%%%%% 7.10.2000
\newcommand{\bea}{\begin{eqnarray}}
\newcommand{\eea}{\end{eqnarray}}
\newcommand{\be}{\begin{equation}}
\newcommand{\ee}{\end{equation}}
\newcommand{\ben}{\begin{enumerate}}
\newcommand{\een}{\end{enumerate}}
\newcommand{\bi}{\begin{itemize}}
\newcommand{\ei}{\end{itemize}}
\newcommand{\bs}{\begin{slide}}
\newcommand{\es}{\end{slide}}
\newcommand{\bmi}[1]{\begin{minipage}{#1 cm}}
\newcommand{\emi}{\end{minipage}}
\newcommand{\itRahmen}[2]{\begin{center}
\fbox{
  \parbox{#1 cm}{\it #2}}
\end{center}}

 
{\catcode`\@=11
\gdef\SchlangeUnter#1#2{\lower2pt\vbox{\baselineskip 0pt \lineskip0pt
  \ialign{$\m@th#1\hfil##\hfil$\crcr#2\crcr\sim\crcr}}}
  % kopiert von \@vereq aus dem TeXbook, Seite 360.
}
\def\gtrsim{\mathrel{\mathpalette\SchlangeUnter>}}
\def\lesssim{\mathrel{\mathpalette\SchlangeUnter<}}

%\documentclass[a4paper,12pt]{article}
\renewcommand{\theequation}{\arabic{section}.\arabic{equation}}
\newcommand{\vectii}[2]{\rund{\begin{array}{c} #1 \\ #2 \end{array} }}
%\usepackage{epsfig,color}
\parskip0.8ex
\parindent0cm
%
%\def\llabel#1{\label{#1}}\marginpar{#1}}
\def\elabel#1{\label{eq:#1}\hbox{REF:} {#1}}
%
\def\ts{\thinspace}
%

\newcommand{\fig}[6]{
\begin{minipage}{#1cm}
\includegraphics[width=#2cm,angle=#3]{#6}
\end{minipage}
\begin{minipage}{#4cm}
#5
\end{minipage}
}

%\begin{document}

\chapter{Introduction}
\section{Dark matter and vacuum energy: grand challenges for modern
physics} 
At the turn of the millenium, the two fundamental fields of physics,
elementary particle physics on the one hand, and cosmology on the
other, are undergoing a great development. Observational evidence is
accumulating that neutrinos have a finite, though tiny rest mass,
signifying the need for a revision or extension of the current
standard model of particle physics. The next generation of particle
accelerators will most likely be able to detect the postulated Higgs
particle, which lies at the heart of the connection between symmetry
groups and particle phenomenology. Furthermore, the LHC at CERN will
reach energies which makes it plausible that the lightest
supersymmetric particle can be discovered, thus providing an essential
test to this theory which aims at the unification between strong and
elektro-weak interaction with the gravitational force, i.e. a theory
of quantum gravity. 

Cosmology, on the other hand, has undergone a phase transition recently. We
are in the process of determining the basic parameters of the
cosmological model with a precision unimaginable only a few years
ago. To wit, several independent methods converge on the value of the
Hubble constant, $H_0\approx 73\pm 10{\rm km/s}$; the very recent
baloon experiments BOOMERANG and MAXIMA measured the fluctuation
spectrum of the Cosmic Microwave Background (CMB), the relic of the
early phase of our Universe, and provided very strong evidence that we
live in a spatially flat spacetime. Finally, two independent groups,
by studying the population of supernovae (SN\ts Ia) at high redshift,
both reported very strong indications for a non-zero value of the
cosmological constant $\Lambda$. The era of precision cosmology is
lurking around the corner, in the form of extending supernovae studies
to even higher redshifts, and future baloon and satellite missions for
measuring the CMB flutuations over the full range of angular scales,
which will determine the fundamental cosmological parameters to within
a few percent.

Already today, there exists very strong evidence for the existence of a
non-zero cosmological constant and for the dominance of dark matter in
the Universe. The baryon density is strongly constrained by
measurements of the light element abundances, most noticibly helium and
deuterium, in connection with Big Bang nucleosynthesis. Baryons cannot
account for the cosmic matter density as determined by combining the
CMB with the SN\ts Ia results, or, completely independently, from the matter
density locked up in clusters of galaxies. Both arguments lead to the
conclusion that $\sim 90\%$ of the mass in the Universe is in the
form of non-baryonic matter. The quest for the dark matter lies at the
center of current interest in cosmology, but also poses a great
challenge to particle physics -- presumably, the dark matter comes in
the form of some as yet unidentified weakly-interacting elementary
particles. Alternatively, dark matter may be in the form of primordial
black holes, presumably formed as a consequence of a phase transition
in the very early Universe. In either case, the nature of dark matter
can only be understood in the frame of an extended theory of
elementary particle physics.

Most surprising is the probable existence of a non-zero cosmological
constant, mainly because its value is some 120 orders of magnitude
smaller than particle theory would estimate the value of the vacuum
energy density. Unless one accepts the idea that we live in a Universe
where some as yet unknown physical constants are subject to extreme
fine-tuning, an understanding of the nearly-zero value of $\Lambda$ is
a most essential step to understanding both cosmology and particle
physics. It is possible, perhaps even likely, that the explanation for
a non-zero $\Lambda$ and the nature of dark matter are intimately
related. These two problems may be seen of the same fundamental nature
as that of discrete energy levels before the discovery of quantum
mechanics, and their solution will most likely have equally profound 
consequences for physics.

