26 | New Scientist | 6 March 2021
Views Columnist
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 modelI
’VE been giving a lot of talks
about my research to a range
of scientific audiences of late.
The listeners range from groups
that are mainly undergraduates
to those made up of specialists
in my field – in other words,
people who are also searching
for and trying to understand
the behaviour of dark matter.
In nearly all of these
presentations, I start by
explaining Vera Rubin and Kent
Ford’s observations of galaxies.
These showed that there was
a mismatch between their
measurements of galactic
masses and what one might
expect the mass to be based on
how many stars are in the galaxies.
I then make an explicit effort to
note that there are two ways to
address this inconsistency: there
is either more matter than we can
see in these galaxies, or we are
interpreting the evidence in the
wrong way – in short, that our
theory of gravity is wrong.
It soon becomes obvious that,
for the rest of the talk, I’m going
to focus on the idea that there is
more matter in galaxies and that
this is comprised of so-called dark
matter. But I do try to make an
effort to highlight that modified
gravity – revisions to our theory
of gravity which would explain
the mismatched data – is also
an active field of research.
To better understand why these
ideas arise at all, it is useful to
spend more time understanding
the first compelling evidence
that there was a problem in need
of a solution in the first place.
Specifically, Rubin and Ford
found that stars were orbiting
the centres of their galactic
homes faster than we would
expect based on how massive
the galaxies are presumed to be,
if we are just counting stars and
adding their masses together.This column appears
monthly. Up next week:
Graham LawtonCosmic conflict There is a mismatch between two ways of
measuring galactic mass. Dark matter is one way to solve it, but so
is rewriting the laws of gravity, writes Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
My book The Disordered
Cosmos launches in
the US in the middle of
March, so I’m actually
looking at this a lot
to make sure I haven’t
forgotten anything.What I’m watching
Well, the first season of
The Real Housewives
of Salt Lake City was
quite a journey.What I’m working on
I’m focused on the secrets
of neutron stars.Chanda’s week
At the time, astronomers
measured a galaxy’s mass
through a combination of
observations. First, they looked
at the typical brightness of stars
in their galaxy and used this to
estimate how massive each star
is. This is possible because the
brightness tells us how much fuel
the star has, which correlates
directly with its mass. Then,
by adding the masses of all of
the stars together, astronomers
came to an approximation for
how massive the galaxy is.
By contrast, Rubin used an
instrument developed by Ford
to test an alternative mechanismfor calculating a galaxy’s mass.
Looking at the speeds of stars and
their distance from the centre of
their galaxies, then combining
them in an equation from
Newtonian physics related to
gravity, one can calculate mass
too. By the way, that equation
is one that in the US we teach to
first year undergraduates – and
even high school students.
However, we have a problem:
these ways of measuring galactic
mass, based on different parts of
physics, give different answers.
Israeli physicist Mordehai
Milgrom first proposed “modified
Newtonian dynamics” (MOND) in
the early 1980s in order to address
the observational data of Rubin
and Ford. He suggested that
perhaps the velocities and radius
were simply going into the wrong
equation. How strong is his case?
There is of course precedent for
thinking Newtonian gravity iswrong: we already know that in
some scenarios, Albert Einstein’s
relativity must be used instead.
However, nearly 40 years later,
the hypothesis that there is dark
matter in galaxies – a type of stuff
that we can’t see – remains a far
more popular solution to the
inconsistency. The existence of
dark matter was first proposed
by astronomer Fritz Zwicky in
the 1930s, with the idea gaining
credence through the later work
of Rubin and Ford.
This idea’s current dominance
is partly because observations
made in recent decades are better
explained by models of dark
matter than by MOND. The most
famous example is the Bullet
Cluster, a set of galaxy clusters
that are colliding. Observations
of this are more consistent with
the presence of dark matter than
with a modified gravity model.
In addition, more recently,
observations of the cosmic
microwave background (CMB)
radiation have become our
strongest evidence for the
existence of dark matter.
The CMB is a form of radiation
that pervades all of the universe
with an ambient temperature of
about 2.73 kelvin (-270.4 ̊C). It has
tiny fluctuations in it on different
scales that are imprints of an
earlier time, when the universe
wasn’t transparent to light. To
make our models of the CMB fit
the data, we have to take dark
matter into account. MOND simply
isn’t as successful at doing that.
For this reason, my talks
proceed on the premise that we
are talking about a dark matter
problem. But we still haven’t
directly detected dark matter, and
that means MOND remains – to
some researchers – a compelling
area of further work. It isn’t an
area that I work on, but I’m glad
others are doing so. ❚“ We have a problem:
two ways of gauging
galactic mass, based
on different parts
of physics, provide
different answers”