Observing Gravitational Lenses: Present and Future

Dear Colleagues,

The year 2019 marks a double anniversary in the field of gravitational lensing: One hundred years since Eddington confirmed General Relativity by measuring the bending of light around the Sun during a solar eclipse, and forty years since Walsh et al. discovered the first gravitationally lensed quasar, thus turning gravitational lensing from a purely theoretical field into a mainstream observational one. Today, all three branches of gravitational lensing are subject to intense observational research, having already significantly enriched our understanding of astrophysics and cosmology.

Almost 200 distant quasars and over 1000 galaxies have now been observed to be strongly lensed into two-, four- or ring-like images around foreground galaxies and clusters. These have been used as tools to study the stellar and dark matter distributions in the lenses, to observe high-redshift sources in otherwise unattainable detail, as well as to cost-effectively measure the Hubble constant and other cosmological parameters.   

First proven to be observationally viable towards the turn of the millennium, weak lensing complements its strong counterpart by enabling the study of the mass profile at large radii from the lens. Currently, all large-scale extragalactic surveys are designed to leverage the power of weak lensing for cosmological studies. 

When the acting lens is of stellar mass, the image separation are in the order of micro arcsec, hence bearing the name microlensing. It was first proposed as a means to constrain the dark matter in compact form by Paczynski, but later on evolved and flourished as a powerful tool to study quasar accretion disks, as well as detect and characterize exoplanets, especially for widely separated planetary companion beyond the snow line, or even free-floating planets that are otherwise beyond the reach of transit and direct imaging techniques.

We look forward to the contributions of the observational community from all three branches to this Special Issue, as they continue to write the next chapters in the study of the Universe through gravitational lenses.

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• gravitational lensing
• dark matter
• cosmological parameters
• quasars
• exoplanets