To summarize, the nature of the dark matter and the non-zero value of
the vacuum energy density constitute the most burning and challenging
problems of fundamental physics, and it is obvious that they can only
be solved in a close collaboration between cosmologists and particle
physicists.

\section{Synthesis of Micro- and Macrocosmos}

Dark matter and cosmological constant (dark energy) are two of the
many phenomena that link the two fields of science that deal with the
explanation of phenomena at the largest and smallest scales:
cosmology and particle physics, respectively.

Collaboration between these two branches of science becomes so
fruitful right now because of the lucky coincidence that both can be
described by what is called a standard model, seemingly describing all
known phenomena in the field.  This indicates that in both fields we
seem to be on the right track and can start further explorations from
a firm base.

The standard model of particle physics has been formulated in the late
sixties and describes physics at subnuclear scales with great
precision.  Matter is composed of quarks and leptons and the
fundamental interactions are mediated by gauge bosons of the
underlying gauge theory: $SU(3)\times SU(2)\times U(1)$ for strong,
weak and electromagnetic forces. All known microscopic phenomena can
be described by this theory. Its prediction concerning the existence
and mass of the weak intermediate bosons $W^{\pm}$, $Z$ and the top
quark have been confirmed experimentally at CERN 1984 and Fermilab
1995, respectively.  The only missing ingredient is the so-called
Higgs boson and its search is a major motivation for the construction
of large accelerators as LEP and LHC (CERN), the Tevatron (Fermilab)
and linear $e^+e^-$ colliders at even higher energies.

The standard model of cosmology is based on general relativity as an
expanding Friedmann-Lemaitre universe originating from a so-called hot
Big Bang.  The evolution of the universe passes through a radiation
dominated phase of high temperature followed by today's matter
dominated phase. That might enter (or has entered already) in a vacuum
energy dominated phase. The theory allows us to understand the
appearance of the universe today with its stars, galaxies, clusters
and superclusters evolving from a hot particle plasma in the early
universe. Its most generic predictions, most notably the existence of
the microwave background and the abundance of light elements, have
been beautifully confirmed by observations. Most recently the
measurement of the energy density of the universe, the observation of
fluctuation in the microwave background and its connection to
structure formation in the presence of cold dark matter have given
further strong support to the standard hot big bang
theory. Observations in the near future will allow a more precise
determination of the cosmological parameters contained in this
standard model.

While these standard models are successful in their description of known
natural phenomena, they do not represent a complete theory yet. But they
can serve as a solid base for exploring the questions that are still
open. Both models contain many parameters that could be (and partially have
been already) determined by observations, 
but the actual values require still some
explanation. It turns out that these questions seem to require a
synthesis of the two fields. As we have seen above, dark matter and
dark energy are such examples. But, as we will argue, there are more
phenomena ``beyond the standard model'' that are of common interest.

The motivations to go beyond the standard of particle physics are
primarily theoretical. One of them is the quest for the unification of
fundamental interactions, including gravity. A second is the search
for mathematical consistency of this unified description in the
framework of quantum field theory.  These considerations lead to the
concepts of supersymmetry, supergravity and (super)string theory with
various predictions for high energy physics and cosmology, that will
be tested in the near future. In fact the notion of a ``Supersymmetric
Standard Model'' has already ermerged. It predicts a host of new
particles, the lightest of which is neutral and stable. The properties
of this particle are such that it could dominate the matter density of
the present universe as cold dark matter. This lightest supersymmetric
particle (LSP) is a so-called WIMP (for Weakly Interacting Massive
Particle) could be the particle physics candidate for the dark matter
seen in astrophysical and cosmological observations. This is of
particular importance since by now it is well known that the neutrinos
of the standard model, contrary to earlier expectations are not able
to play this role.

A further argument to consider physics beyond the standard model is
the absence of baryon number violation.  This, of course, is very
desirable in view of proton stability, but some amount of baryon
number violation is needed in the process of baryogenesis: the
creation of a baryon-antibaryon asymmetry in the early universe. It
cannot be achieved within the physics contained in the particle
physics standard model.

Baryogenesis is as well an ingredient of the phenomena beyond the
standard model of cosmology. While the baryon-to-photon abundance
ratio can be determined from considerations of nucleosynthesis, an
explanation of this ratio of $10^{-9}$ is not available within the
standard model.  Similar statements apply to other cosmological
parameters such as the total matter density $\Omega_M$ and the vacuum
energy density $\Omega_\Lambda$. The great challenge in this model is
the description of the connection of present day astronomical
observations to the fundamental parameters in the early
universe. Mechanism in astrophysics like star formation, galaxy
formation and others will become important aspects in establishing
this relation.  With the new observational results to come in the next
few years, much more detailed questions can be asked and hopefully
answered.

These observations will certainly be very important for the field of
particle physics as well, in particular since we know by now that
neutrinos, although (probably) massive, will not play an essential new
role in cosmology, since their contribution to (hot) dark matter will
not be important. In fact, from the simple observation that the age of
the Universe exceeds $10^{10}$years, from which it follows that the
mean cosmic matter density must not substantially exceed the critical
value, cosmology yields upper limits on the masses of neutrios
($\lesssim 100{\rm eV}$), which for the $\mu$ and $\tau$-neutrino are
several orders of magnitude more sensitive than laboratory constraints. 
Other candidates for (cold) dark matter like the
above mentioned WIMP-neutralino, axions, gravitinos etc. might become
more relevant.

