In 1929 Edwin Hubble calculates the speed of distant galaxies barely visible through the telescopes of the Mount Wilson Observatory. His observations were the first evidence of a theory that became the basis of modern cosmology – the universe is constantly expanding.
If you trace this expansion far enough back in time, you reach a distant point in the past. At that moment, the entire universe was compressed into an incredibly dense particle full of molten subatomic particles. This distant point in the past is colloquially known as hot Big Bang.
So how did this specification expand and form matter as we know it? This is a point of contention between cosmologists and theoretical physicists. In recent years a a theory of the erstwhile periphery – dubbed the Big Bounce – gained popularity in cosmological circles. This suggests that the Big Bang may have happened before and may happen again.
Exploring 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 modelour universe has grown from the smallest things seen under a microscope to something larger than the entire visible universe in a fraction of a second.
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Theory of Cosmic Inflation
The theory of cosmic inflation resolved several questions that were raised by physicists in response to the original Big Bang model. For example, it explains why the matter in our universe, when observed on a large scale, is uniformly distributed. But the theory also depends on principles of quantum physics that have not yet been integrated into classical physics. As a result, some researchers began looking for other answers.
One group of researchers found inspiration in an age-old dichotomy.
“There are really two possibilities for the universe,” says Anna Ijas, a theoretical physicist at New York University. “Either there was a beginning in time and time had a beginning, or there was no beginning to time and no beginning to the universe.”
Since 2017, Ijjas has published ground-breaking research on an alternative to the theory of inflation: The Big Bounce, also known as cyclic universe cosmology. It suggests that our universe has expanded and contracted more than once—perhaps even an infinite number of times.
Read more: This cyclical model of the universe is causing cosmologists to rethink the Big Bang
The Big Bounce: An Alternative to the Big Bang
The Big Bounce continues to gain notoriety as Ijas and her colleagues rationalize the theoretical physics that might explain it. At this point, it’s a formidable opponent to inflation theory. But the idea isn’t really new. Dreams of an infinite 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 student Maimonides, he also believed that “time and motion are unceasing and eternal and not subject to generation and extinction.”
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Big Bang Theory Scientists
Two of the most important Big Bang scientists were almost perfect for Plato and Aristotle. One physicist believed that time and space were created in an instant, while the other believed that the universe existed forever. Georges Lemaitre, 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 there would have been a previous universe that was going through some kind of big crisis,” explains cosmological philosophy expert Daniel Linford. “Once it reaches some minimum size, it expands back into our present, observable universe.”
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The Big Bang vs. The Big Rebound
In 2001, Princeton physicist Paul Steinhardt and Cambridge physicist Neil Turok brought Gamow’s idea into the modern era with document published in Physical examination. They argued that the uniformity of matter in the universe could be better explained by the so-called Big Bounce. Apparently, cosmic inflation tries to explain this, as the universe swings back and forth between expansion and contraction.
A scalar field
Models based on cosmic inflation describe a state early in the formation of the universe where the energy is so high that classical physics no longer applies and quantum gravity takes over. Big Bounce avoids this scenario. Instead, the universe bounces back before the high-energy state is reached. The theory can therefore be described using classical physics plus an additional “scalar field” – a mathematical dimension that assigns a specific energy to each point in space.
After all, the Big Bounce theory requires a “scalar field”, while the inflation theory requires a similar “inflation field”. The Big Leap does not invoke quantum gravity, but it is still complex. Both theories are well-founded and widely supported, and it seems the debate could go on forever. However, one observation could put an end to that — and it could come as early as the end of this year.
Illustration of cosmic microwave background, electromagnetic radiation. (Credit: Declan Hillman/Shutterstock)
Cosmic microwave background
In 2023, Ijas will closely monitor the news coming from Simons Observatory in Argentina in 2023. Soon, a group of researchers, including the eminent cosmologist Brian Keating, will assemble an array of telescopes at the site. Among other things, the researchers will use what’s called the cosmic microwave background, or CMB. Interestingly, the CMB is a universal mosaic of long-wave radiation from the oldest light in the universe!
Inflationary theory predicts that researchers will see a pattern of “primordial gravitational waves” etched into the CMB from the moments after the Big Bang. The Great Rebound, on the other hand, does not. This means that the Big Bounce, along with other theories of a cyclic universe, can be disproved by the mere presence of these waves.
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Gravitational waves or galactic dust?
In 2014, Keating and colleagues believed they had seen primordial gravitational waves in the CMB during 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 be the result of “galactic dust”. Keating hopes to clarify this with the Simons Observatory.
“We need to assemble the equipment, then in about six months we will have all the data we need to confirm or reject the BICEP results,” he says.
Ijjas asserts that there is no predilection for one outcome over another; however, much of her recent work will become obsolete if waves are detected.
“It’s very immediate in that sense,” she explains. “At least we’ll know if the theory is wrong.”
More cosmological questions to come
Whatever the Simons Observatory researchers see, it won’t completely settle cosmological questions. Even if we could somehow know that the theory of inflation was objectively true, the true origin of our universe – and the multiverse – would remain shrouded in mystery and debate.
If you trace this expansion back in time, you reach a distant point in the past. At that moment, the entire universe was compressed into an immeasurably dense blob full of molten subatomic particles. This is known colloquially as the hot Big Bang. these observations provide hints about our primordial past, they will undoubtedly lead to more questions about our universe.
Read more: What came before the Big Bang?