The name in itself holds the poetic flavour of unsolved enigma. Dark matter has fascinated late twentieth century astrophysicists to the point that it has actually made the jump into the imagination of contemporary popular culture. But do we know for sure it is real? The hypothesis certainly does help resolve a number of problems in cosmology, however some astrophysicists still doubt it exists.
Research shows no evidence of unknown, invisible matter around “early-type” galaxies.
source: NASA (The Hubble telescope)
The first mention of dark matter dates back to 1932-33 when Dutch astronomer Jan Oort and Swiss American astronomer Fritz Zwicky discovered, around the same time, that the amount of visible matter was not sufficient to account for the mass necessary to explain the gravitational pull of a spiral galaxy nor the motion of galaxies in clusters. They concluded that there must be another type of invisible substance inside or around the galaxies to explain the difference in mass: the famous dark matter.
With the discovery of the first gravitational lens, the early 1980s saw the real emergence of the dark matter hypothesis, enabling a better estimate of the mass of galaxies. An estimate confirmed and further refined by the analysis of the cosmic microwave background radiation. The latest estimations by astrophysicists attributes to this hypothetical, invisible matter to more than 80% of the total mass of the universe, making it a missing piece of considerable importance, indeed.
The belief in dark matter is widely spread among astronomers. Very few doubt its existence, and many have based their life’s work on the subject. But what if the theory were misguided? The answer too convenient to be true? In this sense, the recent work of two Belgian astrophysicists from the University of Liege, Pierre Magain and Virginie Chantry, [available on MyScienceWork] throws into question the very foundations of the theory.
No evidence of extended dark matter halos around early-type galaxies
The study uses the mass measuring technique of gravitational lensing to assess the assumption of large-scale dark matter halos surrounding galaxies. They concentrated their research on measuring the mass of lenticular and elliptical or so-called early-type galaxies, as opposed to late-type spiral galaxies like our own. (The “early” and “late” designation does not, in fact, designate any kind of evolutionary connection between them, but only their position on the Hubble sequence.) They evaluated the probability of invisible, exotic matter in these galaxies and what they found is that their gravitational pull indicates a mass no more than twice the estimated mass of the luminous, visible matter. The authors believe that this is due to the fact that hidden but nonetheless ordinary (baryonic) matter "might be found in galaxies with a distribution comparable with that of luminous matter”. In other words, nothing exotic there.
The Hubble sequence showing the different types of galaxies
source: Wikipedia Commons
These results could be argued to concern only “early-type” galaxies, particularly because one of the main pieces of evidence for dark matter originates from the rotation curves of spiral galaxies. The study, however, refutes this argument by highlighting the fact that massive elliptical galaxies are generally considered to be the result of the fusion of two or more spiral (later-type) galaxies, “It is thus hard to understand how dark matter halos would be present around spirals and absent after their fusion".
Could there be another explanation for cosmological observations?
Pierre Magain and Virginie Chantry’s work is not the only one, these past few years, to have aimed at bringing to light the failures of the dark matter theory. Pavel Kroupa, astrophysicist at the University of Bonn, wrote a paper called The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology in which he clearly claims that dark matter cannot be real and that “other theoretical solutions to cosmological observations exist”. For instance, it is possible that we do not understand gravity in all of its implications. Pierre Magain believes, for his part, that there might be “a theory that goes beyond Einstein’s theory of relativity, the way that theory goes beyond Newton’s.”
This anti-dark matter movement, as one could call it, perceives the dark matter theory as what could be an overly convenient explanation for cosmologists. Pierre Magain makes the playful comparison that much like fertilizers in agriculture, dark matter's danger lies in the fact that it might seem like a good idea and make the work easier at first, but turn out to be misguided and to create more problems than it solves.
Caution is the watchword in science
James Bartlett, astrophysicist at the University of Paris Diderot, stresses the difficulty, when it comes to scientific research, of not anticipating general conclusions from specific studies. “The choice of the object(s) of the study is fundamental and can result in very different, if not conflicting results” he says. “As serious as this study is, someone else could point a telescope in the same direction and come to a different conclusion.” Just such an example relating to the present research can be found here.
Nothing can be taken for granted in Science, until irrefutably proven (that is, if, philosophically, you even think this is possible). Until that time, science is in a perpetual state of calling previous findings into question. As Karl Popper aptly puts it, “If we are uncritical we shall always find what we want … and we shall look away from, and not see, whatever might be dangerous to our pet theories.” The current state of research in the dark matter field does not allow for the fixed assumption of the existence of dark matter. It constitutes “only”, but nonetheless, a significant step towards the understanding the workings of the Cosmos.