Something like 13.8 billion years ago, all energy in the universe was condensed into a single point. Until suddenly it wasn't. The resulting detonation was the most massive explosion in all of the universe's history, but from it, energy formed into all matter, atoms, molecules, planets and all life on Earth.
This is the Big Bang theory, a model that explains much of what we observe when we look out at the universe. Between all the stars, galaxies and clouds of gas is cosmic background radiation — heat residue from the Big Bang, which is still faintly visible today, and is one of the most glaring pieces of evidence that the universe started from a single point. Measurements using multiple different tools, including satellites and telescopes, indicate this residue is consistent with models of an explosive birth to our universe.
Even Albert Einstein toyed with the idea of a universe that springs back and forth, dying and expanding, over and over.
The universe is still expanding, at a rate of 73 kilometers per second per megaparsec, a metric known as Hubble's constant. You can picture how this works by imagining two dots on a balloon. As the balloon is inflated, the distance between the two dots will increase; fill the balloon with dots, and everything appears to grow further away from everything else over time as the balloon inflates. If the universe is the balloon and the dots are galaxies, this is a good metaphor for how our universe is changing over time.
Many scientists predict that tens of trillions of years from now, the universe will eventually run out of steam and "freeze." This will be the heat death of the universe. Also known as the Big Freeze, this theory describes the ultimate fate of the universe as it approaches maximum entropy. When this threshold is reached, there is no more thermal energy or heat. Stars cannot undergo nuclear fusion, so no life can exist.
But an intriguing alternative, even if it doesn't carry much scientific weight, is that before everything ices over, the universe could fall back again — all the galaxies clumping together, swirling closer and closer instead, until it compacts once again to a point. Astronomers call it the Big Crunch. (Big Bang, Big Crunch… I'm sensing a theme here.) In the distant future, as everything condenses, packing tighter and tighter, it could create the conditions for a Big Bang all over again.
That's the basic premise behind the cyclic or oscillating universe theory, which actually dates back to the 1930s. Even Albert Einstein toyed with the idea of a universe that springs back and forth, dying and expanding, over and over. Not unlike the 1993 romantic comedy "Groundhog Day," starring Bill Murray as a weatherman stuck in a time loop, reliving the same day over and over, our universe could be repeatedly cycling through different iterations. Crunch, bang, crunch, bang.
Around this time, Richard Chace Tolman, an American physicist and cosmologist, was the first to really popularize this idea, but he initially set out to disprove it. In the early 20th Century, the Big Bang theory wasn't mainstream. Most people believed the universe had always been there and always would be. In fact, for many years, "Big Bang" was used derisively, a way to dismiss how ludicrous the idea was to astronomers. But Tolman noticed that the ratio of hydrogen and helium — the two most abundant elements in the universe — could not have happened in a static universe. An explosion most likely kicked things off.
In 1934, Tolman published his book "Relativity, Thermodynamics, and Cosmology," inspired in part by the descriptions of an expanding universe model first proposed by Edwin Hubble in 1929. He and Hubble actually published together once, a paper describing the expansion of the universe. It's quite clear that stars and galaxies are spreading out like in our balloon metaphor. What was less clear to Tolman and other astronomers was whether or not gravity will eventually pull the universe back together. "He took the possibility of an oscillating universe quite seriously," one biography of Tolman said.
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As the Big Bang became accepted scientific theory, the oscillating universe theory faded away. But some physicists, like Paul Steinhardt and Neil Turok, have picked the idea up again, modified it somewhat and given it new life. A central part of the updated theory has to do with dark energy, a mysterious, not-fully-understood aspect of the universe that is thought to be the driving force behind our expanding universe.
In their 2007 book "Endless Universe: Beyond the Big Bang," Steinhardt and Turok describe how they came upon this theory by postulating that dark energy could have existed before the Big Bang and is so powerful that it will eventually pull the universe back together using a "springlike" motion that stretches the "branes," a term used in theoretical physics to describe a type of structure in the universe.
If this were true, that means that our universe is on a seemingly endless loop, a cosmic version of Groundhog Day on a rhythm stretching billions or even trillions of years.
"The potential energy would only be noticeable again after nine billion years of expansion had passed and the density of matter and radiation fell below the potential energy," Steinhardt and Turok wrote. "Only then would the springlike potential energy take over again, just as it did before the bang. Once again, it would act like a source of dark energy that causes the stretching of the branes to accelerate, just what we are witnessing today...."
"Of course, if it could happen once, there is nothing to stop the whole process from happening again, and again, and again. The bangs could continue forever," Steinhardt and Turok continued. "Suddenly and inadvertently, we had revived an ancient idea that we had been taught was impossible: a cyclic universe."
If this were true, that means that our universe is on a seemingly endless loop, a cosmic version of Groundhog Day on a rhythm stretching billions or even trillions of years. Nonetheless, the theory isn't widely accepted in science. It would be pretty hard to test oscillating universe theory, as no information would likely survive cycling through a Big Bang or a Big Crunch, though mathematical physicist Roger Penrose has argued that black holes from previous universes may have survived the transition.
There are many models of the universe, though in order for a model to be useful it needs to be testable. The Big Bang theory is the best model we have of the entire universe, how it formed and where it's going. It could be totally wrong, but good luck disproving it. But until we know more about dark energy — arguably the most mysterious of the constituent matter and energy in the universe — we may not have enough evidence pointing to a repeating cycle of universal death and rebirth.
But intriguingly, there may be other universes with different fundamental constants that do have a cyclical quality to them. Of course, the existence of other universes would require the theory of the multiverse to be real. Incidentally, while there are aspects of our universe that hint that we may be living in a multiverse, this, too, is not provable.
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