Physics
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aps..may..e501f&link_type=abstract
American Physical Society, APS/AAPT Joint Meeting, May 2-5, 1996, abstract #E5.01
Physics
Scientific paper
In a uniform and isotropic Universe, the relative expansion velocity v is proportional to the relative distance r such that v = H × r. Thus a determination of the present-day value of the Hubble constant H0 determines both the expansion timescale and the size scale of the Universe. The Hubble constant also provides constraints on the density of baryons produced in the Big Bang, the amount of dark matter, and how structure formed in the early Universe. The most accurate means of measuring the distances to nearby galaxies has proved to be the application of a relationship between the period and the luminosity for a class of supergiant variable stars known as classical Cepheids. Unfortunately the Cepheid variables are not intrinsically luminous enough to be measured out to distances where the velocities of recession of galaxies are a few thousand km/sec and thus dominate the peculiar velocities due to the gravitational interactions between galaxies (typically a few hundred km/sec). The Hubble Space Telescope Key Project on the Extragalactic Distance has been designed to measure a value of the Hubble constant accurate to ±10% [random + systematic]. The program has been designed to use Cepheid variables to determine the distances to a representative sample of about 20 galaxies both inside and out of small groups and in major clusters. These galaxies are being used to tie into methods with high internal precision ( ~ ±5%) that operate at greater distances, thereby allowing an accurate absolute calibration and an intercomparison of several independent techniques. Our preliminary result is that the value of the Hubble constant is 80 ± 17 km/sec/Mpc footnote Freedman, W. L. et al., Nature, 371, 757, (1994) New results will be presented based on observations of several new galaxies, including NGC 1365 in the nearby Fornax cluster. My collaborators on the HST Key Project team are R. Kennicutt, J. Mould, F. Bresolin, L. Ferrarese, H. Ford, J. Graham, M. Han, P. Harding, J. Hoessel, R. Hill, J. Huchra, S. Hughes, G. Illingworth, D. Kelson, B. Madore, R. Phelps, S. Sakai, A. Saha, N. Silbermann, P. Stetson, and A. Turner, and their enormous contributions to this effort are heartily acknowledged.
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