From the August 2005 issue

Simulating cosmic strings

See cosmic strings writhe and change in the early universe, courtesy of a computer called COSMOS.
By | Published: August 29, 2005 | Last updated on May 18, 2023
One of today’s outstanding cosmological questions is how structure first formed in the universe. Cosmic strings certainly could have done the job. A typical string postulated in grand unification theories (GUTs) might span the visible universe and be a trillion times thinner than a hydrogen atom — yet each 6-mile (10 kilometer) segment could equal Earth’s mass. Such massive objects could be the seeds from which the first galaxies formed.
Cosmic strings actually predate inflation as a model for the beginning of structure. While cosmologists charged ahead with inflationary scenarios, they found strings computationally challenging, making it difficult to make predictions and check them against observations.

That’s changed somewhat in the past few years, thanks in part to supercomputers like the National Cosmology Supercomputer, COSMOS, founded January 1997 in the United Kingdom by a consortium of leading cosmologists brought together by Stephen Hawking. The U.K. Higher Education Funding Council for England and Particle Physics and Astronomy Research Council fund COSMOS with help from the U.S. firms Silicon Graphics, Inc. and Intel Corporation. COSMOS has performed some of the largest and most accurate cosmic-string simulations. Now, in its sixth iteration, the supercomputer boasts 152 Intel Itanium processors and holds 152 gigabytes of memory.

The movies below, courtesy of the cosmology group at the University of Cambridge in England, represent a small fraction of simulations run by COSMOS. Each cube shows a fixed volume of space in the early universe. In the young cosmos, which is small, expanding slowly, and filled with radiation and particles moving at significant fractions of light-speed, the cube brims with strings and loops. Look closely. As strings intersect, they form loops through a two-stage process. The collision creates a large loop, which then decays into many smaller “daughter loops.”

As the universe cools and expansion speeds up, the density of strings and loops declines. In this case, the computational cube’s size remains constant, but the universe has expanded, thinning the number of strings and loops. Models such as these have become powerful tools for exploring the cosmological consequences of cosmic strings.

Downloadable File(s)