Contact and universal relations in dilute quantum systems

The discovery of contact and universal relations is a breakthrough in the field of ultracold atom physics. Since S. Tan first pointed out the concept of contact in 2005, the study of universal relations has become an important subject regarding ultracold atoms, and has had a significant impact on related fields. This article gives an introduction on what are contact and universal relations, and their origins. Recent new developments including contact in high partial wave scattering and contact spectrum are also discussed.