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Is It Possible that the Universe Bounces?
Cyclic cosmological models
Imagine a universe that has been around forever, but is constantly dying and constantly being reborn throughout eternity. One of the absolute most fascinating concepts I've ever come across is that of an "oscillating universe," which is a specific form of a more broad "cyclic model" of the cosmos. The concept is far from unique to physics, although we'll take a look at some of the more eloquent explanations from science. From ancient Hindus to the Mayans of ancient America, to the Greeks more than 2000 years ago, a cyclic universe has been a hot topic of discussion and contemplation for time immemorial.
The oscillating universe
In the 1920s, Edwin Hubble made famous the revelation that the universe is expanding, and Eisntein famously regretted withdrawing his "cosmological constant" from his equations of general relativity. Tracking things back to the beginning, extrapolating backward, Georges Lamaitre famously postulated that the earlier universe must have been hotter and smaller, all the way back to an individual point, or a "primeval atom." Fred Hoyle soon derogatorily called this moment the "Big Bang", and that has stuck ever since. As science presently understands it, and as is capable of making predictions, roughly 13.7 billion years ago there was a rapid expansion of space itself. In far, far less time than the blink of an eye, a point particle with zero dimensions became many, many billions of miles across in a stupendously hot burst of unimaginable energy release, and, ever since, the universe has been expanding ever-outward (and cooling down).
In a brief moment of contemplation, one of the possible models of the universe Einstein considered, among others, was that of a "cyclic model", featuring what is called an "oscillating universe." The idea is that gravity, eventually, will overcome the expansion of space (which is only logical when you think about it; after all, gravity constantly attracts, so everything should be slowing down over time). As the mutual gravity of all matter pulls collectively inward, the universe must eventually collapse in a "Big Crunch", the opposite compliment of the "Big Bang."
Einstein's thought process continued from here. What if the "Crunch" was just a moment when everything crossed the paths of the universal center, and it just kept flying away in the opposite direction? Consider one particle from way, way over there (say, 10 billion light years) accelerating until it crosses the collective center of gravity of the whole universe. Unless said particle was then annihilated in the "Big Crunch", the particle would simply keep going, just like an eternal pendulum, and the process of the "Big Bang" would thus begin again.
This is, in a nutshell, the concept of an oscillating universe.
The ouroboros, a symbol of cycles repeating themselves
Mythology and religion
Of course, we now believe the oscillating universe model isn't likely to be true, thanks to the recent discovery of "dark energy", revealing that the universe isn't being drawn back in under its mutual gravity, but rather, it is accelerating in its expansion. But the cyclic model of the universe isn't anything new. Looking back over time, there were beliefs in the "cycle of life" (birth, death, and rebirth resulting from the death) as applied to our universe across numerous cultures and religions.
Some of the most striking examples of belief in a cyclic model include ancient Egypt, Hindu mythology, the ancient Greeks, and both the Mayans and Aztecs of the ancient Americas.
The symbol of the Ouroboros is probably the most universally recognized symbol of the cyclical universe today. The Ouroboros first appears in history during the 14th century BCE in the tomb of King Tut, although it was almost certainly used long before that. The symbol continues throughout Egypt's history, and then ancient Rome, Greece, and well on into the middle ages in Europe as a symbol of constant renewal.
The scarab in ancient Egypt
Egypt, Greece, Maya, Hindu
In ancient Egypt, the scarab (also called the "dung beetle") came to symbolize birth, death, and rebirth. Ra, the god of the sun, rolls across the sky throughout each day, and similarly, beetles roll around dung (poop) into a ball. Inside of this poop ball, the dung beetles lay eggs, which eventually arise as larvae. This cycle of death and rebirth came to have tremendous influences on numerous future religions as well.
The famous Mayan calendar also suggests a cyclical view of time, and the Maya creation accounts tell a story of the world (and people) being preceded by other worlds. Sacrifice to the gods is one of the things that keeps this current incarnation of the world present, appeasing the gods.
In ancient Greece, the pre-Socratic philosopher Empedocles not only espoused reincarnation, but also a completely cyclic model of the universe centered around a sphere containing forces. If this sounds a little bit like the universe to you, you're not alone. The Stoics in Greece espoused belief in cycles as well, particularly the Ekpyrosis, or the belief in the "periodic destruction of the cosmos by a great conflagration every great year."
Perhaps the most familiar of all to the modern mind is the Hindu religious belief of the 311.04 trillion years of cosmic existence in between cycles of total destruction and rebirth. This "Maha-Manvantara" describes the time of our existence, in the midst of a cycle that repeats itself ad infinitum. The annihilation, or chaos, happens for the same 311.04 trillion year duration as the existence.
Favorite cyclic universe model
Conformal cyclic cosmology
The domain of amazingly unbelievable speculation doesn't only belong to mythology and religion, though. Physicist Roger Penrose's more recent "Conformal Cyclic Cosmology" (CCC) model suggests that the universe expands forever (consistent with all observations) until the matter all decays,eventually turning into photons. This concept is explored in Penrose's recent book, but I first read about it in The Five Ages of the Universe. Essentially, there's nothing left in the universe to distinguish distance or time, as the photons themselves feel no effects of time whatsoever, and they're so far apart that they never interact. Thus, the conditions are identical to those of the very early universe, setting the stage for the next Big Bang, where time and space had no meaning!
There are even stranger speculations about universes being born all the time within our own universe via quantum fluxuations, and for all we know, our own universe may well be the result of such a quantum fluxuation inside of another universe. The multiverse theory and simulated universe theories provides even more fuel for such speculation.