24 | New Scientist | 1 February 2020
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard modelThis column appears
monthly. Up next week:
Graham Lawton“ Figuring out galaxy
structure is one of
the most exciting
questions if you
wonder how the
universe works”How to see the Milky Way Working out what our galaxy looks
like is akin to solving a murder mystery – good sleuthing is needed
to combine all of the clues, writes Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
I’m so pleased I picked up
Alicia Elliott’s powerful
memoir A Mind Spread
Out on the Ground.What I’m watching
The Bachelor has
returned and, yes,
I am watching it.What I’m working on
I just hired a new
postdoctoral researcher
for my group, and I’m
happy to be making plans
for new projects with him.Chanda’s week
Views Columnist
A
LL of the students in my
astrophysics class last
semester had to give a final
presentation. In one, a student
showed everyone an image of our
galaxy, the Milky Way, as viewed
from Earth. The student then
showed another image of a spiral
galaxy, suggesting that it reflected
our own. Because it was a class
focused on stars – the students
are taking on galaxies this
semester – a question came from
the audience: how can we get a
picture of the whole Milky Way
if we are inside it? We can’t.
In this sense, astrophysics
is like a Midsomer mystery.
There is a lot happening, and
some of it is relevant to questions
you might have and some of it
is a distraction. Also, just as with
all of the murders in England’s
fictional most deadly county,
to understand what happened,
you can’t simply reconstruct the
events and watch them happen.
Instead, it is necessary to
piece things together using the
information available. This is in
stark contrast to experimental
sciences, where it is possible to
do an experiment multiple times
to make sure you know what
happened and where you can
control the circumstances of
the physical phenomena you
are studying.
In astrophysics, we have no
control over what we see. We can’t
even leave our own galaxy to get a
good look at it. But we can deduce
some things. First, we know from
looking around that galaxies take
on only a few different shapes.
Almost all galaxies we observe
are either spiral, elliptical or
something between the two,
known as lenticular. There
are also abnormal galaxies,
but these are rarer.
To figure out what kind of
galaxy we live in and what it lookslike, we use circumstantial
evidence. When we see pictures of
the Milky Way, we are seeing only
part of it, from inside it but near
the edge. If you have ever had the
chance to go somewhere that
recognises the sanctity of dark
skies, on a clear night you would
have seen a band of bright dots
that seem closer together as you
get to the centre of the line.
We see the Milky Way like this
not because our galaxy is a line but
because we are looking at it from
the side, rather than the top. This
is a lot like looking at a fancy plate
edge on: it looks almost like a line
and you can only see the textures
that the potter put on the side.You can’t really see whatever
colourful flourishes they put
on the surface of the plate.
However, we can measure how
fast some of those stars are
moving, and the speeds in the
Milk Way – assuming the
presence of dark matter – are
consistent with a spiral galaxy.
Instead of actual pictures
of the Milky Way, what we have
are images of galaxies that we
think are very similar to ours.
For example, we spend a lot of
time studying the Andromeda
galaxy because it is the large
spiral galaxy closest to us, and
its proximity means that we
can get good images of it. The
question, of course, is how do
we know it is at all like ours?
As I said, it is a detective-like
effort. It would be easy for me
to say, as a theorist who thinks
about fundamental physics, thatunderstanding the structure of
galaxies is somewhat simpler
than trying to figure out
dark matter – the missing matter
problem – and dark energy, the
cosmic acceleration problem.
Although we are used to
thinking of galaxies as bright
collections of stars and dust,
they are actually mostly
collections of dark matter with
a smattering of stars and dust
inside. This is because, as I have
mentioned in previous columns,
dark matter makes up the
majority of the matter in the
universe. It is also the case that
the phenomenon of cosmic
acceleration, whereby the
expansion of space-time is
accelerating, changes the
cosmological timeline for how
galaxies form and cluster together.
In reality, figuring out galaxy
structure, and how it is affected
by dark matter, is one of the most
exciting questions to face those
of us who wonder about how the
universe works on large scales.
There are still many questions
about the Milky Way that remain
unanswered. For example,
we know it has small satellite
galaxies: almost 60 of them.
Two have names that you
may have heard before because
they are visible with the naked
eye: the Large Magellanic
Cloud and the Small Magellanic
Cloud. You might wonder
why the number is about 60.
So do astrophysicists.
In fact, I have a PhD student
doing work that seeks to address
exactly this question. We don’t
understand the formation history
of the Milky Way and its dwarf
galaxy satellites, collectively
known as the Milky Way
subgroup. In other words, you
don’t have to go far in the universe
before you run into some of its
biggest cosmological questions. ❚