Lorentz Center - Biased Tracers of Large-Scale Structure from 4 Jul 2016 through 8 Jul 2016
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    Biased Tracers of Large-Scale Structure
    from 4 Jul 2016 through 8 Jul 2016


Biased Tracers of the Large-Scale Structure

4 – 8 July, Snellius


Many of the cosmological constraints from upcoming galaxy surveys (eBOSS, HETDEX, DESI, Euclid, PFS, WFIRST) will rely on an accurate theoretical description of the statistics of biased tracers. This provided strong motivation to bring the leading researchers in this field together in a collaborative workshop environment to evaluate the current state. In particular, the three goals of the workshop were:

  1. Establish the milestones to determine the regime of applicability of a first principle description of biased tracers.
  2. Summarize and critically confront different approaches to the bias problem, identifying where additional prediction or comparisons are necessary.
  3. Determine the necessary precision in bias that will be needed for the current and next generation of LSS experiments and compare with current predictions.


In order to foster discussions and direct collaboration, we only scheduled two overview talks on each day. The rest of the day participants split into four working groups focused on specific topics within the field. We chose the working groups specifically in such a way to allow participants from diverse backgrounds to participate in each (theory vs experiment, high-energy physics vs astrophysics). We believed this was essential to promote interaction across field boundaries, and in fact “nudged” participants to join one group over the other to ensure diversity. The working groups were

  1. Beyond the power spectrum: bispectrum and other higher-point statistics
  2. Probing the early universe with large-scale structure
  3. Neutrinos, Dark Energy, and modified gravity

Instead of having a summary talk on the last day, one representative from each working group reported on the group's activities and results.


A key outcome of the workshop was the realization of which insights into the early Universe and into late-time effects such as massive neutrinos we can realistically achieve with large-scale structure. That is, given the number of galaxies that are realistically observable with survey telescopes in the coming decade, and the fundamental theoretical uncertainties in the modeling of galaxies, there is a certain amount of cosmological information to be extracted from large-scale structure. We now understand this amount at the order-of-magnitude level, though much work needs to be done to work out the details.


Perhaps an “Aha” moment for several participants was the realization that semi-analytical approaches to the biasing of dark matter halos, such as Lagrangian peaks, the “peak-patch picture” and generalizations, are not in competition to the rigorous perturbative bias expansion, but rather complementary: while the latter approach is guaranteed to be correct on large scales, the former can provide a reasonable physical picture for how to connect predictions to non-perturbative scales. Conversely, semi-analytical approaches can benefit from matching the correct perturbative prediction on large scales.


The workshop format, with participants dividing into working groups who report their results, was new to the organizers. However, we deem it highly suitable for workshops of this size (20-30 participants), and a great success in our case. When choosing such a format, it is worth ensuring that the groups are diverse, rather than simply collecting people who collaborate or are in frequent exchange anyway. Further, the organizers went around and joined different working groups each day in order to ensure that discussions stay on track.


Overall, the organizers are very happy with the open environment of the Snellius venue, as well as the helpful and efficient support of the staff (Martijn Fritsen, in our case). It is an ideal environment for a small workshop. What is important to keep in mind to any potential workshop organizers is that, due to the open environment as well as limited time in a 5-day workshop, it is essential for organizers to have a clear plan for what they want to achieve during the week, and to keep the workshop focused.



Daniel Baumann (United Kingdom), Enrico Pajer (the Netherlands), Fabian Schmidt (Germany)