The Cyclic Universe: An Informal Introduction

TL;DR

This paper proposes a cyclic, non-singular cosmology in which the universe undergoes endless cycles of contraction and expansion with finite $T$ and $\rho$, thereby avoiding a true initial singularity. It shows that a four-dimensional field-theory description with a slowly rolling scalar field can drive the cycle, generate scale-invariant perturbations during slow contraction via an exponential potential, and produce a finite-density bounce through brane-inspired dynamics. Key contributions include a concrete mechanism for a finite crunch–bounce, a brane-world interpretation that regularizes the singularity, and a versatile toolkit (negative potentials, scalar-matter couplings, slow contraction) that reproduces standard predictions while offering new cosmological possibilities. The model yields distinctive observational predictions, notably a vanishingly small amplitude of long-wavelength gravitational waves and testable dark-energy dynamics, making it both compatible with current data and testable with future measurements of the stochastic gravitational-wave background and the time-dependence of $P/\rho$.

Abstract

The Cyclic Model is a radical, new cosmological scenario which proposes that the Universe undergoes an endless sequence of epochs which begin with a `big bang' and end in a `big crunch.' When the Universe bounces from contraction to re-expansion, the temperature and density remain finite. The model does not include a period of rapid inflation, yet it reproduces all of the successful predictions of standard big bang and inflationary cosmology. We point out numerous novel elements that have not been used previously which may open the door to further alternative cosmologies. Although the model is motivated by M-theory, branes and extra-dimensions, here we show that the scenario can be described almost entirely in terms of conventional 4d field theory and 4d cosmology.

Figures (2)

• Figure 1: Schematic plot of the potential $V(\phi)$ as a function of the field $\phi$ for (a) inflationary cosmology and (b) cyclic models. The numbered sequence of stages is described in the article.
• Figure 2: Comparison of the sequence of events in the consensus big bang/inflationary and cyclic pictures. The consensus model assumes that the big bang is a beginning, inflation sets the initial conditions, and then the Universe proceeds through the matter and radiation dominated phases. Dark energy is added to explain the current acceleration, but the long-term future is uncertain. The cyclic model proposes that the Universe undergoes an endless sequences of cycles which begin with a bang and end in crunch. Initial conditions for a given cycle are produced during the dark energy-dominated and contraction phases of the previous cycle. The past and future are determined.