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Researchers pursuing an unconventional view of cosmology that dispenses with dark matter have developed a model that can match observations of the cosmic microwave background (CMB), the leftover glow of the big bang [1]. This dark-matter-free model is an extension of the so-called MOND (modified Newtonian dynamics) theory, which assumes that the gravitational force on galaxy scales is different from the standard Newtonian force. Previous MOND-based models could not reproduce the CMB. The researchers say that their model can be further tested with observations of galaxy clusters and gravitational waves.

The MOND theory was devised more than 30 years ago as a way to explain galactic rotation data without invoking the existence of the mysterious dark matter [2]. MOND proponents offered an alternative mystery in which the gravitational force changes for accelerations smaller than a threshold of 1010m/s2. The idea did not spring from any underlying theory, but surprisingly, the same acceleration threshold works for nearly all galaxies—small and large, young and old.

The main reason that dark matter has been favored over MOND is that dark matter is consistent with a much larger range of astrophysical observations. For example, dark matter can explain galaxies’ bending of light from distant sources (gravitational lensing), whereas MOND in its initial form could not. Researchers have devised so-called relativistic MOND models that can fit the lensing observations [3], but until now, none of these revised versions of the theory were able to reproduce CMB data. “If the theory can’t do that, then it’s not worth considering further,” says Constantinos Skordis from the Czech Academy of Sciences in Prague.

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