A fundamental question of particle physics is concerned with a
mathematical consistent description of quantum gravity. No 
satisfactory theory has been found yet, although superstring theory
might be an important step forward in the right direction. The mysteries
of the cosmological constant and the nature of 
black holes will remain enigmatic
as long as this problem is not resolved. Theoretically strings,
membranes, extra dimensions are attempts in this directions, but
particle physics experiments cannot easily explore these unknown
phenomena. It is here that astrophysical observations have to step in.
Only these can reveal the possible existence of a cosmological constant,
which is such an important ingredient of any theory of (quantum)
gravity.

In astronomy the existence of dark matter has been established, but its
origin has still be to clarified since a conventional explanation is not
available. With the upcoming  precision  data it has to be seen
whether a WIMP as a neutral weakly interacting  particle can
explain the phenomenon. Direct and indirect WIMP searches are underway
and will hopefully resolve these puzzles. Detailed question like e.g. the
density profile of dark matter in our galaxy can then be answered with a
common effort in the field of astronomy and physics.

In such a way the two standard models of particle physics and cosmology
might merge to a unified description
of our universe at all the different scales 
during its evolution.





\section{Observational probes of dark matter}
Several deep underground experiments are underway which attempt a
direct detection of weakly-interacting massive particles (WIMPs); one
of them recently claimed a first signal. Such a detection would
provide a major step in the quest for the nature of dark matter, since
it would verify that WIMPs, and not primordial black holes or other
massive bodies, make up the majority of the dark matter in our
Galactic halo. However, it appears that these dark-matter experiments
will be able to prove the sheer existence without providing much
additional information about the physical properties of WIMPs.

On the other hand, there are no strong astronomical arguments (except
Occam's razor) that the dark matter on scales of galaxy halos is the
same as that on larger spatial scales. In fact, the baryon density
determined from primordial nucleosynthesis is sufficiently high to
allow for all dark matter on galactic scales to be in the form of
baryonic matter -- the detection of a significant gravitational
microlensing rate towards the Magellanic Clouds provides some evidence
for the presence of massive compact halo objects (MACHOs) in our
Galaxy, though the interpretation of these results are far from being
obvious. In conclusion, we have no precise idea of how much
non-baryonic matter there is in our Galaxy.

Precision cosmology in the above mentioned sense means that the
statistical properties of our Universe can in principle be predicted
with very high accuracy. As the evolution of weakly-interacting dark
matter is governed solely by gravity, very high resolution numerical
simulations and, to some degree, analytical models, can provide
detailed predictions for the distribution of dark matter and its
evolution with cosmic time. In contrast, modelling the distribution
and evolution of luminous matter, such as galaxies, is tremendously
more difficult owing to the wealth of physical processes which govern
the interplay between gravity, hydrodynamics, star formation and
feedback processes. Such simulations can be compared with
observations to test the underlying theory in more detail, but also to
obtain an empirical handle on the evolution of the interplay between
dark and luminous matter. In particular, discrepancies between generic
predictions of such simulations and observational results will signify
gaps in our understanding of dark matter properties.

One of the results of these simulations of hierarchical structure
formation is that the dark matter halos of galaxies, and of groups and
clusters of galaxies, follow a `universal' density profile. There is
substantial evidence from observations of the rotation curves of
low-surface brightness galaxies, which are dominated by dark matter,
that their dark matter halo does not follow the universal density
profile. To remedy this potential conflict between theory and
observations, the concept of self-interacting dark matter particles
has been suggested; depending on the cross-section, such
self-interactions will affect the central parts of dark matter halo
profiles, but leave the outer regions unaffected. If future
cosmological evidence supports the idea of self-interacting WIMPs, it
would provide an invaluable clue to the nature of fundamental
interactions of matter. Very detailed measurements of rotation curves
of dark-matter dominated galaxies, combining optical and radio
high-resolution data, are needed to verify the discrepancy between
true dark matter mass profiles and theoretically expected ones.

Another generic prediction of hierarchical structure
formation is the existence of many low-mass dark matter halos around
massive galaxies like ours; whereas some 30 faint galaxies are known
to exist in the Local Group, this is about a factor 10 below
predictions from Cold Dark Matter models. If these discrepancies are
confirmed, they would present a serious challenge to our understanding
of structure formation, and thus of dark matter.
  
Whereas on galaxy scales baryonic matter may significantly alter the
profile and appearence of dark matter halos, dark matter on larger
scales provide cleaner laboratories for investigating the dark matter
distribution. Although the defining criterion of dark matter provides
the biggest obstacle to its study, namely that it is dark, it is
generally assumed that the baryonic processes in clusters of galaxies
are sufficiently well understood to allow a detailed investigation of
their dark matter profile. One of the goals of near-future cluster
observations, using new millimeter instruments for measuring the
Sunyaev-Zeldovich effect, the new generation of X-ray telescopes to
map the density and temperature structure of the intra-cluster gas,
and gravitational lensing observations using the new 8-meter class
telescopes and wide-field imaging cameras, will be to compare the mass
density in clusters with the universal density profile predicted by
theory. A serious discrepancy would provide a smoking gun for CDM
models of structure formation.

