[Paleopsych] SW: On the Large Scale Structure of the Universe

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On the Large Scale Structure of the Universe

    The following points are made by David H. Weinberg (Science 2005
    1) In a large-scale view of the Universe, galaxies are the basic unit
    of structure. A typical bright galaxy may contain 100 billion stars
    and span tens of thousands of light-years, but the empty expanses
    between the galaxies are much larger still. Galaxies are not randomly
    distributed in space, but instead reside in groups and clusters, which
    are themselves arranged in an intricate lattice of filaments and
    walls, threaded by tunnels and pocked with bubbles. Two ambitious new
    surveys, the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the
    Sloan Digital Sky Survey (SDSS), have mapped the three-dimensional
    distribution of galaxies over an unprecedented range of scales [1,2].
    Astronomers are using these maps to learn about conditions in the
    early Universe, the matter and energy contents of the cosmos, and the
    physics of galaxy formation.
    2) Galaxies and large-scale structure form as a result of the
    gravitational amplification of tiny primordial fluctuations in the
    density of matter. The inflation hypothesis ascribes the origin of
    these fluctuations to quantum processes during a period of exponential
    expansion that occupied the first
    millionth-of-a-billionth-of-a-trillionth of a second of cosmic
    history. Experiments over the last decade have revealed the imprint of
    these fluctuations as part-in-100,000 intensity modulations of the
    cosmic microwave background (CMB), which records the small
    inhomogeneities present in the Universe half a million years after the
    big bang. Although the visible components of galaxies are made of
    "normal" baryonic matter (mostly hydrogen and helium), the
    gravitational forces that drive the growth of structure come mainly
    from dark matter, which is immune to electromagnetic interactions.
    3) By combining precise, quantitative measurements of present-day
    galaxy clustering with CMB data and other cosmological observations,
    astronomers hope to test the inflation hypothesis, to pin down the
    physical mechanisms of inflation, to measure the amounts of baryonic
    and dark matter in the cosmos, and to probe the nature of the
    mysterious "dark energy" that has caused the expansion of the Universe
    to accelerate over the last 5 billion years. The 2dFGRS, completed in
    2003, measured distances to 220,000 galaxies, and the SDSS is now 80%
    of the way to its goal of 800,000 galaxies.
    4) The key challenge in interpreting the observed clustering is the
    uncertain relation between the distribution of galaxies and the
    underlying distribution of dark matter. If the galaxy maps are
    smoothed over tens of millions of lightyears, this relation is
    expected to be fairly simple: Variations in galaxy density are
    constant multiples of the variations in dark matter density.
    Quantitative analysis in this regime has focused on the spatial power
    spectrum, which characterizes the strength of clustering on different
    size scales [3,4]. The power spectrum describes the way that large,
    intermediate, and small structures -- like the mountain ranges,
    isolated peaks, and rolling hills of a landscape -- combine to produce
    the observed galaxy distribution. The shape of the dark matter power
    spectrum is a diagnostic of the inflation model, which predicts the
    input spectrum from the early Universe, and of the average dark matter
    density, which controls the subsequent gravitational growth. Recent
    analyses have also detected subtle modulations of the power spectrum
    caused by baryonic matter, which undergoes acoustic oscillations in
    the early universe because of its interaction with photons [4,5].
    References (abridged):
    1. M. Colless et al., Mon. Not. R. Astron. Soc. 328, 1039 (2001)
    2. D. G. York et al., Astron. J.120, 1579 (2000)
    3. M. Tegmark et al., Astrophys. J. 606, 702 (2004)
    4. S. Cole et al., http://arxiv.org/abs/astro-ph/0501174
    5. D.J. Eisenstein et al.http://arxiv.org/abs/astro-ph/0501171.
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by Zoltan Haiman (Nature 2004 430:979):
    1) Galaxies are thought to be surrounded by massive haloes of dark
    matter, each outweighing its galaxy by a factor of about eight. The
    visible part of a galaxy, occupying the inner 10% of the halo,
    consists of a mixture of stars and gas. Galaxies harbor a giant black
    hole at their centers, which in some cases is actively fuelled as it
    sucks in surrounding gas. In especially active galaxies, called
    quasars, the fuelling rate is so high that the radiation generated
    close to the black hole outshines the cumulative star-light from the
    entire galaxy. The sequence of cosmic events that leads to this
    configuration is still largely mysterious.(1) How does gas condense
    into the central regions of the dark-matter halo? At what stage of the
    gas condensation process do the stars and the giant black hole light
    2) The formation of massive dark-matter haloes is dictated by gravity,
    and can be described by using ab initio calculations(2). As the
    Universe expanded from its dense beginning, tiny inhomogeneities in
    the distribution of dark matter were amplified through the effects of
    gravity. Regions of space that were slightly denser than average had a
    higher gravitational pull on their surroundings; eventually, these
    regions stopped following the expansion of the rest of the Universe,
    turned around and re-collapsed on themselves. The resulting dense
    knots of dark matter -- forming the intersections of a cosmic web of
    less-dense dark-matter filaments -- are believed to be the sites at
    which galaxies lit up.
    3) Dark matter thus dominates the formation of a galaxy, at least
    initially, and determines the gross properties of the galaxy
    population, such as their abundance, size and spatial distribution.
    But it is the trace amount of gas (mostly hydrogen and helium), pulled
    with the dark matter into the collapsed haloes, that forms the visible
