Unraveling the Mystery: Vera Rubin's Quest for Dark Matter
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In 1969, the American astronomer Vera Rubin puzzled over her observations of the sprawling Andromeda Galaxy, the Milky Way’s biggest neighbour. While mapping out the rotating spiral arms of stars through spectra carefully measured at two different observatories, she had noticed that the stars in the galaxy’s outskirts seemed to be orbiting far too fast. So fast as to leave Andromeda and fling out into the heavens beyond, which they never did. Rubin’s research, which she expanded to dozens of other spiral galaxies, led to a dramatic dilemma: either there was much more matter out there, hidden from sight but holding the galaxies together with its gravitational pull, or gravity somehow works very differently on the vast scale of a galaxy than scientists previously thought.
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Her influential discovery never earned Rubin a Nobel Prize, but scientists began to look for signs of ‘dark’ matter everywhere – among the largest structures in the galaxies in the Universe. By the 1970s, the astrophysicist Simon White at the University of Cambridge argued that the conglomerations of galaxies could be explained using a model in which most of the Universe’s matter is dark, far outweighing all the stars combined. In the following decade, White and others built on that research by simulating the dynamics of hypothetical dark matter particles on the not-so-user-friendly computers of the day.
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But despite those advances, no one has ever directly detected a single particle of dark matter. Physicists searched for dark matter particles with powerful and sensitive tools in places like Antarctica or abandoned mines on other continents; they also tried to produce such particles in particle accelerators. Nothing bore fruit. For a while, physicists hoped to find a theoretical type of matter called weakly interacting massive particles (WIMPs), but they also remained undiscovered. With the WIMP candidacy all but dead, dark matter is apparently the most ubiquitous thing physicists have never found. Therefore, it remains possible that there is no dark matter at all. Which means that, instead of huge amounts of hidden matter, some mysterious aspect of gravity could be warping the cosmos instead.
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The notion that gravity behaves differently on large scales has remained relegated to the fringe since Rubin’s and White’s heyday in the 1970s. But now it’s time to consider the possibility seriously. Scientists and research teams should be encouraged to pursue alternatives to dark matter. Regardless of who turns out to be right, such research on alternatives will ultimately help crystallize the demarcation between what we know and what we don’t. Indeed, as early as in the 1980s, the Israeli physicist Mordehai Milgrom proposed that Isaac Newton’s second law of motion (which describes how the gravitational force acting on an object varies with its acceleration and mass) changes ever so slightly, depending on the object’s acceleration. Naturally, planets such as Neptune or Uranus orbiting our sun, or stars orbiting close to the centre of our galaxy, do not display the difference. But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational pull than previously thought from the bulk of matter in the galaxy; adjusting Newton’s law could provide an explanation for the speeds Rubin measured, without needing to invoke dark matter.
