Major+Open+Questions

= = == =**Setting the Scene**=

Whereas on large scales LCDM seems to agree quite well with theoretical predictions, on smaller scales theory/simulations and observations seem to diverge or are harder to bring into agreement. This program hopes to bring together theorists, simulators and observers of the small-scale structure of the Universe to shed more light on this problem.

**A number of potential issues are:**

 * 1) What is the shape of dN/dM, i.e. the mass function of small-scale structure (both luminous and dark structures/satellites)
 * 2) What is the normalisation of dN/dM? Is there too much/little mass substructure?
 * 3) What is the density profile of mass substructures and does it agree with LCDM?
 * 4) What role do baryons plays in the shaping of (luminous/dark) substructure and satellites?
 * 5) Can warm dark matter explain the current observations better by modifying dN/dM and \rho(r)?
 * 6) Do we need to modify gravity if all else fails?

**Potential solution to shed light on these issues that have been proposed over the last decade have been e.g.:**

 * 1) Feedback
 * Supernova
 * Radiation
 * AGN
 * Winds
 * Tidal interactions/Stirring.Merging
 * MHD + Cosmic Rays
 * 1) Modifications of CDM
 * Warm
 * Interacting
 * 1) Modification of gravity.
 * TeVeS/MOND
 * Otherwise
 * TeVeS/MOND
 * Otherwise

**Possible observations clues that could help to resolve some of the issues are studies of:**

 * 1) The Epoch of Reionization (i.e. feedback)
 * 2) Substructure detections through lensing
 * 3) Visible satellites
 * 4) Streams in the MW and beyond
 * 5) Chemical tracers/tagging

Note that __observations could ultimately also also show degeneracies between distinct models__ (e.g. WDM vs CDM). How can those be disentangled. For example LCDM + feeback might explain the "missing satellite problem", but it also predicts the presence of many dark structures that have no baryons. Would we be satisfied with knowing these structures are there, but not observe them? How can be find these structures w/o baryons and how can they help to further distinguish between models?

These are some of the open question and topics that could be discussed during the program.

**Keeping observers __and__ theorists honest!**
To keep theorists but also observes honest, understanding between both needs to be encouraged. Some un-named theorist has requested to look at HI data-cubes to see how reliable HI models fit the data for LSB galaxies, just to see whether (s)he can trust comparison of data with simulations, related to the cusp/core problem. More such interestions should be encouraged!