DESY WORKSHOP DESY workshop combines gravity and particle physics Gravity and particle physics took centre stage at last year's DESY theory workshop. As chairman of the organizing committee Dieter Liist reports, there was plenty to talk about. The relationship between astro­ the basic ingredients of the physics, cosmology and elemen­ Standard Model. tary particle physics is fruitful •The choice of topics - gravity and has been constantly evolv­ and particle physics - for the ing for many years. Important 2001 DESY workshop (held in puzzles in cosmology can find Hamburg) was largely influ­ their natural explanation in enced by impressive recent microscopic particle physics, astrophysical observations and a discovery in astrophysics showing that the overall mass can sometimes give new and energy density of today's insights into the structure of universe is extremely close to its fundamental interactions. The critical value (Q = 1). Another inflationary universe scenario main theme of the workshop offers a good example. Inflation was string theories, particularly is a beautiful way to understand Wilfhed Buchmulier of DESY (left) in discussion with Hans-Peter the recently developed M-theory the cosmological flatness and Nilles of Bonn University at the DESY theory workshop. (often dubbed "the mother of all horizon problems (see box over­ theories") that underlies string leaf) and apparently induces large-scale density fluctuations con­ theories. In string and M-theory, multidimensional surfaces, rather sistent with experimental observations. Inflation also predicts the than just strings, are also allowed.These higher-dimensional mem­ existence of dark matter elementary particles together with a certain branes (or branes) and one particular type, Dirichlet, or D-branes, amount of dark energy manifested as the cosmological constant A. subject to a particular set of boundary conditions, have proved This has recently become clearthrough fascinating new experimental important in understanding black holes in string theory. results. However, some of the pieces essential for building a theory At DESY theory workshops, introductory lectures covering the main that combines the physics of the macrocosmos with all microscopic topics of the workshop are traditionally given on the first day. On this phenomena in a complete and satisfactory way are still missing. occasion, Costas Bachas of the Ecole Normale Superieure in Paris presented string theory, string dualities, D-branes and M-theory. Quantum gravity Slava Mukhanov of the Ludwig-Maximilians University in Munich dis­ On the theoretical and conceptual level, ttie quest for a theory of cussed inflation. StefanTheisen of the Max-Planck Institute (MPI) in quantum gravity is the most prominent and important problem fac­ Potsdam covered the holographic principle, which asserts that ing theoretical physics. Quantizing gravity will be necessary to information contained in some region of space can be represented describe the physics at regions of very large space-time curvature - as a "hologram" - a theory that lives on the boundary of that region. near or inside black holes, for example, or at extremely short time Finally, Orsay's Pierre Binetruy discussed the cosmological constant. scales after the Big Bang. Any new theory that goes beyond the established Standard Model of particle physics and of cosmology Cosmic inflation must explain known facts in a broader and more unified perspective. Cosmic inflation in the early universe is one of the most appealing At the same time it should not introduce more - and perhaps hidden hypotheses in cosmology. Inflation stretches space to be flat, and - assumptions than there are facts in need of explanation. Finally, it leads naturally to the density of the universe, Q, having its critical must pass experimental tests and be verifiable (or falsifiable), at value of 1. It explains the large-scale smoothness of the cosmic least in principle. Superstrings offer, perhaps for the first time, a microwave background (CMB) and inflates quantum fluctuations promising avenue for constructing a viable theory of quantum grav­ from microscopic scales to the cosmological scale, thereby creating ity, since they contain gravity with a spin 2 graviton field as well as all density fluctuations. In the first talk of the workshop, Paolo 0 CERN Courier April 2002 27 DESY WORKSHOP de Bernardis of the University of Burt Ovrut from the University of Rome, La Sapienza, showed an Pennsylvania.Taking its name from impressive array of new experi­ a Greek word meaning conflagra­ mental CMB data from the balloon tion, the ekpyrotic universe theory experiment BOOMERanG. These explains the rapid expansion of are in complete agreement with the early universe as arising from