All the previously described Multiverse models were considering other Universes being located ‘somewhere’. The Quilted Multiverse model relies upon the infinite Universe with countless worlds similar to one another repeating through space, the Inflationary Multiverse model depicts endless inflation producing countless bubbles each of those becoming a separate Universe and the Brane model gives its own spin to this saying that all the other Universes might well be located millimeters apart from our own but in other dimensions within a higher dimensional space called the “bulk”.
You might have wondered why all these models are giving thought to various types of Multiverse taking into account only spatial dimensions – after all, Einstein taught us that time is just another dimension kind of similar to spatial ones. If you did that you were headed in the right direction, other Universes may indeed be ‘found’ in the past and in the future as well. And this time we are going to consider such a model which is referred to as the Cyclic Multiverse or Oscillating Multiverse. As we are going to see, the Brane Multiverse that has been addressed in the previous article is well connected to this model, which is why they both were described by Brian Greene in his book “The Hidden Reality” under the same chapter.
The cyclic idea has quite a long history. Rotation of the Earth changing day by night and vice versa and motion of our planet about the Sun causing season changes was the reason behind the earliest cyclic models arisen in various cultures in their attempts to understand the Universe. For example, one of the ancient prescientific cosmological institutes – Hinduism was considering the Universe as a complex combination of cosmological cycles lasting, according to various interpretations, millions to trillions years each. Western philosophers – among whom Heraclitus and Cicero – developed various cyclic cosmological theories. Thoughts about dying in fire Universe which then comes into existence again was very popular among various philosophers at that time. With thriving Christianity the concept of the birth of our Universe through a single one-shot act gradually became dominant, but various cyclic models were still sometimes taken into account.
The cyclic model appeared in XX century physics when Alexander Friedmann showed his mathematical solutions to Einstein’s General Relativity equations. According to these solutions expanding Universe might have reached its maximum size and started to contract eventually collapsing to singularity. This singularity then again underwent Big Bang giving birth to another Universe which again expanded up to a certain point and then collapsed again and so on. All of these Universes may have well been home to intelligent living creatures whose civilizations may have been far more powerful than mankind. Nevertheless, they all passed away when their Universe collapsed on itself obliterating any complex structures and any life. Alexander Friedmann dubbed this model Oscillating Universe. When Edwin Hubble and other researchers determined that the Universe is indeed expanding even Einstein himself worked on the cyclic scenario, but the most detailed research on this was done by Richard Tolman in 1930s.
The cyclic model is highly regarded among many physicists partly because it allows them to avoid the question about the origin of the Universe. Indeed, if these cycles change one another for the infinite period of time the very question about the origin loses its applicability. The start of each cycle is caused by the end of the previous one, but according to the model this process lasts forever, whence there is no cycle which was the first.
However, in 1950s Dutch physicist Herman Zanstra showed that earlier works by Tolman led to a contradiction which had not been taken into account before. The problem is in the 2nd law of Thermodynamics which is saying that the entropy of any given system can in no way stay constant; it instead always increases with time. The value of entropy determines how long a given cycle would last. The higher the value of entropy the more erratically moving particles will be squeezed in the process of collapse which will lead to a more intense bang afterwards. Hence, the following Universe would expand more rapidly and the cycle will therefore last longer than the previous one. In this sense, each previous cycle must be shorter; thus we must conclude that in a certain moment in the past there should have been a starting cycle.
This is where the Brane Multiverse scenario comes into play. It was first proposed by Paul Steinhardt, Neil Turok and their colleagues. Their concept is based on the Brane scenario and, as they claim, escapes the trap with the 2nd law of Thermodynamics. Imagine what would happen if two brane worlds were to collide with each other. Although no one knows for sure it is obvious that such a collision would be catastrophic. If two Universes were to collide the amount of energy this collision would produce will certainly heat both of them enormously devastating all complex structures within them. And what’s even more interesting this event would mimic Big Bang. Indeed, after such a collision the Branes will behave exactly the same as our Universe after the Big Bang, they will expand and cool down leading to formation of new galaxies, new stars, and perhaps new forms of life. Moreover, as Steinhardt and colleagues have shown after such a catastrophic crash the Branes would not stick together but rebound of each other and as you might remember from the previous article gravitons from one Brane can be transmitted to the other, thus these two branes would gravitationally attract one another. In this way relative motion of the two would slow down, they would reach the maximum distance and start to move towards each other eventually leading to a new collision. Steinhardt and colleagues estimated the period of a cycle to last approximately trillion years.
This cyclic model gives a solution to entropy conundrum postulating that cycles arise not because the Universe itself is expanding, collapsing and then again expanding, but because the space between two branes is expanding and contracting. This way the overall entropy always increases in accordance with the 2nd law of Thermodynamics as long as Branes themselves always expand, thus it can in no way lead us to a contradiction. What’s decreasing is entropy density. At the end of each cycle entropy density is so low that the conditions on both Branes are similar to the conditions at the start of this cycle that is why these cycles can last forever. Brane Cyclic Multiverse need not have the start.