In 3-D computer movies created by Borrill and his colleague Kevin
Campbell, objects called "semilocal strings" condense out of interacting quantum
fields to form writhing tubes of energy. Some link with other tubes in space-spanning
filaments. Some, like the Worm Ourobouros, join head to tail and devour themselves,
ultimately popping out of existence. These images, redolent of alchemy but firmly grounded
in theoretical physics, may provide insight into the past and present structure of our
universe.
Borrill, a postdoctoral fellow at NERSC, is working with researchers at the Center for
Particle Astrophysics at the University of California, Berkeley, to answer fundamental
cosmological questions. Among them: if the universe began in equilibrium, why is there now
far more matter than antimatter? Why, given its exceedingly smooth beginnings, is the
universe so clumpy, on all scales from galaxies to galactic superclusters?
"Given enough time, gravity can do the job of building stars and galaxies and
larger structures," says Borrill, "so long as the right sort of initial
perturbations occurred in the density of the very early universe. One candidate for
causing those perturbations is the semilocal string."
Borrill stresses that semilocal strings are not to be confused with the fundamental
entities of string theory, which may give rise to the particles of the subatomic world.
Rather they are related to other putative inhabitants of the very early universe, cosmic
strings. While cosmic strings are purely a product of the topology of the vacuum, however,
semilocal strings involve a complex interplay of quantum matter and force fields.
"Semilocal strings are more complicated," says Borrill. "They are like
magnetic tubes with north and south poles. They originate in a four-dimensional vacuum; it
takes eight quantum fields to construct them—four matter fields and four force
fields."
What traditional cosmic strings and semilocal strings have in common is a link to phase
transitions in the early universe. In a way analogous to expanding water vapor, which
condenses to liquid water and then freezes to ice, all the disparate forces seen
today—electromagnetism, the weak force, the strong force, and
gravity—"condensed" from the single, unified force that existed at the
moment of the Big Bang. During these phase changes strings could have been generated, and
with them the primordial density fluctuations that were the seeds of large-scale
structure.
Semilocal strings have theoretical advantages over cosmic strings, however. For one
thing, says Borrill, "they could answer the question of why there is more matter than
antimatter in the universe. One place to look for the generation of this
asymmetry—so-called baryogenesis—is in interactions on the surfaces of these
magnetic tubes."
Until Borrill’s recent work on the Cray T3E, however, the strings were too complex
to model, much less understand. Previous calculations on workstation computers could
handle only a million initial quantum field values, simulating a tiny volume of the
universe—far too small to investigate the strings' properties.
"Some people claimed semilocal strings couldn’t form, or if they did, it
wouldn’t make a difference," Borrill says. "If only a few formed—if
their density was too low—they might just close up on themselves, shrink, and quickly
disappear. "
The NERSC supercomputer allowed Borrill to specify well over 3 billion initial quantum
field values. Once the initial conditions had been set, the Cray was set loose to
calculate the evolution of the system.
To interpret the results of the simulations, Borril worked with Kevin Campbell of
Berkeley Lab’s Visualization Group, generating 3-D images and movies that enabled
them to get a qualitative understanding of the strings' behavior; many of these images and
movies are available on the web at
.
"We couldn’t have known what we were going to see," Borrill says.
"In fact we proved that semilocal strings can exist—enough strings formed that
they tended to join onto their neighbors rather than themselves, so that many of them
rapidly grew, and the network of strings as a whole persisted. But I was surprised that
there was no intercommuting—where two strings cross each other and swap
partners—which is considered a crucial process in the case of cosmic strings."
Having done the initial calculations as a proof of principle, Borrill says, "we
can now address more complicated questions," including further studies of baryon
formation and of the implications for patterns of fluctuations in the cosmic microwave
background radiation—the earliest moment in the history of the universe which can be
directly observed.
"It’s a challenge to try to test theories of the early universe when the only
observations we can make are billions of years after the fact," says Borrill.
"Computers are essential to model the initial conditions and see how they evolve, so
we can compare the results with what we can observe. That’s why we need machines like
the 512-processor Cray T3E at NERSC." Borrill jokes that the computer-generated
strings in his movies are "bigger than the Titanic and a fraction of the cost."
Borril and his colleagues published earlier results on semilocal string formation in Physical
Review D, Volume 57, Number 6, 15 March 1998. Recent results have been
submitted to Physical Review Letters.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research and is managed by the
University of California.