the predictions of inflation. the collision of branes. Through BOOMERanG and COBE show that such a collision, a huge amount of the universe is indeed spatially energy is almost uniformly and flat. Moreover, the matter-energy homogeneously deposited on our density, QM, is clearly dominated universe. Despite offering a fasci­ by a large dark matter component. nating and challenging alternative Most excitingly, QM is not enough to standard inflation, many to flatten the universe, but there is aspects of the ekpyrotic universe now convincing evidence for a need further investigation. non-vanishing contribution QA from dark energy, arising from a The latest results from the BOOMERanG experiment are Challenging branes and strings cosmological constant A. improving our knowledge of the cosmic microwave A particularly compelling picture One of the most burning prob­ background. (BOOMERanG collaboration.) of a 10-dimensional universe has lems is explaining the microscopic been developed over recent years. origin of the cosmological constant A, while at the same time under­ In this picture, observable gauge interactions are confined to a pos­ standing why A is so small compared with the natural scale of grav­ sibly three-dimensional domain wall, whereas the gravitational force ity. In this context it is very important to determine whether A is a is mediated over the entire 10-dimensional space-time. This sce­ static quantity, totally unchanged through time, or whether it is nario would account for the vast difference between the observed dynamic. Quintessence - a "fifth force" that changes with time - strength of the gravitational interaction and nature's other funda­ offers a concrete realization of this idea. It was introduced by Slava mental interactions. It also offers the exciting possibility that the Mukhanov and by Heidelberg's Christof Wetterich, who discussed extra dimensions can be much larger than previously assumed - up how the cosmic coincidence problem (why the cosmological con­ to almost 1 mm. If the extra dimensions are compact, their sizes are stant only recently started to dominate the expansion of the uni­ constrained by high-precision experiments that measure deviations verse) can be explained by some kind of attractor mechanism. from Newtonian gravity below 1 mm, as Joshua Long of Colorado Agreement between the theoretical idea of inflation and experi­ University pointed out. Depending on the coupling strength of grav­ ment is convincing. However, model building is still difficult and ity inside the extra dimensions, the present experimental upper seems to require several assumptions and fine-tuning of parame­ bounds for their size vary between 1 mm and several microns. New ters. This leads to the question of whether there are serious com­ techniques are expected to push these bounds below 1 jxm. In addi­ petitors for inflation, for example, in M-theory. This would be tion, Bonn's Hans-Peter Nilles and Valery Rubakov of Moscow's desirable since some basic arguments state that de Sitter Institute of Nuclear Research discussed more theoretical issues and space-times, which describe an exponentially growing universe, are exotic effects arising from extra dimensions. difficult to implement in supergravity and superstring theories. As One challenge in string theory is to construct brane-world models Fernando Quevedo of Cambridge discussed, there is a nice way to that come as close as possible to the Standard Model of elementary build inflationary models into brane-world models in string theory in particles. Ralph Blumenhagen of Berlin's Humboldt University sug­ such a way as to trigger the graceful exit from inflation.This leads to gested that intersecting brane worlds is a promising approach. Stable a hybrid inflationary scenario being realized in brane-world models. intersecting brane-world models reproducing the Standard Model A more radical approach to explaining the flatness and horizon prob­ can be constructed, but issues such as the correct pattern of Yukawa lems - one that really competes with inflation - was introduced by couplings and gauge coupling unification still need to be addressed. Big Bang problems Despite its success, there are three problems with the Big Bang would steer the universe to a fate between a big chill and a big model which were hotly debated at the DESY theory workshop. crunch. Big Bang cosmology predicts that any deviation from flat­ • The horizon problem Remote regions of the universe that ness in the early universe should have increased as the universe have been out of contact - or beyond each others' horizon - are expanded, which is difficult to reconcile with observation today. nevertheless similar. • The monopole problem Big Bang cosmology predicts that • The flatness