In 2016 we celebrate that is has been a century since Albert Einstein published his theory of General Relativity. This scientific breakthrough describes not only how the space-time of our Universe evolves, but also predicts that light rays are deflected as they pass massive structures. This phenomenon, called gravitational lensing, has become one of the prime tools to study dark matter and dark energy which are the dominant ingredients of our Universe. This has lead to a significant increase in the number of researchers entering this still growing area of research.
Since then, much has changed. Several large imaging surveys aimed at studying the statistics of the large-scale structure through weak gravitational lensing are currently underway, and exciting new results from these are anticipated in 2016. In addition, efforts are building up in preparation for future projects, such as the LSST and the Euclid mission, both of which will yield unprecedented data sets at the start of the next decade. These projects will also revolutionise strong lensing studies, which already have provided important constraints on the dark matter profiles of galaxies and clusters of galaxies and on the cosmic distance scale. Such strong lensing systems are also used as cosmic magnifying glasses to probe the most distant galaxies in the Universe. Weak lensing studies of galaxy clusters have become the preferred way to calibrate their masses, a critical ingredient for galaxy cluster surveys. Recently the lensing distortions in the CMB have been measured. Finally, the impact of gravitational lensing is not limited to extragalactic astronomy, but microlensing studies can be used to study the population statistics of planets in our Galaxy.
Hence 2016 is not only a good moment to celebrate the accomplishments of the field over the past century, but it is an excellent time to showcase the most recent developments in a fast growing field of astronomy and discuss how to prepare for a bright future. A conference on the topic of gravitational lensing is therefore timely. Leiden is an excellent location for both practical and historical reasons. Leiden Observatory hosts a leading research group on gravitational lensing, and its members will make up the LOC. Furthermore, Leiden is located only a 15 minute train ride away from Amsterdam airport. Finally, in the years following the publication of the theory of General Relativity, Einstein visited Leiden regularly to discuss the implications of his work with Paul Ehrenfest, Hendrik Lorentz and Willem De Sitter. In fact, the fountain pen Einstein used to write down this theory of gravity is still in Leiden. We will try to have it displayed during the meeting.