One of the main goals of modern cosmology is therefore to investigate
the spatial distribution of dark matter and its evolution with cosmic
time. Whereas the CMB probes the distribution of matter at very early
times and on large scales, the dark matter distribution `closer to
home' needs to be studied with very different methods. The most
appropriate one is gravitational lensing, since the deflection of
light in gravitational fields is independent of the state and nature
of the matter which causes them. Hence, lensing will be used 
\bi
\item
to study the statistical properties of the large-scale (dark) matter
distribution in the Universe at characteristic redshifts of $z\sim
0.3$, by measuring the so-called cosmic shear,
\item
to measure the mass profile of clusters of galaxies out to large
radii, and compare their mass and light profiles,
\item
to search for matter concentrations with cluster mass which have
failed to produce luminous galaxies and which therefore will escape
more traditional survey methods,
\item
to study the mass properties of the dark matter halos of galaxies and
groups, employing the methods of galaxy-galaxy lensing, in particular
to determine the size of the dark matter halo of galaxies,
\item
to search for substructure in the mass distribution of galaxies which
may signify the presence of the `missing' satellite galaxies predicted
by hierarchical structure formation, by investigating the akward flux
ratios of several multiply imaged quasar system.
\ei
Recent discoveries of a significant cosmic shear signal, reported by 4
independent groups of researchers, have opened up a new field of
research and provided cosmologists with a powerful tool to investigate
the dark matter distribution at intermediate redshifts. Recognizing
the potential of this new handle on dark matter, serious planning has
begun for a dedicated telescope that shall be used entirely or to
a large fraction to cosmic shear obsersation. Three such concepts have
been widely discussed; the perhaps most promising one is the Dark
Matter Telescope, an 8-meter telescope equipped with a 3-degree camera
and $(36000)^2$ pixels which was ranked very highly in the Decadel Report
of US Astronomy.  This ultimate imaging machine presumably can be
realized only in an international effort.


Studying the matter distribution in and around our Galaxy is
complicated due to the lack of luminous tracers at large
Galacto-centric distance. Nevertheless, impressive
progress has been made in the past years, owing to the astrometric satellite
mission Hipparcos, wide-field multi-color photometric surveys like
Sloan, new surveys at 21\ts cm to map the angular and velocity
distribution of hydrogen clouds, and new ultraviolett space missions
to study the absorption of gas in our Galaxy. Here we mention two of
the highlights resulting from these studies:

The new identification of cold streams of stars around our Galaxy are
best interpreted as tidal debris from satellite galaxies which in the
recent past have merged with the Galaxy, following the decay of their
orbits due to tidal friction. In one case, the tidal stream can be
identified with the Sagittarius dwarf galaxy which is currently in the
process of merging. These observations thus lend support to the
hierarchical process of structure formation in which massive galaxies
are built through the merging of less massive entities. In a similar
vain, the Magellanic stream seen as a tail of hydrogen trailing the
orbits of the Magellanic clouds signifies the slow decay of the Clouds
due to tidal interaction with our Galaxy.

The identification of a large number of hydrogen clouds which are
approaching our Galaxy with velocities much larger than characteristic
orbit velocities has recently been interpreted as being due to gas
situated in dark matter halos which are not massive enough to promote
significant star formation. This interpretation is supported by UV
spectroscopy and the failure to detect molecular (e.g. CO) emission,
thus finding that these gas clouds have low metallicity and therefore
are unlikely to be of Galactic origin. If true, the high-velocity
clouds can be the baryonic tracers of the missing satellite halos
predicted in CDM models of struture formation. Testing this
hypothesis is therefore of utmost importance to verify, or otherwide,
generic prediction of this cosmological model.

Upcoming astrometric satellite missions will provide a quantum leap in
accuracy with which we can measure the kinematics of stars in our
Galaxy and thus map its gravitational field out to large
distances. Already today, the stellar tidal debris provides strong
evidence for at most a modest flattening of the dark matter halo of
the Galaxy, and these future missions will firm up these
constraints. Since the shape of the dark matter halo reveals
information about its formation, such constraints are essential for
our understanding of the Galactic history. Wide-field imaging surveys
will detect further tidal streams and thus allow us to reconstruct the
recent accumulation of Galactic mass from satellite
mergers. Higher-resolution and more sensitive 21\ts cm surveys will map
the distribution of high-velocity clouds around our Galaxy and thus
provide clues to their nature, their origin and their relation to dark
matter halos. It is one of the fascinating aspects of modern
astrophysics that very traditional astronomical subjects such as
astrometry and H\ts I surveys are intimately coupled with some of the
most urgent questions of cosmology, i.e. the nature and distribution
of dark matter.

The observational evidence for dark matter could be of great relevance
for elementary particle physics.  It might find its explanation in the
existence of new fundamental particles. Typically such particles
should be neutral and weakly interacting, since otherwise they would
have been observed already in particle physics experiments (as long as
the are not too heavy). They should be stable (or long lived) to be
still around today to account for dark matter. They are classified in
{\it hot} and {\it cold} dark matter, according to whether they are
relativistic or nonrelativistic at their freezeout temperature.  Light
neutrinos would contribute to hot dark matter, while WIMPS are
cold. The present observations greatly favour cold dark matter that
clumps at smaller (e.g. galactic) scales.

