Confidence in Science; more Dark Matter musings

In April of 2012, a team of astronomers at th European Southern Observatory, lead by Christian Moni Bidin of the Universidad de Concepción in Chile, published a paper claiming that there was no gravitational evidence of Dark Matter within about 13,000 ly of the Sun (“In conclusion, the observations point to a lack of Galactic DM at the solar position, contrary to the expectations of all the current models of Galactic mass distribution”). The paper came with a rather confident Press Release. Now I don’t mean to disparage the research. The idea was a very clever one, attempting to detect the gravitational effects of nearby Dark Matter by looking at the orbits of stars around the galactic center. But when a Press Release, and especially a confident Press Release, comes at the same time a paper is made public, I tend to have misgivings about the actual strength of the research. Indeed, in this case, the ESO paper’s results depended on several assumptions, including one about the expected orbital velocities of stars above the galactic plane.

Within a few months of the ESO paper, Jo Bovy and Scott Tremaine from the Institute for Advanced Study published a paper questioning the assumption about the velocities of stars away from the galactic plane. The Bovy and Tremaine paper is generally agreed to be correct, and when the ESO paper’s analysis was redone with the orbital velocities corrected using a more data-driven method suggested by Bovy and Tremaine, the Dark Matter reappeared.

The moral of the story? Even really smart researchers make mistakes. The ESO paper was peer-reviewed and accepted for publication, but peer-review isn’t a guarantee of correctness. Extraordinary claims require extraordinary evidence, and it is always wise to be cautious when strong claims (Dark Matter exists! No, we’ve ruled it out near the Sun!) are made. Let a lot of smart people think over the analysis for a bit, even if the research is peer-reviewed.

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