http://www.skyandtelescope.com.au 31ILLUSTRIS COLLABORATION
W
hen you want to bake a cake, the first ingredients
you grab are the eggs and flour. But it’s the 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 — enabling
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. 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.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. Within a virtual box of space 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
produce 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 in which our galaxy sits.
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. It
adds a huge amount of complexity and, according to Wetzel,
“always makes simulations much more expensive [ie. 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.”BARYONS
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 term for ‘normal’ matter in simulations.Some of the 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.