THE HUBBLE CONSTANT FROM VLA MEASUREMENT OF THE TIME-DELAY IN THE DOUBLE QUASAR 0957+561

IN the double quasar 0957 + 0561, the first detected example of a gravitational lens 1, a quasar at redshift z = 1.41 appears as two images separated by approximately 6 arcsec and accompanied by an extended radio source 2-4. The Hubble constant, H0, can be estimated from the angular separation and the time delay between the appearance of the same brightness variations in the two images-a method that avoids all the usual uncertainties that attend the estimation of distance by the use of standard candles. From 11 years of Very Large Array (VLA) observations we have found the time delay to be l.40 +/- 0.10 yr, a significantly larger and more robust value than has been obtained from optical measurements 5,6. Using a standard lens model 7 and taking the observed velocity dispersion of luminous matter in the lensing galaxy 8 to measure the true gravitational potential, we find H0 = 46 +/- 14 (42 +/- 14) km s-1 Mpc-1 for cosmic density parameter OMEGA-0 = 0 (1), in approximate agreement with Rhee's recent estimate 8. But if, as is likely, there is dark matter in the lensing galaxy, H0 could be as high as 69 +/- 21 (63 +/- 21) km s-1 Mpc-1. The agreement of several completely independent estimates of H0 gives some confidence that the scale of the Universe is indeed known to about a factor of two.