Cyclic Universe: Big-Bang ad infinitum

In the last article (Inflation: Evidence), we looked at the most-commonly supported hypothesis to explain the Big-Bang and how it shaped the modern Universe. In this article we'll be looking at cyclic cosmology. An adjunct to inflation, this model proposes that, rather than expanding faster than light soon after the Big-Bang… the Big-Bang was actually preceded by another Big-Bang. When a universe ends, it’s followed by another Big-Bang, and so on for infinity (hence the term "cycle"). So the Universe is, in a sense, eternal. Although originally formed as an alternative to inflation, some cyclic models actually combine this idea of a cyclic Universe with inflation.

Models

Some examples of the different mathematical variants of the cyclic universe hypothesis includes:

Baum-Frampton Model

This model says that the Universe will continue to expand at an accelerating rate. But less than a thousand-trillion-trillionth (10 to the negative 27^th power) of a second before the would-be Big-Rip (tearing of the fabric of space-time), the Universe starts contracting. After contracting to a point, it will undergo another Big-Bang (or, technically, a Big-Bounce in this case), powered by inflation, which will split our current Universe apart into many small Universes, each ready to start expanding again.

It solves the entropy problem (see Cyclic Cosmology: Evidence) as the near-maximum entropy is divided between the many Universes, so our individual Universe therefore has low entropy. As each Universe contracts, there's no matter or other energy in it except for dark energy, so there is almost no entropy. During inflation, entropy is gradually restored (from the theoretical inflaton field becoming excited and disordered). 

Many cyclic models predict a "Big Bounce", where the Universe contracts to a point then "bounces" back.

However, the likelihood of the model has been called into question, as the Hubble length (13.72 billion light-years) in the model becomes infinite by the time the Universe finishes expanding and begins contracting. 

Aguirre-Gratton Model

A unique model, this one says that inflation will not only continue infinitely into the future, but continued infinitely into the past as well. So it’s a special type of model that believes the Big-Bang began with an infinitely contracting Universe till it got to a point 13.72 billion years ago, at which point it began expanding.

A diagram of de Sitter space-time (vacuum solution of how space is structured, according to General Relativity). To summarize, if one assumes this space to be divided into 2 regions that can't interact with each other... inflation can extend infinitely into the past.

Entropy increases as energy is distributed. And since the Aguirre-Gratton Model says that the Universe was contracting until 13.72 billion years ago, the energy was coming into contact with each other, rather than being distributed. So this model manages to get around the entropy problem by saying that we originally went from a state of high energy to low energy, and now we're going towards high energy again.

Whilst this model is mathematically-possible, the authors themselves admit it is very based on General Relativity, and does not really take into account quantum effects.

Conformal Cyclic Cosmology

This hypothesis says that the Universe will continue to expand exponentially and the heat will dissipate until it becomes too cold for matter to exist. The matter will convert into light (E=mc squared), and with no matter there is no measure of time (the scientific definition of time isn't exactly like our common understanding of time). With no measure of time, there’s nothing to differentiate the future from the past. Thus, another Big-Bang occurs, ad infinitum.

It solves the entropy problem by saying that a certain amount of gravitational radiation carries over from the death of one Universe to the birth of another, but is not distributed-evenly immediately, providing a low amount of entropy every cycle.

Loop Quantum Cosmology

Based on the theoretical field of loop quantum gravity, which assumes that space-time is made up of fundamental units on the smallest scales (kind of like how matter is made up of atoms on the smallest scales), this hypothesis tries to solve the problem of the singularity by saying that the Universe can’t be constrained to a single point. Rather, after expanding for a time then contracting, it will stop at the Planck length (the smallest length physically-possible).

This is because loop quantum gravity predicts (based on quantum geometry) a new repulsive force. At large sizes, this force’s effects are negligible. However, at Planck length, it is magnified to such levels that it overcomes the gravitational strength of the singularity and repels the Universe, forming a “quantum bounce” between Universal end and rebirth. However, it also predicts an inflationary period. So it’s both cyclic and inflationary.

Loop quantum cosmology gets around the entropy problem by taking into account "horizon" entropy. We mentioned in the last article that a cosmological horizon is the furthest distance at which you can receive information (or to put simply, to which we could possibly see). Horizons have their own measure of entropy, which's proportional to their size.

The equations of loop quantum cosmology say that near the bounce, the horizon engulfs the Universe, and the entropy increases to great heights. However, the horizon disappears after the bounce, thereby eliminating the entropy. So, in a sense, the entropy is reset at the Big Bounce, so it never goes below 0.

Other Models

These models, whilst being plausible, are largely-based in mathematics and are difficult and/or uninteresting to describe in simple words. They are:

• Carroll-Chen Model
• Steinhardt-Turok (Ekpyrotic) Model

Interesting: A 3^rd less-popular hypothesis to explain the Big-Bang other than inflationary and cyclic cosmology exists, called the “varying speed of light” hypothesis. It says the speed of light was faster in the past than it is now, thus solving the horizon problem by saying that information came into contact in the past. However, as this doesn't solve any of the other problems in cosmology (and seems to go against relativity), it is not part of mainstream physics.

Acknowledgements

I would, once again, like to thank Hasan Mohammad for co-authoring this article. You can read his own science-related blog here: http://hasansthoughts.blogspot.com.au/