics with a Lorentzian base and Riemannian fiber are described in Newman-Penrose formalism. It is shown that solutions with rank- Ricci tensor are characterized by ?11 +3A = 0 and ?00 ?22 =(?11 - 3A)23. Integrability conditions for the Bianchi identities are studied and it is proved that the sources are null Einstein-Maxwell fields, massless scalar fields or stiff matter fields corresponding respectively to = 0, > 0 or < 0. © 2008 EAS EDP Sciences

Can An Unruh Detector Feel A Cosmic String?

. Unruh's detector calculation is used to study the effect of the defect angle fi in a space-time with a cosmic string for both the excitation and deexcitation cases. It is found that a rotating detector results in a non-zero effect for both finite (small) and infinite (large) time. I. INTRODUCTION Different aspects of cosmic strings are studied in many papers and reviews. One can give References 1,2,3 as a good point to start learning about this ever developing field. Among new physical processes where the effects of cosmic strings are studied one can cite references 4 and 5. Here stimulated and spontaneous emission near cosmic strings are studied. The presence of the cosmic string gives rise to modifications in the rates of these processes. Here we do the similar calculation as in these references for different physical processes, using the model of a particle detection due to Unruh =6 and De Witt =7 . Section II is devoted to the review of the method and the results already ..