Detailed computer simulations have found that a cosmic contraction can generate features of the universe that we observe today.
The standard story of the birth of the universe goes something like this: Some 14 billion years ago, a tremendous amount of energy materialized from anywhere.
In a brief moment of rapid expansion, a burst of energy inflated the universe like a balloon. The expansion straightens the curvature of any mass, creating a geometry that we now call flat. Matter is also well mixed together, so that now the universe appears largely (though not entirely) featureless. Here and there, clumps of particles have formed galaxies and stars, but these are only the minuscule spec on an otherwise uncontrollable cosmic canvas.
The theory, which textbooks have called inflation, matches all observations to date and is preferred by most cosmologists. But it has ideological implications that some bother. In most areas of space-time, rapid expansion will never stop. As a result, inflation cannot help but produce a multiverse - a technological variety that has a variety of pocket universes, one of which we call home. For critics, inflation predicts everything, meaning it ultimately predicts nothing. "Inflation doesn't work because it was intended to work," said Paul Steinhardt, an architect of inflation who has become one of its most prominent critics.
In recent years, Steinhard and others have been developing a different story of how our universe came about. He has revived the idea of a cyclic universe: one that grows and contracts periodically. They hope to replicate the universe we see - flat and smooth - without the baggage that comes with a bang.
To that end, Steinhardt and his colleagues recently worked closely with researchers, who specialize in computational models of gravity. He analyzed how a collapsing universe would change its structure, and he eventually figured out that contraction could beat inflation in his game. No matter how bizarre and twisted the universe looked before it contracted, the collapse would efficiently wipe out a wide range of primordial wrinkles.
"This is very important, which they claim they have done," said Leonardo Senatore, a cosmologist at Stanford University, who analyzes inflation using a similar approach. There are many aspects of the work that have not yet had a chance to investigate, he said, but at first glance "he seems to have done it."
Squeezing the View
Over the past year and a half, a fresh view of the cyclic, or "echophrotic," universe has emerged in Germany from Steinhardt, Anna Izzaz, a cosmologist for the Max Planck Institute for Gravitational Physics, and others, and one fall. Receives without renewal.
When it comes to imagining expansion and contraction, people often focus on a ballooned universe whose shape is described by a "factor". "But a second measure - the Hubble radius, which is the largest distance we can see - is found to have small cracks. The equations of general relativity let them evolve independently, and, crucially, you can either change the universe. Can flatten.
An ant portrait on a balloon. Inflation is like blowing a balloon. This mainly exerts pressure on the swollen universe to be smoothing and flattening. In the cyclic universe, however, smoothing occurs during periods of contraction. During this period, the balloon deflects moderately, but the actual work is done by a very rapidly shrinking horizon. It is as if the ant sees everything through a powerful powerful magnifying glass. The distance that one can see shrinking, and thus its world becomes more and more featureless.
Steinhard and company envision a universe that spans perhaps a trillion years, driven by the energy of a ubiquitous (and imaginary) sphere, which we currently treat as dark energy. When this energy field eventually becomes sparse, the universe slowly deflects. The factor on a contract scale over billions of years brings everything a little closer, but not all down to a single point. A dramatic change occurs from the Hubble radius, which eventually escapes and becomes subtle. The contraction of the universe recharges the energy field, which heats the universe and vaporizes its atoms. A surge ensues, and the cycle begins anew.
In the boom model, the micro Hubble radius ensures smoothness and flatness. And while inflation blows many of the initial flaws into vast plots of multiverse real estate, the slow contraction essentially forces them out of existence. We are left with a universe that has no beginning, no end, there is no eccentricity nor diversity in the Big Bang.
From Any Cosmos to Ours
One challenge for both the inflation and buoyancy of the universe is to demonstrate that their respective energy fields form the right universe, no matter 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 conditions it works."
He and Steinhard criticize inflation for doing its job only in special cases, such as when its energy field is built without notable characteristics and with little speed. Theorists have best explored these situations, as they are the only example with chalkboard mathematics. In a recent computer simulation that Ijjas and Steinhardt described in a pair of preprints posted online in June, the team used pen-and-paper analysis of their slow-contraction model with plenty of baby universes Tested.
The coding code developed by France Pretorius, a theoretical physicist at Princeton University, who specializes in computational models of general relativity, collaborated to discover twisted directions and lumpy fields, fields moving in the wrong direction, even fields. Areas that run in conflicting directions. In almost every case, contraction rapidly made the universe as boring as ours.
"You let it go and - balm! Stanhardt said that in some cosmic moments of slow contraction it smooths like silk.
Katie Clough, a cosmologist at the University of Oxford who also specializes in numerical solutions to general relativity, called the new simulation "very comprehensive". But she also notes that computational advances have recently made such analysis possible, so the entire range of conditions that handle inflationary conditions may remain unchanged.
"It's semi-covered, but it requires a lot of extra work," she said.
While interest in Ijjas and Steinhardt's models differ, most cosmologists agree that the paradigm for defeating inflation remains. "Slow contraction] is not a uniform contender at this point," said Gregory Gabadze, a cosmologist at the University of New York.
The collaboration will end the next boom altogether - a more complex platform that requires novel interactions to push everything again. Ijjas already has a buoyancy theory that elevates general relativity with a new interaction between matter and space-time, and he suspects that other mechanisms also exist. She plans to put her model on the computer soon to understand her behavior in detail.
The group hopes that after observing the contraction and expansion phases together, they will identify the unique characteristics of a bouncing universe that astronomers might perceive.
Cooperation has made no bang and no crunch on every detail of the cyclic universe, much less shown that we live in one. But Steinhardt now feels optimistic that the model will soon offer a viable alternative to multiverse. "The barriers I was most worried about have been overcome," he said. "I am no longer kept at night."
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