The Lorentz workshop Tracing the Cosmic Web” took place from February

17 to February 21, 2014 at the Oort building, Lorentz Center, in Leiden.


The principal driver behind the workshop is to find and define a common language and formalism for identifying the constituents of the Cosmic Web. On Megaparsec scales, galaxies and matter in the universe have arranged them- selves into a complex weblike network of dense inter-compact clusters, elongated filaments, two-dimensional sheets, and huge near-empty voids. The prominent filamentary channels may contain up to 40% of the matter content of the uni- verse.


While the Cosmic Web is one of the most intriguing and striking patterns in nature, its analysis and characterization is far from trivial.  The absence of an objective and quantitative procedure for identifying and isolating clusters, filaments and voids in the cosmic matter distribution  has been a major ob- stacle in investigating the  structure and dynamics of the Cosmic Web.  The overwhelming complexity of the  individual  structures and their connectivity, the huge range of densities and the intrinsic multiscale nature prevent the use of simple tools that may be sufficient in less demanding  problems.


A key aspect of the workshop was the comparison between different meth- ods and formalisms that have been developed for the identification of filaments, voids, and also walls and clusters. The workshop brought together an exten- sive and nearly complete representation of available methods. The workshop contained presentations on geometrical methods, topological methods, Hessian based methods, and new methods explicitly including the phase space structure of developing structure. Part of these methods took account of the multiscale structure of the cosmic mass distribution.


The workshop addressed the comparison between these methods by means of three complementary aspects. The program of the workshop included a range of presentations in which each of the methods described working and results. Each of these extensive  sessions went along with ample discussion, in which the proponents of the various methods discussed the pros and cons of the presented method and compared it with that of their own favorite method. In addition, the workshop included a code comparison  project that had been running over the previous weeks. Noam Libeskind had prepared a set of computer simulations of cosmic structure evolution and had provided the various participating groups with the corresponding datasets. The simulations involved a range of different resolutions. The analysis of the results, involving a range of relevant aspects,

were presented by Noam Libeskind on the first morning of the workshop. It provided an important element in the ensuing discussions.


In addition to the comparison project and the involved methods, the pro- gram also included a range of presentations on key aspects of the cosmic web. In particular noteworthy were the discussions and presentations on galaxy forma- tion and galaxy properties  as a function of weblike environment, involving issues such as the generation of angular momentum, rotation, of galaxies. Another aspect, based on a special discussion  session led by T. Abel (Stanford Univ.) concerned the issue of the pros of assessing the full 6D phase space information. Finally, a range of presentations  focussed on the nature and characteristics of voids in the Megaparsec mass distribution.


The workshop also provided opportunities for specific contributions, usually involving presentations on observational results of which the speaker wanted to see in how far it would connect to the technical issues that formed the core of the workshop.


The context of the Lorentz Center proved ideal for the workshop.  The excellent facilities, at the Center itself as well as at the hotel, lead to long and intense discussions, nearly all days until late at night.  Many discussions were taken home to the hotel and continued at the dinner table and cafe in the hotel.


Rien van de Weijgaert & Noam Libeskind