“Dark matter is interesting. Basically, the Universe is heavier than it should be. There’s whole swathes of stuff we can’t account for.” -Talulah Riley
One of the most puzzling facts about the Universe is that 95% of the energy in it, in the forms of dark matter and dark energy, are completely invisible, and have never been directly detected. Perhaps, the story goes, it’s our theory of gravity that’s to blame, rather than needing new components in the Universe. While dark matter and dark energy can explain a whole slew of observations, gravity modifications do a better job of explaining galactic rotation, but require altering Einstein’s theory of gravity.
The cosmic web is driven by dark matter, with the largest-scale structure set by the expansion rate and dark energy. The small structures along the filaments form by the collapse of normal, electromagnetically-interacting matter. Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).
But merging neutron stars provide a unique test: electromagnetic and gravitational waves both originate from an ultra-distant source over 100 million light years away. The first signals arrive separated by mere seconds, allowing us to constrain models where gravity and light are bent (and delayed) differently by the presence of masses. While theories like Bekenstein’s TeVeS and Moffat’s Scalar-Tensor-Vector predict differing delays by years, the observed arrival time difference was just 1.7s.
The various mass sources in between NGC 4993, where the neutron star-neutron star merger occurred, and the quantified delay that they cause in light/gravitational wave travel time. Image credit: Sibel Boran, Shantanu Desai, Emre Kahya, and Richard Woodard, ‘GW170817 Falsifies Dark Matter Emulators’.
With these new observations, models that attempt to do away with dark matter and dark energy are largely busted, leaving only contrived, non-local modified gravity theories behind. It’s an incredible victory for Einstein and the dark Universe.
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