Comments on a class of similarity solutions of Einstein’s equations relevant to the early Universe

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Abstract
Comments are made on a class of similarity solutions in general relativity that are spherically symmetric and have a negative pressure as required to account for the matter production of certain current models of the early Universe. This class of solutions is defined by one fairly simple ordinary differential equation. This equation was first derived by Henriksen, Emslie, and Wesson [Phys. Rev. D 27, 1219 (1983)], who studied similarity of the second type, with a finite cosmological constant Λ. Now, it is pointed out that the same equation is also derived from similarity of the first type, with a zero Λ. The present work extends some of the results found previously and raises new possibilities about how a model Universe may go over from a state with finite Λ to a state with zero Λ.