In Lopsided Map of the Cosmos, a Glimmer of Its Origins
Image by ESA and the Planck Collaboration
An unexplained feature appears in the Planck satellite image of the early universe: At the largest scales, temperature fluctuations are more extreme in the half of the sky to the right of the gray line than to the left.
If our universe slammed into a neighboring one during a growth spurt in its first second, the collision would have left a mark.
And Matthew Kleban thinks he sees it in the most detailed snapshot yet taken of the dawn of the universe. The satellite image, released by astronomers in March,confirmed what an earlier image suggested: Half of the young cosmos was slightly coarser than the other.
With few other leads about what went on in the early moments of the universe, Kleban is among dozens of theoretical cosmologists trying to piece together a cosmic origin story from the grainy shadow of a new clue.
Natalie Wolchover/Simons Science News
Matthew Kleban, an associate professor of physics at New York University, and graduate student Marjorie Schillo discussing what would happen if two bubble universes collided.
“When they smack into each other, there’s kind of a shock wave that propagates into our universe,” said Kleban, an associate professor of physics at New York University. Such a shock wave — if that’s what the image shows — would be evidence in support of the multiverse hypothesis, a well-known but unproven idea that ours is one of infinite universes that bubbled into existence inside a larger vacuum.
Most of the cosmologists are quick to admit they could be following a false trail.
“This is a high-stakes game,” said Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins University who has proposed several new Big Bang models to explain the asymmetry between the two halves of the cosmos. “We’d really like to learn more about where our universe came from, but nature has not left us with too many hints.”
The asymmetry “might be a statistical fluke,” Kamionkowski said, or “it could really be the tip of the iceberg.”
Only time, and clever tests, will tell.
The asymmetry of our universe appears in the cosmic microwave background — the staticky afterglow from the moment the universe became transparent, 380,000 years after the Big Bang. The fog of charged particles that until then had enshrouded the cosmos cooled down enough to congeal into neutral atoms, freeing light to travel unimpeded through space for the first time. Over the past three years, the European Space Agency’s Planck satellite captured a 50-megapixel image of this light coming from all directions, each photon imprinted with a record of the temperature where it originated more than 13 billion years ago.
Illustration by NASA/WMAP Science Team
Cosmologists think quantum fluctuations at the time of the Big Bang became stretched during a period of exponential growth known as inflation, turning into hot and cold spots that served as the seeds of galaxies and voids.
The cosmic microwave background indicates that the temperature throughout the 380,000-year-old universe was nearly uniform, deviating from average by just 1 part in 100,000. Its marginally “hot” and “cold” spots — the seeds of future galaxies and voids — are believed to have stemmed from quantum fluctuations, or random ripples of energy, that were amplified during a flash of exponential growth within the universe’s first instant, known as inflation.
Cosmologists want to retrace the steps of inflation back to its cause.
Lacking a theory of how physics works at the extremely hot and small scales that existed in the newborn universe, they currently have only a simple “toy model” of the event: An inflation field permeating all of space transitioned to an unstable state approximately 10-36 seconds after the Big Bang, causing space to balloon 1078 times in volume before the inflation field restabilized about 10-30 seconds later. According to this model, the cosmos should have stretched evenly, producing a uniformly random, speckled pattern of hot and cold in the cosmic microwave background. But that’s not what the data suggest.
“On one side, the hot spots and cold spots are hotter and colder than on the other side,” Kamionkowski explained.
The Wilkinson Microwave Anisotropy Probe, or WMAP, first detected evidence that temperature fluctuations were more extreme in one half of the cosmic microwave background than the other in 2007, but it could have been a measurement error. The Planck map strengthened the case for asymmetry and resolved the temperature fluctuations in finer detail, enabling physicists to rule out some explanations and come up with others.
Like topographical differences in the United States, the asymmetry in the temperature fluctuations across the universe is most visible on large scales. A square foot of land in Colorado is no bumpier than a square foot in Indiana, but if you zoom out, mountains and valleys are clearly taller and deeper in Colorado. “You can think of one part of the sky as Indiana and another part as Colorado,” said Donghui Jeong, a postdoctoral researcher in Kamionkowski’s group. “This variation is really odd. It’s hard to imagine what causes it.”
Some cosmologists chalk it up to a statistical fluke. The odds that quantum fluctuations at the birth of the universe could have randomly generated the observed asymmetry are between 0.1 and 1 percent — about the same as a repeatedly tossed coin coming up heads eight times in a row.
“If I were to bet and the odds were even money, I’d bet it was just a fluke,” said Sean Carroll, a cosmologist at the California Institute of Technology. “But the point is that the odds are not even money. If it is telling us something about the early universe, it could be extremely important.”
Cosmologists have already advanced several competing theories to explain how events during and immediately after the Big Bang could have carved this asymmetry into the cosmos.
Few believe the toy model, with its inflation field plopped into place, can fully explain what jumpstarted the universe. Instead, the field could be one of the extra, curled-up dimensions of space that are postulated by a hypothetical “theory of everything” called string theory, which would likely involve more than one inflation field. In a paperposted to the physics preprint site arXiv.org in May, John McDonald, a cosmologist at Lancaster University in the United Kingdom, showed that a two-field model could have caused the asymmetry in the cosmic microwave background as long as the second field, called a curvaton, decayed after inflation ended and after the formation of dark matter.
Alternatively, as described in an article that will appear in the journal Physical Review D, Kamionkowski and colleagues calculate that the asymmetry could have resulted from the variation of certain cosmological parameters across the universe. The most promising model, in which there is a 6-percent drift in a parameter from one side of the universe to the other, “accounts for all the observations fairly comfortably,” Kamionkowski said. The parameter could be anchored to different values at separate defects in the fabric of space-time, which, according to some theories, could be the catalysts of inflation.
Image by ESA and the Planck Collaboration
In March 2013, the Planck satellite produced a 50-megapixel image of the oldest light in the universe, called the cosmic microwave background.
Or, as Kleban and his collaborators argue in a paper published in Physical Review D in February and in a forthcoming paper that incorporates the Planck data, the asymmetry could be the aftermath of a violent collision between two universes or between two points within this universe. In the multiverse scenario, bubbles would frequently pop up close together and collide. Bubbles could also run into themselves while expanding around a curled-up dimension of space (imagine a circle growing on the surface of a cylinder). The collision could then have triggered inflation.
If the shock wave from such a collision were seen cutting through the cosmic microwave background, it would be a smoking gun for the multiverse, Kleban said. But the leading edge of the shock wave is more likely to have moved beyond the horizon of this observable patch of the universe like a ship that passed in the night, trailing gentler turbulence in its wake. The Planck map might depict the stretched-out remnants of such a trail.
Those remnants “would affect the largest scales we can see,” Kleban said. They would have increased in scale as the universe inflated, resulting in an effect similar to the topographical differences between Colorado and Indiana.
Because each of the new inflation models makes its own prediction about the direction in which the ancient light should be polarized, a new “polarization map” of the cosmic microwave background expected to be released by the Planck team next year should help identify which proposal, if any, holds promise.
For now, the theorists must tailor their Big Bang theories around the data in hand. “There are always things which you can’t prove because we just don’t have the technology yet,” Kleban said. “You just have to take your shots and do your best.”