SMEARING
COSMIC
SHEAR
Above: The giant
galaxy in the cen-
ter of the cluster
Abell 3827 is in the
process of devour-
ing several smaller
ones. Galaxies like
this grow by first
capturing and then
later cannibalizing
their neighbors.
R. CARRASCO ET AL., GEMINI
OBSERVATORY/AURA
Left: Galaxy merg-
ers often occur
along filaments in
the cosmic web,
possibly imparting
the end result with
a preferred align-
ment. ESA/HUBBLE AND
NASA; ACKNOWLEDGEMENT:
JUDY SCHMIDT (GECKZILLA)
Cosmic cartography
50°
45°
40°
35°
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15°
3h00m
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22h00m
On this map of the
galaxy distribution
in the region of
the Perseus-Pisces
Supercluster, each
point represents
a galaxy located
150 million to
300 million light-
years from Earth.
Dense clusters
of galaxies, the
metropolises of the
cosmos, dot the
prominent filament
like beads on a
string. MICHAEL WEST
One astronomer’s signal is
sometimes another’s noise.
Charles Messier com-
piled his famous list of
deep-sky objects to spare
fellow comet hunters
from wasting time on
them. Edmund Weiss, a
19th-century Austrian
astronomer, called aster-
oids the “vermin of the sky”
because their trails on long-
exposure photographs
were considered blemishes.
And galaxy alignments turn
out to be a nuisance for
some astronomers, too.
Gravitational lensing,
the deflection of light rays
by gravity, has become a
powerful tool for cosmol-
ogy. As light from faraway
galaxies journeys through
space, the gravitational
pull of matter — both visi-
ble and invisible — alters
its path. This causes galax-
ies near each other in the
sky to appear slightly elon-
gated and aligned.
Astronomers call these dis-
tortions cosmic shear.
Although it’s a minus-
cule effect, by carefully
measuring the shapes and
orientations of large num-
bers of galaxies, astrono-
mers can infer the amount
and distribution of dark
matter, the mysterious
stuff that makes up much
of the universe.
But a crucial assumption
in cosmic shear studies is
that galaxy orientations
are inherently random, so
that any apparent align-
ments are the result of
gravitational lensing.
Physical alignments, such
as those seen in giant ellip-
tical galaxies, masquerade
as cosmic shear and com-
plicate analysis.
So, while intrinsic align-
ments provide important
clues about how environ-
ment shapes galaxies,
they’re a nuisance for scien-
tists whose goal is mapping
the dark matter content of
the universe. — M.W.
Right ascension
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