In the early 1980s, the Israeli physicist Mordehai 'Moti' Milgrom questioned the increasingly popular dark matter narrative. While working at an institute south of Tel Aviv, he studied measurements by Rubin and others, and proposed that physicists hadn't been missing matter; instead, they'd been wrongly assuming that they completely understood how gravity works. Since the outer stars and gas clouds orbit galaxies much faster than expected, it makes more sense to try to correct the standard view of gravity than to conjure an entirely new kind of matter.
Milgrom proposed that Isaac Newton's second law of motion (describing how the gravitational force acting on an object varies with its acceleration and mass) changes ever so slightly, depending on the object's acceleration. Planets such as Neptune or Uranus orbiting our sun, or stars orbiting close to the centre of our galaxy, don't feel the difference. But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational force than previously thought from the bulk of matter in the galaxy; adjusting Newton's law could provide an explanation for the speeds Rubin measured, without needing to invoke dark matter.
Developing the paradigm of a dark-matter-less universe became Milgrom's life project. At first, he worked mostly in isolation on his proto-theory, which he called Modified Newtonian Dynamics (MOND). 'For more than a few years, I was the only one,' he says. But slowly other scientists circled round.
He and a handful of others first focused on rotating galaxies, where MOND accurately describes what Rubin observed at least as well as dark matter theories do. Milgrom and colleagues then expanded the scope of their research, predicting a relationship between how fast the outside of a galaxy rotates and the galaxy's total mass, minus any dark matter. The astronomers R Brent Tully and J Richard Fisher measured and confirmed just such a trend, which many dark matter models have struggled to explain.
Despite these successes, Milgrom's modification of Newton's second law remained just an approximation, causing his ideas to fall short of requirements for a full-fledged theory. [...]
MOND lacked much of a foundation until a few years ago, when the Dutch physicist Erik Verlinde began developing a theory known as 'emergent gravity' to explain why gravity was altered. In Verlinde's view, gravity, including MOND, emerges as a kind of thermodynamic effect, related to increasing entropy or disorder. His ideas build on quantum physics as well, viewing space-time and the matter within it as originating from an interconnected array of quantum bits. When space-time gets curved, it produces gravity, and if it's curved in a particular way, it creates the illusion of dark matter.
Today's seeming dominance of dark matter wasn't inevitable. The processes through which scientists develop theories are heavily influenced by all sorts of historical and sociological factors, a point eloquently made by Andrew Pickering, emeritus philosopher of science at the University of Exeter and the author of Constructing Quarks (1984), a 36-year-old book that's still relevant today.
It's important to pay attention to who decides which phenomena to study, which research earns major government grants, which big experiments get funded, who gets speaking opportunities at scientific conferences, who is media savvy, who wins prominent fellowships and awards, and who gets promoted to high-profile faculty positions. Different choices sometimes can shape the future trajectory of science. And when choices by theorists and experimentalists coincide symbiotically, Pickering argues, it can be challenging for an upstart theory - such as modified gravity - to get a fair hearing.
The astronomers Arno Penzias and Robert Wilson in the 1960s at first misinterpreted their radio telescope's faint static as noise - perhaps due to pigeons roosting and leaving droppings on it. But the signal turned out to be real, and they confirmed their discovery of relic radio waves that date back to soon after the Big Bang. Then in the 1980s and '90s, Soviet and NASA scientists used their own space telescopes, RELIKT-1 and COBE, to spot tiny wiggles in that radiation. John Mather and George Smoot, the physicists who led the COBE research, won the Nobel Prize in Physics 2006 for measuring those little radiation variations, which translate into early density differences that determined where the matter in the Universe collected and structures of galaxies formed.
Mather and Smoot's successors have now measured the wiggles in relic radio radiation to exquisite precision, and any successful theory has to offer an explanation of them. Dark matter physicists have already shown that their theory could reproduce all of those wiggles quite well, but modified or emergent gravity has failed that critical test - so far. Bekenstein died in 2015, but his successors are still trying to make his modified gravity theory consistent with at least some of the measurements. That would be a big leap forward and a compelling one for skeptics of modified gravity, but it's a major task that has yet to be done.
