# Weak-lensing peak counts

### What is weak lensing?

Weak lensing is a physical phenomenon which describes light deflection by gravity. Light rays emitted from distanced galaxies can be slightly perturbed, which distort the images of these galaxies. By measuring the distortion, cosmologists can study the matter distribution and improve our knowledge about the history and the composition of the Universe.

### What are peak counts?

One simple way to study weak lensing is peak counts. Peaks are local maxima of the projected mass distribution. Therefore, they are direct tracers of massive structures, and are related to the mass function of the dark matter halo distribution. Peak statistics have already been shown as a powerful tool to constrain cosmological models.

### A new model for prediction

My work involves the development of a peak-count prediction model with a stochastic forward approach. The summary of the algorithm is as follows:

We have shown that in this way, the prediction agrees well with $N$-body simulations.

Three advantages can be outlined from this model:

• Fast - only few seconds for creating a 25-deg$^2$ field, without MPI or GPU programming
• Flexible - straightforward to include observational effects (e.g. photo-$z$ errors, masks) and additional features (e.g. intrinsic alignment, baryonic effects)
• Full PDF information - allow more flexible constraint methods such as varying covariances, $p$-value evaluation, approximate Bayesian computation, etc.

This new model has already showed its strong potential in mock analyses. We look forward to applying it to the real data.