Number 247, November 2, 1995 by Phillip F. Schewe and Ben Stein|
PROBLEMS WITH BIG-BANG NUCLEOSYNTHESIS? A long with the expansion of
the universe and the cosmic microwave background, the most prominent artifact
of the big bang is the synthesis of several species of light nuclei, namely
D, He-4, He-3, and Li-7, only seconds after the explosion. Cosmologies
which make predictions of the amount of early nucleosynthesis must account
for the present-day abundances of these nuclei, consisting of the primordial
inventory plus any that may have been manufactured (or destroyed) in the
cores of stars. Measurements (particularly of He-4) have improved over
the years to such an extent that various theories can now (or soon) be
put to the test. Not only are cosmological theories at stake but various
features of the standard model of particle physics. For example, the more
species of "light" neutrinos (meaning neutrinos which are massless
or nearly so; particle theory suggests three species: electron, muon, and
tau) there are, the faster the early universe would have expanded, leaving
behind more neutrons, which in turn would lead to a larger amount of He-4.
Although the measurement uncertainties are still considerable, the observed
abundances of He-4 and D seems to be at odds with the main big bang model.
Two groups, publishing papers in Physical Review Letters, 27 November 1995,
assess this discrepancy. One group (N. Hata et al.; contact Gary Steigman,
Ohio State, 614-292-1999) suggests that although the data might be at fault,
one or more factors, maybe betokening "new physics," might be
at work. An example of this would be a tau neutrino with considerable mass.
The other group (Craig J. Copi et al.; contact David Schramm, University
of Chicago, 312-702-8202), however, suggests that within the uncertainties
the data and the standard theory are still consistent with each other.
(Journalists can obtain copies of the articles from AIP Public Information;
X RAYS WERE DISCOVERED 100 YEARS AGO BY WILHELM ROENTGEN. To celebrate
this centennial, the November issue of Physics Today is given over to a
series of articles about the ubiquity of x rays in various research areas.
The following is a sampling. Condensed matter physics: the diffraction
of x rays from a crystal has for decades provided information about the
location of atoms in that crystal. More recently, the trillion-fold increase
in the brilliancy offered by synchrotrons over conventional x-ray sources
makes it possible to use finer beams which are needed to study very pure
crystals or materials under high pressure (in both of these cases the samples
are likely to be tiny). Molecular biophysics: X-ray diffraction studies
helped to elucidate the structure of DNA and numerous proteins. Speeding
up and digitizing this process, a Protein Data Bank has been created. This
compilation of atomic-level x-ray maps of biological structures now grows
at a rate of hundreds of maps per year, directly benefiting the design
of new drugs. Medicine: The half million patients receiving x-ray treatments
and the 300 million x-ray examinations carried out annually in the U.S.
testifies to the importance of x rays in diagnosis and therapy. X-ray computed
tomography (CT) technology continues to provide ever faster and sharper
pictures of tumors. Eventually x ray delivery for diagnosis and therapy
will be integrated into a single gantry unit. Astrophysics: Orbiting x-ray
detectors now look routinely at pulsars, supernova remnants, quasars, and
the flares issuing from our own sun. X-ray imaging is especially valuable
in the effort to demonstrate the existence of black holes and in the study
of how galaxies clump together in clusters.