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The Cosmic Web

Observations and simulations of the intergalactic medium reveal the largest structures in the universe

Robert Simcoe

There is no such thing as empty space. The idea of absolute emptiness realizes its closest approximation in the barren expanses between the stars and the galaxies, but even the most remote corners of the universe are suffused with very low density gas—which becomes increasingly rarefied as one ventures farther away from the places where galaxies consort. Consider this fact: In the air we breathe, each cubic centimeter contains roughly 5 X 1019 atoms. In contrast, the intergalactic medium has a density of only 10–6 particles per cubic centimeter—each atom inhabits a private box a meter on each side. This would seem to suggest that there is not much matter in the intergalactic medium. But, given the enormous volume between the galaxies, it quickly adds up: The combined atomic mass of intergalactic gas exceeds the combined atomic mass of all the stars and galaxies in the universe—possibly by as much as 50 percent! There is indeed something in empty space.

As cosmologists construct new narratives of the universe's evolution from its beginning—the Big Bang—to the present day, it is becoming clear that we must understand the physics of intergalactic matter if we are to write the history of how the galaxies, stars and planets formed. In the past decade, rapid advances in both the design of telescopes and computing power have allowed us to study the remote corners of intergalactic space in unprecedented detail. These new results deepen our understanding of how the grandest structures in the universe formed and evolved.

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