hen Galileo turned his spyglass on the Milky Way he made a marvellous discovery by resolving its misty light into countless stars, distributed in clusters. At a stroke he discovered the vast cosmos that sprawls beyond the solar system. From his survey he decided that the fainter stars were more distant, which was a profound cosmological conclusion in his day. Four centuries later astronomers continue with systematic surveys that are designed to reveal the content and structure of the universe. Two Hot Papers in this period, #2 and #7, bring the latest news on the content of the visible universe.

Hot Paper #2 describes the 2MASS survey of the whole sky made at the infrared wavelength 2 microns. Our galaxy, and the universe as a whole, is much more transparent in the infrared than in the optical part of the spectrum, where obscuration by interstellar dust plays havoc with any attempts to find the true distribution of luminous matter. The survey, conducted from June 1997 to February 2001, used two highly automated 1.3 m telescopes, one at Mt. Hopkins, Arizona, and the other at Cerro Tololo, Chile, which together covered 99.998% of the sky with a sensitivity 5 x 10⁴ greater than previous infrared surveys.

2MASS has delivered 25.4 Tbytes of raw data on uniform precise photometry and astrometry in three near-infrared bands at 1.25, 1.65, and 2.16 µm. The all-sky data release from the project lists a staggering 470,992,970 point sources (mostly galactic stars) and 1,647,599 extended sources (mainly galaxies). Paper #2 is a for-the-record paper, describing the technical aspects of the survey and the data reduction, which is being heavily cited in numerous papers devoted to astrophysical results spanning objects from dim brown dwarfs to blue supergiants. The first-ever "bird’s eye view" of our home galaxy was made using 2MASS data for 30,000 carbon stars.

A survey of a different kind features in #7, which describes cosmological results from a survey of galaxy redshifts conducted with a two-degree field (2dF) spectrograph on the Anglo-Australian Telescope. This device is capable of observing up to 400 galaxies simultaneously, giving a throughput of as many as 3,600 spectra in one night. The 2dF survey is designed to get the redshifts, and therefore the distances, to 250,000 galaxies. That’s sufficiently large to yield a fair sample of the universe.

Conditions in the early universe form the backdrop to paper #7. In the expanding universe, gravity-driven evolution imprints characteristic scales that depend on the average matter density. In effect, sound waves stir the matter distribution, leading to structural features that survive as clustering of galaxies. The power-spectrum of that clustering is dependent on the matter density at the time the structures began to form. What is remarkable is that the cosmological structure evident in the universe today began with small density perturbations when the universe was much younger, before the formation of the first galaxies.

NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) has already performed superbly in mapping fluctuations on the cosmic microwave background, showing the emergence of structure.¹ Paper #7 asks if the distribution of galaxies retains the imprint of emergent structure. The answer is yes.

Power-spectrum analysis is a statistical technique that quantifies properties such as the patterns of galaxy filaments and clusters. In #7, Shaun Cole (University of Durham, U.K.) and the 2dF team adopt a cold dark matter model. Their analysis does indeed show evidence of the baryon oscillations (sound waves) expected in CDM models. The fraction of matter in baryonic form (such as stars and galaxies) is 18.6%. By combining the 2dF data with WMAP, the team arrive at an energy budget for the universe: 23% of the mass of the universe is composed of baryons and dark matter, with the balance of 77% residing in dark energy.