Our traditional interpretative picture of the internal dynamics of Globular Clusters (GCs) has recently been revolutionised by a series of discoveries about their chemical, structural, and kinematic properties. The empirical evidence that their velocity space is much more complex than expected encourages us to use them as novel dynamical laboratories for some long-forgotten aspects of collisional gravitational dynamics. Coupled with the recent discovery of star-to-star chemical abundance anomalies, suggesting that not all stars in a cluster were born in a single homogeneous population, this makes them challenging chemo-dynamical puzzles. GCs may also soon reveal themselves as black hole (BH) cradles. They may harbour massive BH that would provide a “missing link” in the populations and growth of cosmic supermassive BHs. Recent evidence also suggests that GCs may host populations of smaller (i.e. stellar-mass) BHs, potential progenitors for sources of gravitational wave emission. Finally, we can consider GCs as Galactic beacons. Their ages, kinematics and chemical properties, as well as the stars escaping from their outskirts, carry invaluable information about the assembly history of the Milky Way, including its dark matter component.

The European space observatory Gaia is now measuring the distances, positions and velocities of thousands of stars in the GCs of our Galaxy, with unprecedented accuracy. This new generation of data, coupled with astrometric measurements by the Hubble Space Telescope (e.g. in the high- density cluster cores) and other state-of-the-art photometric and spectroscopic ground-based surveys (e.g. Gaia-ESO), will enable us to unlock, for the first time, the full six-dimensional position and velocity “phase-space” properties of individual stars in GCs. We are therefore truly about to enter a new “golden age” for the study of the internal dynamics of these stellar systems. The overarching goal of our ISSI international team is precisely to leverage the richness of such new-generation astrometric data (especially from Gaia DR2) in order to gain a fundamental understanding of this emerging phase-space complexity and ultimately to reveal the dynamical evolution of the clusters, their elusive stellar populations and putative BHs, and their role as tracers of the history of our Galaxy.

Our multi-disciplinary international ISSI team brings together experts on a range of topics of relevance to the dynamical evolution of GCs and their stellar populations, both from the observational and modelling/theoretical point of view. In the short term, we plan to capitalise on the imminent arrival of the game-changing second Gaia data release (DR2 – April 2018) by delivering science-ready Gaia data products for each Milky Way GC to the scientific community, along with results from dynamical mass models for a large sample of GCs. This project will have an invaluable legacy value for a variety of applications by the scientific community. It will be an essential step to tightly constrain GC formation models, supply an independent measurement of the mass of the Milky Way, and establish the role of GCs as gravitational wave factories.