The deepest enigma of modern physics is whether or not there are any  fundamental  scalar fields in nature. Scalar fields have long been part  of the standard model of  particle physics, most notably through the  Higgs particle, which is thought to be responsible for endowing the  building blocks of matter with mass (as well as making the theory gauge-invariant). On the other hand, Einstein gravity does not make  use of any scalar fields. This is a remarkable fact, because almost  any consistent gravitational theory that one can think of (from  Newtonian gravity to models with extra dimensions, such as string  theory) will do so. Indeed, the fact that the only gravitating field  in the universe is a rank-two tensor (the metric) is, to some extent, what defines Einstein gravity.

Recent developments suggest that scalar fields are just as important in cosmology. They are preferred candidates to solve key puzzles, including the exponential expansion of the early universe (inflation) and the dark energy that is accelerating it again today. They might also contribute to the formation of structure in the universe and the possible variation of what have been considered fundamental constants of nature.

Although there are widely accepted theories on both sides which rely on scalar fields, neither has so far produced any definitive evidence for them. String theory, for example, is arguably the most beautiful mathematical theory ever built, but so far there isn't a shred of observational or experimental evidence that it applies to the real world. Researchers in string theory and related areas don't often make a serious effort to connect to researchers in cosmology and astrophysics, which offer the only realistic hope of confirming or disproving it. This is one example of a more general problem, whose origin is that there is a striking difference in the way both camps tend to approach the subject. Particle physicists, even when they develop their models to considerable detail, tend to consider only the simplest cosmological consequences (typically focusing on the order-zero, background solutions, if at all). Cosmologists often disregard any particle physics motivation (or lack thereof) and treat models as toy models, which they can put through the sieve of their preferred cosmological tests.

This Physics--Astronomy divide has deep historical reasons, but it is clearly not ideal: models that are well motivated from the particle physics point of view may fail detailed cosmological tests (or sometimes even fairly simple ones), while toy models that can fit observational data quite well may be deeply un-natural from a particle physics point of view. This workshop aims to foster an Europe-wide effort to bridge this debilitating gap, and identify a range of key astrophysical and cosmological tests of fundamental physics, that might be carried out in the coming decades.

This is an intrinsically interdisciplinary issue, and the range of opportunities afforded by the wealth of high precision data that will gradually become available means that it will inevitably be at the forefront of research in astrophysics and fundamental physics in the coming decades. However, its very inter-disciplinarity implies that it is technically very demanding, and hence there is a large initial inertia in the research community that needs to be overcome. Moreover, as we mentioned above there are also sociological and historical reasons that slow the process, in particular the fact that there is commonly a sharp organizational separation between the particle physics and astrophysics communities (for example, at European level, CERN vs ESO/ESA). A workshop of this kind, sponsored by a cross-disciplinary body like ESF, can have a strong steering effect on both communities and help bridge the gap.

In a novel area like this, it is crucial to maintain the breadth of the field and a diversity of approaches, rather than focusing all efforts on a single theoretical approach or experimental setup to tackle a particular issue that is deemed (possibly erroneously) to be the crucial one. On the contrary, it is important to, in some sense, spread the risk and thereby enlarge the serendipity potential of forthcoming theoretical and observational endeavours. This means that it is imperative to stimulate the interactions between researchers approaching the problem from different angles, as opposed to letting each sub-community proceed on its own. By the same token, it is also important to foster individuality, as opposed to defaulting to an excessive centralization that will leave European research in this area in a frail state for a generation if the one chosen approach turns out to be non-optimal.

This is a timely moment to open this new direction since organizations like ESA and ESO are now actively planning the next generation of experiments. Notice, in particular, the recently released ESA's Cosmic Vision 2015-2025 (for which some calls for proposals are ongoing and others due to be issued shortly), while ESO has recently released its E-ELT book, with a call for further VLT instrument proposals due to be issued in 2008. Various senior European scientists have expressed concern at the lack of vision and ambition of these documents, which can perhaps be seen as confirmation of the loss of European leadership in this area. Clearly, only a Europe-wide effort can stop and reverse this decline. Again, a workshop of this kind, that can bring together the top European researchers in this area, can have a strong impact in promoting leadership and identifying the key strategic decisions that the European research and funding organizations should consider.