    parts of galaxies and determines their observable properties. In
    particular, to condense to the core of the dark halo, the gas must
    cool continuously so as to deflate the pressure acquired by its
    compression. A fraction of the gas (typically 10% by mass) eventually
    turns into stars, and a much smaller fraction (typically 0.1%) into
    the central massive black hole(3).
    4) The composition of the gas inside the galaxy can be studied through
    the spectrum of radiation that it absorbs and emits. Primordial gas is
    essentially a pure mix of hydrogen and helium, but the spectra of all
    of the quasars discovered so far have shown the presence of various
    heavier elements (such as carbon, nitrogen, oxygen and iron). This
    indicates that the gas has been enriched by the nucleosynthetic yields
    from previous generations of stars. Even the most distant quasars,
    including those that existed about a billion years after the Big Bang
    (a mere 5% of the current age of the Universe), show a significant
    heavy-element content(4). This suggests that vigorous star-formation
    is a necessary condition for any quasar activity. On the other hand,
    star formation seems to be occurring on relatively small scales, close
    to the galactic center. A natural inference would then be the
    following sequence of events: the cosmic gas first contracts to the
    inner regions of the halo, and only then forms stars --but this is
    still before the formation (or at least activation) of any central
    quasar black hole.
    5) Not necessarily so, according to Weidinger et al(1). They have
    detected the faint glow of hydrogen emission enveloping a distant
    quasar at a radius equivalent to about 100,000 light years --several
    times the size of the visible part of a typical galaxy. Such emission
    has a simple physical origin. The hydrogen atoms falling through the
    halo are ionized by the quasar's light, then recombine with electrons
    to become atoms again. Each recombination results in the emission of a
    so-called Lyman-photon (a photon with energy equal to the difference
    between the ground and first excited states of a hydrogen atom). As a
    result, when viewed through a filter tuned to the Lyman-alpha
    frequency, a faint "fuzz" can be seen to surround quasars(5). This
    fuzz can serve as a diagnostic of whether or not a spatially extended
    distribution of infalling gas is present around the quasar. If most of
    the gas has already cooled and settled at the center of the halo, the
    extended fuzz would be absent.
    References (abridged):
    1. Weidinger, M., Mueller, P. & Fynbo, J. P. U. Nature 430, 999-1001
    2. Navarro, J. F., Frenk, C. S. & White, S. D. M. Astrophys. J. 462,
    563-575 (1996)
    3. Magorrian, J. et al. Astron. J. 115, 2285-2305 (1998)
    4. Fan, X. et al. Astron J. (in the press); preprint at
    http://arxiv.org/abs/astro-ph/0405138 (2004)
    5. Rees, M. J. Mon. Not. R. Astron. Soc. 231, 91-95 (1988)
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by Gregory D. Wirth (Nature 2004
    1) Over the past two decades, astrophysicists have been spectacularly
    successful in explaining the early evolution of the Universe. Existing
    theories can account well for the time span from the Big Bang nearly
    14 billion years ago until the Universe began to cool and form the
    first large structures less than a million years later. But detailed
    explanations of how the original stew of elementary particles
    subsequently coalesced over time to form the stars and galaxies seen
    in the present-day Universe are still being refined. Glazebrook et
    al(1) and Cimatti et al(2) have recently discovered the most distant
    "old" galaxies yet. and the existence of these objects at such an
    early epoch in the history of the Universe seems inconsistent with the
    favored theory of how galaxies formed.
    2) That favored theory is the so-called hierarchical model, in which
    smaller structures gradually accumulate into ever larger structures,
    ultimately forming galaxies of the sort we see today(3). The most
    massive galaxies are expected to have formed relatively late in the
    process, with few existing before the Universe was half its present
    age. Such predictions can be tested in principle through the
    observations made of distant galaxies.
    3) We have a powerful means of observing the history of the Universe:
    because the speed of light is finite, as we look out into space we
    actually peer back in time, seeing distant objects not as they are
    now, but as they were when their light was emitted millions or
    billions of years ago. Unfortunately, galaxies more than 6 billion
    light years away are not only exceedingly faint, but are also
    particularly difficult to identify. The visible galaxy spectra are
    "redshifted" to longer, near-infrared wavelengths as a consequence of
    the expansion of the Universe; at these wavelengths, the Earth's
    atmospheric emission obscures the key spectral "fingerprints" that are
    commonly used to identify galaxies.
    4) For these reasons, virtually all of the galaxies known from the
    early days of the Universe are those that are still forming new stars,
    and hence emitting copious amounts of light(4). Although easier to
    find, such galaxies are not particularly useful for testing theories
    of galaxy formation because it is impossible to set strong lower
    limits on how old they are. However, finding significant numbers of
    massive, evolved galaxies (which finished forming stars long ago) at
    distances that correspond to half the present age of the Universe
    would indicate that such galaxies formed much earlier than the leading
    theory predicts.(5)
    References (abridged):
    1. Glazebrook, K. et al. Nature 430, 181-184 (2004)
    2. Cimatti, A. et al. Nature 430, 184-187 (2004)
    3. Blumenthal, G. R. et al. Nature 311, 517-525 (1984)
    4. Steidel, C. C., Adelberger, K. L., Giavalisco, M., Dickinson, M. &
    Pettini, M. Astrophys. J. 519, 1-17 (1999)
    5. Dickinson, M., Papovich, C., Ferguson, H. C. & Budávari, T.
    Astrophys. J. 587, 25-40 (2003)
    Nature http://www.nature.com/nature

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