Has dogma derailed the scientific search for dark matter?
Pavel Kroupa is professor of astrophysics at University of Bonn, in Germany, where he heads the Stellar Populations and Dynamics research group. His research interests include the nature of dark matter and planetary, stellar, and galactic dynamics. He lives in Germany.
According to mainstream researchers, the vast majority of the matter in the Universe is invisible: it consists of dark-matter particles that do not interact with radiation and cannot be seen through any telescope. The case for dark matter is regarded as so overwhelming that its existence is often reported as fact. Lately, though, cracks of doubt have started to appear. In July, the LUX experiment in South Dakota came up empty in its search for dark particles – the latest failure in a planet-wide, decades-long effort to find them. Some cosmic surveys also suggest that dark particles cannot be there, which is especially confounding since astronomical observations were the original impetus for the dark-matter hypothesis.
The issues at stake are huge. Acceptance of dark matter has influenced scientific thinking about the birth of the Universe, the evolution of galaxies and black holes, and the fundamental laws of physics. Yet even within academic circles, there is a lot of confusion about dark matter, with evidence and interpretation often conflated in misleading and unproductive ways.
The modern argument for dark matter begins with the assumption that the Universe is described by Albert Einstein’s field equation of general relativity, and that Newtonian gravitation (that is, gravity as we measure it on Earth) is valid in all places at all times. It further assumes that all the matter in the Universe was produced at the Big Bang. Simulations based on that scenario make specific predictions about how quickly cosmic structures form, and also about the motions of galaxies and stars within galaxies. When compared with observations, those simulations indicate that gravitational effects in the real world must be stronger than can be accounted for by the matter we know. Dark matter provides the additional gravitational pull to bring model and reality broadly into alignment. Researchers now routinely take this model – Einstein plus dark matter, often called the ‘null hypothesis’ – as their starting point and then perform detailed calculations of galactic systems to test it.
This is how I stumbled into the field in the late 1990s. I was studying the dynamics of small satellite galaxies as they orbit our galaxy, the Milky Way. From observation, we expected that these satellite galaxies must contain a lot of dark matter, from 10 to 1,000 times as much as their visible, normal matter. During my calculations, I made a perplexing discovery. My simulations produced satellite galaxies that look much like the ones actually observed, but they contained no dark matter. It seemed that observers had made wrong assumptions about the way the stars move within the satellite galaxies; dark matter was not required to explain their structures.
I published these results and quickly learned what it meant to not follow the mainstream. Despite the critiques I received, I followed up on these results some years later and uncovered another major inconsistency. The known satellite galaxies of the Milky Way are distributed in a vast polar disk running perpendicular to the orientation of our galaxy. But dark-matter dominated models predict that primordial dwarf galaxies should have fallen into the Milky Way from random directions, so should follow a spheroidal distribution. This finding set off a major debate, with the mainstream researchers arguing that this disk of satellites does not really exist; that it is not significant; or that it cannot be used to test models.
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