— A map of the cosmic microwave background made by NASA's WMAP satellite shows density fluctuations in the early universe – regions of higher density were slightly warmer than voids. Similar maps by Planck may reveal the existence and shape of extra spatial dimensions (Image: NASA/WMAP Science Team)
Our 3D universe may be floating in a higher dimensional space. A six-dimensional object would leave a cross section similar to the one above (Illustration: Andrew J Hanson/Indiana U)
A satellite to be launched next year could see signs of extra dimensions in the afterglow of the big bang, a new study says.
Some theories such as string theory that attempt to unify all known forces into a single "theory of everything" posit the existence of extra spatial dimensions beyond the three familiar ones.
But string theory has proven stubbornly resistant to experimental tests (although some physicists say it could be tested in the Large Hadron Collider scheduled to open by the end of 2007).
Now, Gary Shiu and Bret Underwood, both physicists at the University of Wisconsin in Madison, US, say the shape of the extra dimensions could leave an imprint in the afterglow of the big bang. This glow, called the cosmic microwave background, reveals the structure of the universe about 370,000 years after the big bang.
They use a popular model of the universe's early growth called Dirac-Born-Infeld (DBI) inflation, which is inspired by string theory. It is one of a class of ideas called braneworld models, which state that our universe is like a sheet of paper floating in a higher dimensional space.
Because we are confined to our 3D universe, we ordinarily have no way of seeing the extra dimensions. But Shiu and Underwood show that in this scenario, the big bang's afterglow is affected by the precise shape that the extra dimensions take.
The effects are subtle, however, and Underwood points out that other inflation theories that do not require extra dimensions could probably produce similar signals. So the effects could not be used on their own as proof that extra dimensions are present.
But if some other evidence is found that the DBI inflation scenario is correct, the new study shows that the detailed shape of those extra dimensions could be distinguished by Europe's Planck satellite, which is set to be launched in July 2008 (see the blog Precision cosmology with Planck).
Previous work by Shiu and others has indeed shown that DBI inflation could be distinguished from other scenarios through another effect on the cosmic microwave background by causing the measured clumpiness of matter in the early universe to deviate in a characteristic way from a random distribution.
"The really exciting thing is that it's possible in principle extra dimensions can actually show up in observations," Underwood told New Scientist. "If you know that you're in this particular model, then these details of the geometry can actually be seen."
Licia Verde, a cosmic microwave background researcher at the University of Pennsylvania in Philadelphia, US, says the predicted effects are big enough to be noticeable by Planck. "The prospects are exciting," she told New Scientist.
String theorist Robert Myers of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, cautions, however, that DBI inflation is just one of many scenarios proposed to explain the early universe, each of which could produce subtle effects in the microwave background.
"DBI is one variant which seems to be consistent with what we know," he told New Scientist. "But it will be very difficult to use CMB observations alone to single out the correct story."
Journal reference: Physical Review Letters (vol 98, 051301)