For decades, a band of rebel theorists has waged war with one of cosmology’s core concepts—the idea that an invisible, intangible form of matter forms the universe’s primary structure. This dark matter, which seems to outweigh the stuff we’re made of five to one, accounts for a host of observations: the tight cohesion of galaxies and packs of galaxies, the way light from faraway galaxies will bend on its way to terrestrial telescopes, and the mottled structure of the early universe, to name a few.
The would-be revolutionaries are seeking an alternative cosmic recipe. In place of dark matter, they’re substituting a subtly modified force of gravity. But attempts to translate their rough idea into precise mathematical language have always run afoul of at least one key observation. Some of their formulations get galaxies right; some get the contortion of light rays right. Yet none have pierced dark matter’s most bulletproof piece of evidence: precise maps of ancient light, known as the CMB, or the cosmic microwave background. “A theory must do really well to agree with this data,” says Ruth Durrer, a cosmologist at the University of Geneva. “This is the bottleneck.”
Now, theorists Tom Złosnik and Constantinos Skordis, from the Central European Institute for Cosmology and Fundamental Physics, say they’ve finally squeezed an alternative theory of gravity past that obstacle. Their work—which was posted online in late June as a preprint and has not yet passed peer review—uses a tweaked version of Einstein’s theory of gravity to reproduce an iconic map of the early universe, a feat that even some rebels feared to be impossible. “For 15 years we’ve just been dead in the water,” says Stacy McGaugh, an astronomer at Case Western Reserve University and a longtime advocate for modified-gravity theories who wasn’t involved in the research. “It’s a huge leap forward.”