摘要：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, says Chanda Prescod-Weinstein
I’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.
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 mechanism for 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.
“We have a problem: two ways of gauging galactic mass, based on different parts of physics, provide different answers”
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 is wrong: 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.