[Paleopsych] SW: On the Large Scale Structure of the Universe
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On the Large Scale Structure of the Universe
http://scienceweek.com/2005/sw050819-5.htm
The following points are made by David H. Weinberg (Science 2005
309:564):
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:
COSMOLOGY: ON THE FIRST GALAXIES
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
up?
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
(2004)
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:
COSMOLOGY: ON THE FORMATION OF GALAXIES
The following points are made by Gregory D. Wirth (Nature 2004
430:149):
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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