Sky & Telescope - January 2022 - 40

FILAMENT: ROLAND BACON / DAVID MARY / ESO / NASA; XRISM TELESCOPE: NASA
Cosmic Structure
A Bright Future
The most surprising part of Bacon's work is that the photons
exciting the hydrogen gas appear to come from large numbers
of extremely small, star-forming galaxies hidden in the
filaments. According to Bacon, the dwarf galaxies weigh in at
just a million solar masses or so. " If our interpretation is correct, "
he says, " the number of galaxies in the early universe
that are forming stars is huge. " Billions of these tiny galaxies
may have been responsible for re-ionizing the universe at the
end of the so-called Dark Ages, a few hundred million years
after the Big Bang.
The new results provide additional observational constraints
to cosmologists like van de Weijgaert, who try to
model the evolution of the large-scale structure of the universe.
" But don't forget that dark matter is the main component
of the cosmic web, " he says. " The big question is to what
extent the gas distribution is representative for the web as a
whole. " Building on work by Francisco-Shu Kitaura (now at
the Institute for Astrophysics in the Canary Islands, Spain),
van de Weijgaert's PhD student Johan Hidding has reconstructed
the dark matter distribution in the local universe out
to some 300 million light-years by meticulously " tweaking "
supercomputer simulations until they exactly reproduce the
FILAMENT One of the hydrogen filaments (blue) discovered using the
MUSE instrument in the Hubble Ultra-Deep Field (background image).
The structure stretches across 15 million light-years.
XRISM TELESCOPE Scheduled to launch in 2022, this X-ray telescope
may help astronomers explore cosmic structure.
observed distribution of real galaxies. Eventually, his eyecatching
reconstructions (see page 34) may provide much
more detailed information on the evolution and the current
locations of cosmic-web filaments and on the density of
intergalactic gas in the local universe, Hidding says.
Massive galaxy surveys will also yield a wealth of detailed
information about the cosmic web, enabling cosmologists to
explore the precise way in which dark matter and dark energy
have shaped the large-scale structure of the universe. Prime
examples are the recently completed Dark Energy Survey
(DES), the ongoing Dark Energy Spectroscopic Instrument
(DESI) survey, the upcoming Legacy Survey of Space and Time
(LSST), and the future space-based galaxy surveys of ESA's
Euclid and NASA's Nancy Grace Roman Space Telescope.
Next-generation ground-based telescopes like the European
Southern Observatory's 39-meter Extremely Large Telescope,
outfitted with sensitive integral-field spectrographs, will
vastly surpass the recent achievements of the VLT's MUSE
instrument. And future X-ray observatories - in particular
the Japanese-American X-Ray Imaging and Spectroscopy
Mission (XRISM) and the European Advanced Telescope for
High-Energy Astrophysics (Athena) - have high enough sensitivity
and spectral resolution to detect hundreds of tenuous
intergalactic filaments.
As theories and simulations become more sophisticated
over time, and observations ever more detailed, the study of
the elusive cosmic web will likely remain a fecund interaction
between the two approaches for quite some time to come. In
the past, theoretical insights have usually guided cosmologists'
interpretation of their sparse observational data. But in
the end, the real universe has the last word. Before long, we
could be witnessing how theories follow observations, instead
of the other way around.
¢ Contributing editor GOVERT SCHILLING's new book on the
search for dark matter, The Elephant in the Universe, will be
published this spring by Harvard University Press.
40 JANUARY 2022 * SKY & TELESCOPE
Sky & Telescope - January 2022

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