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.