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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.
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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).
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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.
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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.
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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)
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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.
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