weak lensing

How to tease out the tiniest distortions of galaxy shapes to probe the secrets of the Universe

Jan 5, 2022
One of the key measurements of a galaxy’s shape is its ellipticity, which, for those who remember their geometry, is zero for a perfect circle and approaches one for something flat like a pancake. In the case of a galaxy, its ellipticity is a measure of how round it appears to us as we view it through a telescope. One might guess that, for an array of galaxies, the average of all galaxies’ ellipticities would be zero over a large volume, because galaxies are seemingly randomly oriented. That is, it would look as if a galaxy is just as likely to be slanted in one direction as any other. To the naked eye, this generally seems to be true. However, this is not quite the case!