Cosmology with gravitational lenses: time delay and microlensing

Astronomers study quasars because of their huge distances; they sample the universe when it was only 1/10 of the present age. In about a dozen known cases, a random galaxy along the line of sight causes the quasar's image to be double or multiple, providing cosmologists with a tool for study of young galaxies and the structure of space and time itself. In these gravitational lens alignments, the images do not arrive at the same time, and measurement of the quasar's irregular brightness fluctuations allows determination of the light travel time differences. For QSO 0956+561 A,B a 15-year data record of brightness fluctuations (Schild & Thomson, 1995) has been analyzed for time delay. Bad weather, telescope availability, and seasonal effects cause sub-optimal data sampling, but all time scales from a day to 15 years are reasonably sampled. The authors find the cosmologically interesting time delay to be 405+/-10 days, implying a universe about as old as the oldest known stars. Complications to the data analysis come from discoveries that the quasar has internal structure in the form of reflecting regions, and planetary mass objects in the lens galaxy introduce additional brightness fluctuations. The quasar also shows periodic variability at multiple frequencies, probably due to oscillations of the luminous disc surrounding the black hole.