April 2022, ScientificAmerican.com 65gest that these dense spinning stars produce axions when
protons and neutrons collide in their cores. We might be
able to observe these axions as they decay into photons
and escape from the stars. And as neutron stars release
the dark matter over tens to hundreds of years, they
would cool down in a pattern that we may be able to mea-
sure—if we look long enough. Another hot topic of study
right now is whether nonaxion dark matter collects in
neutron stars, affecting the structure of the star. As a
member of the nasa Neutron Star Interior Composition
Explorer (NICER) collaboration, I am leading a research
project that is using data from NICER, a little telescope
on the International Space Station that is up for renewal
later this year. Our project is looking for evidence that
dark matter is inside or enveloping neutron stars.
We can also learn more about the nature of dark
matter by studying the best evidence we have for its
existence so far—the cosmic microwave background
(CMB) radiation. This light is a radio signal that origi-
nated in the early universe, and it is inescapably every-
where, all around us. It provides a snapshot of a
moment early in cosmic history, and the patterns we
see in the frequencies of its light reflect the makeup of
the universe when it was created. It turns out that we
can only explain the patterns we see in the CMB by
assuming that dark matter was present—if there were
no dark matter, the CMB data would make no sense.
The patterns in the data tell us what fraction of the total
mass and energy dark matter contributed; they even
help constrain the possible masses of the dark matter
particles. As I write, the CMB-Stage 4 collaboration is
preparing to use a collection of telescopes in Chile’s
Atacama Desert and at the South Pole to take the most
detailed measurements yet of the CMB.ON THE HORIZON
sinCe that 2009 Women in Astronomy conference, Rubin
and Roman have both passed away, but their legacies live
on through projects that will seek to better understand
our universe. The nasa Nancy Grace Roman Space Tele-
scope will launch in the mid-2020s, and although it is
primarily focused on studying cosmic acceleration (the
“dark energy problem”) and exoplanets, it will also pro-
vide insight into dark matter. At the same time, here on
Earth, the Vera C. Rubin Observatory in the Atacama Des-
ert will support research on many questions, including
the search for the dark matter that made Rubin famous.
In other words, we have lots to look forward to in
the coming years. One reason is that almost any large-
scale astronomical observation has something to tell
us about dark matter. For example, a team in Mexico
led in part by Alma X. Gonzalez-Morales and Luis
Arturo Ureña-López showed that we can use the phe-
nomenon of gravitational lensing, where large masses
bend spacetime so much that it acts like a fun-house
mirror, to place constraints on the mass of fuzzy dark
matter. Gonzalez-Morales and Ureña-López are both
active participants in the Rubin Observatory's Legacy
Survey of Space and Time program, working on gravi-
tational lensing and participating in the dark matter
working group. Within the group, we are discussing
observations that will capture more detailed informa-
tion about dark matter halos that can then be com-
pared with computer simulations of proposed dark
matter candidates. Similarly, Roman telescope surveys
of large-scale structure will provide insight into dark
matter’s behavior on cosmic scales.
In the future, proposed x-ray observatories such as
the nasa Spectroscopic Time-Resolving Observatory
for Broadband Energy X-rays (STROBE-X) can help ustake a closer look at neutron star structure in ways that
will enhance our understanding of dark matter’s pos-
sible properties. Other proposed future projects such
as nasa’s All-sky Medium Energy Gamma-ray Obser-
vatory, or AMEGO (not to be mistaken for AMEGO-X),
will do the same in a different wavelength.
I will be an active participant not just as a scientist
but as one of three conveners, alongside Alex Drlica-
Wagner and Hai-Bo Yu, for the Snowmass Cosmic Fron-
tier’s topic Dark Matter: Cosmic Probes. It is our respon-
sibility to describe the excitement and possibilities of
astrophysical searches for dark matter to the funding
decision-makers. The document I will help produce may
influence guidance that is given to the National Science
Foundation and the U.S. Department of Energy about
what research we conduct over the next decade.
Coincidentally, the astronomy community just
recently completed a similar process known as the
2020 Decadal Survey on Astronomy and Astrophysics.
The resulting report sidestepped substantively address-
ing the dark matter problem, but it still offered strong
support for efforts to better map the CMB, instruments
to study neutron stars and x-ray observatories—three
goals that will help us understand dark matter.
Doing science is never just about calculations, obser-
vations and experiments; it is also about working col-
laboratively with other people, including policy mak-
ers. How much progress we make will depend in part
on what kind of support we get from lawmakers. Of
course, this is stressful to think about. The good news
is that there is a universe to wonder about, and trying
to understand dark matter is a great distraction.FROM OUR ARCHIVES
Dark Matter’s Last Stand. Clara Moskowitz; April 2021.
scientificamerican.com/magazine/saDoing science is never just about
calculations, observations and
experiments; it is also about
working collaboratively with other
people, including policy makers.