Was There a Beginning

TL;DR: In this paper, the authors construct nonsingular cyclic cosmologies that respect the null energy condition, have a large hierarchy between the minimum and maximum size of the universe, and are stable under linearized fluctuations and argue that Casimir energy contributions associated to the compact spatial slices can become important at short scales and may lift nonperturbative decays towards vanishing size.

TL;DR: The question of whether the multiverse is past-eternal or if there was a beginning was discussed by Bousso and Vilenkin this paper, and given three people, there were three answers.

TL;DR: In this article, the authors considered two different types of the tachyonic potential, one having only a quartic term and the other having both the quartic and quadratic terms, and showed that there exist several kinds of new O(4)-symmetric Fubini instanton solution, which are possible only if gravity is taken into account.

TL;DR: In this paper, the fundamental parameters of a theory are probabilistically localised around the critical value and the universe finds itself at the edge of a phase transition, which is called self-organised localisation.

TL;DR: In this paper, the authors discuss three candidate scenarios which seem to allow the possibility that the universe could have existed forever with no initial singularity: eternal infation, cyclic evolution, and the emergent universe.

TL;DR: In this article, Harlow, Shenker, and Stanford explain how the tree-like structure of eternal inflation, together with the existence of terminal vacua, leads to an arrow-of-time.

TL;DR: In this article, the authors introduce a simple discrete stochastic model of eternal inflation that shares many of the most important features of the continuum theory as it is now understood, and apply the model to the problem of the cosmological constant.

TL;DR: Conditions are formulated for a spacetime to describe an eternally inflating universe without a beginning, and it is shown that these conditions cannot be satisfied and a rigorous proof is given for two-dimensional spacetime and a plausibility argument for four dimensions.