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## Non-Linear Structure in the Modified Universe |

The
future of cosmology lies in understanding the late time universe. The
accelerated expansion happening today is still a mystery. Since gravity is the
only force we know that acts on scales as large as the entire universe, it
seems obvious to consider modifications of gravity in order to understand the
accelerated expansion. Gravity however acts on all cosmological scales, varying
from the radius of the entire universe to the size of planets. Modifications
that explain the accelerated expansion, could therefore potentially be
distinguished from the General theory of Relativity in the history of the growth
of structures. The
aim of the workshop was therefore: "Constructing a road map for testing
modified gravity through linear and non-linear cosmological structures."
Following
the advise of the Lorentz Center,
we opted for a programme with more time allocated for discussions than for
talks. In the plenary sessions, we chose to let two speakers who represent
potentially opposite approaches speak directly after each other, followed by a
discussion lead by a third expert. The format proved to be a very successful
way to provoke discussion. With renowned scientists as discussion leaders
rather than speakers, discussions immediately focussed on fundamental
questions.
Prior
to the workshop, we stated that the workshop would be a success if N-body
simulators learned about the key aspects of modified gravity in late time
structure formation, and if gravity theorists learned in which regimes
modifications of gravity can matter at all. Given this condition, the workshop
was more than a success. The
exchange of knowledge between various experts was tremendous. However, as with
all progress in science, the number of questions raised grows faster than the
number of questions answered. Throughout
the workshop there were many `aha'-moments, most of them in the category of an
extra degree of uncertainty on (or lack of understanding of) one phenomenon or
another, about which people had not worried enough. For example, an important
step forward is the acknowledgement of the fact that baryonic physics does play
a crucial and not well-understood role on the formation of structure, on much
larger scales than what many of the participants assumed. Vice versa,
modifications of gravity can play significant roles on much smaller scales then
assumed, depending on the type of screening, and the possibility of the absence
of Birkhoff's theorem.
The
workshop was attended by 49 participants from 11 countries. Practically each of
them thanked the organizers for an exceptionally fruitful workshop, but most of
their gratitude must be forwarded to the Lorentz Center,
which made the workshop an extremely smooth experience. The organising
committee could fully focus on the scientific content of the workshop, and even
there, the help from the Lorentz Center with the
design of the programme certainly had great positive impact on its success. The
workshop received additional support from the “Leiden University Fund” (LUF).
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