It’s been more than 50 years since astronomers first proposed “dark matter,” which is believed to be the most common form of matter in the universe. Despite this, we have no idea what it is – no one has seen it directly or produced it in the lab.
So how can scientists be so sure it exists? Should they be? It turns out that philosophy can help us answer these questions.
In the 1970s, a seminal study by astronomers Vera Rubin and Kent Ford of the rotation of our neighboring galaxy Andromeda revealed a surprising inconsistency between theory and observation. According to our best gravitational theory for these scales – Newton’s Laws – stars and gas in a galaxy should spin slower and slower as they move away from the center of the galaxy. This is because most stars will be close to the center, creating a strong gravitational pull there.
Rubin and Ford’s result showed that this was not the case. The stars on the outer edge of the galaxy were moving about as fast as the stars around its center. The idea that the galaxy must be embedded in a large halo of dark matter has basically been proposed to explain this anomaly (although others have suggested it before). This invisible mass interacts with the outer stars by gravity to increase their speeds.
This is just one example of several anomalies in cosmological observations. However, most of them can also be explained by modifying the current gravitational laws of Newtonian dynamics and Einstein’s theory of general relativity. Perhaps nature behaves slightly differently at certain scales than these theories predict?
One of the first such theories, developed by Israeli physicist Mordehai Milgrom in 1983, suggested that Newtonian laws might work slightly differently when there is extremely low acceleration, such as at the edge of galaxies. This setting was perfectly compatible with the observed galactic rotation. Nevertheless, physicists today massively favor the dark matter hypothesis incorporated in the so-called ΛCDM (Lambda Cold Dark Matter) model.
This view is so strongly rooted in physics that it is widely referred to as the “standard model of cosmology.” However, if the two competing theories of dark matter and modified gravity can also explain galactic rotation and other anomalies, one wonders if we have good reason to prefer one over the other.
Why does the scientific community have a strong preference for the dark matter explanation over modified gravity? And how can we ever decide which of the two explanations is correct?
Philosophy to the rescue
This is an example of what philosophers call “the underdetermination of scientific theory” by available evidence. This describes any situation in which the available evidence may be insufficient to determine which beliefs we should hold in response. This is a problem that has long puzzled philosophers of science.
In the case of the strange rotation of galaxies, the data alone cannot determine whether the observed velocities are due to the presence of additional unobservable matter or to the fact that our current gravitational laws are incorrect.
Scientists are therefore looking for additional data in other contexts that will eventually settle the question. One such example in favor of dark matter comes from observations of how matter is distributed in the galaxy cluster Bullet, which is made up of two colliding galaxies about 3.8 billion light-years from Earth. . Another is based on measurements of how light is deflected by (invisible) matter in the cosmic microwave background, the light left behind by the big bang. These are often taken as compelling evidence in favor of dark matter because Milgrom’s original theory cannot explain them.
However, following the publication of these results, other modified gravity theories have been developed over the past decades to account for all the observational evidence of dark matter, sometimes with great success. Yet the dark matter hypothesis still remains the preferred explanation of physicists. Why?
One way to understand this is to employ the philosophical tools of Bayesian confirmation theory. It is a probabilistic framework for estimating the extent to which assumptions are supported by available evidence in various contexts.
In the case of two competing hypotheses, what determines the final probability of each hypothesis is the product of the ratio between the initial probabilities of the two hypotheses (before proof) and the ratio of the probabilities that the proof will appear in each case (called the ratio of likelihood).
If we accept that the more sophisticated versions of modified gravity and dark matter theory are also supported by evidence, then the likelihood ratio equals one. This means that the final decision depends on the initial probabilities of these two hypotheses.
Determining what exactly counts as the “initial probability” of a hypothesis, and possible ways of determining those probabilities, remains one of the most difficult challenges in Bayesian confirmation theory. And this is where philosophical analysis comes in handy.
At the heart of the philosophical literature on this topic is the question of whether the initial probabilities of scientific hypotheses should be determined objectively based solely on probabilistic laws and rational constraints. Alternatively, they could involve a number of additional factors, such as psychological considerations (whether scientists favor a particular hypothesis based on interest or for sociological or political reasons), background knowledge, success of a theory scientist in other fields, etc. .
Identifying these factors will ultimately help us understand why dark matter theory is overwhelmingly favored by the physics community.
Philosophy ultimately cannot tell us whether astronomers are right or wrong about the existence of dark matter. But it can tell us if astronomers actually have good reason to believe it, what those reasons are, and what it would take for modified gravity to become as popular as dark matter.
We still don’t know the exact answers to these questions, but we are working on it. More research in philosophy of science will give us a better verdict.