Fredagskollokvium: Weighing Planck’s monsters

The most massive clusters of galaxies represent the culmination of structure formation in the universe. 
This makes them sensitive probes of the cosmological parameters which govern the growth of structure. 
Some recent observations reveal moderate tension between the amplitude of density fluctuations in the universe
measured by Planck from the primordial cosmic microwave background fluctuations and the abundance of massive 
clusters. In this talk I will describe an ongoing effort to understand this discrepancy, using weak gravitational lensing 
to measure the masses of a sample of distant massive clusters of galaxies found by Planck.