To see the light, you sometimes have to journey through darkness. That aphorism, it seems, applies not only to journeys of the heart but also to excursions through the history of the universe. In the largest and most detailed computer simulation of this cosmic saga, something utterly dark shapes the universe as it unfolds over some 13.7 billion years.
That new simulation traces the fate of the universe’s original stocks of energy and matter from just a few hundred thousand years after the Big Bang to the present.
To make sense of the arrangement of starlit galaxies and brilliant quasars across the sky, Volker Springel of the Max Planck Institute for Astrophysics in Garching, Germany, and his colleagues based their work on dark matter. That invisible material accounts for more than 90 percent of the gravity within the universe.
Although no one knows what dark matter is made of, researchers suspect that it’s responsible for pulling galaxies and galaxy clusters into the gargantuan, filamentary structures seen in the sky today. Because dark matter doesn’t seem to interact with any force other than gravity, it’s relatively simple to model. Springel’s team built 10 billion clumps of the stuff into its simulation.
The modelers laid onto this canvas of dark matter a rough approximation of some of the messy and complex interactions between galaxies, such as the eruption of supernova explosions and the trajectories of powerful intergalactic winds. With that, the researchers could explore how the larger structures in the universe—both invisible ones such as dark matter and visible ones such as galaxies—evolved over billions of years.
As described in the June 2 Nature, the model confirms recent findings that the expansion of the universe has sped up. It also suggests a scenario for the surprisingly rapid growth of supermassive black holes—the powerhouses that fuel quasars—early in the history of the cosmos.
The new work “gives us the most detailed and accurate theoretical predictions so far of the properties of galaxies from the dawn of cosmic time to the present day,” comments Nickolay Gnedin of the University of Colorado in Boulder.
In their model, known as the millennium simulation, Springel and his collaborators trace cosmic history in a cube more than 2 billion light-years on a side. That’s large enough to portray the formation of some 20 million galaxies along with the rare supermassive black holes.
Previous dark matter simulations by the same group of researchers depicted a smaller volume of the universe and focused on its biggest visible objects, giant clusters of galaxies (SN: 5/29/99, p. 344). That simulation included only one-tenth as many clumps of dark matter as the new version does.
Because the new model can reveal cosmic features one-thirtieth the width of the smallest features shown in the earlier simulation, astronomers can now depict the growth of individual galaxies and so compare the model with actual telescope observations.
“It’s an impressive technological achievement,” says cosmologist David Weinberg of Ohio State University in Columbus. “They had to do lots of very clever things in order to be able to carry out this big a simulation, even with the very powerful computer resources they have at their disposal. This wasn’t just something that became possible because computers are getting faster.”
Indeed, improvements in computing power weren’t the main motivation for making the new model, notes Gus Evrard of the University of Michigan in Ann Arbor, a member of the millennium-simulation team. The researchers were reacting to the several mammoth surveys of the heavens performed since the late 1990s. The Sloan Digital Sky Survey and the infrared 2-MASS study, among others, have charted the distribution of galaxies and black holes and the structure of the universe in unprecedented breadth and detail.
Theoretical accounts of the large-scale structure of the universe and simulations were falling behind, says Evrard. The survey data “really drove us to try to create a simulation that would match,” he says.
Scientists need to know that their theories and data are consistent with one another “if we are to use these surveys effectively to learn about the origin and nature of our world,” adds team member Simon White of the Max Planck Institute of Astrophysics.
The new study, with its much greater detail than previous models, is at “a qualitatively different level, which allows an analysis of many physical questions which could not be well addressed with previous simulations,” says Springel. Among those questions: Why do galaxies form where they do, and what factors underlie the emergence of quasars, which can shine with the light of trillions of suns?
The simulation required one of the world’s fastest supercomputers and consists of 25 million megabytes of data—enough to fill some 36,000 CDs.
The model assumes that structure began in the universe as random, subatomic ripples in the density of an otherwise uniform soup of material and radiation. Radio telescopes have observed such ripples in snapshots of the early universe.