Particle Physics candidates for dark matter are manifold and fall into
3 different classes: (i) those particles that are known to exist
(e.g. light neutrinos), (ii) those that have been conjectured for
reasons other than dark matter considerations and (iii) others.  The
existing neutrinos are not likely to contribute much to dark matter
(contrary to earlier expectations), because their masses do not seem
to lie in the relevant range and since hot dark matter would clump
predominantly at larger scales than observed.  The second class (ii)
contains already many candidates. Most of them are connected to the
supersymmetric extension of the standard model.  Supersymmetry
predicts many new particles that should be in the range of TeV or
below. A consistent supersymmetric extension of the standard model
requires a new symmetry (usually called R-symmetry or R-parity) to
assure a long lifetime of the proton. Such a symmetry implies that
supersymmetric particles can only be produced in pairs: {\it thus the
lightest particle (LSP) has to be stable}. This is very similar to
fermion number conservation: it is at the origin of the stability of
the electron. The LSP should be neutral: depending on the model it
could be the gravitino (partner of graviton), neutralino (partner of
gauge boson), higgsino (partner of higgs boson), sneutrino (partner of
neutrino) etc. In the minimal model (MSSM) it is the neutralino: the
partner of the neutral gauge boson Z and/or $\gamma$. Its mass should
be in the range of 50 - 300 GeV with weak interactions. It could
easily lead to dark matter with the required abundance $\Omega_M \sim
0.3 - 0.4$. At the moment this seems to be the most likely WIMP
candidate. It is searched for by current particle physics experiments
e.g. at LEP at CERN (Geneva), that provide the current lower limit on
its mass of around 50 GeV. Future searches will extend this bound to
higher energies. Other experiments search for neutral particles in our
environment (galaxy), one of which (DAMA) claims to have found a
signal, but this observation is still controversial.

Another cold dark matter candidate in class (ii) is the axion, 
a very light pseudoscalar
boson. Its existence was conjectured in connection 
with a solution of the strong
CP-problem. Now it could serve as cold dark matter as well. Its
supersymmetric partner, the fermionic axino might as well be a realistic
candidate for cold dark matter.

In the third class (iii) of candidates  there is not much predictive power
since these particles have been conjectured for the purpose of dark matter
considerations.
Here we might find heavy neutrinos, heavy neutral scalars, ultra heavy
particles (like Wimpzillas), just to name a few.

Very often particle physics models tend to predict an
embarrasingly large contribution to the matter density of the
universe.
In string theories and theories with extra dimensions,
for example, we usually find a
host of new candidates: the so-called moduli fields that parametrize the
vacuum degeneracies of these theories. In fact, very often they might
contribute too much to the energy density of the universe, a property
that might be shared by gravitino as well. More detailed calculations
are needed to clarify these questions, as these problematic 
abundances might originate from a nonthermal production in the early
universe, a mechanism that is not yet undestood very well. 

\section{The young Universe}
The past five year have witnessed an impressive widening of our
visible horizon into the early phases of the Universe:
\bi
\item
By now, nearly a thousand galaxies have been detected at redshifts
$z\sim 3$, demonstrating that galaxies were alive and well at a time
where the Universe had only $\sim 15\%$ of its current age. Already by
then, galaxies were strongly clustered and the precursors of current
day massive galaxy clusters were in place.
\item
Similarly, about 100 galaxies with redshifts $\sim 4$ are known, some
of which have been studied with great detail, partly making use of
natural telescopes provided by the gravitational lens action of
intervening clusters of galaxies. 
\item 
These findings have allowed the reconstruction of the cosmic star
formation history, yielding that most of the stars have been formed well
in the past; the Universe was a more exciting place back then.
\item
Sub-millimeter surveys have uncovered a population of deeply
dust-enshrouded galaxies, probably very actively forming stars.
\item
Galaxies and quasars with redshifts close to 6 have been discovered,
showing that the Universe is transparent at least out to these
distances (i.e., reionization must have occurred at even earlier
epochs) and that it found a way to form massive galaxies and
supermassive black holes merely one billion years after the big bang.
\item
Gamma-ray bursts, these mysterious flashes of very high-energy
photons, once believed to origin on the surface of neutron stars in
the backyard of our Galaxy, are now known to occur at very high
redshifts, implying energies apparently exceeding the rest mass energy
of a whole star.
\ei
This, by no means exhaustive, list of achievements demonstrates that
the young Universe can be studied in detail, thus allowing us to
obtain the history of structure formation from very early times on.
Presently, though, these findings raise more questions than provide
answers: When did the first galaxies form? Why was star formation
more efficient in the past than today? Why does there exist a `quasar
era', i.e. why is nearly all quasar activity confined to a brief time
period aound 2 billion years after the big bang? How do massive black
holes form? What is the relation between dark matter halo formation,
galaxy formation, and the formation of black holes which seem to
reside at the center of almost every galaxy?

The last question may bear the answer to many of the previous
ones. Since dark matter cannot be probed directly at high redshift, one
has to rely on understanding better than today the relation between
dark and luminous mass to use galaxies as markers of the underlying
dark matter distribution. But progress is rapid: the new 8-meter class
telescopes will allow the measurement of the rotation curves of
high-redshift galaxies and therefore study their dark matter halo
profile. The same telescopes, together with a new generation of
instruments on the Hubble Space Telescope, will identify the optical
counterparts of sub-millimeter sources. The sub-millimeter sky itself
will be reopened with ALMA, the sub-millimeter array to be in
operation in 2005, after which sub-millimeter astronomy will achive
similar sensitivity and resolution as optical astronomy. High-redshift
clusters will be found, using the Sunyaev-Zeldovich effect, the X-ray
emission from intracluster gas, or weak lensing signatures. 

