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A view of the center of the Milky Way galaxy. Theories of modified gravity have had a hard time describing the universe from relatively small scales like this all the way up to the scale of the universe as a whole. NASA/JPL-Caltech/ESA/CXC/STScI

Modified gravity theories have never been able to describe the universe’s first light. A new formulation does.

Charlie Wood July 28, 2020

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 5-to-1, 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 seek an alternative cosmic recipe. In place of dark matter, they substitute 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 formulations get galaxies right, some get the contortion of light rays right, but none have pierced dark matter’s most bulletproof piece of evidence: precise maps of ancient light, known as the cosmic microwave background (CMB). “A theory must do really well to agree with this data,” said Ruth Durrer, a cosmologist at the University of Geneva. “This is the bottleneck.”

 

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