The Mystery of the Missing Matter According to the best cosmological models, the total mass of the Universe (inferred from its gravitational force) appears to vastly exceed the mass of matter we directly observe. Estimates of the atomic (or “baryonic”) mass of the Universe based on measured primor- dial ratios of hydrogen, helium, and deuterium can be made. These estimates still exceed the amount that we can actually see in stars and interstellar gas by a factor of ten, suggest- ing that a large component of normal matter is hidden in some way. But the gravitational mass of the Universe is much larger still, implying that much of the mass of it is not even in the form of atoms or their nuclear constituents. This “non-baryonic” matter would neither emit nor absorb light of any form and would reveal its presence only through gravity. Determining the nature of this non-baryonic dark matter is one of the central goals of modern physics and astronomy. To keep their stars and hot gas from flying away, we infer that galaxies must be sur- rounded by halos of non-baryonic dark matter that provide additional gravitational attraction. Elliptical galaxies contain hot X-ray emitting gas that extends well beyond where we can see stars. By mapping this hot gas, which has been one focus of X-ray missions such as Chandra, XMM-Newton, and soon Astro-E2, we can develop a reliable model of the whole galaxy, showing where the dark matter lurks. Constellation-X will give a dynami- cal handle on the problem. Gravitational lensing provides yet another probe of dark mat- ter. The missing baryonic matter is also important and elusive. Although some could be hidden from us in collapsed gas clouds or cold stars too dim to see, most is now believed to lie between the galaxies in the form of very tenuous and nearly invisible clouds of gas. Some may be associated with galaxies themselves, and some may follow the intergalactic web defined by non-baryonic matter. We want to find this missing matter to understand why so little of it was used to build stars and galaxies. By 2010, surveys will have outlined the distribution of luminous baryonic matter in and around galaxies in fine detail, but the intergalactic component will still be largely unexplored. An efficient way to locate missing baryonic matter in the darkness of intergalactic space is to look for absorption of light from distant quasars. The Lyman α line is an exquisitely sensitive probe for cold hydrogen gas. If the baryonic dark matter is mainly primordial, such an ultraviolet detection strategy would be the only option. If the gas is hot and chemically enriched, then Constellation-X and large next-generation X-ray and ultraviolet telescopes will be able to see absorption lines from heavier elements. These efforts are difficult and just beginning on HST, FUSE, and Chandra. Constellation-X and new generation ultraviolet and X-ray telescopes will be needed to complete the task. The fluctuations of the cosmic microwave background radiation are a powerful tool for assessing the total mass content of the Universe. First detected by the COBE a decade ago, these fluctuations have a scale size that will be characterized by the recently launched Microwave Anisotropy Probe (MAP). ESA’s future Planck mission will extend this to smaller scales and look for polarization signatures. The most important fluctuations are on scales of arcminutes, so it is essential to map the distribution of dark matter on a comparable scale. The Beyond Einstein program includes an Inflation Probe that will measure the polarization of this back- ground. This polarization will reveal gravitational lensing by intervening matter, light or dark.
Posted on: Sun, 26 Jan 2014 09:47:18 +0000
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