Number 172, April 7, 1994 by Phillip F. Schewe and Ben Stein THE COLD DARK MATTER (cdm) theory attempts to account for the observed rotation of spiral galaxies and interactions between galaxies by invoking the presence of considerable amounts of nonluminous, slowly-moving particles. Some experiments have sought to observe cdm particles (lurking in a halo around our galaxy) either directly in detectors or indirectly by measuring the presumed gravitational microlensing effect of such dark matter (at least that part of it consisting of baryons) on the starlight coming to us from the Large Magellanic Cloud, the nearby satellite galaxy. Evalyn Gates and Michael Turner of the University of Chicago work with a model in which the dark matter near the Milky Way resides in a roughly spherical halo. They calculate that the cdm halo has a density of at least 10**-25 g cm**-3. They also assert that current microlensing experiments cannot sensitively probe the local cdm density. (Upcoming article: Physical Review Letters, 18 April 1994.) POROUS SILICON , unlike bulk silicon, can emit light, a fact that may lead to new opto- electronic devices. Studies over the past few years have determined that the luminescence arises not from any surface residue left behind by the silicon electrochemical etching process but rather from some quantum effect relating to the small size of the silicon structures responsible for light emission. A new experiment at AT&T Bell Labs (Louis Brus, 908-582-3119) shows that these structures are not wire-like, as was previously thought, but more particle-like with a typical size of only 2 or 3 nm. (Upcoming article: S. Schuppler et al., Physical Review Letters, 18 April.) THE SURVEY OF GALAXY REDSHIFTS published by Harvard-Smithsonian astronomers (Margaret Geller, 617-495-7409) a decade ago, a survey which revealed that the universe is filled with immense sheets and clumps of galaxies, as well as regions nearly devoid of galaxies, has now been extended to cover parts of the southern celestial hemisphere. The new survey, supplementing northern observations at the Whipple Observatory in Arizona with southern- hemisphere observations in Chile, Argentina, and South Africa, shows that southern skies are also clumpy on a scale of 100 million light years. (Astrophysical Journal Letters, 20 Mar. 1994). FEMTOSECOND LASERS keep getting better. In just the past three years typical laser power has increased by a factor of ten and pulse size has shrunk by a factor of three. Practical devices can now generate 10-20 fsec pulses. Light chopped up in parcels this small has a variety of uses such as for observing chemical reactions or melting in real time, or for studying photosynthesis. Henry Kapteyn and Margaret Murnane of Washington State University believe that the pulse-size limit for titanium-doped sapphire lasers, commonly used in this research, will be 3 fsec, which constitutes about 1 cycle of light at a wavelength of 800 nm. To produce shorter pulses, they say, will require the use of shorter-wavelength light such as x rays. (Optics and Photonics News, March 1994.)