NEW MEASUREMENTS OF THE HUBBLE CONSTANT AND ITS IMPLICATIONS

One of the key projects on the Hubble Space Telescope (HST) is to dete rmine the distance to the Virgo cluster of galaxies using classical Ce pheid variables as standard candles, The refurbished HST has resolved and measured the periods and magnitudes of twenty odd Cepheids in M100 , a spiral galaxy near the core of the Virgo cluster, Coincidentally, ground-based observations from Hawaii with the help of a high resoluti on camera, under subarcsecond seeing conditions, have also succeeded i n the identification of a few Cepheids in NGC 4571, another Virgo spir al, The distance to the Virgo cluster from these two sets of measureme nts turns out to be 17.1 +/- 1.8 Mpc and 14.9 +/- 1.2 Mpc respectively and implies a value of the Hubble Constant H-o equal to 80 +/- 17 km s(-1) Mpc(-1) and 87 +/- 7 km s(-1) Mpc(-1), The result implies that t he age of the Universe is in the neighbourhood of 10 +/- 2 Gyr, Some o f the globular clusters in the Milky Way system are known to be older (age similar to 14-18 Gyr) from stellar evolutionary calculations, Sec ondly, another important standard candle, the Type Ia supernova, has n ow been accurately calibrated using HST data on Cepheids in IC 4182 an d NGC 5253, the host galaxies for SN 1937C and SN 1972E respectively, These calibrations yield a much lower value of H-o from the Hubble dia gram of SNe Ia. Though there may still be uncertainties of the order o f 10-20% in the measurements, the low value of H-o from SN Ia measurem ents focuses attention on the class of objects classified as SN Ia, an d their suitability as standard candles, Significantly, the high value of H-o from the measurements of classical Cepheids may indicate that we live in a Universe somewhat different from the one we have been thi nking we do.