What if the Big Bang was in fact a big bounce?
Steinhardt and company imagine a universe that develops for perhaps a trillion years, animated by the energy of an ubiquitous (and hypothetical) field, the behavior of which we currently attribute to dark energy. When this energy field eventually becomes sparse, the cosmos begins to slowly deflate. Over billions of years, a contractual scale factor brings everything together a bit, but not completely at some point. The dramatic change comes from the Hubble ray, which rushes in and eventually becomes microscopic. The contraction of the universe recharges the energy field, which heats the cosmos and vaporizes its atoms. A rebound ensues and the cycle begins again.
In the rebound model, the microscopic Hubble radius ensures smoothness and flatness. And as inflation explodes many initial imperfections into giant plots of multiverse real estate, a slow contraction essentially excludes them from existence. We end up with a cosmos that has no beginning, no end, no Big Bang singularity, no multiverse.
From any cosmos to ours
A challenge for inflation and rebound cosmologies is to show that their respective energy fields create the right universe no matter how they started out. “Our philosophy is that there shouldn’t be a philosophy,” said Ijjas. “You know it works when you don’t have to ask what conditions it works.”
She and Steinhardt criticize inflation for only doing its job in special cases, such as when its energy field forms without noticeable features and with little movement. Theorists have explored these situations in more depth, in part because they are the only examples treatable with the mathematics of the table. In recent computer simulations, which Ijjas and Steinhardt describe in a pair of preprints Uploaded in June, the team tested their slow-twitch model with a range of baby universes too wild for paper and pen analysis.
Adapting code developed by Frans Pretorius, a theoretical physicist at Princeton University specializing in computational models of general relativity, the collaboration explored twisted and bumpy fields, fields moving in the wrong direction, even born fields with halves that run in opposite directions. In almost all cases, the contraction quickly produced a universe as boring as ours.
“You gave up and… bam!” In a few cosmic moments of slow twitch, it looks as smooth as silk, ”said Steinhardt.
Katy Clough, a cosmologist at the University of Oxford who also specializes in numerical solutions of general relativity, called the new simulations “very complete”. But she also noted that advances in IT have only recently made this kind of analysis possible, so the full range of conditions inflation can handle remains unexplored.
“He’s been semi-covered, but he needs a lot more work,” she says.
Although interest in Ijjas and Steinhardt’s model varies, most cosmologists agree that inflation remains the paradigm to beat. “[Slow contraction] is not an equal competitor at this point, ”said Gregory Gabadadze, a cosmologist at New York University.
The collaboration will then flesh out the bounce itself – a more complex step that requires new interactions to pull everything apart again. Ijjas already has a rebound theory which improves general relativity with a new interaction between matter and space-time, and she suspects that other mechanisms exist as well. She plans to put her model on the computer soon to understand her behavior in detail.
The group hopes that after gluing the stages of contraction and expansion together, they will identify the unique characteristics of a bouncing universe that astronomers could spot.
The collaboration hasn’t worked out all the details of a cyclical cosmos without a bang and a crunch, let alone show that we live in one. But Steinhardt is now optimistic that the model will soon offer a viable alternative to the multiverse. “The roadblocks that worried me the most have been passed,” he said. “I am no longer kept awake at night.”