# A rising tide: intrinsic alignments since the turn of the millennium

## TL;DR

This paper reviews how galaxy alignments affect cosmological studies and how they can be used to understand the universe's structure and evolution.

## Contribution

The paper provides a comprehensive review of intrinsic alignments, their modeling, observations, and cosmological implications over the past 25 years.

## Key findings

- Intrinsic alignments are modeled across various scales using different approaches.
- Observational constraints and simulations help predict and mitigate their effects on gravitational lensing.
- Open problems remain in fully understanding and applying intrinsic alignments in cosmology.

## Abstract

The alignments of galaxies across the large-scale structure of the Universe are known to be a source of contamination for gravitational lensing, but they can also probe cosmology and the physics of galaxy evolution in many ways. In this review, I cover developments in our understanding of intrinsic alignments over the past 25 years on: (1) different approaches to model intrinsic alignments across a range of scales, (2) existing observational constraints, (3) predictions from cosmological numerical N-body and hydrodynamical simulations, (4) mitigation strategies to account for their contamination to lensing observables and (5) cosmological and astrophysical applications. While the review focuses mostly on two-point statistics of intrinsic alignments, I also give a summary of other statistics beyond two-point. Finally, I point out some of the open problems hindering the understanding or application of intrinsic alignments and how they might be overcome in the future.