Concerning the X-ray detection of high-redshift clusters, the current
generation of X-ray telescopes will measure the brightness and
temperature profiles of many high-redshift clusters. Since clusters
provide perhaps the cleanest probe of dark matter, these new
observations will allow us to measure the dark matter profile of halos
at $z\sim 1$, thereby enabling one to test the universal dark matter
profile at earlier times. The following generation of X-ray telescopes
will have the sensitivity to measure clusters out to redshift $z\sim
5$, if they exist, thus probing the tail of the density contrast
population and its Gaussian nature, as predicted by inflationary
theories. However, these measurements are hampered by absorption from
Galactic gas: Even the iron-K line at $\sim 6.4$\rm keV is then
redshifted to below 1\ts keV, making it difficult to unambiguously
identify it as such. In order to make best use of these future
facilities, the angle-dependent absorption, i.e. the distribution of
column density of Galactic gas, needs to be understood in great
detail, and a new all-sky survey at 21\ts cm is badly needed.

The evolution of the quasar population with redshift directly probes
the formation of supermassive black holes. To understand the relation
between galaxy formation and black hole formation, the formation and
merging of dark matter halos needs to be understood in much more
detail than at present. Are supermassive black holes formed in the
process of galaxy mergers, or are they formed in the first generation
of galaxies? If so, what is the fate of the individual black holes in
merging events? In order to answer questions like this, the census of
quasars must be further extended to fainter flux limits, as is already
happening with the very first data from the Sloan survey (which
yielded several quasars with redshift $\ge 5$). In parallel,
quasars need to be searched from radio surveys, as traditionally they
have yielded the highest redshift examples of this energetic
phenomenon. Whereas the relation between galaxy and black hole
formation is currently not understood, it is most likely that a very
tight connection exists; this connection can then be used to study
galaxy mass-scale dark matter halos to larger redshifts than possible
with the stellar light from galaxies alone.

\section{The very young universe}
%
Whereas the standard model of cosmology, obtained through combining
the laws of General Relativity with known particle physics, has been
extremely successful and can be regarded one of the greatest triumphs
in physics, there are questions which need to addressed before we can
understand the origin of the Universe:
\bi
\item
Why is the Universe so close to being spatially flat?
\item
Why is the CMB so incredibly isotropic?
\item
What is the origin of the density fluctuations seen in tiny CMB
temperature fluctuations, from which all structure in the Universe
developed? 
\item
Why is the cosmological constant so much smaller than generic
predictions for the vacuum energy density?
\ei
All of these questions can be attributed to special `initial
conditions' at a time when $T\sim 10^{13}{\rm K}$, but a more
scientific view would be a natural explanation of these phenomena
through physical processes in the very early universe by extrapolating
as yet unknown physical laws further back in time.

The fundamental scale of the particle physics standard model is the
scale of electroweak symmetry breakdown: a few hundred GeV, and the
physics beyond the standard model deals with energies of a TeV or
larger, as e.g. the grand unified scale $M_X \sim 10^{16}$ GeV or even
the Planck scale of $M_{\rm Pl} \sim 10^{19}$ GeV.  These phenomena
will be relevant for the very early universe with temperatures $T \geq
10^{15}$ K or redshifts $z \geq 10^{15}$.  Todays astronomical
observations cannot directly probe these phenomena and only provide
indirect information about the nature of the electroweak phase
transition, baryogenesis, leptogenesis, just to name a few.
Nonetheless today's universe is believed to originate from such a hot
universe and we should find ways to connect the different phases of
evolution. One of the past successes of the hot big bang scenario is
the prediction of the microwave background as well as the prediction
of the abundance of light elements in the framework of
nucleosynthesis, which e.g. is very sensitive with respect to the
number of light neutrino species.  In fact one of the strong
constraints on the physics of the early universe is the requirement
that the successful predictions of nucleosynthesis should not be
spoiled by new phenomena.

A key ingredient of the connection between the early and the late
universe is the mechanism of structure formation. It should provide
the seeds of density perturbations and describe their evolution to the
structures observed today: galaxies and clusters of galaxies.

It is the mechanism of inflation that is at the basis of this
description; the generic predictions of inflation address several of
the aforementioned problems so that it by now has become part of the
`standard model' of cosmology, despite our lack of understanding of
its details.  One assumes a phase of exponential expansion in the
early universe.  Originally this conjecture was made to solve some
conceptual problems of the naive hot big bang scenario, such as the
flatness and horizon problem. New degrees of freedom (as e.g. a scalar
field called inflaton) are needed to implement this mechanism. The
inflationary expansion takes place while the inflaton field is
dislocated from the minimum of its potential and the total energy of
the universe is stored as potential energy of this field. At the end
of inflation this potential energy is transformed into radiation
energy of a reheat temperature $T_R$. Possible values of $T_R$ range
from 100 GeV to $10^{19}$ GeV.  After the inflationary phase the
universe will then enter a radiation dominated expanding universe with
starting temperature $T_R$.

The inflationary scenario makes some definite general predictions of
which the most important one concerns the energy density of the
universe: $\Omega_{\rm M}+\Omega_\Lambda = 1$, i.e. critical total
density, implying a spatially flat universe.  Recent measurements
conducted by balloon experiments, BOOMERANG and MAXIMA, seem to
confirm this prediction: a big support to the theoretical conjecture
of an inflationary early universe.

