Astrophysics in 2006

The fastest pulsar and the slowest nova; the oldest galaxies and the youngest stars; the weirdest life forms and the commonest dwarfs; the highest energy particles and the lowest energy photons. These were some of the extremes of Astrophysics 2006. We attempt also to bring you updates on things of which there is currently only one (habitable planets, the Sun, and the Universe) and others of which there are always many, like meteors and molecules, black holes and binaries.     

Order out of Randomness: Self-Organization Processes in Astrophysics

Self-organization is a property of dissipative nonlinear processes that are governed by a global driving force and a local positive feedback mechanism, which creates regular geometric and/or temporal patterns, and decreases the entropy locally, in contrast to random processes. Here we investigate for the first time a comprehensive number of (17) self-organization processes that operate in planetary physics, solar physics, stellar physics, galactic physics, and cosmology. Self-organizing systems create spontaneous “order out of randomness”, during the evolution from an initially disordered system to an ordered quasi-stationary system, mostly by quasi-periodic limit-cycle dynamics, but also by harmonic (mechanical or gyromagnetic) resonances. The global driving force can be due to gravity, electromagnetic forces, mechanical forces (e.g., rotation or differential rotation), thermal pressure, or acceleration of nonthermal particles, while the positive feedback mechanism is often an instability, such as the magneto-rotational (Balbus-Hawley) instability, the convective (Rayleigh-Bénard) instability, turbulence, vortex attraction, magnetic reconnection, plasma condensation, or a loss-cone instability. Physical models of astrophysical self-organization processes require hydrodynamic, magneto-hydrodynamic (MHD), plasma, or N-body simulations. Analytical formulations of self-organizing systems generally involve coupled differential equations with limit-cycle solutions of the Lotka-Volterra or Hopf-bifurcation type.     

EXTRAGALACTIC DISTANCE SCALES: it H 0 FROM HUBBLE (EDWIN) TO HUBBLE (HUBBLE TELESCOPE)

In the first fifty years after Edwin Hubble announced a linear relationship between distances and redshifts of external galaxies, the accepted value of his constant dropped by (or the Universe expanded and aged by) a factor of 5 to 10. More recently, different groups, often using nearly the same data, have passionately defended distance scales that differ by about a factor of two. The sections of this review explore (1) the history of extragalactic distance scales, (2) the relationships between the Hubble constant, H ₀, and other cosmological parameters, (3) types of distance indicators, (4) ways of measuring distances in practice, (5) values of H ₀ reported recently on the basis of these methods, (6) the continuing discrepancies between the 'long' and 'short' distance scales, and (7) prospects for future convergence on a single, global value of H, so that we can all get back to doing other things. The units of the Hubble constant are km s^-1 Mpc^-1 (or reciprocal time), and no one now strongly favors any value outside the range 40–90 km s^-1 Mpc^-1 (time scales of 11–25 Gyr).