In 1929, Edwin Hubble calculated the velocity of distant galaxies barely visible through the telescopes at the Mount Wilson Observatory. His observations were the first evidence of a theory that has become a foundation of modern cosmology — the universe is continuously expanding.
If you trace that expansion far enough back in time, you reach a distant point in the past. At this point, the entire universe was squeezed into an unfathomably dense spec filled with molten subatomic particles. This distant point in the past is known colloquially as the hot Big Bang.
So how did this spec expand and form matter as we know it? It’s a point of contention among cosmologists and theoretical physicists. In recent years, a once-fringe theory — dubbed the Big Bounce — has gained traction in cosmological circles. It suggests that the Big Bang may have happened before, and may happen again.
Probing the Origins of the Big Bang
In 1981, physicist and then newly-appointed MIT professor Alan Guth proposed a new model called cosmic inflation. The theory, which quickly became dominant in cosmology circles, describes a period of exponential expansion at the beginning of time. According to the model, our universe grew from the smallest things seen under a microscope to something greater than the entire observable universe within a fraction of a second.
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Cosmic Inflation Theory
Cosmic inflation theory solved several questions that had been raised by physicists in response to original Big Bang model. For instance, it explained why the matter in our universe, when observed at a grand scale, is evenly distributed. But the theory also hinges on quantum physics principles that have yet to be integrated into the classical physics. As a result, some researchers have gone looking for other answers.
One group of researchers found inspiration in an age-old dichotomy.
“There are really two options for the universe,” says Anna Ijjas, a theoretical physicist at New York University. “Either it had a beginning in time and time had a beginning, or there was no beginning to time and there was no beginning to the universe.”
Since 2017, Ijjas has published ground-breaking research on an alternative to inflation theory: the Big Bounce, also known as cyclic universe cosmology. It proposes that our universe has expanded and contracted more than once — perhaps even an infinite number of times.
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The Big Bounce: An Alternative to the Big Bang
The Big Bounce continues to gain notoriety as Ijjas and her peers streamline the theoretical physics that might explain it. At this point, it is a formidable rival for inflation theory. But the idea is not actually new. Dreams of an unending universe are older than the field of cosmology itself.
Unlike his teacher Plato, who believed in divine creation, Aristotle believed that the world was eternal. According to his disciple Maimonides, he also believed that “time and motion are perpetual and everlasting and not subject to generation and passing-away.”
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Big Bang Theory Scientists
Two of the most important Big Bang Theory scientists were near perfect foils for Plato and Aristotle. One physicist believed that time and space were created in an instant, while the other believed the universe existed in perpetuity. Georges Lemaître, who proposed the Big Bang model of cosmology in 1927, was a Catholic priest who imagined God as the architect behind the Big Bang. Meanwhile, George Gamow, the first person to model the primordial formation of atoms, had other ideas.
“[Gamow] thought that there would be a prior universe that goes through some sort of big crunch,” explains cosmological philosophy expert Daniel Linford. “Once it reaches some minimum size, then it re-expands into our present, observable universe.”
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The Big Bang vs. the Big Bounce
In 2001, Princeton physicist Paul Steinhardt and Cambridge physicist Neil Turok brought Gamow’s idea into the modern age with a paper published in Physical Review. They argued that the uniformity of matter in the universe could be better explained by the so-called Big Bounce. Apparently, cosmic inflation attempts to account for this as the universe seesaws back and forth between expansion and contraction.
Models based on cosmic inflation describe a state, early on in the formation of the universe, where energy is so high that classical physics no longer applies and quantum gravity takes over. The Big Bounce avoids this scenario. Instead, the universe bounces back before the high-energy state is reached. Because of this, the theory can be described using classical physics plus an additional “scalar field” — a mathematical dimension that assigns a specific energy to every point in space.
At the end of it all, Big Bounce theory requires a “scalar field,” while inflation theory requires a similar “inflationary field.” The Big Bounce does not invoke quantum gravity, but it is still complex. Both theories are well-reasoned and broadly supported, and it seems the debate could go on forever. However, one observation could end it entirely — and it could come as early as the end of this year.
Cosmic Microwave Background
In 2023, Ijjas will keep a close watch on news coming from the Simons Observatory in Argentina in 2023. Soon, a group of researchers, including prominent cosmologist Brian Keating, will assemble an array of telescopes at the site. Among other things, the researchers will use what is called a cosmic microwave background, or CMB. Interestingly, CMB is a universal patchwork of long-wave radiation from the oldest light in the universe!
Inflationary theory predicts that the researchers will see a pattern of “primordial gravitational waves” etched across the CMB from the moments after the Big Bang. The Big Bounce, on the other hand, does not. That means the Big Bounce, along with other cyclic universe theories, could be disproved by the mere presence of these waves.
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Gravitational Waves or Galactic Dust?
In 2014, Keating and colleagues thought they saw primordial gravitational waves in the CMB through the BICEP Program, an international collaboration using telescopes located near the South Pole. However, when they published their results, other cosmologists pointed out that the observation could have been the result of “galactic dust.” Keating hopes to clear this up with the Simons observatory.
“We have to assemble the equipment, then in about six month’s time we’ll have all the data we need to confirm or deny the BICEP results,” he says.
Ijjas claims not to have a predisposition towards one result versus the other; however, much of her recent work will become obsolete if waves are found.
“In this sense it’s very immediate,” she explains. “We will at least know if the theory is wrong.”
More Cosmological Questions to Come
Whatever the researchers at the Simons Observatory see, it won’t settle cosmological questions entirely. Even if we could somehow know that inflation theory is objectively true, the true origins of our universe — and the multiverse — would remain shrouded in mystery and debate.
If you trace that expansion back in time, you reach a distant point in the past. At this point, the entire universe was squeezed into an unfathomably dense speck filled with molten subatomic particles. This is known colloquially as the hot Big Bang. these observations provide hints of our primordial past, they will no doubt lead to more questions about our universe.
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