Gravitational waves in a spatially closed de Sitter spacetime Amir H. Abbassi, J. Khodagholizadeh and Amir M. Abbassi Eur. Phys. J. C, 73 10 (2013) 2592 Published online: 09 October 2013 DOI: 10.1140/epjc/s10052-013-2592-6