GETTING THE WORD OUT

COVER STORY
SMASH
AND GRAB
The Milky Way’s dwarf satellites were violently
acquired – and that spells trouble for established
ideas of gravity, says Stuart Clark
THE end of the Milky Way is already
scheduled, and will be marked with
fireworks. Some 4 billion years from
now, the night skies will be lit by the glow of
hundreds of billions of stars as the nearby
Andromeda galaxy bears down on us. The
two celestial giants will become one and stars,
planets and gas clouds will be hurled into
intergalactic space by titanic gravitational
forces. Surviving stars and planets will be
pitched into a jumbled cloud flaring up with
new stars – floating into a long future not
in the Milky Way, nor Andromeda, but a
monstrous “Milkomeda” galaxy.
It’s a well-established picture of our galaxy’s
cataclysmic future. More controversially,
it might also be a vision of its past.
Observations indicate that the eviscerated
remains of a past encounter between two
celestial giants encircle our galaxy’s
neighbourhood. Forbidden alignments of
satellite galaxies, globular clusters and
streams of stars trailing in our galactic wake
all hint that our local cosmic history needs
a rewrite. And not only that: to explain what
our telescopes are telling us, we may need to
rethink that most mysterious of substances,
dark matter – and perhaps our entire
conception of how gravity works, too.
Like many big problems, this one started
out small: in a strange configuration of tiny
dwarf galaxies surrounding the Milky Way.
In 2012, astronomer Marcel Pawlowski, then
of the University of Bonn in Germany, dubbed
it the “vast polar structure”. This was for the
way the dwarfs line up in a ring that circles the
galaxy at right angles to the main disc of stars,
which contains our sun and everything else.
But he was by no means the first to see it.
That was Donald Lyndon-Bell of the University
of Cambridge, who in 1976 pointed out that
the satellite galaxies surrounding the Milky
Way are not scattered randomly, but look as if
something has corralled them into a distinct
alignment. “They thought it could be the
break-up of a larger galaxy, making it some
form of debris stream,” says Pawlowski,
“There was an open discussion at the time,
but then the topic became unpopular.”
The thing that made it unpopular was
the rise of dark matter. Dark matter became
30 | NewScientist | 2 April 2016