The Cosmic Web

摘要

There is no such thing as empty space. The idea of absolute emptiness real-izes 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 10~(19) 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.