For a technical introduction see Misner, Kip S. Thorne and John Archibald Wheeler, Gravitation, Ch. 34, esp. 34.4, 34.6; see also R. Penrose, "Gravitational Collapse and Space-Time Singularities," Phys. Rev. Lett. 14 (1965): 57-59, S. W. Hawking and R. Penrose, "The Singularities of Gravitational Collapse and Cosmology," Proc. R. Soc. London A 314 (1969): 529-48; Stephen Hawking and George F. R. Ellis, The Large Scale Structure of Space-Time, Cambridge Monographs on Mathematical Physics Series (Cambridge: Cambridge University Press, 1973). These theorems only apply if certain reasonable conditions hold for the model, namely ρ+3p/c>0. These conditions are violated in many inflationary models (see below).