A Cyclic Model of the Universe

We propose a cosmological model in which the universe undergoes an endless sequence of cosmic epochs each beginning with a `bang' and ending in a `crunch.' The temperature and density are finite at each transition from crunch to bang. Instead of having an inflationary epoch, each cycle includes a period of slow accelerated expansion (as recently observed) followed by slow contraction. The combination produces the homogeneity, flatness, density fluctuations and energy needed to begin the next cycle.Comment: 15 pages, 1 figure, revisions as publishe

Beyond Inflation: A Cyclic Universe Scenario

Inflation has been the leading early universe scenario for two decades, and has become an accepted element of the successful `cosmic concordance' model. However, there are many puzzling features of the resulting theory. It requires both high energy and low energy inflation, with energy densities differing by a hundred orders of magnitude. The questions of why the universe started out undergoing high energy inflation, and why it will end up in low energy inflation, are unanswered. Rather than resort to anthropic arguments, we have developed an alternative cosmology, the Cyclic universe, in which the universe exists in a very long-lived attractor state determined by the laws of physics. The model shares inflation's phenomenological successes without requiring an epoch of high energy inflation. Instead, the universe is made homogeneous and flat, and scale-invariant adiabatic perturbations are generated during an epoch of low energy acceleration like that seen today, but preceding the last big bang. Unlike inflation, the model requires low energy acceleration in order for a periodic attractor state to exist. The key challenge facing the scenario is that of passing through the cosmic singularity at t=0. Substantial progress has been made at the level of linearised gravity, which is reviewed here. The challenge of extending this to nonlinear gravity and string theory remains.Comment: 27 pages, 6 figures, talk given at the Nobel Symposium `String Theory and Cosmology', 2003. To appear, Physica Script

Singular Instantons Made Regular

The singularity present in cosmological instantons of the Hawking-Turok type is resolved by a conformal transformation, where the conformal factor has a linear zero of codimension one. We show that if the underlying regular manifold is taken to have the topology of $RP^4$, and the conformal factor is taken to be a twisted field so that the zero is enforced, then one obtains a one-parameter family of solutions of the classical field equations, where the minimal action solution has the conformal zero located on a minimal volume noncontractible $RP^3$ submanifold. For instantons with two singularities, the corresponding topology is that of a cylinder $S^3\times [0,1]$ with D=4 analogues of `cross-caps' at each of the endpoints.Comment: 23 pages, compressed and RevTex file, including nine postscript figure files. Submitted versio

Cosmic Texture from a Broken Global SU(3) Symmetry

We investigate the observable consequences of creating cosmic texture by breaking a global SU(3) symmetry, rather than the SU(2) case which is generally studied. To this end, we study the nonlinear sigma model for a totally broken SU(3) symmetry, and develop a technique for numerically solving the classical field equations. This technique is applied in a cosmological context: the energy of the collapsing SU(3) texture field is used as a gravitational source for the production of perturbations in the primordial fluids of the early universe. From these calculations, we make predictions about the appearance of the anisotropies in the cosmic microwave background radiation (CMBR) which would be present if the large scale structure of the universe was gravitationally seeded by the collapse of SU(3) textures.Comment: 28 pages, latex, 11 figures, submitted to Phys. Rev.

A Causal Source which Mimics Inflation

How unique are the inflationary predictions for the cosmic microwave anisotropy pattern? In this paper, it is asked whether an arbitrary causal source for perturbations in the standard hot big bang could effectively mimic the predictions of the simplest inflationary models. A surprisingly simple example of a `scaling' causal source is found to closely reproduce the inflationary predictions. This letter extends the work of a previous paper (ref. 6) to a full computation of the anisotropy pattern, including the Sachs Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure