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Europhysics
News (2001) Vol. 32 No. 6
Dark
Matter
Yorck Ramachers
Oxford University, NAPL, Keble Road, OX1 3RH, UK
Dark matter is among the hottest
topics of research in astrophysics. Although the phenomenon has been
noticed the first time almost seventy years ago by F. Zwicky, in recent
times dark matter research entered a new era. Its existence is practically
accepted due to independent and converging observations in astrophysics
(see also the articles in this special issue by J.M. Lamarre and J.L.
Puget and by P.D. Sackett). However, the actual composition of dark
matter is yet to be determined.
Dark matter as a puzzle inspires astrophysicists
and particle physicists, amalgamating these research areas into the
rather young discipline of astroparticle physics. To summarize roughly
the present status, the overwhelming majority of mass in the universe
neither emits nor absorbs light and nobody knows what it is made of.
The exercise is clear: to reveal the nature of dark matter and its role
in the universe.
Mass and energy
budget of the Universe
Fig. 1 shows the mass and energy budget of the universe as known today.
Two dark matter problems can be found there (labeled gap I and II).
- there are dark baryons (gap I), hence missing normal matter. So
far, astronomers did not find all the normal matter that should exist
in the universe due to the very successful theory of primordial nucleosynthesis.
- There has to be a non-baryonic dark matter contribution (gap II).
The observations today agree on a universe matter content of about
35% of the critical density (the mean matter density to have a flat
universe) on average. The maximum allowed amount of baryonic matter,
however, is just about 5%. This discrepancy leads to the notion of
non-baryonic dark matter – the small syllable ‘non’ has far-reaching
consequences.
Non-baryonic is synonymous with an exotic form of matter that we do
not know. It is therefore a main playground for particle physicists
entering the field of dark matter. A more detailed presentation of Fig.
1 and the terms used there will be given below.
The main motivation for current experiments to reveal the nature of
dark matter or at least some specific properties different than just
mass can be understood from Fig. 2. The idea is that all luminous matter
is embedded in a huge halo of dark matter. Shown in the right panel
one can find the basis for such a model: measurements of the rotation
velocity of test bodies (stars, gas clouds) around the center of a galaxy
yield its rotation curve. As indicated in the right panel, our galaxy
has an approximately flat rotation curve, inconsistent with the expected
Keplerian decline (dotted curve) in case all the mass was luminous (velocity
µ 1/√(distance to center)). Instead, a constant
rotation velocity implies a linearly increasing galaxy mass distribution
and in fact this has been measured (left panel) up to very large distances
of about 200 kpc (our sun is located at the outskirts of the Milky Way
galaxy at a radius of about 8 kpc).
Baryonic or exotic?
These observations motivate us on Earth to search for dark matter either
in our local (galactic) neighbourhood or even in laboratory experiments.
Plenty of candidates for dark matter have been proposed. They can be
classified as baryonic and exotic (or non-baryonic).
This distinction is crucial with respect to the two dark matter (DM)
problems of Fig. 1. Baryonic candidates can at best fill gap I. These
candidates consist of gas, planetary objects and stellar remnants, for
example white dwarfs. They would be too dim to be observed with telescopes
and therefore would be dark matter by definition. However, the clever
idea of microlensing can help to detect such candidates and has been
successfully applied.
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Fig 1 Mean
matter/energy density in the universe normalised to the critical
density. A value of W=1 means a flat
universe topology as the boundary case between a spherical and
hyperbolic topology (for simple cosmologies this means closed
and open universe). Summaries of measurements for this important
parameter are indicated. Below the logarithmic axis is shown an
overall account of matter and energy; above the composition of
the matter components. The dotted red rectangles, labeled gap
I and II indicate the two dark matter problems: dark baryons and
non-baryonic dark matter. Colored regions: yellow – range of luminous
matter, brown – required range of baryonic matter due to primordial
nucleosynthesis, light green – range of non-baryonic cold dark
matter (CDM), red – total account of matter/energy density consistent
with W=1, dark green – total matter
content in the universe, blue – dark energy content adding up
with matter to the total content. The dashed green rectangle shows
the allowed amount of matter density due to neutrinos. Lower bound
due to successful neutrino mass measurements, upper bound from
failure of structure formation scenarios using neutrinos as hot
dark matter. The picture has been adopted from M.S. Turner’s dark
matter review in Phys. Reports (see references).
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This technique looks for light amplification of distant
stars of the two Magellanic Clouds by massive compact halo objects (MACHOs)
from our Milky Way galaxy. This amplification is a generic effect of
the gravity field of the MACHO that bends the light and eventually acts
as a lens (see also P.D. Sackett’s article) for an observer on Earth.
The probability that a given star is amplified at a given time is very
low; millions of stars have to be monitored for years in order to ever
be able to detect such a rare event. The MACHO and the EROS (Expérience
pour la Recherche d'Objets Sombres) collaboration have indeed found
microlensing events but their interpretation poses some considerable
problems. They are consistent with a roughly 50% dark baryon halo but
the masses of the lenses needs to be around half a sun mass which would
mean they are probably white dwarfs. That turns out to be inconsistent
with several astronomical observations, so the latest status is that
the nature of the lenses is unknown. Nevertheless, something has caused
these microlensing effects and it remains a fascinating challenge to
explain them.
