The conventional story of the universe’s creation goes somewhat like this: nearly 14 billion years ago, a tremendous amount of energy materialized as if from nowhere.

In a brief moment of rapid expansion, that burst of energy inflated the cosmos like a balloon. The expansion straightened out any large-scale curvature, resulting in what we now call flat geometry. Matter has also been fully mixed together, so that the universe seems essentially (though not entirely) featureless. Clumps of particles have formed galaxies and stars here and there, but these are minuscule specks on an otherwise pristine cosmic canvas.

This theory, known as inflation in textbooks, matches all data to date and is preferred by the majority of cosmologists. However, it has conceptual implications that some people find disturbing. The rapid expansion of space-time will never stop in most locations. As a result, inflation cannot help but result in a multiverse – a technicolor existence with an infinite number of pocket universes, one of which we call home. According to critics, inflation predicts everything, which ultimately means nothing. “Inflation doesn’t work as it was intended to work,” said Paul Steinhardt, an architect of inflation who has become one of its most prominent critics.

Steinhardt and others have been creating a new story of how our cosmos came to be in recent years. They have revived the concept of a cyclical cosmos, one that grows and contracts on a regular basis. They hope to create a cosmos that is flat and smooth, without the baggage that comes with a bang.

To that goal, Steinhardt and his colleagues recently collaborated with experts who specialize in computational gravity models. They investigated how a collapsing universe might change its own structure, and they discovered that contraction can outperform inflation. The collapse would effectively erase a wide spectrum of primordial wrinkles, no matter how strange and twisted the cosmos appeared before it contracted.

“It’s very important, what they claim they’ve done,” said Leonardo Senatore, a cosmologist at Stanford University who has analyzed inflation using a similar approach. There are aspects of the work he hasn’t yet had a chance to investigate, he said, but at first glance “it looks like they’ve done it.”

Squeezing the View

Over the last year and a half, a cooperation between Steinhardt, Anna Ijjas, a cosmologist at the Max Planck Institute for Gravitational Physics in Germany, and others has produced a new picture of the cyclic, or “ekpyrotic,” cosmos — one that accomplishes renewal without collapse.

When it comes to visualizing expansion and contraction, many people imagine a balloon-like world whose size changes are defined by a “scale factor.” However, a second measure — the Hubble radius, which is the greatest distance we can observe — is overlooked. The equations of general relativity allow them to evolve separately, and, crucially, altering either can flatten the cosmos.

Consider an ant on a balloon. Inflation is similar to blowing up a balloon. It places the primary burden of smoothing and flattening on the expanding universe. However, in the cyclic universe, the smoothing occurs during a time of contraction. The balloon deflates a little during this time, but the actual job is done by a sharply diminishing horizon. It’s as if the ant is looking at everything through a magnifying glass that gets stronger and stronger. The distance it can see shrinks, and its world becomes increasingly featureless.

Steinhardt and colleagues imagine a cosmos that extends for trillions of years, propelled by the energy of an omnipresent (and hypothetical) field, the behavior of which we now attribute to dark energy. When this energy field becomes sparse, the universe begins to deflate gradually. A declining scale factor brings everything closer over billions of years, but not all the way down to a point. The Hubble radius rushes in and eventually becomes microscopic, causing the abrupt change. The contraction of the universe recharges the energy field, which warms up the universe and vaporizes its atoms. A bounce ensues, and the cycle starts anew.

The microscopic Hubble radius assures smoothness and flatness in the bounce model. And, although inflation expands many early imperfections into massive plots of multiverse real estate, slow contraction squeezes them out of existence. We have a universe with no origin, no end, no singularity at the Big Bang, and no multiverse.

From Any Cosmos to Ours

One issue for both inflation and bounce cosmologies is demonstrating that their respective energy fields produce the correct cosmos regardless of how they begin. “Our philosophy is that there should be no philosophy,” Ijjas said. “You know it works when you don’t have to ask under what condition it works.”

She and Steinhardt accuse inflation of only doing its job in extraordinary cases, such as when its energy field forms with no visible features and little motion. These scenarios have received the most attention from theorists, in part because they are the only ones tractable with chalkboard mathematics. The team stress-tested their slow-contraction model with a variety of infant universes too wild for pen-and-paper analysis in recent computer simulations, which Ijjas and Steinhardt detail in a pair of preprints published online.

The collaboration investigated twisted and lumpy fields, fields moving in the wrong direction, and even fields born with halves racing in opposing directions, using code developed by Frans Pretorius, a theoretical physicist at Princeton University who specializes in computational models of general relativity. In almost every case, contraction produced a cosmos as dull as ours.

“You let it go and — bam! In a few cosmic moments of slow contraction it looks as smooth as silk,” Steinhardt said.

The new simulations were described as “very comprehensive” by Katy Clough, a cosmologist at the University of Oxford who also specializes in numerical solutions to general relativity. She did, however, point out that computer advances have just lately enabled this type of analysis, thus the full spectrum of conditions that inflation can manage remains uncharted.

“It’s been semi-covered, but it needs a lot more work,” she said.

While there is some interest in Ijjas and Steinhardt’s concept, most cosmologists think that inflation is still the paradigm to beat. “[Slow contraction] is not an equal contender at this point,” said Gregory Gabadadze, a cosmologist at New York University.

The collaboration will then flesh out the bounce itself, which will be a more complex stage requiring innovative interactions to push everything apart again. Ijjas already has one bounce theory, which adds a new interaction between matter and space-time to general relativity, and she expects that more processes exist as well. She intends to run her model on the computer shortly in order to fully comprehend its behavior.

The team expects that by connecting the contraction and expansion stages, they may be able to uncover distinctive aspects of a bouncing universe that astronomers might notice.

The cooperation has not worked out every detail of a cyclic cosmos without a bang or a crunch, much alone demonstrated that we live in one. However, Steinhardt currently believes that the model will soon provide a viable alternative to the multiverse. “The roadblocks I was most worried about have been surpassed,” he said. “I’m not kept up at night anymore.”