The cyclic model of the universe
By Nida Ghani
What is an ending?
In general , our brains comprehend it to be a finite stopping point. A conclusive dead-end . This definition, however, doesn’t fit into every situation. For example: when a chapter in a book ends, it doesn’t mean that we are truly stopping our intake of content. No, we are just Transitioning to a different chapter where we continue our reading journey. What if our universe was like this? An entity that does not necessarily die but rather emerges as a phoenix from its own ashes. This is a far-fetched theory without any doubt but the most agreed upon alternative would be permanent death, trillions of years into the future, in a universe devoid of any light. The former hypothesis glows in the backdrop of such a sinister future but that obviously was not the reason why humanity’s research on this has progressed. The real reason was to comprehend what happened before the beginning.
Confused yet? Don’t worry, everybody is.
The question that has mocked cosmologists in their face. “Open or Close?”
There are multiple ways to predict the end of the universe but the two paths that lay the foundation for more are The Closed and Open universe models.
A closed universe can be considered as one that eventually stops expanding at some point. The Big Bang started off as an infinitely dense point or a singularity that had an extremely high temperature. As the universe started expanding , due to dark energy, the temperature started cooling down. According to this model, the universe will eventually cool down to such an extent where the expansion as a whole will stop taking place and the effects of gravity will surpass that of dark energy. This , in accordance with Einstein’s general relativity, will lead to the universe contracting in on itself. The large gravitational attraction between celestial objects will be extremely strong causing the objects to be pulled in together and leading the average density of the cosmos to be greater than the critical density. Eventually, the gravitational attraction will be so high that the universe will collapse in on itself and drag everything into a highly dense point. This hypothesis is known as the “Big Crunch” , exactly opposite to the Big Bang.
This theory however has a major flaw : As we have discovered in the past few years, the expansion of the cosmos is actually accelerating and hence, there is no evidence of it ever stopping. There is still hope for this hypothesis being a reality though, as it serves as the foundation for the “Ekpyrotic Theory” or “The Big Bounce Theory” which explain some of the phenomenon that the inflationary theory couldn’t.
An open universe is one that never stops expanding. As the evidence suggests, the expansion is accelerating and due to dark energy, all celestial objects will constantly keep moving apart from another. According to this model, star formation will cease to exist in the next 100 trillion years because most of the material left will be locked up by dense stellar remnants such as white dwarfs or black holes and there basically won’t be anymore matter left to fuel them. As time goes by, our universe will be filled with blackholes and even these black holes will finish evaporating, about a googol years from now, due to Hawking Radiation. This will then, fling the cosmos into the Dark Era where matter is nothing but a nostalgic concept. According to the second law of thermodynamics, the entropy in a system will keep on increasing until it reaches a maximum level. So, at some point the universe will reach a state where all the energy is uniformly distributed and its final temperature will just be a smidge above absolute zero. Cosmic death, a state that is quite unimaginable will be the new reality. Paused in a dark, lifeless chasm of oblivion. This theory is known as the “Big Freeze” or “Heat Death” and it is the most widely accepted one.
There is indeed more space in space
The two models above definitely gave a potential answer to what the death of the universe would look like but they don’t provide an answer to the beginning. Another theory that built its base on the foundations of a closed universe was the “Ekpyrotic Theory” which not only gave us an answer to how the universe came into being but solved the three major issues that the inflationary theory had: The Flatness Puzzle, Homogeneity and Monopoles.
According to this theory, The beginning of this universe was actually brought about due to the collision of two Three-spatial-dimensional branes in a space (bulk) that consisted of an extra spatial dimension. When this took place, the kinetic energy involved in the collision gave rise to the quarks, electrons etc that we know today, which are confined to move along only in four dimensions (3-spatial and 1 time). In this theory, since the temperature is finite and incredibly high, the big bang took place without the formation of a singularity.
The critical density is the density required in our universe to maintain the balance between gravity and the seemingly expanding force, which is often referred to as dark energy, from either collapsing the cosmos or tearing it completely apart. A miraculous coincidence , however, comes into the picture when it was observed that the energy density in our universe is exactly equal to the critical density. It was impossible to determine why this unusual phenomenon existed and why our universe is largely homogenous in nature. Along with being homogenous, it is also geometrically flat. Expanding at an accelerated pace. This Homogeneity and flatness puzzle was solved with the help of the ekpyrotic theory because here , the collision and initiation of the big bang phase occurs nearly simultaneously everywhere. The energetically preferred geometry for the two worlds is flat, so their collision produced a flat big bang universe. Hence, the total energy density is equal to the critical density and our universe is seemingly flat. Similarly, The big bang theory predicts the existence of an overabundance of monopoles for which we haven’t found any evidence but in the ekpyrotic theory, the monopoles are not produced at all because the temperature after the collision is far too low to formulate such large objects.
The three major issues of the inflationary theory being solved, the next step was to determine what happens at the end.
What is life if not a circle?
The Big Bounce is one such theory that specifies what happens to the universe if it had the ability to be re-born again after its “end”. Keep in mind, here the end refers to a transition rather than a permanent stop. According to it, the universe has a life-cycle and constantly keeps expanding and contracting. After the big crunch takes place, The small density (not singularity) has a gigantic amount of energy accumulated in it. This energy heats up the universe and vaporizes all the atoms in it. So, the universe, again starts expanding in the form of a seemingly new big bang. The cycle continues and more and more universe are born, each being hotter and bigger than the previous. On the surface, this is a satisfying theory because it finally gives us an explanation for life after death.
Humans are designed to be curious with questions leading to a more knowledgeable future. Even if we answer how the singularity before the big bang came into existence with the ekpyrotic theory, the next question will be targeted as to how the branes leading to our creation were brought into existence. So, the search continues. Hopefully one day, we will find a unified theory that has an answer to all our questions.
References:
https://www.space.com/17594-string-theory.html https://www.youtube.com/watch?v=__0Y5SyEVUI https://astronomy.com/news/2021/08/what-would-this-cyclic-model-of-theuniverse-mean-for-the-big-bang https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe#Theories_about _the_end_of_the_universe https://www.space.com/38982-no-big-bang-bouncing-cosmology-theory.html https://astronomy.com/news/magazine/2021/01/the-beginning-to-the-endof-the-universe-the-big-crunch-vs-the-big-freeze https://en.wikipedia.org/wiki/Big_Crunch#:~:text=The%20Big%20Crunch%20is %20a,universe%20starting%20with%20another%20Big https://en.wikipedia.org/wiki/Big_Bounce https://astronomy.com/news/2020/09/the-big-freeze-how-the-universe-willdie https://en.wikipedia.org/wiki/The_Five_Ages_of_the_Universe https://en.wikipedia.org/wiki/The_Five_Ages_of_the_Universe https://wwwphy.princeton.edu/~steinh/npr/#:~:text=The%20Ekpyrotic%20Mo del%20of%20the,extra%20(fourth)%20spatial%20dimension. https://arxiv.org/abs/hep-th/0103239 https://www.wired.com/story/what-if-the-big-bang-was-actually-a-bigbounce/ https://physicsworld.com/a/ekpyrotic-cosmology-resurfaces/
