A prime example of the intimate connection between particle physics and cosmology is the problem of Dark Matter. Dark matter apparently accounts for most of the matter density of the Universe, but it has been proved that dark matter is not a known type of matter. Particle Physics offers a variety of candidates for the dark matter particle, and the existence of dark matter is one of the most compelling indicators that the Standard Model of particle physics is incomplete in a fundamental and non-trivial way. Evidence from observational cosmology and astrophysics gives information about the nature of the dark matter particle (e.g., that it behaves more like "cold" dark matter than "hot" dark matter) and these constraints impact on the development of theories of particle physics which go beyond the Standard Model.
Recently, observations have hinted that the dark matter particle may have interactions not expected from fashionable particle physics candidates. The effort to reconcile this puzzle benefits from a multipronged approach and intensive collaboration between particle theorists and cosmologists. At NYU research ranges from constructing theories with interacting dark matter and investigating their phenomenological and observational implications to mapping the dark matter density of the Universe using galaxy evolution, weak lensing, and the large scale structure of the Universe.
Dark matter is not the only unidentified ingredient of the Universe. In the past decade, observations have shown that the expansion of the Universe is accelerating rather than decelerating as had been predicted. In standard cosmology based on Einstein's General Relativity, this expansion is due to a tiny but non-vanishing vacuum energy density. This is called the Cosmological Constant if it is not dynamical, but is more generally known as "Dark Energy". The observations lead to extremely perplexing questions:
1. Why is the vacuum energy determined by a momentum scale much smaller than any reasonable cut-off scale in an effective field theory of particle interactions?
2. Why are the vacuum energy and the matter energy comparable today? Do we live in a special epoch?
Faculty, research scientists and students in the CCPP are trying to answer these questions. The solution my lie in a modification of standard gravitational laws at very large (super-horizon) scales. This modification may be a manifestation of new hidden dimensions of the Universe, or of some other -- as yet unknown -- physical laws. The new picture is expected to have a major impact on various cosmological issues, such as inflation and the large scale structure of the Universe. NYU is a leading center of research in this area.
Other phenomena at the intersection of particle physics and cosmology which are being studied at NYU include the origin of the baryon asymmetry of the Universe, the nature of inflation, and the possible non-constancy of fundamental couplings such as the fine structure constant.