Scientists
take a peek
behind a
black holeEINSTEIN’S THEORY OF general
relativity has been confirmed several
times in recent years. And last year, it
was reinforced yet again, with the first
observation of X-rays coming from
behind a black hole, bent in our direc-
tion as the black hole’s gravity warps the
shape of space-time around it.
Although light cannot escape from
inside a black hole’s event horizon, we
can see signals emitted from outside this
boundary, where a superheated, fast-
moving accretion disk of material swirls
inward. This is the region astronomers
were looking at within the heart of
galaxy I Zwicky 1, located 800 million
light-years away, using two X-ray space
telescopes: NASA’s NuStar and the
European Space Agency’s XMM-Newton.
There, they were watching the super-
massive black hole for bright f lares of
X-ray light from its corona — a spherical
region of energetic particles around the
black hole.
But a short time after each f lare, the
telescopes recorded a fainter X-ray f lash
at a different wavelength. These f lashes,
the team determined, were the echoes of
each f lare ref lecting off the portion of the
accretion disk blocked from us by the99
A black hole sits in the middle of a swirling maelstrom of hot gas (red) in this artist’s concept. In 2021,
researchers captured X-ray light, shown in white, reflected off gas on the farside of a black hole. The black
hole’s extreme gravity had warped space-time around it, bending the X-rays around itself and into our line
of sight. DAN WILKINSTHE COSMIC WEB is a vast net-
work of filaments of dark matter and
normal matter (stars and gas) strung
through the universe like a spiderweb.
Galaxies lie along these filaments, with
larger groups and clusters of galaxies
congregating where filaments cross and
intersect.
Although it is all around us, this
cosmic web is notoriously difficult to
observe. Astronomers can only see itwhen hydrogen gas, either inside or
outside galaxies strung along these fila-
ments, glows. But this hydrogen needs
something to light it up: either a galaxy’s
stars from within, or a bright light source
such as a quasar or galaxy cluster shining
on it from the outside. Researchers have
seen the latter — but these are rare,
specific cases, and seeing only unique
circumstances limits our understanding
of the universe as a whole.
This sparse picture is now beginning
to change. A paper published March 18 in
Astronomy & Astrophysics highlights a
new image of the cosmic web. Taken by
staring at a single region of the sky for
140 hours over a period of eight months,the groundbreaking shot was captured
with the Multi Unit Spectroscopic
Explorer (MUSE) on the European
Southern Observatory’s Very Large
Telescope in Chile. After a full year of
data processing, the resulting image
reveals light from hydrogen strung along
filaments of the cosmic web as they
appeared 10 billion to 12 billion years ago.
In all, the image shows five filaments
at different distances, says study co-
author Thibault Garel at the University
of Geneva. These filaments are glowing
because, embedded within each struc-
ture, he says, are thousands of small,
faint galaxies. These galaxies are too
small to spot individually, but together,77
The cosmic
web shows
off its glowblack hole’s shadow. The change in wave-
length of the light indicated the ref lec-
tions had been twisted and stretched
by gravity, until they’d arced around
the black hole and back toward Earth.
It is the first time such behavior, long
predicted by general relativity, has been
directly observed. “We think that any-
time we’ve seen these ref lections coming
from the accretion disk, there should be
some part of that signal that is coming
from behind the black hole, but it’s not
been possible to separate it out,” says
Dan Wilkins of Stanford University, lead
author of a paper published July 28 in
Nature describing the observations.“Now we’ve been able to actually tease
out that signal.”
Seeing that signal gives astronomers
one more peek into how black holes truly
work. Most of these extreme objects can-
not be imaged directly, so the trick is
working out ways to see them without
actually seeing them. And that’s where
this new find fits in: X-ray f lares provide
key information about the region imme-
diately around the black hole, Wilkins
says. “What we’re hoping is that we’ll be
able to take what we’ve learned and use
this to actually be able to reconstruct an
image of other black holes right across
the universe.”18 ASTRONOMY • FEBRUARY 2022