Peering
backward in time to an instant after the big bang, physicists at the University
of Wisconsin-Madison have devised an approach that may help unlock the hidden
shapes of alternate dimensions of the universe.
A computer-generated
rendering of a possible six-dimensional geometry similar to those studied by
UW-Madison physicist Gary Shiu. (Image: courtesy Andrew
J. Hanson, Indiana
University) A new study demonstrates that the shapes of extra dimensions can be
"seen" by
deciphering their influence on cosmic energy released by the
violent birth of the universe 13 billion years ago. The method, published today
(Feb. 2) in Physical Review Letters, provides evidence that physicists can use
experimental data to discern the nature of these elusive dimensions - the
existence of which is a critical but as yet unproven element of string theory,
the leading contender for a unified "theory of everything."
Scientists
developed string theory, which proposes that everything in the universe is made
of tiny, vibrating strings of energy, to encompass the physical principles of
all objects from immense galaxies to subatomic particles.
Though currently
the front-runner to explain the framework of the cosmos, the theory remains, to
date, untested.
The mathematics of string theory suggests that the world we
know is not complete. In addition to our four familiar dimensions -
three-dimensional space and time - string theory predicts the existence of six
extra spatial
dimensions, "hidden" dimensions curled in tiny geometric
shapes at every single point in our universe.
Don't worry if you can't
picture a 10-dimensional world. Our minds are
accustomed to only three
spatial dimensions and lack a frame of reference for the other six, says
UW-Madison physicist Gary Shiu, who led the new study. Though
scientists use
computers to visualize what these six-dimensional geometries could look like
(see image), no one really knows for sure what shape they take.
The new
Wisconsin work may provide a long-sought foundation for measuring this
previously immeasurable aspect of string theory.
According to string theory
mathematics, the extra dimensions could adopt any of tens of thousands of
possible shapes, each shape theoretically
corresponding to its own universe
with its own set of physical laws. For our universe, "Nature picked one - and we
want to know what that one looks like," explains Henry Tye, a physicist at
Cornell University who was not involved in the new research.
Shiu says the
many-dimensional shapes are far too small to see or measure through any usual
means of observation, which makes testing this crucial aspect of string theory
very difficult. "You can theorize anything, but you have to
be able to show
it with experiments, " he says. "Now the problem is, how do we test it?"
He
and graduate student Bret Underwood turned to the sky for inspiration. Their
approach is based on the idea that the six tiny dimensions had their strongest
influence on the universe when it itself was a tiny speck of highly compressed
matter and energy - that is, in the instant just after the big
bang.
"Our idea was to go back in time and see what happened back then," says
Shiu. "Of course, we couldn't really go back in time."
Lacking the requisite
time machine, they used the next-best thing: a map of cosmic energy released
from the big bang. The energy, captured by satellites such as NASA's Wilkinson
Microwave Anisotropy Probe (WMAP), has persisted
virtually unchanged for the
last 13 billion years, making the energy map basically "a snapshot of the baby
universe," Shiu says. The WMAP experiment is the successor to NASA's Cosmic
Background Explorer (COBE) project, which garnered the 2006 Nobel Prize in
physics.
Just as a shadow can give an idea of the shape of an object, the
pattern of cosmic energy in the sky can give an indication of the shape of the
other six dimensions present, Shiu explains. To learn how to read telltale signs
of the six-dimensional geometry from the cosmic map, they worked backward.
Starting with two different types of athematically simple geometries, called
warped throats, they calculated the predicted energy map that would be seen in
the universe described by each shape. When they compared the two maps, they
found small but significant differences
between them. Their results show
that specific patterns of cosmic energy can hold clues to the geometry of the
six-dimensional shape - the first type of observable data
to demonstrate
such promise, says Tye. Though the current data are not precise enough to
compare their findings to
our universe, upcoming experiments such as the
European Space Agency's Planck satellite should have the sensitivity to detect
subtle variations between different geometries, Shiu says.
"Our results with
simple, well-understood shapes give proof of concept that the geometry of hidden
dimensions can be deciphered from the pattern of cosmic energy," he says. "This
provides a rare opportunity in which string theory
can be tested."
Technological improvements to capture more detailed cosmic maps should help
narrow down the possibilities and may allow scientists to crack the code of the
cosmic energy map - and inch closer to identifying the single geometry that fits
our universe.
The implications of such a possibility are profound, says Tye.
"If this shape can be measured, it would also tell us that string theory is
correct."
The new work was funded by grants from the National Science
Foundation, the U.S. Department of Energy, and the Research Corp. Note: This
story has been adapted from a news release issued by University of
Wisconsin-Madison.