Designing Cyclic Universe Models

We study an inflation model with a flat scalar potential supported by observations and find that slow-roll inflation can emerge after a quasi-cyclic phase of the Universe, where it undergoes repeated expansions and contractions for a finite time period. The initial conditions and the positive spatial curvature required for such nontrivial dynamics align with the quantum creation of the Universe. The key ingredients that trigger inflation are dissipative interactions of the in...

We propose a new type of cosmological model in which it is postulated that not only the temperature but also the curvature is limited by the mass scale of the Hagedorn temperature. We find that the big bang of this universe is smoothly connected to the big crunch of the previous universe through a Hagedorn universe, in which the temperature and curvature remain very close to their limiting values. In this way, we obtain the picture of a cyclic universe. By estimating the entr...

We examine the behaviour of a closed oscillating universe filled with a homogeneous scalar field and find that, contrary to naive expectations, such a universe expands to larger volumes during successive expansion epochs. This intriguing behaviour introduces an arrow of time in a system which is time-reversible. The increase in the maximum size of the universe is closely related to the work done on/by the scalar field during one complete oscillatory cycle which, in turn, is r...

We revisit the cyclic Universe scenario in scalar field FRW cosmology and check its applicability for a nonminimally coupled scalar field. We show that for the most popular case of a quartic potential and the standard nonminimal coupling this scenario does work. On the other hand, we identify certain cases where cyclic model fails to work and present corresponding reasons for this.

I give a brief review of the status of research on the nature of initial conditions required to obtain a period of cosmological inflation. It is shown that there is good evidence that in the case of large field models, the inflationary slow-roll trajectory is a local attractor in initial condition space, whereas it is not in the case of small field models.

I will discuss the development of inflationary theory and its present status, as well as some recent attempts to suggest an alternative to inflation. In particular, I will argue that the ekpyrotic scenario in its original form does not solve any of the major cosmological problems. Meanwhile, the cyclic scenario is not an alternative to inflation but rather a complicated version of inflationary theory. This scenario does not solve the flatness and entropy problems, and it suff...

We discuss how a cyclic model for the flat universe can be constructively derived from Loop Quantum Gravity. This model has a lower bounce, at small values of the scale factor, which shares many similarities with that of Loop Quantum Cosmology. We find that quantum gravity corrections can be also relevant at energy densities much smaller than the Planckian one and that they can induce an upper bounce at large values of the scale factor.

We present analytic solutions to a class of cosmological models described by a canonical scalar field minimally coupled to gravity and experiencing self interactions through a hyperbolic potential. Using models and methods inspired by 2T-physics, we show how analytic solutions can be obtained in flat/open/closed Friedmann-Robertson-Walker universes. Among the analytic solutions, there are many interesting geodesically complete cyclic solutions in which the universe bounces at...

Being able to reliably track perturbations across bounces and turnarounds in cyclic and bouncing cosmology lies at the heart of being able to compare the predictions of these models with the Cosmic Microwave Background observations. This has been a challenging task due to the unknown nature of the physics involved during the bounce as well as the technical challenge of matching perturbations precisely between the expansion and contraction phases. In this paper, we will presen...

In this work, we investigate how a smooth transition from a constant-roll to a slow-roll inflationary era may be realized, in the context of a canonical scalar field theory. We study in some detail the dynamical evolution of the cosmological system, and we investigate whether a stable attractor exists, both numerically and analytically. We also investigate the slow-roll era and as we demonstrate, partially compatibility of the resulting scalar theory, may be achieved, with th...