CYCLIC MODEL





Cyclic model refers to several cosmological models in which the universe follows infinite, self-sustaining cycles (for example: an eternity of Big Bang-Big crunches).



Overview
In the 1930s, theoretical physicists, most notably Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the Big Bang. However, work by Richard C. Tolman showed that these early attempts failed because of the entropy problem that, in statistical mechanics, entropy only increases because of the Second law of thermodynamics. This implies that successive cycles grow longer and larger. Extrapolating back in time, cycles before the present one become shorter and smaller culminating again in a Big Bang and thus not replacing it. This puzzling situation remained for many decades until the early 21st century when the recently discovered dark energy component provided new hope for a consistent cyclic cosmology.[citation needed]
One new 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 of Princeton University and Neil Turok of Cambridge University. The theory describes a universe exploding into existence not just once, but repeatedly over time.
The theory could potentially explain why a mysterious repulsive form of energy known as the "cosmological constant", and which is accelerating the expansion of the universe, is several orders of magnitude smaller than predicted by the standard Big Bang model.[citation needed]
A different cyclic model relying on the notion of phantom energy was proposed in 2007 by Lauris Baum and Paul Frampton of the University of North Carolina at Chapel Hill.[citation needed] Another cyclic model, one in which time itself is cyclic, was proposed by Peter Lynds in 2007.
[edit] The Steinhardt-Turok model
In this 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.[citation needed]
As Richard C. Tolman showed, the earlier cyclic model failed because the universe would undergo inevitable thermodynamic heat death. However, the newer 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. [1]
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.[citation needed]
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.[citation needed]
[edit] The Baum-Frampton model
This more recent cyclic model of 2007 makes a different technical assumption concerning the equation of state of the dark energy which relates pressure and density through a parameter w. It assumes w < -1 throughout a cycle, including at present. (By contrast, Steinhardt-Turok assume w is never less than -1.) In the Baum-Frampton model, a trillion-trillionth (or less) of a second before the would-be Big Rip a turnaround occurs and only one causal patch is retained as our universe. The generic patch contains no quark, lepton or force carrier only dark energy and its entropy thereby vanishes. The adiabatic process of contraction of this much smaller universe takes place with constant vanishing entropy and with no matter including no black holes which disintegrated before turnaround.[citation needed]
The idea that the universe "comes back empty" is a central new idea of this cyclic model, and avoids many difficulties confronting matter in a contracting phase such as excessive structure formation, proliferation and expansion of black holes, as well as going through phase transitions such as those of QCD and electroweak symmetry restoration. Any of these would tend strongly to produce an unwanted premature bounce, simply to avoid violation of the second law of thermodynamics. The surprising w < -1 condition may be logically inevitable in a truly infinitely cyclic cosmology because of the entropy problem. Nevertheless, many technical back up calculations are necessary to confirm consistency of the approach. Although the model borrows ideas from string theory, it is not necessarily committed to strings, or to higher dimensions, yet such speculative devices may provide the most expeditious methods to investigate the internal consistency. The value of w in the Baum-Frampton model can be made arbitrarily close to, but must be less than, -1.[citation needed]
[edit] Lynds Cyclic Time model
Peter Lynds has proposed a model in which time is cyclic, and the universe repeats exactly an infinite number of times. Because it is the exact same cycle that repeats, however, it can also be interpreted as taking place just once. Lynds argues that his model addresses a number of problems in cosmology that other models are unable to, including the question of the origin of the universe. A key feature of Lynds' model is his treatment of thermodynamic time reversal. Rather than the second law of thermodynamics possibly be violated and entropy decrease, the order of events reverse so that entropy can continue to increase. As the laws of physics are time reversible (with the exception of the second law of thermodynamics), Lynds argues that this causes no problems, while also avoiding some. [2] [3]
[edit] Distinguishing models
The Planck Surveyor mission should provide a measurement of w to unprecedented accuracy, discover whether w < -1 or not, and thereby discriminate between the models