The majority of current cosmological observations, such as the scale size of fluctuations of the Cosmic Microwave Background radiation (CMB), the measurements of clustering mass on large scales, and the magnitude-redshift relation of type Ia supernovae (SNe Ia), are interpreted to give consistent estimates of the amount of dark matter. This situation has given rise to a spectacular popularity of the so-called ΛCDM (Cosmological constant + Cold Dark Matter) model of structure formation. In this framework, baryons, which can be detected in the form of, for example, luminous
objects such as stars, galaxies and hot clouds, would only constitute some 5% of the total mass in the universe. The rest is made of entities about which little is understood: dark matter and dark energy. Dark matter, in form of non-baryonic elementary particles, would dominate the ～30% of the total mass of the universe. Despite great efforts, the direct detection of the cosmological dark matter particles in laboratory experiments or sky searches is still lacking and becoming worrisome.
In order to interpret observations within the standard cosmological framework which includes the non-baryonic dark-matter, it has been necessary to introduce another substance, which has been termed dark energy, which behaves as if it has a gravitational negative pressure. This mysterious form of energy would cause the accelerating re-expansion of the universe and should account for about 70% of the mass-energy in the Universe. In the ΛCDM model, the amount and properties of these different kinds of dark matter can only be defined a posteriori: however they are supposed to provide more than 90% of the total fraction of the mass-energy in universe, playing a crucial dynamical role both in the early and in the present-day universe.
Many successes of the ΛCDM model to fit different observations can actually be traced back to very strong a-priori assumptions, a large number of free parameters and ad-hoc hypotheses. Recently a growing number of astrophysical papers are being published on specific cosmological observations which significantly differ from the predictions of ΛCDM and which challenge the standard ΛCDM model in fundamental ways. These observations include: the large scale flows, the sizes and amplitude of galaxy large scale structures, the systematic effects biasing the analysis of the CMB data provided by the WMAP satellite and the lack of large-angle correlations, the
anisotropy of the Hubble flow, the evolution of galaxy size, and the failure to find the sub-halo building blocks left over from the primordial fluctuation spectrum. Also the observation of older and older galaxies and black holes with apparent enriched chemistry seems to “confound” the dark age of the model.
While each of these observations can be seen as an anomaly that the model would possibly explain, the bulk of them calls for a more careful analysis of the model foundations, particularly the amount and role of dark substances. From the theoretical side it is by now known that the role of hydrodynamics at the end of the plasma epoch can be more important that often assumed, and that CDM could not have such a central role in the theory of structure formation as is currently thought. Moreover recently there has been a considerable theoretical effort to develop a coherent picture in General Relativity which appropriately takes into account matter inhomogeneities. In this
perspective the problem of dark energy would be intimately related to the correct understanding of observational anomalies, in particular, the observed abundance, size scales, and emptyness of voids.
A workshop that puts together these many different approaches and results would be very timely and can provide a substantial advance in the comprehension of the large scale universe. Progress can be hoped for by combining the insights from the various groups and researchers studying the different observational and theoretical problems. The workshop offers a unique opportunity to
discuss conceptual and methodological problems of the modern cosmological paradigm.
A measurement of the success of this workshop could be a consensus on or a way out of the dilemma that ΛCDM is both too good to be ignored but fails too often to be correct.
During the workshops there have been 28 talks which can be divided into 4 main different groups: (i) large scale structures in the universe, (ii) cosmic microwave background anisotropies, (iii) general relativistic models with inhomogeneities and (iv) structure formation.
Each of these topics has been then discussed in a round table. For the topic (i) the chairman was Michael Joyce, for (ii) Ruth Durrer, while topic (iii) and (iv) where discussed in the longer round table at the end of the meeting chaired by Theo M. Nieuwenhuizen and Rudolph E. Schild. The aim of the round table was to confront different views and to identify the open problems and the directions to consider for further investigations.
From the discussion of the question of the size and amplitude of structures it has been concluded that it is necessary to test all the different assumptions which are used in the data analysis. Indeed, the final information about the size and amplitude of structures depend on a number of assumptions which enter, implicitly or explicitly, in the statistical methods and in the treatment of observational selection effects. It was agreed in particular that the different groups working on the problem would consider the same data sets and consider in detail the properties of the statistical estimators used
in the data analysis.
The point in the discussion of the CMB fluctuations is that it is necessary that different groups of researchers analyze the rough data from the basis. The data from the WMAP satellite were subject of intense discussions as during the workshop four different approaches with respect to the standard data analysis were presented. Each of the speakers has found important differences with the published data analysis. This situation clearly points out that it is extremely important that different teams may have access to the rough data and consider the whole data analysis from scratch.
In the discussion about structure formation several points were discussed. The first concerns the understanding of the numerical resolution, the second the formation of structures at smalll scales and the third the effect of hydrodynamics in the gravitational clustering. The first two are closely related, as the issue of numerical resolution is behind the predictions of the models at small enough scales. The role of hydrodynamics in gravitational structure formation was largely discussed as it is generally not included in the models while it can have an important impact, especially at small scales.
Finally the problem of how to model inhomogeneities in general relativity was discussed along two different lines of approach. The first involves the consideration of an average solution to the Einstein field equations together with the effect of inhomogeneities on the synchronization of clocks. The second uses solutions to a spherically symmetric distributions without fluctuations. Different tests of model were discussed together with their predictions for future observations.
It was generally felt that being together with many diverse opinions and having much time for discussions, will enhance the understanding of the physics that lies behind the data.
• Dr. Theo M. Nieuwenhuizen (ITFA,UvA)
• Dr. Rudolph E. Schild (Harvard Smithsonian Center for Astrophysics, Cambridge, USA)
• Dr. Francesco Sylos Labini (Centro Enrico Fermi, Institute for Complex System, CNR, Rome,
• Dr. Ruth Durrer (Theoretical Physics, Geneva, Switzerland)