My work



Light from very distance sources (galaxies, quasars...) is deflected by the matter intervening along the line of sight. This phenomenon is known as gravitational lensing and it is one of the most powerful way to study the dark universe. In the figure you can see one of the most spectacular effect of gravitational lensing namely the arcs and arclets nearby a galaxy cluster.


First Results from the KiDS Survey (montage)

Left, a group of galaxies mapped by KiDS. Right, the same area of sky, but with the invisible dark matter rendered in pink. (credit: ESO, based on Viola et al. 2015)

Understanding galaxy formation is one of the most challenging and difficult topic in modern cosmology. Infact it is necessary to explain the interplay between dark matter and baryonic matter and moreover it is necessary to understand the physics of the gas. The processes involved are often dissipative and non-linear and generally are poorly understood. I use weak gravitational lensing to weight galaxies and galaxy clusters and I study the correlations between the mass of galaxies and their luminosity and stellar mass content. The strength of these correlation contains key information about physical processes regulating star formation in galaxies, such as mergers, feedback mechanisms from supernovae or AGN and environmental effects.


Dark matter distribution as seen using weak gravitational lensing. Credit: van Waerbeke, Heymans and the CFHTLenS collaboration

Measurements of temperature fluctuations in the cosmic microwaves background, large scale structures in the distributions of galaxies, number counts of galaxy clusters, supernovae data, provide compelling evidence that the expansion history of the Universe and the growth of structures can be described by a simple six-parameter model, known as LCDM. However our understanding of the very nature of two key ingredients of this model, namely “dark matter” and “dark energy” is still very limited. 

I study the “dark Universe” by mapping the dark matter distribution at different cosmic time, using weak gravitational lensing,  allowing me to study the effect of dark energy on the growth of structure. 


Left: two typical low signal-to-noise galaxies as observed in the KiDS survey. Their ellipticity has to be accuratly measured to infer the gravitational shear. Right: same galaxies but observed at higher signal-to-noise and resolution by HST.

One of the most challenging aspect of weak lensing is to measure with high accuracy the shape of galaxies, from which it’s possible to derive information about shear and flexion. This is a difficult task since the galaxies are usually very faint, they are convolved with the PSF of the telescope and their images are noisy.

There are different  techniques used to recover the correct shape of an observed galaxy. Recently I proposed a way to improve one of them (the KSB methods) and developed a new method, called DEIMOS, based on the measurements of moments of the light distribution.

© Massimo Viola 2015