To discard the theory of dark matter, “you’ll need to replace it with something even more bizarre: a force of gravity that, at some distances, pulls massive objects together and, at other distances, pushes them apart.” That’s how Science magazine describes a new study, adding that “The analysis underscores how hard it is to explain away dark matter” — even though “after decades of trying, physicists haven’t spotted particles of dark matter floating around.” [T]o do away with dark matter, theorists would also need explain away its effects on much larger, cosmological scales. And that is much harder, argues Kris Pardo, a cosmologist at NASA’s Jet Propulsion Laboratory, and David Spergel, a cosmologist at Princeton University. To make their case, they compare the distribution of ordinary matter in the early universe as revealed by measurements of the afterglow of the big bang — the cosmic microwave background (CMB) — with the distribution of the galaxies today…. Pardo and Spergel derived a mathematical function that describes how gravity would have had to work to get from the distribution of ordinary matter revealed by the CMB to the current distribution of the galaxies. They found something striking: That function must swing between positive and negative values, meaning gravity would be attractive at some length scales and repulsive at others, Pardo and Spergel report this week in Physical Review Letters. “And that’s superweird,” Pardo says… In a paper posted in June to the preprint server arXiv, theoretical cosmologists Constantinos Skordis and Tom Zlosnik of the Czech Academy of Sciences present a dark matter-less theory of modified gravity they say jibes with CMB data. To do that, researchers add to a theory like general relativity an additional, tunable field called a scalar field. It has energy, and through Einstein’s equivalence of mass and energy, it can behave like a form of mass. Set things up just right and at large spatial scales, the scalar field interacts only with itself and acts like dark matter… Skordis’s and Zlosnik’s paper is “very exciting,” Pardo says. But he notes that in some sense it merely replaces one mysterious thing — dark matter — with another — a carefully tuned scalar field. Given the complications, Pardo says, “dark matter is kind of the easier explanation.”

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