The opening words of the book of Genesis make a declarative statement. God created Heaven and Earth, and so begins cosmic history. Although not all creation myths have a beginning act, most do. Humans are storytellers, and we like stories with a beginning. This original need is deeply rooted in us and is even part of our scientific worldview. As we say so often in science, effects have causes. This process of cause and effect is a powerful tool for understanding the world around us, but it’s not without its problems, especially with the origin of the universe.
In the standard model of cosmology, the universe begins with a big bang. A first warm and dense state from which our modern universe comes. Technically, this means we can trace cosmic history back to a big bang. But if effects have causes, then what caused the big bang? What existed before the universe? The short answer is that we don’t know.
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For some, this leaves the door open to a divine creator. A large pumpkin that lies outside of time and space. But it could also be asking what existed before the universe is a nonsensical question, like asking what is north of the north pole. Although effects in the universe have causes, the universe as a whole may not have or need them. But this answer does not seem very satisfactory.
There is, however, an alternative. This is called the cyclic universe model, and it holds that our current observable universe is one of a series of universes. Our universe is currently expanding because of the Hubble parameter, and if this parameter remains constant, it will expand forever. But if Hubble’s parameter is cyclical, then the universe will start to contract at some point in the future. It will contract until it reaches a new hot and dense state. A new big bang for a new universe.
The wheel of time turns and the ages pass…
A cyclic universe does not need a beginning. It always has been, always is and always will be. But the model is not without its problems. One of the main ones is the entropy problem. Entropy is a measure of disorder in a system, and according to the laws of thermodynamics, it can never decrease. In a simple cyclic universe model, the entropy of a given universe must be at least a little higher than its parent universe. So if the universes cycle to an infinite past, the current universe would have infinite entropy, which it does not. So there must have been an initial universe with low entropy, and we’re back to the beginning.
There is a way around this problem. The universe could have a global scale factor. If this scale factor increases with each universe, then the entropy problem disappears. Interestingly, this scale factor is conformally invariant. That means it doesn’t change how the universe appears. Each universe may be twice as large as the previous one, but everything in that universe evolves by the same factor. If you double the income of everyone on the planet, but also the cost of everything, then nothing has really changed.
This conformally invariant scale factor allowed the cyclic universe to exist beginningless. Every universe has a cause, and they’re turtles all the way. But a new study has found a flaw in that idea. The team examined the mathematical structure of cyclic universe models within general relativity and found that they are all geodetically past-incomplete. In other words, within the limits of general relativity, you cannot trace a universe like ours through an infinite cycle of universes. There may have been many universes before ours, but there must still have been a first universe.
There is no beginning or end to the Wheel of Time. But it was a start.
Thus, the cyclic universe model can provide a cause for our universe, but that just solves the problem of beginnings on the road. Even though our universe was not the first, some universes were. At least for the usual cyclic universe models. As the authors point out, their work does not apply to the conformal cyclic cosmology proposed by Roger Penrose. In this model, the scale of each universe is infinitely larger than the previous cycle. The authors plan to examine this model next.
And so begins a story for another time.
Reference: Kinney, William H., and Nina K. Stein. “Cyclic Cosmology and Geodetic Completeness.” Journal of Cosmology and Astroparticle Physics 2022.06 (2022): 011.