Sky & Telescope - October 2019 - 17

THE GREAT ATTRACTOR
The ﬁrst gravitational sink identiﬁed with cosmic ﬂows was the Great Attractor. It's a concentration of galaxy
clusters that lies between the Milky Way and the larger Shapley Supercluster. It was found by a gang of seven
astronomers dubbed "The Seven Samurai" and led by Donald Lynden-Bell (1935-2018), one of the great
astronomers of the past half century.

measure of its distance will be - and the more inaccurate
the distance, the less sure we are of what the Hubble ﬂow
velocity for that galaxy should be.
Furthermore, the universe didn't bless every single galaxy
with a standard candle as accurate as a Type Ia explosion.
Only about 400 of the 40,000 or so supernovae we have
observed in the billions of galaxies out there provide good,
reliable distances. While there are other ways of measuring
the distance to galaxies without supernovae, these methods
tend to have errors of around 20% at great distances.
But it turns out that we don't need to precisely measure
every galaxy's distance and motion in order to map the cosmic
landscape. All the galaxies in a given volume of space inhabit
the same gravitational environment, so they're all attracted
toward the same concentrations of matter. Thus, charting
only a fraction of the galaxies traveling along the currents in
the cosmic ocean tells us the cosmography: You only need to
measure the velocity of a few ﬁsh to know the speed of the
entire school, and you only need to measure the motion of a
few schools to infer where the hunting grounds are.
We can use this cosmographic approach on scales of
hundreds of millions of light years, much larger than a single
galaxy cluster. On these scales, deviations from the Hubble
ﬂow are small, and galaxies bob in relatively smooth currents
- think of water gently ﬂowing around a small obstacle. This
is what's known as a laminar ﬂow, as opposed to a turbulent
ﬂow, which is when a river rushes so fast that it foams and
froths and creates eddies.
The broad-brush atlas of the heavens that we build with
these smooth ﬂows reveals quite a lot. We have charted the
cosmic landscape out to distances of roughly half a billion
light-years, using a mere 8,100 galaxies. These maps indicate
that two structures dominate the local ﬂow: an association
of massive clusters of galaxies called the Shapley Supercluster, and an immense cosmic void coined the Dipole Repeller
(S&T: May 2017, p. 8). Both lie about 650 million light-years
away from the Milky Way, in almost diametrically opposing directions. One of us (YH) and colleagues also used the
technique to discover that many features of the local universe
actually belong to a much larger system called Laniakea, our
home supercluster. Laniakea spans more than 500 million
light-years and contains 100,000 galaxies.

The Gravitational Time Machine
In laminar cases, it is fairly easy to trace the ﬂow back in
time, even all the way back to when the universe was young

and galaxies had not yet formed. For example, if two galaxies are converging towards a common center, we may infer
not only that there is some huge mass there that's attracting
them, but that they were farther away from that common
center in the past.
This time-traveling enables us to reconstruct what our
section of the early universe once looked like, such that,
under the inﬂuence of gravity, it would grow to produce
the cosmographic landscape we observe today. We can then
use this picture as the starting point of a computer simulation. Computer simulations allow us to ﬁll in the blanks, to
"guess" what the universe looks like on the scale of individual

Reversing galaxies' paths in the
heart of a cluster would be like
trying to track footprints in the
snow on a town square.
clusters, where galaxies' motions are too turbulent for us to
infer the distribution of matter from peculiar velocities. In
fact, Zeldovich and Doroshkevich were among the ﬁrst to
realize that reversing galaxies' paths in the heart of a cluster
would be like trying to track footprints in the snow on a town
square: easy to do for a few tracks right after snowfall, but an
impossible task when half the town has gone to market.
If you want to simulate gravity in a computer, it's not that
hard. You ﬁll an imaginary box with particles, arranged in
some way. You then compute the gravitational force that each
particle feels due to all the other particles and have the computer code move them accordingly. For example, if your initial
setup is a uniform sea of particles with a big spherical blob
in the center, then all the particles will feel a net attraction
to the blob and collapse towards it. Gravity is the ultimate
capitalist force: The rich get richer and the poor get poorer. By
that we mean, the denser regions get more dense and massive
as they attract more stuff, at the expense of the empty, less
massive regions. It happens in a kind of gravitational avalanche. By today, most of the mass of the universe is concentrated in blobs - galaxies and clusters - whose density can be
up to a few hundred times the mean density of the universe.
This leaves behind vast, under-dense volumes whose typical
density is about 10% of the mean (S&T: Feb. 2015, p. 20).
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Sky & Telescope - October 2019

Table of Contents for the Digital Edition of Sky & Telescope - October 2019