Warp and Woof

Science  04 Jan 2008:
Vol. 319, Issue 5859, pp. 46
DOI: 10.1126/science.319.5859.46

With all respect to the poet Dylan Thomas, who depicted the moonless sky as “starless and bible-black,” a clear night is actually ablaze with specks of light. For a long time, we thought those isolated stars were the whole story. But researchers now realize that they are embedded in a filamentary structure of matter both dark and visible, called the cosmic web. In this issue, a News feature and three Perspectives bring us up to date on just what we know about the cosmic web and what we still want to know.

For starters, Adrian Cho (p. 47) surveys the many techniques astronomers are using to trace the web. Their goal is both simple and ambitious: to map everything they can see. Their tools include large-scale surveys of galaxies and galactic clusters, which probe the interplay of dark matter and dark energy; weak gravitational lensing, which detects matter by analyzing how it affects passing light; and techniques for viewing the universe through microwaves and other radiation outside the usual optical and infrared wavelengths. To make real progress, however, theorists must clear up some fundamental mysteries about how galaxies form and evolve.

As Ibata and Lewis explain (p. 50), the cosmic web shows up in our own galactic backyard. Galaxies are distributed not randomly but along the tendrils of the cosmic web, yet this pattern can only be explained by large amounts of connective dark matter. “Just like Swift's big fleas and little fleas,” the authors write, these structures should occur at all distance scales, including within the Milky Way. Astronomers have marshaled a large observational effort to understand the web at various scales, but how do they make sense of the data?

This is where advances in modeling and theory can help. Faucher-Giguère et al. (p. 52) discuss supercomputer simulations that have enhanced our understanding of where the cosmic web came from. Tiny fluctuations in the very early universe have grown over billions of years into the massive cosmological structures we see today. The distribution of galaxies predicted by computer models has been dramatically confirmed by observational surveys, but much more needs to be done. New simulation techniques will have to provide higher resolution while modeling the cosmic web over immense distances.

CREDIT: MARK SUBBARAO/ADLER PLANETARIUM & UNIVERSITY OF CHICAGO

These questions connect at the smallest scales too, as Nicastro et al. describe (p. 55). The exact nature of the dark matter that makes up 95% of the cosmic web remains baffling, but things aren't much better for the remaining 5% that we can see. These are the baryons—protons and neutrons—that make up ordinary matter, yet we can account for only about half of the baryon mass predicted by the standard cosmological model. The missing baryons might be lurking in the cosmic web, and perhaps we could detect the warm-hot intergalactic matter, or WHIM, that could be their signature. Detecting these WHIMsical filaments will require both closer analysis of existing data and future observational missions.

Astronomers and physicists have given us a good picture of how the elaborate filigree of matter in the cosmic web arose from the whispering echoes of the Big Bang, and the view is stunning. Their continuing efforts to map the cosmic web should begin to nail down the nature of the missing matter, whether dark or light, and tell us more about how the universe came to be and how it is evolving.

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