Researchers are using high-powered computer simulations to help understand one of the most puzzling phenomena in science–how the universe is expanding faster than ever.
Ever since 1929, when Edwin Hubble first theorized the Big Bang, cosmologists have been hard at work to define what it all means. Over the course of the 20th century, scientists have come to realize that the universe is comprised of numerous galaxies made up of stars, planets, black holes, gas, and dust, as well as mysterious and invisible dark matter with gravitational properties. More recently, they have hypothesized that the expansion is accelerating due to dark energy, a little-understood cosmic force. The universe is estimated to be made up of approximately 70% dark energy, 25% dark matter, and only 5% normal matter, also called baryons (that includes the galaxies themselves, Earth and we humans).
Despite this explosion of new theories, cosmologists are still struggling to understand the real causes of this accelerated pace of expansion. One way for them to arrive at reasonable and informative conclusions about the true nature of the universe is through the application of sophisticated computer codes and technology that simulates the cosmos. They can then compare such simulations to telescopic observations.
Now, a team of eight U.S. Department of Energy researchers has run a cosmological computer simulation that provides one of the largest, high-resolution portrayals of the expanding universe ever displayed, called the Q Continuum Simulation. The results of their work were outlined in a study subtitled “Harnessing the Power of GPU Accelerated Supercomputers,” published in August by the Astrophysical Journal Supplement Series of the American Astronomical Society.
The unique simulation was created on the DOE’s Oak Ridge National Laboratory’s Titan supercomputer—one of the world’s most powerful that became available to researchers in early 2013— using a code developed since 2008, called the Hardware/Hybrid Accelerated Cosmology Code (HACC), also known for being the only cosmological simulation code that can run on multiple supercomputer platforms worldwide.
“We wanted to create a very detailed synthetic map of the universe that we can use as a cosmic laboratory,” says Katrin Heitmann, an Argonne physicist who led the project. “Also, we wanted to answer questions about how the structures that we observe actually formed. From the observational information we have right now, we set up initial conditions that we put on a computer. We ran this simulated model and can compare the results to observations from telescopes.”
Overall, the simulation shows the formation of structure over time and how gravity acts on dark matter, causing a clumping that leads to so-called halos, where galaxies form. “This is a very rich simulation,” Heitmann adds. “We can use the data to look at how galaxies are distributed, as well as the fundamental physics of structure formation itself.”
“Now we have a synthetic sky map catalog of the galaxies that we can compare to what we see,” she notes, adding that her team was ultimately able to simulate numerous galaxies of much bigger volume in a cosmologically precedent-setting high resolution. The entire process involved 90% of the supercomputer’s power and half a trillion particles that were divided into cube-like images labeled as the “cosmic web,” showing the distribution and evolution of dark matter under the influence of dark energy.
All of the Q Continuum sky maps are under deep analysis and will be for some time. With its combination of large volume and high mass resolution, the entire Q Continuum run claims to be a valuable resource for precision cosmological studies of large-scale structure formation as well as a testbed—providing both theoretical predictions and synthetic sky catalogs for end-to-end survey analyses. In particular, they will be compared to results from the Dark Energy Survey (DES) as well as the Large Synoptic Survey Telescope (LSST), two of the world’s most thorough cosmological surveys.
As noted on the DES site, this survey is probing the origin of the accelerating universe, aiming to uncover “the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 120 scientists from 23 institutions in the United States, Spain, the United Kingdom, Brazil, and Germany are working on the project. This collaboration has built and deployed an extremely sensitive 570-megapixel digital camera, DECam. This new camera has been mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes.”
The LSST project is currently under construction. Plans are to “conduct a 10-year survey of the sky that will deliver a 200-petabyte set of images and data products that will address some of the most pressing questions about the structure and evolution of the universe and the objects in it.” LSST includes building the facilities, also located in Chile, to house a special optical imaging telescope that will create “an exceptionally wide field of view,” with “the ability to survey the entire sky in only three nights.”