Large metallicity variations in the Galactic interstellar medium.

The interstellar medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic and molecular species, and dust particles. The neutral ISM is dominated by neutral hydrogen and has ionization fractions of up to eight per cent. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars, because they formed at different times.

Numerical simulations of the Princeton magnetorotational instability experiment with conducting axial boundaries.

We investigate numerically the Princeton magnetorotational instability (MRI) experiment and the effect of conducting axial boundaries or endcaps. MRI is identified and found to reach a much higher saturation than for insulating endcaps. This is probably due to stronger driving of the base flow by the magnetically rather than viscously coupled boundaries.

Universal bound on N-point correlations from inflation.

Models of inflation in which non-Gaussianity is generated outside the horizon, such as curvaton models, generate distinctive higher-order correlation functions in the cosmic microwave background and other cosmological observables. Testing for violation of the Suyama-Yamaguchi inequality τ(NL) ≥ (6/5f (NL))(2), where f(NL) and f(NL) denote the amplitude of the three-point and four-point functions in certain limits, has been proposed as a way to distinguish qualitative classes of models. This inequality has been proved for a wide range of models, but only weaker versions have been proved in general.

Confirmation of general relativity on large scales from weak lensing and galaxy velocities.

Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Such a test has recently been proposed, using a quantity, E(G), that combines measures of large-scale gravitational lensing, galaxy clustering and structure growth rate. The combination is insensitive to 'galaxy bias' (the difference between the clustering of visible galaxies and invisible dark matter) and is thus robust to the uncertainty in this parameter.

Discrete-dipole approximation for periodic targets: theory and tests.

The discrete-dipole approximation (DDA) is a powerful method for calculating absorption and scattering by targets that have sizes smaller than or comparable to the wavelength of the incident radiation. The DDA can be extended to targets that are singly or doubly periodic. We generalize the scattering amplitude matrix and the 4 x 4 Mueller matrix to describe scattering by singly and doubly periodic targets and show how these matrices can be calculated using the DDA.

The legacy and large-scale distribution of active galaxies.

If accretion onto massive black holes is the power source for active galaxies, then nearby galaxies should contain 'dead quasars': black holes that do not shine, either because they are starved for fuel or because they accrete with low radiative efficiency. This article briefly reviews the evidence that most inactive galaxies contain black holes at their centres, and how the local distribution of black holes is related to the population of active galaxies.

Discrete-dipole approximation with polarizabilities that account for both finite wavelength and target geometry.

The discrete-dipole approximation (DDA) is a powerful method for calculating absorption and scattering by targets that have sizes smaller than or comparable with the wavelength of the incident radiation. We present a new prescription-the surface-corrected-lattice-dispersion relation (SCLDR)--for assigning the dipole polarizabilities while taking into account both target geometry and finite wavelength. We test the SCLDR in DDA calculations for spherical and ellipsoidal targets and show that for a fixed number of dipoles, the SCLDR prescription results in increased accuracy in the calculated cross sections for absorption and scattering.

Characterization of extrasolar terrestrial planets from diurnal photometric variability.

The detection of massive planets orbiting nearby stars has become almost routine, but current techniques are as yet unable to detect terrestrial planets with masses comparable to the Earth's. Future space-based observatories to detect Earth-like planets are being planned. Terrestrial planets orbiting in the habitable zones of stars-where planetary surface conditions are compatible with the presence of liquid water-are of enormous interest because they might have global environments similar to Earth's and even harbour life.

A lightweight universe?

How much matter is there in the universe? Does the universe have the critical density needed to stop its expansion, or is the universe underweight and destined to expand forever? We show that several independent measures, especially those utilizing the largest bound systems known-clusters of galaxies-all indicate that the mass-density of the universe is insufficient to halt the expansion. A promising new method, the evolution of the number density of clusters with time, provides the most powerful indication so far that the universe has a subcritical density. We show that different techniques reveal a consistent picture of a lightweight universe with only approximately 20-30% of the critical density.

Outstanding problems and instrumental prospects for the coming decade (A Review).

After a review of the discovery of external galaxies and the early classification of these enormous aggregates of stars into visually recognizable types, a new classification scheme is suggested based on a measurable physical quantity, the luminosity of the spheroidal component. It is argued that the new one-parameter scheme may correlate well both with existing descriptive labels and with underlying physical reality.Two particular problems in extragalactic research are isolated as currently most fundamental.