“I think I know how the universe was born,” said Andrei Linde, Russian-American theoretical physicist and the Harald Trap Friis Professor of Physics at Stanford University. Linde is one of the main authors of the inflationary universe theory, as well as the theory of eternal inflation and inflationary multiverse.
According to quantum models, galaxies like the Milky Way grew from faint wrinkles in the fabric of spacetime. The density of matter in these wrinkles was slightly greater compared to surrounding areas and this difference was magnified during inflation, allowing them to attract even more matter. From these dense primordial seeds grew the cosmic structures we see today.
Galaxies are children of random quantum fluctuations produced during the first 10^-35 seconds after the birth of the universe.” Andrei Linde
Late one summer night in 1981, while still a junior research fellow at Lebedev Physical Institute in Moscow, Andrei Linde was struck by a revelation, according to a Stanford University article about India and the birth of inflation theory, The Fractal Universe. Unable to contain his excitement, he shook awake his wife, Renata Kallosh, and whispered to her in their native Russian, “I think I know how the Universe was born.”
Kallosh, a theoretical physicist herself, muttered some encouraging words and fell back asleep. “It wasn’t until the next morning that I realized the full impact of what Andrei had told me,” recalled Kallosh, now a professor of physics at the Stanford Institute for Theoretical Physics.
Linde’s nocturnal eureka moment had to do with a problem in cosmology that he and other theorists, including Stephen Hawking, had struggled with for months.
In less than a billionth of a trillionth of a trillionth of a second, space-time doubled more than 60 times from a subatomic speck to a volume many times larger than the observable universe.
A year earlier, a 32-year-old postdoc at SLAC National Accelerator Laboratory named Alan Guth shocked the physics community by proposing a bold modification to the Big Bang theory. According to Guth’s idea, which he called “inflation,” our universe erupted from a vacuum-like state and underwent a brief period of faster-than-light expansion. In less than a billionth of a trillionth of a trillionth of a second, space-time doubled more than 60 times from a subatomic speck to a volume many times larger than the observable universe.
Guth envisioned the powerful repulsive force fueling the universe’s exponential growth as a field of energy flooding space. As the universe unfurled, this “inflaton field” decayed, and its shed energy was transfigured into a fiery bloom of matter and radiation. This pivot, from nothing to something and timelessness to time, marked the beginning of the Big Bang. It also prompted Guth to famously quip that the inflationary universe was the “ultimate free lunch.”
As theories go, inflation was a beauty. It explained in one fell swoop why the universe is so large, why it was born hot, and why its structure appears to be so flat and uniform over vast distances. There was just one problem – it didn’t work.
To conclude the unpacking of space-time, Guth borrowed a trick from quantum mechanics called “tunneling” to allow his inflation field to randomly and instantly skip from a higher, less stable energy state to a lower one, thus bypassing a barrier that could not be scaled by classical physics.
But closer inspection revealed that quantum tunneling caused the inflation field to decay quickly and unevenly, resulting in a universe that was neither flat nor uniform. Aware of the fatal flaw in his theory, Guth wrote at the end of his paper on inflation: “I am publishing this paper in the hope that it will … encourage others to find some way to avoid the undesirable features of the inflationary scenario.”
The “New Inflation”
Guth’s plea was answered by Linde, who on that fateful summer night realized that inflation didn’t require quantum tunneling to work. Instead, the inflation field could be modeled as a ball rolling down a hill of potential energy that had a very shallow, nearly flat slope. While the ball rolls lazily downhill, the universe is inflating, and as it nears the bottom, inflation slows further and eventually ends. This provided a “graceful exit” to the inflationary state that was lacking in Guth’s model and produced a cosmos like the one we observe. To distinguish it from Guth’s original model while still paying tribute to it, Linde dubbed his model “new inflation.”
