That was it more than 50 years since astronomers first proposed “dark matter,” which is considered the most common form of matter in the universe. However, we have no idea what it is – no one has seen it directly or produced it in the laboratory.
So how can scientists be so sure it exists? Should they be? It turns out that philosophy can help us answer these questions.
As early as the 1970s, a fundamental study by astronomers Vera Rubin and Kent Ford about the rotation of our neighboring galaxy Andromeda revealed a surprising discrepancy between theory and observation. According to our best gravitational theory for these scales – Newton’s laws – the stars and gas in the galaxy should rotate slower and slower the farther they are from the center of the galaxy. This is because most of the stars will be close to the center, creating a strong gravitational force there.
The results of Rubin and Ford showed that this is not the case. The stars at the outer end of the galaxy moved about as fast as the stars around its center. The idea that the galaxy should be embedded in a large halo of dark matter was mainly proposed to explain this anomaly (although others have suggested it before). This invisible mass interacts with outer stars through gravity to increase their velocities.
This is just one example of several anomalies in cosmological observations. Most of them, however, can be explained equally by changing the current gravitational laws of Newtonian dynamics and Einstein’s general theory of relativity. Maybe nature behaves a little differently on a certain scale than these theories suggest?
One of the first such theories, developed by the Israeli physicist Mordechai Milgrom in 1983, suggests that Newton’s laws may work a little differently when there is extremely little acceleration, such as at the edge of galaxies. This setting was fully compatible with the observed galactic rotation. However, today physicists predominantly prefer the dark matter hypothesis included in the so-called ΛCDM model (Lambda Cold Dark Matter).
This view is so entrenched in physics that it is widely called the “standard model of cosmology.” However, if the two competing theories of dark matter and modified gravity can equally explain galactic rotation and other anomalies, one may wonder if we have good reason to prefer one over the other.
Why does the scientific community have a strong preference for explaining dark matter over modified gravity? And how can we even decide which of the two explanations is correct?
The philosophy of help – This is an example of what philosophers call the “uncertainty of scientific theory” from the available evidence. This describes any situation in which the available evidence may be insufficient to determine what beliefs we should have in response to them. This is a problem that has puzzled philosophers of science for a long time.
In the case of strange rotation in galaxies, the data alone cannot determine whether the observed velocities are due to the presence of additional unobservable matter or due to the fact that our current laws of gravity are incorrect.
Therefore, scientists are looking for additional data in other contexts that will ultimately resolve the issue. One such example in favor of dark matter comes from observations of how matter is distributed in a cluster of Bullet galaxies, which consists of two colliding galaxies about 3.8 billion light-years from Earth. Another is based on measurements of how light is diverted from (invisible) matter in the cosmic microwave background, the light left over from the Big Bang. They are often seen as indisputable evidence in favor of dark matter, as Milgrom’s original theory could not explain them.
However, following the publication of these results, additional theories of modified gravity have been developed in recent decades to account for all evidence from observations of dark matter, sometimes with great success. Yet the hypothesis of dark matter still remains a favorite explanation of physicists. Why?
One way to understand this is to use the philosophical tools of Bayesian theory for confirmation. This is a probabilistic framework for assessing the extent to which hypotheses are supported by available evidence in different contexts.
In the case of two competing hypotheses, what determines the ultimate 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 occurs in each case (called probability coefficient) .
If we assume that the most complex versions of the modified theory of gravitation and the theory of dark matter are equally supported by the evidence, then the probability ratio is equal to one. This means that the final decision depends on the initial probabilities of these two hypotheses.
Determining what exactly is considered the “initial probability” of a hypothesis and the possible ways in which such probabilities can be determined remains one of the most difficult challenges in Bayesian confirmation theory. And it is here that philosophical analysis proves useful.
At the heart of the philosophical literature on this subject lies the question of whether the initial probabilities of scientific hypotheses should be determined objectively on the basis of probabilistic laws and rational constraints only. Alternatively, they could include a number of additional factors, such as psychological considerations (whether scientists prefer a particular hypothesis based on interest or for sociological or political reasons), basic knowledge, 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 highly preferred by the physics community.
Philosophy, after all, cannot tell us whether astronomers are right or wrong about the existence of dark matter. But it can tell us whether astronomers really have good reason to believe what these reasons are and what it takes to make modified gravity as popular as dark matter.
We do not yet know the exact answers to these questions, but we are working on them. More research in the philosophy of science will give us a better verdict.
This article was originally published on The conversation by Antonis Antoniou of the University of Bristol. Read the original article here.