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Fig 2 The
right panel shows a schematical rotation curve of our galaxy (from
http://www.astro.psu.edu/users/niel/psiwa/darkmatter/mw-rotation-curve.jpg,
on Penn State Inservice Workshops in Astronomy). The important
observation is that the velocities of stars and gas spinning around
the center of the galaxy remains constant up to the largest measured
distances. If nothing else but luminous matter was there, the
rotation curve should decrease as indicated (dashed line). Therefore
there must be much more mass inside and around our galaxy than
visible, the dark foundation. In fact, the flat rotation curve
implies a linear increase of the total mass with increasing distance
from the center. That has been observed as is shown in the left
panel up to large distances (from S.M Faber and J.S. Gallagher,
Ann. Rev. Astron. Astrophys. 17 (1979) 135, or http://ned.ipac.caltech.edu/level5/Faber/Faber_contents.html).
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Hot
and cold non-baryonic dark matter
Non-baryonic candidates are classified as hot and cold DM (also an intermediate
state, called warm DM has been proposed), depending on their kinematical
state in the early universe at the time of decoupling of light and matter.
Hot DM candidates would have been relativistic, cold DM ones non-relativistic.
The reason for such a classification originates from the fundamentally
different consequences for structure formation in the universe, like
formation of superclusters and clusters of galaxies. Hot DM would form
huge structures first (top-down approach), cold DM vice versa. The important
efforts of research groups using supercomputers for N-body simulations
of huge parts of the universe showed that structure formation cannot
be understood with dominantly hot DM. A consistent scenario results
for a substantial cold DM component and a rather negligible hot DM component.
However, it also turned out that realistic scenarios
for structure formation need one mysterious part in addition to cold
DM: dark energy. The nature of dark energy is unknown but it can be
described as a smooth component which contributes at least 60% of the
energy density in the universe (see Fig. 1). It must evolve more slowly
than matter not to interfere with structure formation at early times.
In addition, it has to have a negative pressure! In order to explain
recent cosmological observations (see also J.M. Lamarre and J.L. Puget’s
article), especially the distant supernova Ia surveys, the universe
accelerates instead of slowing down or keeping a constant expansion
rate. Initially, this observation led to a revival of Einstein’s cosmological
constant but soon it has been realized that also time dependent variants,
termed quintessence, could explain the dark energy. No conclusion can
be given so far about the dark energy but huge efforts are underway
to measure the equation-of- state of this phenomenon in order to learn
about its nature.
Search techniques
The prime candidate for a hot DM contribution is a massive neutrino.
The new results from the Superkamiokande experiment and recently from
SNO (Sudbury Neutrino Observatory) seem to prove for the first time
that neutrinos are in fact massive although extremely light. Their possible
contribution to dark matter is summarised in Fig. 1 as well. Nevertheless,
they will not be able to fill exotic matter gap II. Searches for non-baryonic,
cold dark matter exist in a large variety of techniques. One can classify
them as three different concepts: production in laboratory experiments
(typically at accelerators), indirect and direct detection. These techniques
presume that this exotic form of matter consists of unknown particles.
Indirect searches look for products of reactions
of these particles. It might be that they decay or annihilate or have
inelastic reactions with normal matter, meaning that they might not
be ‘dark’ in the strict sense but shine in ‘some’ form. Satellite experiments
look for radiation from such reactions, for example in the form of high
energy gamma rays or antiprotons. Additionally, neutrino ‘light’ (neutrinos
as reaction products) is seen as a promising signal to learn more about
non-baryonic dark matter. Large experiments, termed neutrino telescopes,
are either in the process to start soon or even measure already.
The direct detection technique seeks to find rare
energy depositions from elastic scattering events. One direct search
experiment announced evidence for the detection of dark matter. The
main category of particle candidates for non-baryonic dark matter are
called WIMPs (weakly interacting massive particles). One important signature
of WIMP interactions would be an annual modulation of these rare events.
The DAMA collaboration, operating almost 100 kg of low background NaI
scintillator detectors in the Gran Sasso laboratory in Italy has published
evidence for the detection of such a modulation, consistent with the
WIMP hypothesis. Now other experiments are bound to confirm or exclude
that evidence which promoted large efforts and interest worldwide in
inventing alternative and far more sensitive detectors. That compelling
development happens at the moment and new results are expected very
soon.
To conclude, dark matter appears to be well established
as a phenomenon in astrophysics, a fascinating puzzle. Precision cosmological
measurements determine the matter and energy content of our universe
and reveal the necessity for a substantial amount of unknown matter,
a dark side of the universe. Astronomical techniques like supernova
searches seem to have detected dark energy, an unexplained phenomenon
so far, hinting at new physics beyond current borders. About matter
in the universe, we now have a clear picture that the familiar luminous
matter represents only a negligible contribution with respect to dark
matter. Normal, known forms of matter evaded detection so far and so
did an even larger contribution from an exotic form of dark matter.
The near future will most probably reveal some pieces of the grand mosaic
and it’s exciting to follow that development.
General References:
M.S. Turner, Physics Reports 333-334 (2000) 619-635
B. Sadoulet, Reviews of Modern Physics 71 (1999) S197
L. Bergstroem, Rep. Progr. Phys. 63 (2000) 793-841
Articles by M.S. Turner, J. Ellis, M. Spiro and E.W. Kolb in Physica
Scripta, vol. T85 (2000)
About the author
Yorck Ramachers has been a research assistant at Oxford University
since 1998, working for the direct detection experiment CRESST (cryogenic
rare event search using superconducting thermometers) at the Gran Sasso
laboratory in Italy in the group of Prof. H. Kraus.
PhD. thesis 1998 at the Max-Planck Institute for Nuclear Physics in
Heidelberg on building the HDMS (Heidelberg dark matter search) direct
detection experiment in the group of Prof. H.V. Klapdor-Kleingrothaus.
Copyright EPS
and EDP Sciences,
2001
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