Do We Need to Revise General Relativity?
The idea that our Universe is filled with dark matter has been around for nearly a century. When astronomers noticed that orbital speeds towards the edges of spiral galaxies remain the same or even increase slightly, rather than decrease, they surmised that either there must be some huge unseen mass driving the rotation, or that the laws of gravity given by Einstein's General Relativity need to be changed. They elected the first option.
Over that time, cosmologists have accumulated boatloads of evidence in favor of the notion that this invisible, "dark" matter -- which neither interacts with nor emits light -- comprises roughly 84% of the mass of the Universe. So compelling is this story that millions and millions of dollars have been spent on ingenious experiments to actually detect the stuff, but thus far, the particles have remained elusive.
It is partly because of dark matter's inherent ability to not be found that, in 1983, Israeli physicist Mordehai Milgrom proposed an upstart theory to challenge its dominance. Modified Newtonian dynamics, or MOND for short, dares to go where physicists of the past dared not: It slightly tweaks the laws of gravity put forth by Einstein's General Relativity. While the changes are subtle, only affecting Einstein's equations at very low accelerations, the ramifications are massive. General Relativity has remained essentially unscathed for over a century.
And yet MOND matches its audacity with surprising veracity. It successfully accounts for galaxy rotation curves just as well, and in some cases, even a little bit better than dark matter. Moreover, no evidence has come to light that conclusively disproves MOND. That's quite an accomplishment, as the annals of physics are littered with the corpses of theories that challenged General Relativity and failed.
"The idea is sound," cosmologist Ethan Siegel writes in his recent book Beyond the Galaxy. "Surely hypothesizing that 80-85% of the matter in the Universe is of some hitherto undiscovered type... represents a greater leap than making a tweak to our theory of gravity. After all, tweaking our theory of gravity to explain Mercury's orbital motion was what led to General Relativity in the first place!"
But, as Siegel notes, full-fledged cosmological theories built from MOND cannot fully account for many findings arising from the theory of dark matter.
"Gravitational lensing, the cosmic web of structure, and cosmic microwave background observations all go unexplained in all the modified gravity theories put forth so far."
"A compelling physical basis for MOND is still lacking. But then, it took Newton twenty years to realize there was a good geometric reason for the inverse square law, and centuries to develop our modern understanding of gravity. These things only seem crystal clear with the benefit of hindsight. So it no doubt shall be with MOND, whatever the underlying physics."
As Sabine Hossenfelder reported last year, one potential way to test MOND could soon become available. According to the modified gravity theory, an offshoot of MOND, a black hole's shadow should be ten times larger compared to what general relativity predicts. The Event Horizon Telescope aims to image a black hole and its shadow for the first time in 2017.
Siegel succinctly sums MOND's current scientific standing in his book.
"MOND remains an attractive avenue of investigation, as it is still more successful at predicting the rotation curves of individual galaxies, overall, than the theory of dark matter is. But its failure to meet the criteria of reproducing the successes of the already-established leading theory means that it has not yet risen to the status of scientifically viable."
When it comes to MOND, McGaugh is a strict adherent to empiricism, but he also has a flair for the philosophical.
"Is our universe an unfamiliar darkness filled with invisible mass, with the 'normal' matter of which we are composed no more than a bit of queer flotsam in a vast sea of dark matter and dark energy? Or is our inference of these dark components just a hint of our ignorance of some deeper theory?"
"Ultimately, what we want is irrelevant. Science is not a consensus endeavor: the data rule."
Primary Source: Ethan Siegel. Beyond the Galaxy: How Humanity Looked Beyond Our Milky Way and Discovered the Entire Universe. 2015. World Scientific.