1. Bousso, R. (2002). Reviews of Modern Physics 74, 825 [hep-th/0203101].
2. Carniero, S. (2003). Grav. Res. Found. 2003 Special issue of the IJMPD [gr-qc/0305081].
3. Frolov, A., and Kofman, L. (2003). Journal of Cosmology and Astroparticle Physics 0305, 009 [hep-th/0212327].
4. Gerlach, U., and Sengupta(1978) U. Physical Review D18, 1773.
5. Islam, J. (2002) An Introduction to Mathematical Cosmology, 2nd edition, Cambridge University Press, pp. 43–44, Cambridge, U.K.