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Life Cycles of Matter and Energy

The Next Steps: The Space Astronomy Imperative

The Mystery of the Missing Matter

NGC 2300 group of galaxies
The NGC 2300 group of galaxies contains a large reservoir of million-degree gas glowing in X rays. A false-color X-ray image of the hot gas (blue cloud) taken by ROSAT is superimposed here on an optical picture of the galaxy group. Gravity from the luminous parts of the galaxies alone is not enough to keep the gas in its place. There must be large quantities of dark matter whose gravity is preventing the gas from escaping.
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 primordial 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, suggesting 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 surrounded 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 dynamical handle on the problem. Gravitational lensing provides yet another probe of dark matter.

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 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 background. This polarization will reveal gravitational lensing by intervening matter, light or dark.

Once we understand the missing baryonic matter, we will have the first glimpses into the role that it plays in the evolution of our Universe.

Bullets of the Cosmos

Simulation of Particle Showers
Simulations of particle showers produced by 1020 eV cosmic rays in the Earth’s atmosphere.
The origin of cosmic rays is a 90-year-old mystery. Most of these high energy nuclei are thought to be hurled at us by supernova shock fronts, perhaps from collisions with dust grains. Future measurements of the abundances of trace elements in cosmic rays can determine the nature of their sources and the time between the elements' creation and acceleration.

The distribution of cosmic-ray energies is remarkable in that it is almost a constant power law over at least 13 decades in energy. A small steepening, or "knee," in the power law near 1015 eV is thought to represent the limit to energies achievable by supernova shock acceleration. A mission designed to measure the composition of these cosmic rays will explore their connection to supernovae by identifying these high energy nuclei.

At higher energies, the mystery deepens. In fact, we have detected cosmic rays up to about 1020 eV, where individual atomic particles have the energy of a well-hit baseball! About the only conceivable sources for these particles are galactic nuclei, giant extragalactic double radio sources, or the mysterious sources that give rise to gamma-ray bursts. Scattering off cosmic background photons should make the Universe fairly opaque to these highest-energy particles, so they must come from nearby sources. It has been suggested that the highest-energy particles could come from the annihilation of topological defects formed in the early Universe. The detection rate of these particles is so low that we see too few to describe their properties well. The DOE/NSF/UNESCO Pierro Auger Observatory now under construction is the next step in understanding these exceedingly rare but energetic particles. Results from it will inform the design of space instruments capable of monitoring still larger areas of the Earth's atmosphere for the showers these rare particles produce, to determine their energy spectrum and source directions.



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