The simulation predicts that traces of these ripples are imprinted on the modern-day distribution of galaxies as seen in the large telescope surveys. Earlier this year, two teams of astronomers reported that they had indeed spotted reverberations of such ripples in the universe today (SN: 1/15/05, p. 35: Ultimate Retro: Modern echoes of the early universe). The evidence suggests that over billions of years, gravity amplified the ripples to produce the clusters of galaxies seen today.
The millennium model predicts that galaxies start out small and then grow by gravitationally capturing more material. This bottom-up, or hierarchical, prescription of galaxy building dovetails with several directly measured features of galaxies, including their color, brightness, and clustering tendencies.
“I cannot help but be stunned that the whole picture of hierarchical galaxy formation based on a dark matter universe works so well,” says Springel.
In addition to probing the formation of galaxies, the simulation seeks to test a startling view of the universe. Since the late 1990s, investigators have collected evidence that 70 percent of the universe currently consists of an entity even more mysterious than dark matter. Dubbed dark energy, this proposed force field provides a cosmic push that could account for the apparent acceleration in the expansion of the universe.
The millennium simulation has confirmed this view, along with the proposal that dark matter and regular matter account for about 25 percent and 5 percent of the universe’s mass, respectively.
But the simulation also permits theorists to test alternative descriptions of the universe. “You can bring your own rules to the sandbox and find out how well they reproduce the evolution of galaxies,” notes Evrard.
Doing this with rigor, says Weinberg, would require complete mathematical descriptions of star formation and galaxy interactions. “But you can make a lot of progress by sticking [the team’s approximate] recipes into the dark matter simulation,” he adds.
Or perhaps, even better, by sticking in data from direct observations. Over the past 4 years, the Sloan Digital Sky Survey has found several ancient quasars that glow with superlative brilliance. For the quasars to be that bright so early in the history of the universe, the black holes that fuel them must have been a billion times as massive as the sun at a time when the cosmos was only 870 million years old, less than a tenth its present age.
“Many astronomers thought this impossible to reconcile with the gradual growth of structure predicted by the standard dark matter picture,” notes Springel.
When he and his colleagues took a close look at what their model indicated about the young universe, they found indications that a few massive black holes could indeed have formed early enough to account for the rare quasars. The simulation indicates that in unusually dense regions of the early cosmos, black holes grew at an accelerated rate. The model also suggests that the resulting supermassive black holes ultimately became the core of massive galaxies that lie at the centers of the biggest galaxy clusters in the cosmos today, notes Evrard.
Extending the model
Even as the millennium simulation confirms and clarifies existing theories and data, it has produced some unexpected results. Springel, White, and Liang Gao of the Max Planck Institute for Astrophysics used the model to examine the clustering of dark matter halos, the vast envelopes of invisible material whose gravity draws galaxies together into clusters.
The team found that among halos with the same mass, those that formed earlier in the history of the universe bunch together more tightly than those formed later do. Because the distribution of galaxies mimics the distribution of the halos, the finding suggests that galaxies clump more or less tightly, depending on when they formed. The finding, recently reported online (http://xxx.lanl.gov/astro-ph/0506510), indicates that old galaxies cluster more tightly than younger ones do.
Such clustering contradicts a key assumption of a rival theoretical scenario of how the present-day distribution of galaxies emerged, says Springel. In that model, the density of galaxies within a dark matter halo depends only on the mass of the halo.
The most far-reaching applications of the millennium model are still to come, the researchers say. “The data set is so rich that we haven’t yet uncovered all the surprises,” notes Evrard.
One of the biggest immediate challenges, says White, is to share the wealth. The team plans to make the model publicly available so that all astronomers can test their theories of galaxy and quasar formation. With 25 million megabytes of data comprising the model, that’s no easy task.
But by the end of the year, the group says, cosmologists everywhere should have a brand-new—and far-reaching—computer model to play with.