By the time Linde and Kallosh moved to Stanford in 1990, experiments had begun to catch up with the theory. Space missions were finding temperature variations in the energetic afterglow of the Big Bang – called the cosmic microwave background radiation – that confirmed a starting prediction made by the latest inflationary models. These updated models went by various names – “chaotic inflation,” “eternal inflation,” “eternal chaotic inflation” and many more – but they all shared in common the graceful exit that Linde pioneered.
The inflationary universe is not just the ultimate free lunch, it’s the only lunch where all possible dishes are served.”
Inflation predicted that these quantum fluctuations would leave imprints on the universe’s background radiation in the form of hotter and colder regions, and this is precisely what two experiments – dubbed COBE and WMAP – found. “After the COBE and WMAP experiments, inflation started to become part of the standard model of cosmology,” Shamit Kachru said.
These parts are so large that for all practical purposes they look like separate universes.”
Linde and others later realized that the same quantum fluctuations that produced galaxies can give rise to new inflating regions in the universe. Even though inflation ended in our local cosmic neighborhood 14 billion years ago, it can still continue at the outermost fringes of the universe. The consequence is an ever-expanding sea of inflating space-time dotted with “island universes” or “pocket universes” like our own where inflation has ceased. “As a result, the universe becomes a multiverse, an eternally growing fractal consisting of exponentially many exponentially large parts,” Linde wrote. “These parts are so large that for all practical purposes they look like separate universes.”
Linde took the multiverse idea even further by proposing that each pocket universe could have differing properties, concluding that some string theorists were also reaching independently. “It’s not that the laws of physics are different in each universe, but their realizations,” Linde said. “An analogy is the relationship between liquid water and ice. They’re both H2O but realized differently.”
Linde’s multiverse is like a cosmic funhouse filled with reality-distorting mirrors
Linde’s multiverse is like a cosmic funhouse filled with reality-distorting mirrors. Some pocket universes are resplendent with life, while others were stillborn because they were cursed with too few (or too many) dimensions, or with physics incompatible with the formation of stars and galaxies. An infinite number are exact replicas of ours, but infinitely more are only near-replicas. Right now, there could be countless versions of you inhabiting worlds with histories diverging from ours in ways large and small. In an infinitely expanding multiverse, anything that can happen will happen.
While disturbing to some, this eternal aspect of inflation was just what a small group of string theorists were looking for to help explain a surprise discovery that was upending the physics world – dark energy.
The Last Word –Avi Loeb, astrophysicist, Harvard University
“According to the latest quantum-mechanical incarnation of cosmic inflation, the multiverse is inevitable. Inflation’s pioneer, Alan Guth, stated: “in the multiverse, everything that can happen, will happen an infinite number of times”. This statement has immediate consequences.
“For example, students who take it seriously will not worry about getting a bad grade in class, because there is a version of them getting an “A” somewhere else in the multiverse. Also, cosmologists could “explain” any observational fact they uncover in the context of reverse-engineering inflation, since “everything that can happen, will happen”.
I am not a fan of the multiverse despite its popularity among those who prefer not to be proven wrong.
“However, scientific knowledge is supposed to incorporate ideas that can be falsified based on evidence. The rationale is simple: humans are attracted to virtual realities that flatter their ego and so science must eliminate unrealistic possibilities. For example, evidence shows that the Earth is not the center of the Solar system, let alone the Universe, even though this idea of us being at the center flattered the ego of ancient philosophers who wanted to believe in it.
“The bottom line is simple. Whatever remains from the possible world of virtual realities under the guillotine of data, is under no obligation to flatter our ego. I therefore choose to dedicate my scientific passion to ideas that can be falsified. As a result, I am not a fan of the multiverse despite its popularity among those who prefer not to be proven wrong.”
Nevertheless, perhaps scientists have yet to realize a robust test of the multiverse theory. Just because a theory cannot be currently tested and falsified with today’s technology, data, and ideas does not automatically make the theory untrue or non-scientific.
Image credit: Shutterstock License
Maxwell Moeastrophysicist, NASA Einstein Fellow, University of Arizona via Avi Loeb, Scientific American, The Fractal Universe and Stanford University
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.