Sky and Telescope - May 2017 - 36

Universe 2.0
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ously bind together large-scale dark matter structure growth
with smaller-scale baryonic objects and processes - things
like black hole feedback, gas dynamics, and star formation. It
all boils down to computational cost: the effort, complexity,
and time required to complete a realistic simulation.
"The community has bifurcated a bit now," says Wetzel.
"Groups doing large-volume simulations that encompass
thousands of galaxies at a time gain excellent statistics to
compare against observational surveys. But the trade-off is
that these simulations are not able to resolve the relevant
physical processes within a galaxy."
In contrast, groups conducting "zoom-in" simulations
start with a large-volume box but only have high resolution in
the region immediately around a single galaxy. "This means
that such simulations only model one galaxy at a time, but
they resolve it exquisitely well," he says.
Zoom-in simulations that focus on a smaller cosmic box
can be likened to home baking. Bakers can pick as many ingredients as they want and take their time over the bake, because
the simulation is covering a smaller spatial area and thus can

q COSMIC SCALES Cosmological simulations try to replicate the universe and its evolution, and early ones produced remarkable matches
to large-scale structures (left-hand boxes in this illustration). But for many years astronomers were forced to fudge what was happening at
the smallest scales (other illustrations) - processes that, it turns out, heavily influence the larger structures. (A parsec is 3.26 light-years.)

~1010 parsecs

~107-108 parsecs

~104-105 parsecs

Observable
universe

Clusters, large-scale
structure

Galaxy

M A Y 2 0 1 7 * SK Y & TELESCOPE

BOLSHOI SLICES: N. MCCURDY (UC -HIPACC) / R. K A EHLER & R. WECHSLER
(STA NFORD) / M. BUSH A (UNIV. ZURICH) / SDSS

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While the dark-matter-only simulation experts offer insights
into how the universe evolves as a whole - albeit without
directly including anything in the simulations that is observable - those running cosmological simulations that include
ordinary matter have faced a dilemma akin to trying to make
a big cake with a delicate, complex taste: how to simultane-

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Small Beginnings

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[i.e., time-consuming] than dark-matter-only simulations."
"It's easier to simulate dark matter than to simulate baryons," conﬁrms Claude-André Faucher-Giguère (Northwestern
University). "This is because dark matter is postulated to
interact only via gravity, whereas baryons experience many
other forces, like electricity and magnetism. So we have to
solve many more equations, including those of hydrodynamics, to simulate the baryons."
As a result, the cosmological simulations community has
long been waiting for a time when computers and algorithms
would be powerful enough to handle the complexity of baryonic physics. That time is now.

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COSMIC PIE
The Bolshoi simulation's
predictions for what large-scale
cosmic structure looks like (left, teal)
closely match the real distribution of galaxies
observed by the Sloan Digital Sky Survey (right) -
in fact, statistically they're virtually identical.
Each wedge spans a quarter of the way around the sky.

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Sky and Telescope - May 2017

Table of Contents for the Digital Edition of Sky and Telescope - May 2017