Hot pursuit of missing matter

摘要

Astronomers are going to extraordinary lengths in the quest to tot up the 'ordinary' matter in the Universe. The latest initiative has probed hot gas in intergalactic space by means of an X-ray lighthouse. As cosmological measurements have become feasible and more precise, astronomers have undertaken 'cosmic inventories' of mass and energy in all their forms. Nature is quite devious, hiding matter in a range of astronomical objects (stars, planets, galaxies), in intergalactic gas at temperatures of 10~4 K to 10~7 K, and in even more exotic forms — so-called dark matter and dark energy. However, recent inventories suggest that at least 40% of ordinary matter is missing. Using spectrographs aboard the Chandra X-ray Observatory, a group of X-ray astronomers believe they have detected this missing matter, hiding in gas at temperatures of around a million kelvin, and spread throughout vast volumes of intergalactic space (Nicastro et al, page 495 of this issue).