[Paleopsych] SW: Dark Matter and the Early Universe

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Cosmology: Dark Matter and the Early Universe

    The following points are made by J. Diemand et al (Nature 2005
    1) The Universe was nearly smooth and homogeneous before a redshift of
    z = 100, about 20 million years after the Big Bang[1]. After this
    epoch, the tiny fluctuations imprinted upon the matter distribution
    during the initial expansion began to collapse because of gravity. The
    properties of these fluctuations depend on the unknown nature of dark
    matter[2-4], the determination of which is one of the biggest
    challenges in present-day science[5].
    2) The cosmological parameters of our Universe and initial conditions
    for structure formation have recently been measured via a combination
    of observations, including the cosmic microwave background (CMB),
    distant supernovae, and the large-scale distribution of galaxies.
    Cosmologists now face the outstanding problem of understanding the
    origin of structure in the Universe from its strange mix of particles
    and vacuum energy.
    3) Most of the mass of the Universe must be made up of a kind of
    non-baryonic particle[1] that remains undetected in laboratory
    experiments. The leading candidate for this "dark matter" is the
    neutralino, the lightest supersymmetric particle, which is predicted
    to solve several key problems in the standard model for particle
    physics[5]. This cold dark matter (CDM) candidate is not completely
    collisionless. It can collide with baryons, thus revealing its
    presence in laboratory detectors, although the cross-section for this
    interaction is extremely small. In a cubic-meter detector containing
    10^(30) baryon particles, only a few collisions per day are expected
    from the 10^(13) dark-matter particles that flow through the
    experiment as the Earth moves through the Galaxy.
    4) The neutralino is its own anti-particle, and can self-annihilate,
    creating a shower of new particles including gamma-rays[5]. The
    annihilation rate increases as the density squared; the central
    regions of the Galaxy and its satellites will therefore give the
    strongest signal. However, the expected rate is very low -- the flux
    of photons on Earth is the same as we would receive from a single
    candle placed on Pluto. Numerous experiments using these effects are
    under way that may detect the neutralino within the next decade.
    Furthermore, in the next few years the Large Hadron Collider (LHC) at
    CERN will confirm or rule out the concepts of supersymmetry (SUSY).
    5) The authors report supercomputer simulations of the concordance
    cosmological model, which assumes neutralino dark matter (at present
    the preferred candidate), and find that the first objects to form are
    numerous Earth-mass dark-matter haloes about as large as the Solar
    System. They are stable against gravitational disruption, even within
    the central regions of the Milky Way. The authors expect over 10^(15)
    to survive within the Galactic halo, with one passing through the
    Solar System every few thousand years. The nearest structures should
    be among the brightest sources of gamma-rays (from particle particle
    References (abridged):
    1. Peebles, P. J. E. Large-scale background temperature and mass
    fluctuations due to scale-invariant primeval perturbations. Astrophys.
    J. 263, L1 L5 (1982)
    2. Hofmann, S., Schwarz, D. J. & Stöcker, H. Damping scales of
    neutralino cold dark matter. Phys. Rev. D 64, 083507 (2001)
    3. Berezinsky, V., Dokuchaev, V. & Eroshenko, Y. Small-scale clumps in
    the galactic halo and dark matter annihilation. Phys. Rev. D 68,
    103003 (2003)
    4. Green, A. M., Hofmann, S. & Schwarz, D. J. The power spectrum of
    SUSY-CDM on sub-galactic scales. Mon. Not. R. Astron. Soc. 353, L23
    L27 (2004)
    5. Jungman, G., Kamionkowski, M. & Griest, K. Supersymmetric dark
    matter. Phys. Rep. 267, 195 373 (1996)
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by Sean Carroll (Nature 2004 429:27):
    1) Humans seem to be extremely unimportant in the grand scheme of the
    Universe. This insight is often associated with Copernicus
    (1473-1543), who suggested (although not for the first time) that the
    Earth was not the center of the Solar System. A bigger step towards
    calibrating our insignificance was taken by Edwin Hubble (1889-1953),
    who determined that astrophysical nebulae are really separate galaxies
    in their own right. We now think there are about one hundred billion
    such galaxies in the observable Universe, with perhaps one hundred
    billion stars per galaxy.
    2) But a metaphysically distinct blow to our importance came with the
    introduction of the idea of dark matter -- we are not even made of the
    same stuff that comprises most of the Universe. The need for dark
    matter, in the sense of "matter we cannot see", was noticed in 1933 by
    Fritz Zwicky (1898-1974), when studying the dynamics of the Coma
    cluster of galaxies. When galaxies are orbiting each other, their
    typical velocities will depend on the total mass involved, but when we
    observe clusters of galaxies, the velocities are consistently much
    higher than we would expect from the mass we actually see in stars and
    gas. Vera Rubin and others have driven the point home by examining
