Probing dark matter caustics with weak lensing

Caustics are high-density structures that form in collisionless media. Under self-gravity, cold dark matter flows focus onto caustics which are yet to be resolved in numerical simulations and observed in the real world. If detected, caustics would provide strong evidence for dark matter and would rule out alternative models such as those with modified dynamics. Here, we demonstrate how they might be observed in weak lensing data. We evaluate the shear distortion and show that its radial profile is marked by a characteristic sawtooth pattern due to the caustics in dark matter haloes that form by selfsimilar accretion. We discuss the observational complications, mainly due to the poor knowledge of the virial radii of the haloes and demonstrate that a superposition of about 600 cluster-size haloes would give a signal-to-noise ratio which is sufficiently large for the detection of caustics with ground-based observations. This number is reduced to 200 for space-based observations. These bounds can be easily achieved by the ongoing wide field optical surveys such as CFHTLS and the future space-based projects SNAP and DUNE which have to be accompanied by an X-ray follow-up of the selected clusters for a precise determination of their virial radius.