Free Astronomy Magazine July-August 2015

COSMOLOGY F rom left to right: an image taken by Hubble of the galaxy pro- ducing the lensing on SDP.81 (the ring is practically invisible); a shot of the ring taken by ALMA (the in- terposed galaxy is largely invisible); an image of SDP.81 reconstructed with a mathemat- ical model. [ALMA (NRAO/ESO/NAOJ) /Y. Tamura (The University of To kyo)/Mark Swin- bank (Durham University)] Left, a diagram sum- marizing SDP.81 case. [ALMA (ESO/ NAOJ/NRAO)/ C. Collao/Japan Meteorological Agency] brightness and density is not uniform. One of these structures has recently at- tracted much attention from groups of re- searchers, such that in the last months at least seven of them published no less than eight scientific articles regarding it. We are referring to the ring produced by the distortion of galaxy HATLAS J090311.6 +003906, more simply called SDP.81, spot- ted 11.7 billion light-years away from Earth in the constellation of Hydra. It is located those alterations in various forms, one of which is the so-called “Einstein ring”, where the image of the source, typically a remote galaxy, appears overall more bright, elongated and redistributed along a circle centred on the mass causing that alteration, known as “gravitational lens- ing”. More often than not, Einstein’s rings are incomplete, thus formed by one or more arcs of different lengths, and only rarely we can see full ones, although their