1. S. R. Das, Mod. Phys. Lett. A20, 2101 (2005)
2. B. Craps, Big Bang models in string theory, arXiv:hep-th/0605199
3. M Gasperini and G. Veneziano, Phys. Rep. 373, 1 (2003), arXiv:hep-th/0207130
4. H Liu, G W Moore and N Seiberg, The challenging cosmic singularity, arXiv:gr-qc/0301001
5. E Silverstein, Singularities and closed string tachyons, arXiv:hep-th/0602230