(ORDO NEWS) — It is supposed to be the most common form of matter in the universe, but no one has ever actually seen it.
It’s been more than 50 years since astronomers first hypothesized the existence of “dark matter,” considered the most common form of matter in the universe. Despite this, we have no idea what it is – no one has seen it directly and has not received it in the laboratory.
Why are scientists so sure of its existence? And should they? It turns out that philosophy can help us answer these questions.
Back in the 1970s, astronomers Vera Rubin and Kent Ford did a fundamental study of how our neighboring Andromeda galaxy rotates and found a surprising discrepancy between theory and observations.
According to our best-suited gravitational theory for this scale, Newton’s laws, stars and gas in a galaxy should rotate more and more slowly the farther they are from the center of the galaxy. This is because most stars are near the center, creating a strong gravitational force there.
Rubin and Ford’s result showed that this was not the case. The stars at the outer edge of the galaxy were moving about as fast as the stars around its center.
To explain this anomaly, the idea that the galaxy must be encased in a large halo of dark matter has been proposed (although other scientists have suggested it before). This invisible mass interacts with outer stars through gravity, increasing their speed.
This is just one example of several anomalies in cosmological observations. However, most of them can be explained by correcting the existing gravitational laws of Newtonian dynamics and Einstein’s general theory of relativity. Perhaps, on certain scales, nature behaves a little differently than these theories predict?
One of the first such theories, developed by the Israeli physicist Mordechai Milgrom in 1983, suggested that Newtonian laws might work slightly differently at extremely low acceleration, such as at the edge of galaxies.
This correction turned out to be quite compatible with the observed galactic rotation. However, physicists today overwhelmingly prefer the dark matter hypothesis included in the so-called CDM (Lambda Cold Dark Matter) model.
This view is so firmly rooted in physics that it is widely referred to as the “standard model of cosmology.” However, if two competing theories of dark matter and modified gravity can equally explain the rotation of galaxies and other anomalies, one might wonder if we have good reasons to prefer one over the other.
Why does the scientific community prefer the concept of dark matter over the modified gravity model? And how can we 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 the available evidence. This describes any situation in which the available evidence may not be sufficient to determine what beliefs we should hold in response to them. This is a problem that has puzzled philosophers of science for a long time.
In the case of strange rotations in galaxies, the data alone cannot determine whether the observed speeds are due to the presence of additional unobservable matter or because our current gravitational laws are wrong.
Therefore, scientists are looking for additional data in other contexts that will ultimately resolve this issue. One such example in favor of dark matter comes from observations of the distribution of matter in the Bullet Galaxy Cluster, which consists of two colliding galaxies about 3.8 billion light-years from Earth.
Another theory is based on measurements of how light is deflected by (invisible) matter in the cosmic microwave background, the light left over from the Big Bang. These results are often seen as undeniable evidence for dark matter, as Milgrom’s original theory fails to explain them.
However, since the publication of these results, other modified gravity theories have been developed over the past decades to account for all the observational evidence in favor of dark matter, sometimes with great success. However, the dark matter hypothesis is still the favorite explanation of physicists. Why?
One way to understand this is to use the philosophical tools of Bayesian confirmation theory. It is a probabilistic framework for assessing the extent to which hypotheses are supported by the available evidence in different contexts.
In the case of two competing hypotheses, the final probability of each hypothesis is determined by the product of the ratio of the initial probabilities of the two hypotheses (before evidence appears) and the ratio of the probabilities of evidence in each case (called the likelihood ratio).
Assuming that the most advanced versions of modified gravity and the theory of dark matter are equally supported by the evidence, then the likelihood ratio will be equal to one. This means that the final decision depends on the initial probabilities of these two hypotheses.
Determining what counts as the “initial probability” of a hypothesis, and how such probabilities can be determined, remains one of the most difficult tasks 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 objectively determined solely on the basis of probabilistic laws and rational constraints.
Alternatively, they may include a number of additional factors such as psychological considerations (whether scientists favor one hypothesis over another out of interest or for sociological or political reasons), background knowledge, the success of scientific theory in other fields, and so on.
Identifying these factors will ultimately help us understand the reasons why the theory of dark matter is overwhelmingly supported by the physics community.
Philosophy cannot ultimately tell us whether astronomers are right or wrong about the existence of dark matter. But it can tell us if astronomers really have good reasons to believe in it, what those reasons are, and what needs to be done to make modified gravity as popular as dark matter.
We still do not know the exact answers to these questions, but we are working on it. Additional research in the field of philosophy of science will allow us to make a more accurate verdict.”
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