The small distortions of distant galaxy images due to the gravitational deflection of light by the intervening clumpy distribution of matter are potentially very powerful probes of the state of the Universe. By probing the growth of structure over cosmic time, and also being sensitive to the global geometry of the Universe, they can shed light on the nature of dark energy and also probe the theory of gravity. The sensitivity of the distortions to these may allow us to determine the cause of the observed acceleration of the Universe. Theoretically very appealing, observationally this weak gravitational lensing effect is a very challenging technique, with many aspects of the problem that need to be very well understood. For the normal method of study, so-called cosmic shear, which uses galaxy image shapes, these include intrinsic alignments of galaxies, good modelling of the small-scale matter power spectrum, exquisite measurements of shapes, good knowledge of the posterior distributions of the galaxy redshifts, and so on. With future surveys, such as ESA’s Euclid satellite, statistical errors will be very small, and the main concern is systematic errors. In this regime, having a complementary measurement technique is valuable.

The group of people we have assembled are studying weak lensing magnification, a promising method using the changes in size and brightness of galaxy images caused by the lensing effect. This information is collected at the same time as the galaxy shapes, with little extra effort, and can be used to analyse the matter distribution in a similar way, giving us an independent probe of dark energy and modified gravity. It adds signal to what we can measure from weak lensing, but more importantly, it acts as a cross-check. However, as the development of size and flux magnification lags far behind that of cosmic shear, there is a lot of work that needs to be done, to show that it can provide accurate and precise results in the small-signal regime on a cosmic scale. We work on the theoretical as well as the observational side of this field to make significant progress in the exploitation of magnification in forthcoming large galaxy surveys.