INTERPRETATION OF THE SIKLOS SOLUTIONS AS EXACT GRAVITATIONAL-WAVES IN THE ANTI-DE-SITTER UNIVERSE

The Siklos class of solutions of Einstein's field equations is investi gated by analytical methods. By studying the behaviour of free particl es we reach the conclusion that the spacetimes represent exact gravita tional waves propagating in the anti-de Sitter universe. The presence of a negative cosmological constant implies that the 'background' spac e is not asymptotically Rat and requires 'rotating' reference frames i n order to fully simplify and view the behaviour of nearby test partic les. The Kaigorodov spacetime, which is the simplest representative of the Siklos class, is analysed in more detail. It is argued that it ma y serve as a 'cosmological' analogue of the well known homogeneous pp- waves in the flat universe.