Astronomy

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WWW.ASTRONOMY.COM 49

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.

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)

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

Cosmic cartography