Number 223, April 24, 1995 by Phillip F. Schewe and Ben Stein MACHOs MAKE UP LESS THAN 20% OF THE PRESUMED DARK MATTER HALO shrouding our galaxy. The presence of massive compact halo objects (MACHOs), such as non- radiating neutron stars or white dwarfs and substellar objects such as planets, is invoked to partially explain the rapid rotation of the outer parts of the Milky Way. Several groups search for MACHOs by scanning stars in the central galactic bulge and in the overhead Large Magellanic Cloud (LMC). They look for instances of microlensing, a phenomenon in which the star's light is brightened by the gravitational focusing effect of a foreground MACHO. (On an extragalactic scale, large lensing effects have been observed in which a distant quasar's image is split in two by the gravity of foreground galaxies.) One of the groups, the "MACHO collaboration," has now finished its full analysis of lensing events. A representative of this group, Livermore physicist Kem Cook, reported at last week's American Physical Society meeting in Washington, DC that he and his colleagues were now convinced that the LMC events are indeed related to the influence of truly nonluminous objects. They therefore assert that these measurements constitute the first definitive observation of dark matter in our galaxy. Furthermore, they calculate that the mass of the MACHOs in the halo added up to about 7.6 x 10**10 solar masses and that the MACHO fraction of the dark halo was about 19%. Unlike the LMC events, the lensing events seen in the direction of the galactic bulge are probably caused by ordinary stars and not dark matter objects. Still, the bulge events are of interest partly because they may offer a way of looking for extra-solar planets. One observed lensing event entailed a double-cusped brightening, suggesting to the MACHO scientists that some lensing objects are binary systems; some of these might be star-planet systems. NEW MEASUREMENTS OF THE NEWTONIAN GRAVITATIONAL CONSTANT G , the number that determines the strength of gravity, depart significantly from the accepted value established in the 1980s. G is the least well known of all the fundamental constants; the accepted value of 6.6726 x 10**-11 m**3/kg-s**2 is known with a relatively high uncertainty of 0.01%. G is arguably the most difficult constant to measure because, among other reasons, gravity is the weakest of all forces and it is impossible to shield delicate measurements from the gravity influences of buildings and other nearby objects. Underscoring this difficulty, three scientists from international labs (the German Bureau of Standards, the Measurement Standards Laboratory of New Zealand, and the University of Wuppertal in Germany) reported at the APS Meeting new measurements of G which disagreed widely with one another and with the standard value. The Wuppertal value was 0.7% below the accepted value (corresponding to 7 standard deviations), the New Zealand measurements were 0.07-0.08% below (7-8 standard deviations) and the German Bureau of Standards value was a whopping 0.6% above (60 standard deviations). Although the techniques differed, the groups all essentially determined G by measuring the gravitational effects of cylindrical masses acting on objects suspended above the ground. Researchers at Los Alamos, the lab which helped set the 1980s standard, are undertaking a new measurement of G which may be five times as precise as current measurements, and may shed light on these puzzling results. A firmly established value of G will be important for future grand unified theories that attempt to relate G to fundamental constants associated with the other three physical forces.