Cyclic model
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The cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. It was proposed in 2001 by Paul Steinhardt and Neil Turok.
The model
In the cyclic model, two parallel orbifold planes or M-branes collide periodically in a higher dimensional space. The visible four-dimensional universe lies on one of these branes. The collisions correspond to a reversal from contraction to expansion, or a big crunch followed immediately by a big bang. The matter and radiation we see today were generated during the most recent collision in a pattern dictated by quantum fluctuations created before the branes. Eventually, the universe reached the state we observe today, before beginning to contract again many billions of years in the future. Dark energy corresponds to a force between the branes, and serves the crucial role of solving the monopole, horizon, and flatness problems. Moreover the cycles can continue indefinitely into the past and the future, and the solution is an attractor, so it can provide a complete history of the universe.
An earlier cyclic model of Richard Tolman failed because the universe would undergo inevitable thermodynamic heat death. However, the cyclic model evades this by having a net expansion each cycle, preventing entropy from building up. However, there are major problems with the model. Foremost among them is that colliding branes are not understood by string theorists, and nobody knows if the scale invariant spectrum will be destroyed by the big crunch, or even what happens when two branes collide. Moreover, like cosmic inflation, while the general character of the forces (in the ekpyrotic scenario, a force between branes) required to create the vacuum fluctuations is known, there is no candidate from particle physics. Moreover, the scenario uses some essential ideas from string theory, principally extra dimensions, branes and orbifolds. String theory itself is a controversial idea in physics.
Originally, ekpyrotic models described two branes separated along a fifth dimension which collide once. Crucially, both the ekpyrotic and cyclic models create the fluctuations we observe today in a contracting "ekpyrotic" phase. However, in the ekpyrotic model, while a future collision with a different brane could conceivably happen in the future, ending our epoch in a conflagration, this happens randomly, not periodically. There were problems with the old ekpyrotic picture having to do with the very special, nearly supersymmetric initial state required in order to end up with a nearly homogeneous universe: the problems solved by cosmic inflation, such as the monopole, flatness and homogeneity problems were shifted to a set of fine-tuned initial conditions. The ekpyrotic picture was not connected to the issue of dark energy.
There are other technical differences having to do with the nature of the branes. For example, in the ekpyrotic model, they are D-branes; while in the cyclic model, they are orbifold planes.
External links
- Paul J. Steinhardt, Department of Physics, Princeton University http://feynman.princeton.edu/~steinh/