The discovery of the accelerating expansion of the Universe arguably provides the most profound puzzle in contemporary physics. It implies that either the Universe is dominated by a component, dubbed “Dark Energy”, that turns gravity's pull into a push, or that General Relativity, our understanding of gravity itself, breaks down on cosmological scales. Either way, it deeply challenges our knowledge of the basic laws of Nature. Characterizing and understanding the origin of the accelerated expansion is a truly global and interdisciplinary enterprise.
As general structure for the programme, we aim at discussing probes of cosmic expansion, tests of structure growth and at understanding the implications of combining them. Different competing theoretical scenarios, both of the “modified matter” and “modified gravity” kinds, will be compared against each another. Both the theoretical and experimental/observational viewpoints will be represented and discussed.
Key scientific questions to be addressed include:
• Testing models of cosmic acceleration. The general picture emerging from observations is that Dark Energy is very close to a simple Cosmological Constant, with w=p/rho=-1 to within a few percent. In order to disentangle competing theoretical scenarios, massive observational efforts are needed. Most alternative models to a cosmological constant predict a time varying equation of state. Constraining the evolution of the equation of state is the next natural step to unravel the riddle of cosmic acceleration. Furthermore, models which extend Einstein’s gravity on large scales typically lead to a different evolution of the cosmological density perturbations and to a different amplitude of gravitational lensing. Hence, probing the growth of cosmic structures is also very important for understanding the nature of cosmic acceleration. Different observational methods are affected by different systematics, so complementary approaches are required to control them. Comparing results and prospects from these different approaches allows to tackle key questions on the nature of the accelerated expansion.
Do we understand the systematics that affect the different methods? What are the implications of the current results for fundamental physics? Can Dark Energy really be accurately described as a simple Cosmological Constant, or are there evidences for it varying in space and/or time? Do the observational data require an extension of Einstein’s gravity? What input do experiments need from theory to limit the available parameter space and to guide the interpretation of the observational evidences? What is the most efficient observational strategy to detect deviations from standard LCDM in the first place?
• The theoretical framework. A plethora of models have been put forward to explain Cosmic Acceleration. However, no agreement has emerged yet on a comprehensive framework for discussing the landscape of models. This, in turn, hampers further observational efforts, for which no guidance can be provided as what to look for.
Can we start to define a comprehensive theoretical framework? How can modified matter models be distinguished from modified gravity models? Are there generic methods to parameterize linear and non-linear density fluctuations? What is the role of cosmological simulations in guiding observations? What input does theory need from observations? What interesting models are still available after the constraints from the Gravitational Wave event GW170817? What steps need to be taken so that numerical codes of dynamical Dark Energy and Modified Gravity reach the same level of accuracy as those for standard LCDM? Can we test Dark Energy and gravity in a model-independent way?