Number 200, October 26, 1994 by Phillip F. Schewe and Ben Stein|
ACCURATELY MEASURING THE HUBBLE CONSTANT (H) , the parameter describing
the universe's rate of expansion, depends on having a reliable yardstick.
For gauging distances to objects outside our galaxy, astronomers often
use Cepheid variable stars, stars whose light emissions vary with a period
proportional to their intrinsic luminosity; one can compare the intrinsic
to the observed luminosity of the Cepheid to determine the distance to
the galaxy in which it lies. The effort to extend this method over a larger
distance scale would result in a more precise value for H because it would
lessen the effect of local gravitational interactions. The trouble with
this method, however, is that telescopes have trouble making out faint
Cepheids in distant galaxies and following their pulsations unambiguously.
Nevertheless, two sets of observations have now been made of Cepheids in
galaxies in the faraway Virgo cluster, at a distance twice as far away
as for Cepheids previously used for the purpose of finding H. One group,
using the Canada-France-Hawaii telescope on Mauna Kea, monitored three
Cepheids and worked out the distance to galaxy NGC4571. This results in
a value for H of 87 km per second per megaparsec (Michael J. Pierce et
al., Nature, 29 Sept.). A second team, using the (appropriately named)
Hubble Space Telescope (HST) and looking at 20 Cepheids in galaxy M100,
found a value of 80 for H (results announced at a NASA news conference
today). Converting a measurement of H into an estimate for the lifetime
of the universe is as not as easy as it once was since astronomers began
to suspect that large amounts of dark matter lurk in and around galaxies.
The presence of this matter can distort the cosmological "flow"
of galaxies and complicate any determination of the expansion rate of the
universe. Still, these new H values suggest a young universe; the HST results
provide a lifetime estimate of 8 to 12 billion years.
THE THEORY OF SUPERSTRINGS seeks to account for all four of the known
physical forces, including gravity. It holds that space has ten dimensions
and that all matter, including the elementary particles recognized by the
Standard Model---quarks and leptons---are really no more than tiny strings
with a characteristic length of 10**-35 m, a size so small that it has
a special name, the Planck length. Investigating matter on that level requires
a powerful microscope, one in which the probe particles would have an energy
of 10**19 GeV (the Planck energy), an energy so far beyond present or foreseeable
accelerators as to preclude all thought of direct experimentation. Indeed,
Texas physicist Steven Weinberg believes that the "intellectual investment
now being made in string theory without the slightest encouragement from
experiment is unprecedented in the history of science." (Scientific
American, Oct. 1994.) Although it has stood up well to experimental tests,
the Standard Model remains unsatisfactory as a "theory of everything,"
since for one thing is leaves out gravity and, for another, it requires
the use of 19 different input parameters whose values must be derived from
measurements. Superstring theory, if it could ever be made to work, would
surmount these problems; it would include gravity and have no input parameters.
But that the task has proven difficult. According to Lance Dixon of SLAC,
the superstring framework is not so much a theory as it is a theory of
theories. Dixon believes that some physicists are going back to the drawing
board of general principles and giving up their work on specific string
models. (Beamline, Summer 1994.)