Sky & Telescope - October 2019 - 19

Virgo Cluster

Locall Filament
am
Local Group

Local
Void

NOAM LIBESKIND (2)

tp SIMULATED UNIVERSE Simulations of large-scale structure can
reproduce both our local cosmic structure (left) and the interactions of
galaxies similar to the Milky Way and Andromeda (right).

So, on large enough scales we can use our time-reversal
machine to go back in time, reversing the evolution of a
giant region of the observable universe from what it looks
like today to the near-uniform sea of particles that was its
initial state. Once at the beginning, we ﬁ ll in the details
that the large, gentle ﬂows gloss over, by performing an
educated guess on what's there. That's where computers
come in handy. We then feed this smooth, nearly uniform
early universe with our guesses into a computer and simulate forwards in time. In short, we go from the ﬁ nal state
to the initial state, statistically ﬁ ll in the details of what we
missed, and then go forwards again.
These simulations all look similar on large scales and provide a good proxy to the real universe. They have to: They're
built to reproduce galaxies' observed peculiar velocities and
the underlying matter distribution, which is why we call them
constrained simulations. But unlike our cosmographic maps,
the simulations are not limited to the zoomed-out view. In
the computer we are free to simulate down to whatever scale
we want (computational limitations notwithstanding) and
see how the galaxies move there.
What's remarkable is that on scales of roughly 10 million light-years - similar to the distances between individual
galaxies - the virtual universes reproduced in our computers don't differ that wildly. That suggests that even with the
rough, zoomed-out view that peculiar velocities provide, we
can predict what the universe actually looks like on scales onetenth as grand. By averaging many constrained simulations
together, we are able to probe the movements of galaxies on
these scales and simultaneously make some of the most accurate cosmographic maps in existence of the nearby universe.

Using this approach, we have been able to identify
speciﬁc features hitherto unseen in the local universe.
These features include bridges of dark matter, connections
between clusters of galaxies, and voids that push galaxies outward - a network of structures that is commonly
dubbed the cosmic web. Observers have also found such
bridges, using the lensing effect the structures' gravity has
on the light of galaxies behind them.
Certainly Doroshkevich and Zeldovich were right:
Gravity, once it causes high-density regions to form, can
never be reversed; the knot can never be untied. However,
with good observational data and careful calculations, we
can turn back the clock, not only creating rough maps of
today's local universe but also reversing gravity to ﬁ ll in
the ﬁ ne-grained details of the history of our galaxy's
neighborhood. And someday, these charts of the cosmic
currents will help us understand why the Milky Way
looks the way it does.
We don't think Zeldovich, buried alongside other Russian
giants such as Gogol, Chekhov, and Prokoﬁev in Moscow's
Novodevichy Cemetery, would be turning in his grave. We
hope that he would be pleased.

¢ NOAM LIBESKIND is a computational cosmologist at the
Leibniz Institute for Astrophysics Potsdam in Germany and
a professor at the University of Lyon-1 in France. YEHUDA
HOFFMAN is a professor of Physics at the Hebrew University
of Jerusalem.
Watch a detailed video on how cosmologists create maps of
cosmic ﬂows: http://irfu.cea.fr/cosmography.
sk yandtele scope.com * OCTOBE R 2 019

http://irfu.cea.fr/cosmography http://www.skyandtelescope.com

Sky & Telescope - October 2019

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