It’s supposed to be the most common form of matter in the universe, but nobody has ever actually seen it.
It has been more than 50 years since astronomers first proposed “dark matter”, which is thought to be the most common form of matter in the universe. Despite this, we have no idea what it is – nobody has directly seen it or produced it in the lab.
So how can scientists be so sure it exists? Should they be? It turns out philosophy can help us answer these questions.
Back in the 1970s, a seminal study by astronomers Vera Rubin and Kent Ford of how our neighbour galaxy Andromeda rotates 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 rotate slower and slower the further away they are from the galaxy’s centre. That’s because most of the stars will be near the centre, creating a strong gravitational force there.
Rubin and Ford’s result showed that this wasn’t the case. Stars on the outer edge of the galaxy moved about as fast as the stars around its centre.
The idea that the galaxy must be embedded in a large halo of dark matter was basically proposed to explain this anomaly (though others had suggested it previously). This invisible mass interacts with the outer stars through gravity to boost their velocities.
This is only one example of several anomalies in cosmological observations. However, most of these can be equally explained by tweaking the current gravitational laws of Newtonian dynamics and Einstein’s theory of general relativity. Perhaps nature behaves slightly differently on certain scales than these theories predict?
One of the first such theories, developed by Israeli physicist Mordehai Milgrom in 1983, suggested that Newtonian laws may work slightly differently when there’s extremely small acceleration involved, such as at the edge of galaxies. This tweak was perfectly compatible with the observed galactic rotation.
Nevertheless, physicists today overwhelmingly favour the dark matter hypothesis incorporated in the so-called CDM model (Lambda Cold Dark Matter).
This view is so strongly entrenched in physics that is widely referred to as 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 might wonder whether we have good reasons to prefer one over another.
Why does the scientific community have a strong preference for the dark mater explanation over modified gravity? And how can we ever decide which of the two explanations is the correct one?
Philosophy to the rescue
This is an example of what philosophers call “underdetermination of scientific theory” by the available evidence. This describes any situation in which the available evidence may be insufficient to determine what beliefs we should hold in response to it. It is a problem that has puzzled philosophers of science for a long time.
In the case of the 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 gravitational laws are incorrect.
Scientists therefore look for additional data in other contexts that will eventually settle the question. One such example in favour of dark matter comes from the observations of how matter is distributed in the Bullet cluster of galaxies, 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 over from the big bang. These are often seen as indisputable evidence in favour of dark matter because due Milgrom’s initial theory can’t explain them.
However, following the publication of these results, further theories of modified gravity have been developed during the last decades in order to account for all the observational evidence for dark matter, sometimes with great success. Yet, the dark matter hypothesis still remains the favourite explanation of physicists. Why?
One way to understand it is to employ the philosophical tools of Bayesian confirmation theory. This is a probabilistic framework for estimating the degree to which hypotheses are supported by the 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 evidence) and the ratio of the probabilities that the evidence appears in each case (called the likelihood ratio).
If we accept that the most sophisticated versions of modified gravity and dark matter theory are equally supported by the evidence, then the likelihood ratio is equal to one. That means the final decision depends on the initial probabilities of these two hypotheses.
Determining what exactly counts as 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 where philosophical analysis turns out to be useful.
At the heart of the philosophical literature on this topic lies the question of whether the initial probabilities of scientific hypotheses should be objectively determined based solely on probabilistic laws and rational constraints.
Alternatively, they could involve a number of additional factors, such as psychological considerations (whether scientists are favouring a particular hypothesis based on interest or for sociological or political reasons), background knowledge, the success of a scientific theory in other domains, and so on.
Identifying these factors will ultimately help us understand the reasons why dark matter theory is overwhelmingly favoured 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 whether astronomers do indeed have good reasons to believe in it, what these 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.
Antonis Antoniou, PhD candidate in Philosophy of Science, University of Bristol
‘October Surprise’: Russia To Launch Nukes in Space
The ‘national security threat’ announced on Wednesday is
about Russia planning to launch nuclear weapons in space, causing some
to speculate whether it’s really an election year ploy.
The panic began when House Intelligence Committee Chair Mike Turner
(R-Ohio) asked President Biden to declassify information about a
“serious national security threat”.
Modernity.news reports: The weapon would reportedly be designed to be used to take out satellites.
Speaker Mike Johnson (R-La.) responded by telling reporters he wanted “to assure the American people, there is no need for public alarm.”
The big, scary threat is serious business and involves a space-based nuke controlled by evil dictator Putin, but it’s also “not an immediate crisis,” according to what three members of the U.S. House Intelligence Committee have told Politico.
Okay, then. Just for election season, is it?
Zero Hedge reports: “So, the question is – was this:
a) a distraction from Biden’s broken brain, or
2) a last desperate attempt to get more funding for anything-but-the-US-border, or
iii) a path to pitching Putin as the uber-bad-guy again after his interview with Tucker Carlson.”
Just by coincidence, Mike Turner recently returned from Ukraine having lobbied for billions more in weapons and aid for Zelensky’s government.
Some questioned the timing, suggesting it might all be a deep state plot to keep American voters afraid when they hit the ballot box.
Speculation will now rage as to whether this is “the event,” real or imagined, that billionaires and elitists the world over have been building underground survival bunkers in preparation for.
Earth has built-in protection from asteroids
Asteroids are not just wandering space rocks, but a potential threat
to Earth. But what if the Earth already has its own built-in defenses
against them? Recent research published on the preprint server arXiv puts forward an unusual theory: Earth’s gravitational forces may serve as its secret shield against asteroids.
planet uses powerful gravitational interactions with other celestial
bodies to break apart asteroids that approach it. These tidal forces,
akin to those that explain Earth’s tides caused by the Moon, can be so
intense that objects undergo tidal disruption, causing them to be torn
Observations of fragments of Comet Shoemaker-Levy 9 after
its collision with Jupiter in 1994 provided the first confirmation of
this phenomenon. However, for decades astronomers have been looking for
evidence that Earth or other terrestrial planets could have a similar
effect on asteroids and comets.
Planetary scientist Mikael Granvik
from the Swedish University of Technology, Luleå, led the research that
came closer to solving the above phenomenon.
discovery is linked to the search for gravitationally disrupted
near-Earth asteroids (NEAS), and provides compelling evidence that our
planet’s gravitational forces are not just an abstract concept, but a
factor capable of breaking asteroids into small pieces.
modeling of asteroid trajectories, Grunwick and colleague Kevin Walsh of
the Southwest Research Institute found that collisions with rocky
planets can cause asteroids to lose a significant portion of their mass,
turning them into debris streams.
New data shows that small
asteroid fragments, while not posing a threat to life on the planet, may
nevertheless increase the likelihood of local collisions like those
that occurred in Tunguska and Chelyabinsk.
Granwick assures that
asteroids smaller than 1 km in diameter are not a critical threat, but
increase the likelihood of incidents. However, it is worth remembering
the additional risks that may arise due to the formation of new debris
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