THE universe was born as a space-time bubble that popped up inside an even bigger metaverse. So hint tiny anomalies in our best maps yet of the infant cosmos.
Our most widely accepted theory of cosmic origins says that the very young universe inflated from a pinprick to a huge expanse in a sliver of a second – a process called inflation.
Ten years ago the first maps of the cosmic microwave background (CMB) – the light emitted shortly after the big bang – gave strong support to this theory. The maps showed that the soup of matter filling the early universe varied slightly in density. These variations were the seeds of the stars, galaxies and galaxy clusters that form the large-scale structure we see today.
The fluctuations look nearly the same in any random sample of the sky, leading astronomers to conclude that all matter was once in contact and then got suddenly shoved apart – just as inflation theory predicts.
But the first maps also showed that the density variations are more marked on one side of the sky than the other. In March 2013, the Planck probe made the highest-resolution CMB map yet, and it too shows this asymmetry.
Andrew Liddle of the University of Edinburgh, UK, and colleagues sought an explanation. They went back to a theory published in 2008 by Sean Carroll and colleagues at the California Institute of Technology in Pasadena, which suggested that the small variations are superimposed on a disturbance spanning the observable universe, like small waves carried on a big ocean wave.
"The trouble with that is they just made it up," says Liddle. "There's no reason why that should be." But then he recalled a cosmological model called bubble nucleation, which he had worked on in the 1990s. In this picture, our universe arose from quantum fluctuations in a much bigger cosmos called a metaverse. The quantum effects caused a phase transition in the fabric of the metaverse, and our universe popped into being, like an air bubble forming in boiling water.
Weaving the two ideas together, Liddle and colleagues have shown that when inflation happens in a bubble universe, it naturally gives rise to large disturbances in space-time that could account for the lopsided CMB (Physical Review Letters, doi.org/ns2).
The bubble model always makes a universe that can be thought of as saddle-shaped, whereas the density of visible matter in the CMB maps suggest it should be flat. That's OK, the team concludes. The cosmos could appear flat up until the edge of the observable universe, where the saddle's curves would begin.
Some cosmologists argue that the CMB asymmetry might be a statistical fluke. "But if the anomaly does turn out to be there, this physical model explains it more naturally than any of the ones I am aware of," says Dragan Huterer of the University of Michigan, who was not involved in the work.
So what might be outside our bubble? Can we ever escape it, or could stuff come in from beyond?
Probably not, says Marc Kamionkowski, a co-author of the 2008 paper and now at Johns Hopkins University in Baltimore, Maryland. The bubble walls are expanding at the speed of light, so we could never catch up with them.
And if something could enter our universe from the metaverse, from our perspective it would seem to appear at the moment of the big bang, and would get mixed with the rest of the early matter. Today such an intruder would only be noticeable as an oddly large cluster of galaxies.
This article appeared in print under the headline "Cosmic baby snaps hint at bubbly birth"
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