In addition, the inflationary scheme gives rise to seeds for structure
formation. Quantum fluctuations in the very early universe (before the
inflationary epoch) lead to density perturbations. During inflation
these pertubations leave the event horizon due to the fast expansion.
As a result their size is not washed out but frozen in. After
inflation they will eventually reenter the event horizon and provide
the seeds for structure formation with a Harrison-Zeldovich type of
spectrum. A first sign of these perturbations can be seen as the tiny
fluctuations in the cosmic microwave background spectrum as first
observed in the early 90's by the COBE experiment.

At later time these fluctuations will grow and turn into galaxies and
clusters of galaxies. This mechanism of structure formation depends, of
course, very much on the parameters of our universe. As such, it can be
used as an indirect tool to determine these parameters. At the moment it
is believed that cold dark matter must be present to give a satisfactory
account for structure formation. This is an argument for cold dark
matter independent of the astronomical observations explained earlier.
More detailed studies of this mechanism will most probably reveal new
insights in particle physics, astrophysics and astronomy.

Thus the inflationary paradigm gives us a mechanism that links the
very early universe to today's astronomical observations, one of the
examples that makes the synthesis of particle physics and cosmology so
successful.


\section{Supermassive black holes}
The discovery of quasars some 37 years ago, and studies of their
properties quickly led to the hypothesis that they are powered by
accretion onto a central, supermassive ($\sim 10^8 M_\odot$) black
hole. Only much more recently, mainly due to high-resolution
observations with HST, came the realization that basically every
spheroidal galaxy, or spheroidal galaxy component (such as the bulge
in spiral galaxies) contains a supermassive black hole. The latest
evidence convincingly shows that the mass of the black hole is very
tightly correlated with the velocity dispersion of the spheroidal
stellar component in which it is contained. This remarkably tight
correlation may provide important hints on the nature of dark matter: 

As numerical simulations predict a cuspy density profile of dark
matter halos, the mass distribution in the center of a newly formed
galaxy will initially be dominated by dark matter; hence, the current
density profile and the corresponding velocity dispersion must have
originated from the initial dark matter profile which determined the
fate of the baryonic, star-forming material. Also, the initial dark
matter profile, in conjunction with (as yet not understood) baryonic
processes, must have led to the formation of supermassive black
holes. The observed tight correlation between black hole mass and the
kinematic properties of the stellar population requires a joint
explanation for their formation and their evolution, e.g. in the
merger processes of subhalos. 

The formation of supermassive black holes is far from being
understood; a dense central (neutron?) star cluster could become
dynamically unstable and collapse to a black hole; alternatively, a
stellar mass black hole could grow via accretion of baryonic material,
and supermassive black holes are formed by merging the holes of the
progenitor galaxies in galaxy merging. If the dark matter particles
are self-interacting, black hole growth can also occur via accretion
of WIMPs, without the emission of electromagnetic radiation.


\section{The highest-energy cosmic rays}
Some of the most energetic phenomena in the Universe occur in the
direct vicinity of black holes. Many Active Galactic Nuclei (AGN) have
been found to emit very hard $\gamma$-radiation, which varies on time
scales of days or even shorter. Since this type of AGN is expected to
host black holes with mass $\sim 10^8M_\odot$, the variability time
scale is not appreciable larger than the event horizon crossing time,
so the radiation, or better: the radiators, must come from very close
to the Schwarzschild radius. The processes leading to these $\gamma$
rays are not understood in detail, but it is likely that they
originate from very high energy electrons. Since the energetic
particle production process in AGNs has not yet been uniquely
identified, it is unknown whether there exists a corresponding
population of ultra-energetic protons, but for many acceleration
processes, protons are more easily accelerated than electrons.

In any case, it is clear that near the black hole of AGNs there
exist a production site of particles with energies unattainable in any
of the future man-made accelerators. Direct evidence for such a
production is provided by the highest-energy cosmic rays with $E\ge
10^{11}$\ts GeV. Although their flux near the Earth is too small to
allow controlled experiments with them, near their production site
they undergo interactions which can be observed -- e.g., the TeV
emission from AGNs. The current revolution of astronomical
intrumentation will rapidly improve our capability to observe these
accelerator sites and to study the high-energy processes occurring
therein. 

The origin of the most energetic cosmic rays remains one of the big
puzzles. Whereas cosmic rays with energies $\lesssim 10^{17}{\rm eV}$
are believed to be produced by shock acceleration in supernova
explosions in our Galaxy, those with $E>10^{19}{\rm eV}$ are of
different origin. One  suggestion for their origin is the production
in energetic radio galaxies; however, cosmic rays at such high
energies cannot propagate through intergalactic space without
dramatically loosing energy through photon-pion pruduction through
collisions with the CMB photons. Owing to their high energy, these
cosmic rays are not substantially deflected by magnetic fields, so
that their arrival direction should point towards their source, but no
obvious candidate sources have been identified within $\sim 100
{\rm Mpc}$. 

An alternative explanation for the origin of ultra high energetic
cosmic rays employs particle physics beyond the standard model. For
example, they may be generated by the decay of some supermassive `X'
particle. If the `X' particle is a metastable relic from the
inflationary phase, its decay time should then be comparable to the
age of the Universe and may be one of the dark matter
candidates. Alternatively, the `X' particle could be produced by
topological defects, like cosmic strings of magnetic monopoles, that
were created at a phase transition at the end of inflation. 

