On the bounce (Image: Richard Megna/Fundamental Photographs)

OUR universe rose from the ashes of a dying cosmos, thanks to a push from a ghostly force. So says a model supporting the idea that the universe was born not with a bang, but a bounce.

What’s more, the theory would do away with the popular notion that the infant cosmos rapidly ballooned in size during a period called inflation.

The earliest light emitted in the universe, the cosmic microwave background (CMB), dates to about 380,000 years after our cosmic birth. Oddly, it is roughly uniform everywhere we look. This would not be the case if the universe grew up slowly from a single burst. Far-flung regions of the sky should have developed very differently because they were not in contact with each other.


The most widely accepted explanation is that the universe went through a period of rapid expansion in the first slivers of a second after the big bang. This inflation would cause cosmic evenness, because spots that look similar today would have been in contact initially, before suddenly getting shoved apart. Quantum fluctuations in the very early cosmos would have seeded large-scale structures like galaxies (see diagram).

Features of the dying universe survive the bounce, seeding new structures like galaxies

Despite its popularity, inflation has a few holes. For one, no one knows what caused the universe to inflate. And some versions of the theory predict areas inflating at different rates, giving rise to an infinite number of bubble universes alongside ours.

Inflation is more of an explanation than a predictive model of our universe, says Paul Steinhardt of Princeton University. So for about a decade, he and his colleagues have advocated the cyclic universe as an alternative. In their theory, a previous universe went through a phase of slow contraction, crunching space-time. Then something reversed the process and it expanded again to make a new universe.

Compression explains cosmic smoothness without the need for inflation, because high pressures during the crunch would iron out most wrinkles. However, tiny quantum fluctuations could be carried over from the previous universe to provide the seeds of large-scale structure.

Previous studies had shown that these “before” and “after” pictures work mathematically. But no one was sure what was happening during the bounce. Some models require the universe to shrink to a singularity before rebounding, and we would need something we don’t have – a complete theory of quantum gravity – to describe it.

Now Steinhardt and his colleagues have built a model where, before the universe can collapse to a point, an unknown field with negative pressure shoves everything back outwards. The team used a “ghost” field, a physically unrealistic but mathematically simple field with negative kinetic energy.

“Programming a more realistic energy source would not change the outcome, but would require more complicated equations that would slow down the simulation,” says Steinhardt.

The ghost field is weak enough that it can be ignored, except during the bounce, when the universe is very small and dense. Using the ghostly equations, computational tools that describe space-time under general relativity show that tiny fluctuations from the dying universe can indeed be carried over into the reborn cosmos (Physical Review D, doi.org/pcd).

“Paul’s paper shows that things go through beautifully,” says Burt Ovrut at the University of Pennsylvania in Philadelphia. “It means that these alternatives to inflation are alive and well.”

Steinhardt admits that this bounce model has its demons. For one, the ghost field is just a placeholder, and the true nature of the one that gave us a push is as mysterious as whatever would have driven inflation.

Further scrutiny of our most detailed maps of the CMB, made by the European Space Agency’s Planck satellite, could help determine which model wins out. For instance, only the rapid outward force of inflation could have produced ripples in space-time called primordial gravitational waves, and the hunt is on to spot them in the CMB.

This article appeared in print under the headline “Bouncing baby cosmos gets a push”