Gravitational lensing occurs when a massive object — such as a galaxy — warps space and causes light from a more distant object to be deflected, as predicted by Einstein’s theory of general relativity. Astronomical lensing can result in a background galaxy being lensed in to two or more separate images by a foreground galaxy. PKS 1830-211 is a gravitational lens, with a quasar at a redshift of 2.5 being lensed by a galaxy at a redshift of 0.89, forming two images of the background galaxy Due to the different paths taken by the light from the quasar, there is a time delay between the two images and as the background quasar is a variable radio source, efforts have been made to measure this time delay, as it can be used to provide an independent measure of the Hubble constant. This was done by Lovell et al., who used the ATCA to measure a time delay of 26(+5/-4) days. Biggs has recently reanalysed the available ATCA data and combined it with unpublished VLA data to measured a time delay of 25.3 ± 2.0 days — consistent with the previous value, but with an uncertainty that is smaller by more than a factor of 2. However, VLBI imaging and long-term radio monitoring suggests the system is more complex that just a single lensing galaxy, and more detailed modelling is required in order to accurately measure a value of the Hubble constant. The figure above shows the archival data considered by Biggs, showing the variability in the two components of the lens.