FEATURE ARTICLE
The Cosmic Web
Observations and simulations of the intergalactic medium reveal the largest structures in the universe
Robert Simcoe
Spinning a Cosmic Web
When the absorption lines of quasars were first studied, it was not at all clear how to interpret them, particularly without the benefit of the high-quality data we have today. From the late 1970s to the early 1980s, Wallace Sargent's team at Palomar Observatory made a series of measurements that convinced most astronomers that these absorption lines represent intergalactic matter. However, a number of theoretical explanations were consistent with the available data, and most models explained the lines as clusters of discrete spherical clouds of gas.
In recent years, the advances in observing techniques have been joined by increasingly powerful computer models, which together deliver a more sophisticated picture of the intergalactic medium. This work involves several collaborations and requires months of supercomputer time. In these simulations, an imaginary box is designed to resemble a large representative volume of the universe. The box is divided up into a three-dimensional grid of cells, and matter is distributed throughout the grid in an initial state—according to conditions set by observations of the early universe. All of the physical processes that affect the evolution of the intergalactic medium are dialed into the model. Then the simulation is "turned on," allowing matter and energy to flow from cell to cell in the box, governed simply by the physics. The final product resembles a cosmic time-lapse movie with millions of years compressed into each frame. The computer code examines the distribution of matter in the box at each frame, or time step, and calculates the total force acting on each particle to determine where it should move in the next step. At regular intervals, the computer records the density of the gas throughout the intergalactic medium, and these results are compared with actual observations of quasar spectra to test the accuracy of the physical models.
One such output, from a simulation run by Jeremiah Ostriker and Renyue Cen of Princeton University, is shown in the top panel of Figure 1. This particular view shows the universe when it was about 15 percent of its present age, or about 2 billion years old. The most striking feature seen is a tendency for gas to collapse into a network of filamentary tendrils that crisscross through vast, low-density voids. This pattern is a common feature of the new computational models and has been nicknamed "the cosmic web."
To test this depiction of the universe against concrete observations, large numbers of artificial quasar spectra are generated by drawing random lines through the simulation box. By evaluating the variations of gas density along any single line, astronomers can calculate the amount of absorption that would be observed in a spectrum measured along that line of sight (Figure 3). It is as though an observer stood on one side of the box and measured the spectrum of a quasar on the other side.
Statistically, the "spectra" from these artificial universes are nearly indistinguishable from the spectra of real quasars. The models accurately predict the number of absorption lines, the distribution of their strengths and widths, and their evolution through time. At a basic level, these models have captured the physical processes that dominate the evolution of the universe on the largest scales.
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