If these cosmic rays have an astrophysical origin, e.g. being produced
near the event horizon of a spinning black hole, the study of their
arrival direction and the energy-dependent arrival times can be used
to put constraints on magnetic fields outside of our Galaxy about
which very little is known. 

\section{Challenges in theoretical physics}

Mostly cosmological constant and quantum nature of black holes...

Could get new input from cosmology that is not available
otherwise...

Strings and extra dimensions...... brane worlds and 
cosmological consequences.



\section{The necessity for founding an SFB}
\section{Timing}
As argued above, we are currently witnessing the completion of a great
milestone in particle physics and astrophysics, where in both areas
`standard models' have been developed and verified within their range
of validity, owing to very recent observational, experimental, and
theoretical progress. By reaching this important milestone, several
new fundamental physical questions have been uncovered whose solutions
will most likely affect the foundations of modern physics. At the same
time, prospects for a thorough investigation of these questions are
most favourable, given the current outline for future observatories
and experimental facilities (such as a whole new generation of collider
experiements, vast advances in detector developments, new astronomical
observatories in space and on the ground). 

Progress in uncovering the nature of the dark matter and of the vacuum
energy, and the puzzle of the highest energy cosmic rays, can
undoubtedly be
made only in a close interaction and collaboration between
experimental physicists, theoreticians, and astronomers. The insights
contributed by each of these groups, whereas valuable on its own, must
be combined in order to attack the aforementioned fundamental
questions. 

In contrast to the need of close interactions between physicists and
astronomers is the lack of a strong tradition in Germany for such
collaborations. Cross-disciplinary research is often difficult to
carry out as they do not fit well within the current structure and
organization of research. To mention one example, only recently did
experimental astro-particle physics become part of the
Verbundforschung. (Could mention here labs like Gran Sasso and
Kamiokande to illustrate the activities in other countries...)

Given the large attention that the interplay between physics and
astrophysics receives in several other countries (e.g., the USA and
UK), measures are needed to allow German scientists to contribute to
the big endevour that will eventually lead to the understanding of 
the origin, nature and properties of dark matter. The SFB-375 in
Munich (where to of us have been involved decisively)
has been a fruitful example how an SFB can positively affect
the research of formerly unrelated research groups. At the time of its
creation, the quest for the properties of neutrinos has been the key
question which combined experimental physicists, theoreticians, and
astrophysicists. The impressive progress made demonstrates the
positive effects the installment of the SFB had.

Similarly today, the time is ripe to make a coherent effort towards
dark matter research. The planned SFB will be centered around this
basic theme, and is intended to catalyze cross-disciplinary research
in this and closely related areas. It will strongly encourage to
create collaborations between currently separated teams and to
capitalize on their joint expertise, and it will be invaluable in
making optimized use of the current and near-future research
facilities. The SFB is not centered on funding new experimental
hardware, at least not initially, but on `brainware', in an effort to
maximize the scientific output from experiments in which the
applicants are involved.

Given the large public outreach effect questions related to
fundamental interactions, cosmology, and dark matter have, the SFB
will have a positive impact on the attractiveness of the physics
program at the University of Bonn. With the current dramatic decrease
in the number of physics students, attracting young and talented
people will be essential to keep up the scientific standards of
research on fundamental physics. The SFB will provide a framework for
joint seminars and discussions to achieve a mutual understanding of
hitherto separated research fields and to obtain a common ground for
future research, in particular also for young researchers like
postdocs and PhD students.


\section{Location}
Bonn hosts one of the biggest concentration of astonomy and physics
research in Germany, together with Munich and Heidelberg. The physical
research at the University of Bonn covers a wide range of areas,
including elementary particle physics and field theory, nuclear
physics, applied physics ....HP.... Likewise, astronomy in Bonn covers
a large diversity of fields, including cosmology, galactic and
extragalactic research, star formation and evolution, cosmic ray
physics, active galactic nuclei, interstellar chemistry etc. Bonn
hosts the only two radio-astronomical institutes in Germany, one being
the Max-Planck-Institut f\"ur Radioastronomie which operates the 100-m
antenna in Effelsberg. Researchers within the SFB  are centrally involved in
major national and international research projects, including ZEUS at
HERA, OPAL at LEP, ATLAS at the LHC at CERN ....HP....., and on 
the astronomical
side, ALMA, the large millimeter array in the Atacama desert in Chile,
DIVA, a German astrometric satellite recently approved by DARA ...

The traditionally excellent relations between physicists and
astronomers in Bonn provides an ideal framework for our research
program. The recent appointments of two former members of the Munich
SFB (H.P. Nilles and P. Schneider) has brought additional experience
in inter-disciplinary research and its coordination to Bonn.
In addition the presence of K. Menten as a Director of the
Max Planck Institute and the newly appointed professor
H. Dreiner at the PI strengthens the program (should we mention
NF Dietz?). 

Recognizing the importance of dark matter research, several courses
and seminars on cross-disciplinary topics have been organized,
including a seminar on astro-particle physics (U. Klein \& N. Wermes)
and a lecture course on this topic (H.P. Nilles). In agreement with
what was mentioned above, these attracted large and active
participation by students.




%\end{document}
