Universe: The Movie
When we look up into a clear night sky, we’re looking into the past. Even with the naked eye, we can see the Andromeda galaxy as it was 2.3 million years ago. Earth- and space-based telescopes provide us with a plethora of snapshots of the history of the Universe. But none of these snapshots show how the Universe or any of the objects within it evolved.
“Turning the snapshots into movies is the job of theorists,” says Joel Primack, Professor of Physics at the University of California, Santa Cruz. “With computational simulations, we can run the history of stars, galaxies, or the whole Universe forward or backward, testing our theories and finding explanations for the wealth of new data we’re getting from instruments on the ground and especially in space.”
Primack is one of the originators and developers of the theory of cold dark matter, which has become the standard theory of structure formation in the Universe because its predictions generally match recent observations. His research group uses NERSC’s Seaborg computer to create simulations of galaxy formation. Their work here has two major thrusts: simulations of large-scale structure formation in the Universe, and simulations of galaxy interactions.
How did the Universe evolve from a generally smooth initial state after the Big Bang (as seen in the cosmic microwave background radiation) to the chunky texture that we see today, with clusters, filaments, and sheets of galaxies? That is the question of large-scale structure formation. In the cold dark matter theory, structure grows hierarchically by gravitational attraction, with small objects merging in a continuous hierarchy to form more and more massive objects.
However, “objects” in this context refers not only to ordinary “baryonic” matter (composed of protons and neutrons), but also to cold dark matter—cold because it moves slowly, and dark because it cannot be observed by its electromagnetic radiation. In fact, the latest simulation results 1 suggest that galaxy clustering is primarily a result of the formation, merging, and evolution of dark matter “halos.”
The concept of dark matter halos came from the realization that the dark matter in a galaxy extends well beyond the visible stars and gas, surrounding the visible matter in a much larger halo; the outer parts of galaxies are essentially all dark matter. Scientists came to this conclusion because of the way stars and galaxies move, and because of a phenomenon called gravitational lensing: when light passes through the vicinity of a galaxy, the gravity of the dark matter halo bends the light to different wavelengths, acting like imperfections in the glass of a lens.
In the view of Primack and his colleagues, the large-scale structure of the Universe is determined primarily by the invisible evolution and interactions of massive halos and subhalos, with ordinary matter being swept along in the gravitational ebbs and flows. They have used Seaborg to create the highest-resolution simulations of dark matter halos to date, producing 10 terabytes of output and models with around 100,000 halos. These models can be used to predict the types and relative quantities of galaxies, as well as how they interact. Figure 1 shows the simulated distribution of dark matter particles and halos at two different stages of structure formation. Interestingly, there are more subhalos in this model than there are visible objects in corresponding observational data. Observations from the DEEP (Deep Extragalactic Evolutionary Probe) and ESO/VLT (European Southern Observatory/Very Large Telescope) sky surveys should soon confirm or refute the predictions of this model.
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| Figure 1 Simulated distribution of dark matter particles (points) and halos (circles) at redshift 3 (left) and redshift 0 (right). (Higher redshift signifies greater distance and age.) Image: Kravtsov et al., astro-ph/0308519. |
Elliptical galaxies, galactic bulges, and a significant fraction of all the stars in the Universe are thought to be shaped largely by galaxy interactions and collisions, with dark matter still playing the major role but with bright matter interactions adding some details. For the first half of the Universe’s history, stars were formed mostly in spectacular “starburst” events resulting from the compression of gases by the gravitational tides of interacting galaxies. A starburst is a relatively brief event on a cosmic scale, lasting perhaps 100 million years, while the merger of two galaxies might take 2 to 3 billion years. The Hubble space telescope in 1996 captured spectacular images of two colliding galaxies and the resulting starburst (Figure 2).
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| Figure 2 |
By creating galaxy interaction simulations (Figure 3) and comparing them with images and data from a variety of telescopes and spectral bands, Primack’s research team tries to understand the evolution of galaxies, including phenomena such as starbursts. They recently focused their attention on how much gas is converted to stars during the course of a major galaxy merger, and found that previous studies had overestimated the amount of starburst. Their results showed that galaxy mergers produce large outflows of gas that inhibit star formation for a while after the merger, an effect that they are adding to their models of galaxy formation.
Like many physicists and astronomers, Primack believes that we are now living in the golden age of cosmology. “Cosmology is undergoing a remarkable evolution, driven mainly by the wonderful wealth of new data,” he says. “The questions that were asked from the beginning of the 20th century are now all being answered. New questions arise, of course, but there’s a good chance that these answers are definitive.”
Research funding: HEP, NSF, NASA
(Organizational acronyms are spelled out in Appendix G)
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| Figure 3 |
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1 A.V. Kravtsov, A. A. Berlind, R. H. Wechsler, A. A. Klypin, S. Gottlöber, B. Allgood, and J. R. Primack, "The dark side of the halo occupation distribution," Astrophysics Journal (submitted); astro-ph/0308519 (2003).
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