FEATURE ARTICLE
The Cosmic Web
Observations and simulations of the intergalactic medium reveal the largest structures in the universe
Robert Simcoe
Fiat Lux
It seems preposterous that something as small as a star could affect the universe on intergalactic scales. After all, a star is only a few light seconds across, whereas the filaments of the cosmic web may extend for billions of light-years. How can a relatively tiny object impact such a large volume? The answer lies in how stars work, where they live and what happens when they die.
Before there were stars, the normal matter in the universe was composed almost entirely of hydrogen and helium. Astronomers refer to this mixture as a chemically pristine gas because it reflects the chemical composition of the cosmos just after the Big Bang. Since then, nearly every atom of every other element—from argon to zinc—was forged inside a star. Stars are effectively nuclear fusion reactors: They gravitationally compress gas to such high densities that light atomic nuclei smash together to form heavier elements. Such stellar nucleosynthesis releases enormous amounts of energy, and that's what makes the stars shine.
Nucleosynthesis had several important effects on the intergalactic medium. First, it generated starlight, which escaped into intergalactic space and interacted with the neutral atoms. Later, the newly minted heavy elements were ejected into the intergalactic medium by strong galactic winds—powerful expulsions of hot gas—that stirred up and "polluted" vast regions of the universe.
Let's consider these processes in more detail by returning to the cosmic web (Figure 1, top). Because galaxies are more than 10,000 times denser than the cosmic average, we would expect to find systems like the Milky Way within dense regions of the web itself, which contain the raw materials (gas reservoirs) needed to build the stars and galaxies.
In simulations, the densest regions are found within the web's filaments, especially where several intersect. Therefore, on cosmic scales, galaxies should behave like tiny particles trapped in the strands of the web, actually tracing the much larger structures outlined by intergalactic gas. Recent three-dimensional galaxy surveys, such as the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey, have indeed revealed a filamentary pattern in the way that galaxies cluster (Figure 6). Research groups, led by Max Tegmark at the University of Pennsylvania and Rupert Croft at Carnegie Mellon University, are currently investigating the clustering statistics of galaxies relative to those of the intergalactic gas as seen in quasar spectra. Their early results suggest that the same physics underlies the assembly of the intergalactic gas network and large-scale galaxy structures.
As the galaxies coalesced out of the web and began to shine, the universe was filled with the first new light since the Big Bang—the dark era had ended. And the stars dutifully began to churn out heavy elements. When enough stars had formed, the cumulative production of light and chemicals began to alter the nature of the intergalactic medium itself. Astronomers refer to these collective effects as "galaxy feedback," because the galaxies act on the surroundings from which they formed. Here I'll only consider two types of feedback, radiation and chemical pollution.
The first agent of galaxy feedback was starlight, which reionized the intergalactic medium. Recall that normal matter began to form large structures during the era of recombination, when the protons and electrons teamed up to form hydrogen atoms, so the gas in the universe was, for a time, entirely neutral. It was also very cold, reaching gas temperatures only a few tens of degrees above absolute zero. When the first stellar photons leaked out from galaxies, they interacted with the hydrogen atoms, stripping away the electrons that had been in place since the era of recombination and reheating the resulting plasma up to temperatures near 10,000 kelvins. Reionization was initially confined within bubbles centered on the fledgling galaxies, because the starlight had not yet traveled far out into intergalactic space. As more galaxies began to shine, the ionized bubbles grew outward until those from adjacent galaxies began to overlap. Soon the entire volume of the universe was once again ionized (Figure 5).
We now believe that the universe finally emerged from its "dark ages" and was reionized when it was less than 1 billion years old, or about 10 percent of its present age. Today, only about 1 hydrogen atom in 10,000 is in a neutral state and the average temperature of intergalactic gas is still very near 10,000 kelvins.
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