    individual galaxies. As we move away from the central galactic region,
    the velocity of orbiting gas becomes systematically higher than it
    should be. These observations imply the existence of an extended,
    massive halo of dark matter. Indeed, the picturesque galaxies we see
    in astronomical images are really just splashes of visible matter
    collected at the bottom of these more substantial, yet invisible,
    3) Of course, the air we breathe is invisible and transparent, just
    like dark matter. A sensible first guess might be that the extra mass
    we infer is ordinary matter, just in some form we cannot see. But we
    have independent ways to measure the amount of ordinary matter,
    through its influence on the early-Universe processes of primordial
    nucleosynthesis and the evolution of density perturbations. These
    constraints imply that ordinary matter falls far short of what is
    needed to explain galaxies and clusters (perhaps one-fifth of the
    total). Not only is dark matter "dark", it is a completely new kind of
    particle --something outside the standard model of particle physics,
    something not yet detected in any laboratory here on Earth.
    4) And we have not even mentioned dark energy -- the mysterious form
    of energy that is smoothly distributed throughout space and (at least
    approximately) constant through time. Independent observations of
    high-redshift supernovae, the microwave background radiation, and the
    distribution of large-scale structure all require the existence of
    dark energy. The featureless, persistent nature of dark energy
    convinces us that it is not even a particle at all. About 70% of our
    current Universe is dark energy and 25% is dark matter. This leaves
    all the stuff we have directly observed at a paltry 5% of the whole
    1. Krauss, L. Quintessence: The Mystery of the Missing Mass (Basic
    Books, New York, 2001)
    2. Peebles, P. J. E. From Precision Cosmology to Accurate Cosmology
    online at http://arxiv.org/abs/astro-ph/0208037
    3. Rees, M. Our Cosmic Habitat (Princeton Univ. Press, Princeton,
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by K. Zioutas et al (Science 2004
    1) Astrophysical observations reveal that galaxies and clusters of
    galaxies are gravitationally held together by vast halos of dark
    (i.e., nonluminous) matter. Theoretical reasoning points to two
    leading candidates for the particles that may make up this mysterious
    form of matter: weakly interacting massive particles (WIMPs) and
    theoretical particles called "axions". Particle accelerators have not
    yet detected either of the two particles, but recent astrophysical
    observations provide hints that both particles may exist in the
    Universe, although definitive data are still lacking. Dark matter need
    not consist exclusively of only one of these two types of particles.
    2) Precise measurements of the cosmic microwave background have shown
    that dark matter makes up about 25% of the energy budget of the
    Universe; visible matter in the form of stars, gas, and dust only
    contributes about 4%. However, the nature of dark matter remains a
    mystery. To explain it, we must go beyond the standard model of
    elementary particles and look toward more exotic types of particles.
    3) One such particle is the neutralino, a WIMP that probably weighs as
    much as 1000 hydrogen atoms (henceforth, we refer to the neutralino as
    a generic WIMP). Neutralinos are postulated by supersymmetric models,
    which extend the standard model to higher energies. To date, no
    neutralinos have been created in particle accelerators, but in the
    future they may be produced in the world's most powerful particle
    accelerator, the Large Hadron Collider currently being built at CERN.
    A recent precise measurement of the magnetic dipole moment of the muon
    favors the existence of new particles such as neutralinos.
    4) Another possibility for the direct detection of neutralinos is to
    seek evidence for the tiny nuclear recoils produced by interactions
    between neutralinos (created when the Universe was very young and very
    hot) and atomic nuclei. Because such interactions are rare and the
    effects small, they can only be detected in experiments that are
    conducted underground, where the high-energy cosmic radiation is
    suppressed by several orders of magnitude.
    5) Astrophysical observations could provide indirect evidence for
    neutralinos. On astrophysical scales, collisions of neutralinos with
    ordinary matter are believed to slow them down. The scattered
    neutralinos, whose velocity is degraded after each collision, may then
    be gravitationally trapped by objects such as the Sun, Earth, and the
    black hole at the center of the Milky Way galaxy, where they can
    accumulate over cosmic time scales. Such dense agglomerates could
    therefore yield an enhanced signal for the postulated neutralinos of
    cosmic origin.(1-5)
    References (abridged):
    1. P. Jean et al., Astron. Astrophys. 407, L55 (2003)
    2. F. Aharonian et al., Astron. Astrophys. 425, L13 (2004)
    3. R. Irion, Science 305, 763 (2004)
    4. R. D. Peccei, H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977)
    5. R. D. Peccei, H. R. Quinn, Phys. Rev. D 16, 1791 (1977)

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