The Small-Scale Structure of Cold(?) Dark Matter

Coordinators: Laura Baudis, Graciela Gelmini, and Julio Navarro

Scientific Advisors: Mike Boylan-Kolchin, Simona Murgia, and Josh Simon

Our understanding of cosmology rests on two fundamental puzzles—“dark matter” and “dark energy”— whose solution will almost certainly redefine the way we look at the Universe and the way we do Physics. The nature of “dark matter”, in particular, is key to extending the Standard Model of Particle Physics and to understanding the hierarchical nature of structure formation. Studies of the relic Cosmic Microwave Background radiation and of galaxy clustering have led to a true paradigm, where dark matter takes the form of weakly-interacting elementary particles that emerge from the early Universe with negligible thermal velocities. This Lambda-Cold-Dark-Matter (LCDM) scenario has now matured into a full theory without tunable parameters, and where detailed theoretical predictions are possible.

LCDM predictions for the clustering of dark matter on small scales make the paradigm eminently falsifiable and have fueled strong interest from astronomers and high-energy astrophysicists, as well as theoretical and experimental particle physicists. Small scales are also where the LCDM paradigm has so far met its most severe challenges; from the overall abundance of faint galaxies (far fewer than naively expected in LCDM), to the presence/absence of density cusps in their dark matter halos, to the surprisingly low halo masses inferred for nearby dwarfs. However these issues are eventually resolved, it is clear that any solution would have immediate repercussions on dark matter theoretical models, as well as on dark matter detection experiments.

Indeed, dark matter clustering on the smallest scales plays a critical role in direct detection experiments, in indirect searches, and in observations of dwarf galaxies. These different fields are interlinked to such degree that each community would benefit from a meaningful exchange of ideas where the current status and future prospects of each discipline can be discussed and appreciated by the community at large. The methods and backgrounds of researchers in each of these fields differ, so the main goal of the program is to facilitate this communication, allowing information to flow from one discipline to another, enriching all.