NEWS FROM CARNEGIE INSTITUTION OF WASHINGTON
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FOR IMMEDIATE RELEASE: 1 March 1996
CALL:
* Allan Sandage (818-304-0246) or
* Abi Saha (410-338-4806 or saha@stsci.edu) or
* G.A. Tammann (61-271-77-11/12) or
* Tina McDowell (Carnegie news office, 202-939-1121 or tmcdowell@ciw.edu)

HUBBLE CONSTANT MEASURED FROM SUPERNOVAE TYPE IA
--Older Age of Universe Indicated--

Astronomers of the Carnegie Institution of Washington, the Space Telescope Science Institute, and the University of Basel, Switzerland,* today announced recent observations and summarized results to date in their extended study of the expansion rate of the universe. The issue is centrally important in cosmology, critical in many ways for understanding the history of the universe, including its distance scale, age, and destiny.

Writing in the current (March 20) issue of The Astrophysical Journal Letters, the investigators offer a value for Hubble Constant of 57 km/sec/Mpc, with a formal uncertainty of 7%. (This means that an object is seen to recede from us by 57 km/sec for each Megaparsec of distance from us. One Megaparsec equals 3.26 million light years.) This result supports the longer distance scale of the universe and a high value, at least 15 billion years, for the age of the universe. Key to the result was the success of the investigators in pinning down the characteristic peak absolute luminosity of type Ia supernovae through measurements of Cepheid variable stars. This value was then applied in determining distance to very distant supernovae.

It has been known for three decades that type Ia supernovae, exploding stars observable at great distances, are potentially the best "standard bombs" known in astronomy. Until now, however, the absolute value of the emitted light at the maximum explosive phase was uncertain. Different values of peak luminosity led to contradicting values of the universe's expansion (i.e., Hubble Constant), with major consequences for the big bang model of cosmology.

The present investigators set out to determine the absolute energy flux of type Ia supernovae at maximum light. A foremost first task was to pin down distance to several type Ia supernovae situated in galaxies close enough for measurement of their Cepheid variable stars. The observations began in 1992 prior to the Hubble Telescope repair mission, were resumed after the repair, and still continue.

Cepheids were identified from among the millions of observable stars in each target galaxy, and their periods of brightening and dimming were determined in repeated observations. The investigators then obtained the true luminosity of each Cepheid from the well-known Cepheid period-luminosity relation. Distance to each Cepheid (and thus to the galaxy harboring the known supernova) was obtained by comparing true and apparent Cepheid luminosity.

The current report includes the group's newest measurements, to Cepheids in the galaxy NGC 4639. Situated 82 million light years from us, this galaxy is 1.5 times more distant than any galaxy previously measured by the Cepheid method. The group visited NGC 4639 fourteen times during a 70-day interval ending in July 1995; twenty Cepheids were discovered and their distances determined, thereby establishing distance to the galaxy. Then, using this distance along with past measurements of the apparent luminosity of the galaxy's supernova, Supernova 1990N, the peak absolute luminosity of the latter was determined.

By the same method, the investigators had previously obtained the peak absolute luminosities of five other type Ia supernovae, situated in other galaxies. Their present analysis also drew on data previously reported by other investigators for one additional supernova.

Having determined the characteristic peak luminosity of their standard bombs, the investigators then applied this value to other, historical supernovae situated at much greater distances. Even though Cepheids are brighter than most stars, type Ia supernovae are a million times brighter in absolute luminosity than Cepheids and thus can be observed at distances 1000 times greater. Some observable type Ia supernovae are so remote that their recessional velocities are very large compared with local velocity perturbations; in effect, they define the universal expansion rate.

Team members Abhijit Saha and G.A. Tammann explained: "This method of calculating the expansion rate circumvents many of the pitfalls of other methods. Once the peak luminosity is calibrated locally with Cepheids, the calibration can be applied to supernovae at large redshifts at remote distances where the peculiar motions due to local perturbations induced by clustering of galaxies are small compared to the overall expansion flow. Thus one does not need to make uncertain assumptions about how any locally measured redshifts are connected to the remote distances that define the global expansion rate. Issues such as the distance to the nearby Virgo Cluster and the relation of the Virgo Cluster spirals to the cluster core are also eliminated, and so this method cuts to the core of the distance-scale problem."

Commenting on the group's determination of the Hubble Constant, team leader Allan Sandage said: "The results of the first four years of this supernova experiment support the long distance scale for the universe. The low Hubble Constant requires a high age for the creation event of at least 15 billion years. This age is longer than the age of globular clusters in our own Galaxy, now believed to be as short as 13 billion years."
The present investigators contend that "...the ratio of the age from the expansion rate (Hubble Constant) and the age of the Galaxy from its globular clusters is now consistent with the standard big bang model of cosmology, eliminating the idea of a crisis in cosmology in which the universe has been said to be younger than the known age of the oldest objects in it."

In coming years, the continuing Hubble Telescope experiment using supernovae will yield more calibrations, thereby showing further details of possible subtle differences among prototypical Type Ia supernovae. If prototypical type Ia supernovae continue to test as superb standard bombs, astronomers believe that type Ia supernovae discovered at extremely large distances can be used to determine the past change of the expansion rate. Such a change would show the deceleration of the expansion caused by the self-gravity of the cosmos. It may eventually be possible to "weigh the universe"--by determining the total mass of the universe from its braking effect on the expansion. This possibility, known since the 1960s, only now seems a realistic possibility.

Carnegie Institution of Washington was founded in 1902 as Andrew Carnegie's institution for discovery. It now has five research centers: the Department of Embryology in Baltimore, the Department of Plant Biology in Stanford, California, the Geophysical Laboratory and Department of Terrestrial Magnetism, both in Washington, D.C., and the Carnegie Observatories at Pasadena, California and Las Campanas Observatory, Chile. The Institution's president is the biologist Maxine Singer. The director of the Observatories, holding the Crawford H. Greenewalt chair, is Augustus Oemler. Allan Sandage has been a Carnegie staff member since 1952. Abhijit Saha was a fellow at the Carnegie Observatories 1985- 1988. Astronomer Edwin Hubble, whose name is attached to the Hubble Constant and Hubble Space Telescope, was a Carnegie staff member throughout his distinguished scientific career. More on Carnegie Institution is available by clicking at web site http://www.carnegieinstitution.org/

*Allan Sandage of The Observatories of the Carnegie Institution of Washington (team leader), Abhijit Saha of Space Telescope Science Institute, Gustav A. Tammann and Lukas Labhardt, both of University of Basel, Switzerland, and Nino Panagia and F. Duccio Macchetto, both of Space Telescope Science Institute on assignment from the European Space Agency.
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note: The full text of the article to appear in Astrophysical Journal Letters can be read by clicking at http://www.noao.edu/apjl/apjl.html