A BUBBLE of space that expanded differently to the rest of the early universe could explain a strange “cold spot” in the afterglow of the big bang. Such bubbles might have formed just fractions of a second after the universe came into existence, when it grew dramatically in size.
“It would be an unexpected gift, because we would all of a sudden have a very good window on inflation,” says Anže Slosar of the Brookhaven National Laboratory in Upton, New York, part of the team that put forward the idea.
Much of our insight into the early universe comes from light emitted when it was just 380,000 years old, the oldest light we can still detect. In 2001, the was launched to map this cosmic microwave background (CMB). Data collected in the probe’s first year revealed something unexpected: an anomalous cold spot, which cosmologists have struggled to explain.
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Perhaps the spot is a hint of another universe, or maybe just an artefact of analysis. A large void that sits between our galaxy and the CMB might also explain the cold spot, but hunts for such cosmic holes have come up dry.
Now there’s another option. What if the expansion of the universe was not uniform, contrary to the prevailing theory? Bubble-shaped regions could have expanded out of sync with the rest of the universe, says of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, along with Slosar and another colleague.
Such bubbles would have a lasting effect on the density distribution of the universe. While their centres would look the same as the rest of the universe, the density of regions near their edges would vary from the average. Afshordi’s team calculate that had such a bubble wound up between the CMB and Earth, it would interact with the photons from the CMB to create the appearance of a cold spot ().
It is “an exciting if exotic possibility”, says of the University of Michigan in Ann Arbor, who was not involved with the study. A lumpy inflation is not as elegant as a smooth one, he notes. “It sort of breaks Occam’s razor, but then again there’s no reason to think that inflation was the simplest model.”
Evidence of such a bubble could be found in future large-scale galaxy surveys or show up as a hot ring around the cold spot in data from the European Space Agency’s Planck telescope, which is also mapping the CMB.
