Research

One of the most stunning examples of direct evidence for dark matter is the "bullet cluster": the visible matter (red and yellow) only contributes little to the total mass of the two colliding clusters (density contours in green). Picture taken from Clowe et al '06. A plethora of independent cosmological observations consistently point at most of the matter in the universe being dark, i.e. not detectable by the light it emits (like stars, nebulas, hot gas etc.). In fact, more than 80% of all the matter in the universe cannot even be "ordinary" (i.e. baryonic) matter – the stuff that we, the earth and all we know from the laboratory is built of. While the total amount of this puzzling dark matter can be determined to a great accuracy, its nature so far remains a mystery. Many scientists believe, however, that dark matter consists of a new type of elementary particle. The most favorite candidates are WIMPs (weakly interacting massive particles), particles that arise – almost as a by-product – in many modern particle physics theories. Already in the near future, they could be detected by measuring their (very scarce) interaction with huge detectors on earth or even be produced at large colliders like the CERN LHC. When these particles meet in the galactic halo, they can pair-annihilate to ordinary particles; another promising way to indirectly detect dark matter is therefore to spot possible annihilation products in the spectrum of cosmic rays (like gamma rays, positrons or antiprotons).
This is how small, curled-up extra dimensions might look like. There are numerous popular accounts on EDs on the web, see e.g. this one. The idea that spacetime may consist of more than four dimensions, with spatial extra dimensions compactified on such a small scale that we cannot directly observe them, goes back to the works of Kaluza and Klein at the beginning of the 20th century. Today, scenarios involving extra dimensions are mostly motivated by string theory and cosmology provides an important testing ground for such theories. A generic prediction is for example that the fundamental coupling constants should vary with the volume of the internal space - but since there are tight observational constraints on such a variation, one has to find mechanisms that effectively stabilize the size of the extra dimensions during the cosmological evolution. Ordinary particles with extra-dimensional momentum may even be the explanation for the nature of the mysterious dark matter, which contributes six times as much to the matter content of the universe as ordinary, baryonic matter. It is mainly this latter connection that that has caused the recent boost in (cosmological) interest in extra-dimensional scenarios.
The universe today exhibits a rich variety of structures that can all be traced back to tiny density fluctuations in the distant past. (Picture taken from the HDF project) According to the present paradigm of cosmology, all the structure that we can see in the universe today (like galaxies, clusters of galaxies, etc.) can be traced back to tiny density fluctuations in the otherwise homogeneous early universe. Such primordial density fluctuations are believed to have their origin in an era of so-called inflation, where the universe underwent a period of accelerated expansion. If these fluctuations grow big enough, they are expected to gravitationally collapse and thereby form primordial black holes (PBHs). In contrast to astrophysical black holes, these objects may in principle be arbitrarily small and thus possibly even provide a probe of quantum gravity effects. No PBHs have yet been observed, but there exist strong constraints on their abundance - both from their gravitational contribution to the total matter content of the universe and from effects due to their Hawking-radiation - and these constraints provide valuable information about the primordial density fluctuations (especially at very small scales) and thus about the physics of the early universe.