Sky and Telescope - May 2017 - 35

Baking
a Universe

Some of our best cosmological simulations
don't even include what we're made of -
ordinary matter - but maybe this is just
what's needed to shine a light on the
universe's mysteries.

hen you go to bake a cake, the ﬁ rst ingredients
you grab are the eggs and ﬂour. But it's the
muscovado sugar, the vanilla pod, or maybe the
orange zest that really determines what kind of cake you
end up with.
Similarly, until recently, most cosmological simulations of
the evolution of galaxies and the universe were basic bakes,
including only the big main ingredients - dark matter and
gravity. Dark matter is computationally "simple," immune to
everything but gravity. So, limited by how many calculations
their supercomputers could run simultaneously, researchers have long chosen to use just dark matter and gravity and
assume that "normal" matter follows along.
At ﬁrst glance this makes sense: Dark matter is ﬁve times
more abundant than normal matter, meaning it should have
a much larger impact on what the universe looks like. And
these vast, dark-matter-only simulations have managed to
mimic in stunning detail the intricacy and scale of the cosmic web, the large-scale invisible structure of galaxy clusters
that ﬁlls the universe. Most importantly, they also very successfully match the abundance, sizes, and distances between
galaxies with those of real observations - allowing insights
and predictions to be made of how the universe ﬁts together
at the largest of scales.
But on smaller, galactic scales, these simulations have
had their problems. For example, they predict hundreds of
dwarf galaxies should swarm around larger systems like the
Milky Way, yet observers have found only dozens (S&T: Mar.
2017, p. 16). They also predict that dark matter is denser in a
galaxy's core than observations suggest.
Now, astronomers are discovering that the solution to
understanding what's really going on in the universe lies
with the very thing they were leaving out: ordinary matter.

ILLUSTRIS COLL ABOR ATION

In the Dark
Bolshoi, for instance, is one of the most accurate cosmological simulations of the evolution of the large-scale structure
of the universe (S&T: July 2012, p. 28). Within a box roughly
1 billion light-years across, the Bolshoi team followed the
development of 8.6 billion lumps of dark matter, all gravitationally interacting with one another, from 20 million years
after the Big Bang to the present day. As time passed, these
lumps combined to create galaxies and the larger cosmic web.
The simulation has had many successes. For instance, it
reproduces the number of massive satellite galaxies (like the
Large and Small Magellanic Clouds) around Milky Way-like
galaxies, which in turn constrains the properties and mass of
the Milky Way's dark matter halo, the big cloud of dark matter that our galaxy sits in (S&T: Apr. 2017, p. 22).
Yet, there is one gaping omission - the complete lack of
what humans, stars, gas, and galaxies are made of: ordinary
matter (or in astronomers' slang, "baryons").
"Baryonic physics can affect the properties of the galaxies
that form and even the distribution of dark matter around a
galaxy, even though baryons do not dominate the mass budget
of the universe," says Andrew Wetzel (Caltech). "So any real
connection to observations must include baryons, full stop."
Unfortunately, adding ordinary matter is no easy task. Normal matter adds a huge amount of complexity and, according
to Wetzel, "always makes simulations much more expensive

BARYONS
º Technically, baryons are a class of subatomic particle that
includes protons and neutrons but not all non-dark matter
(such as electrons). But astronomers use the term as a
catch-all for "normal" matter in simulations.

t UNIVERSE AGLOW This snapshot from the Illustris simulation shows structure in a modern-day universe, revealing both dark matter density (left,
blue) and gas density (right). The clump in the middle is a massive galaxy cluster.
s k y a n d t e l e s c o p e .c o m

* MAY 2017

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Sky and Telescope - May 2017

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