Sky & Telescope - January 2022 - 36

Cosmic Structure
also the " push " of dark energy - the mysterious force behind
the observed acceleration of cosmic expansion (S&T: May
2018, p. 14). According to Joop Schaye (Leiden University,
The Netherlands), the principal investigator of the EAGLE
simulation, no one questions the existence of
intergalactic material in the cosmic web. " But
of course, observers always want to really see it
first, " he says.
Background Beacons
While the web-like pattern is clearly visible in
large 3D galaxy surveys, observing its intergalactic
content is a real challenge. Remember that
most of it is dark matter, which is invisible by
definition. In the early universe, the baryonic gas
in the filaments has a very low density (in a terrestrial laboratory,
we would call it a perfect vacuum), and temperatures are
on the order of 10,000 kelvin. Over time, the structures grow
larger and more massive, and shocks further heat the gas. But
even at temperatures of millions of degrees, this plasma is
generally much too tenuous to be easily seen.
Only at the endpoints of the tendrils, close to galaxy
clusters and individual galaxies (where it's more commonly
known as the circumgalactic medium), the cosmic web has
a significantly higher density, up to hundreds of times the
average density in the universe. Here, primordial gas flows
into galactic halos and disks, ultimately feeding the birth of
new stars. Through stellar winds and supernova explosions,
galaxies also blow processed gas back into space. Some of this
material ends up in the cosmic web again, enriching the filaments
with heavy elements (metals in astronomical parlance)
that are produced by stellar nucleosynthesis. " The details of
this feedback mechanism are still not well understood, " says
Schaye. " We don't know how far these metals can end up
from their parent galaxy. "
One way to detect relatively cold, tenuous gas in intergalactic
space is by looking at the absorption fingerprint it
leaves in the light of background beacons, such as bright quaGROWING
WEB Over time (left to right), matter in the universe has
collected in a web-like structure. Redder colors indicate hotter gas
temperatures, while more intense color indicates higher gas density. The
strip spans about 300 million light-years vertically.
10%
Fraction
of the
universe's
baryons in
galaxies
sars. Neutral hydrogen atoms preferentially absorb ultraviolet
photons with a wavelength of 121.6 nanometers, which provide
the right amount of energy to help the atom's single electron
jump from its ground state to the next quantum level
up. The resulting absorption line in the quasar's
spectrum will be observed at a longer wavelength
here on Earth, depending on the distance to the
absorber and the corresponding redshift due to
cosmic expansion. Usually, quasar spectra contain
a " forest " of these Lyman-alpha lines, produced by
a large number of absorbers at various distances
along the line of sight.
The problem is figuring out the true nature
of the absorbers. Most of the detectable ones are
probably individual galaxies or galactic halos
(indistinguishable because of their remoteness), which produce
a relatively strong absorption signal. Because only neutral
(that is, cool) hydrogen produces the Lyman-alpha line,
the approach only works as a way to detect the cosmic web
at very large distances, corresponding to early times when
the web's temperature was still really low. Closer to home, in
more recent cosmic epochs, the tendrils of the cosmic web are
expected to be much hotter - astronomers sometimes call
it the warm-hot intergalactic medium, or WHIM - and thus
there's little neutral hydrogen. Here, at distances of a few billion
light-years, other detection techniques are needed.
That's where X-ray astronomy comes in. As mentioned
above, the evolved cosmic web contains a smattering of heavy
elements, including oxygen atoms - the most abundant
metal in the universe. Because of the gas's high temperature,
the oxygen atoms are highly ionized: Many of them have only
one or two of their original eight electrons left. Ionized oxygen
absorbs X-ray photons at a handful of particular energies,
leaving characteristic dips in the brightness of background
X-ray sources.
Because this technique focuses on the relatively nearby
universe, it's easier to check whether intervening galaxies
cause the absorption features. If not, the features may be due
to tenuous patches of the cosmic web. Using NASA's Chandra
X-ray Observatory, launched in 1999, astronomers made
the first tentative WHIM detections more than 20 years ago.
" But it's incredibly hard, " notes cosmic-web expert Rien van
36 JANUARY 2022 * SKY & TELESCOPE
EAGLE PROJECT

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
Sky & Telescope - January 2022 - Contents
Sky & Telescope - January 2022 - 3
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Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
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