Using Supercomputers to Probe the Early Universe

Artist's depiction of the WMAP satellite gathering data to understand the Big Bang. Source: NASA.

For decades physicists have been trying to decipher the first moments after the Big Bang. Using very large telescopes, for example, scientists scan the skies and look at how fast galaxies move. Satellites study the relic radiation left from the Big Bang, called the cosmic microwave background radiation. And finally, particle colliders, like the Large Hadron Collider at CERN, allow researchers to smash protons together and analyze the debris left behind by such collisions.

Physicists at Los Alamos National Laboratory, however, are taking a different approach: they are using computers. In collaboration with colleagues at University of California San Diego, the Los Alamos researchers developed a computer code, called BURST, that can simulate a slice in the life of our young cosmos.

While BURST is not the first computer code to simulate conditions during the first few minutes of cosmological evolution, it can achieve better precision by a few orders of magnitude compared to its predecessors. Furthermore, it will be the only simulation code able to match the precision of the data from the Extremely Large Telescopes currently under construction in Chile. These new telescopes will have primary mirrors that range in aperture from 20 to 40 meters, roughly three times wider than the current very large telescopes, and an overall light-collecting area up to 10 times larger.

A few seconds after the Big Bang, the universe was composed of a thick, 10-billion degree "cosmic soup" of subatomic particles. As the hot universe expanded, these particles' mutual interactions caused the universe to behave as a cooling thermonuclear reactor. This reactor produced light nuclei, such as deuterium, helium, and lithium — all found in the universe today. "Our code, developed with Evan Grohs, who at the time was a graduate student at UCSD, looks at what happened when the universe was about 1/100 of a second old to a few minutes old," says Los Alamos physicist Mark Paris of the Theoretical Division. "By determining the amount of helium, lithium and deuterium at the end of those first few minutes of life, BURST will be able to shed light to some of the existing puzzles of cosmology."

One such puzzle is dark matter: physicists know that such matter exists because of the way galaxies rotate, but they haven't been able to detect it because it does not radiate in any known spectrum. Physicists have theorized that dark matter is made of so-called "sterile neutrinos", which do not interact with any other particle and are responsible for these unobservable interactions. "Once we start getting data from the Extremely Large Telescopes," Paris explains, "we will model sterile neutrinos into the BURST code. If we get a good description, we will be able to prove their existence."

Measurements of the cosmic microwave background radiation have led physicists to theorize "dark radiation," a speculative form of energy that may have acted in the early universe. BURST could possibly reveal whether or not dark radiation is real and caused by sterile neutrinos. "The universe is our laboratory," Paris enthusiastically concludes. "BURST will help us answer questions that are currently very difficult to address with particle colliders like the one at CERN."

Ongoing support for the project is provided by the National Science Foundation at UCSD and the Laboratory Directed Research and Development program through the Center for Space and Earth Sciences at Los Alamos. BURST will be running on the supercomputing platforms at Los Alamos National Laboratory.

Reference
Grohs, E., Fuller, G., Kishimoto, C., Paris, M., & Vlasenko, A. (2016). Neutrino energy transport in weak decoupling and big bang nucleosynthesis Physical Review D, 93 (8) DOI: 10.1103/PhysRevD.93.083522