What Even is Gravity? Seriously, Are We Missing Something?

May 19, 2021 | Daily Space, Dark Matter, Galaxies, Physics

There are actually a lot of really cool theories out there involving black holes, gravity, the expansion of the universe, and other really awesome ideas that make me glad that I studied planetary science because some of those ideas seem designed to trigger a migraine.

IMAGE: ESO 325-G004, shown here, is a giant elliptical galaxy. A new study explores a sample of ellipticals to better understand dark matter and alternative theories. CREDIT: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); J. Blakeslee (Washington State University)

Gravity, for instance, every time we think we finally understand it, someone comes along and throws out an idea that just might be a better match to reality than we have right now. It all started with Newton. He watched that mythical apple fall, looked to the Moon, and realized it’s also falling but just missing the Earth, and a whole lot of math later, including the development of calculus, he had nice sets of equations that allowed accurate predictions of both how objects would fall on Earth and how planets should orbit the Sun. 

And, for the most part, those equations matched reality. But there was a problem: Mercury. Mercury’s orbit was a few seconds off, and as the years went by and technology got better, it became impossible to ignore that Mercury just didn’t quite match. This discrepancy was finally explained away when Einstein’s theories of relativity clarified that time and space twist to force the speed of light to always appear the same. Explaining Mercury was just one of the myriad observational pieces of evidence we have saying that relativity works.

But, and there is always that but, Newton’s theories work at lower speeds and in lower gravity environments than what Mercury experiences. Einstein’s theories also have their breaking point; specifically, they don’t work at quantum scales, and everything breaks down inside a black hole. This means we still have more to learn.

When Vera Rubin discovered in the middle of the last century that the outer parts of our galaxy just don’t orbit at the expected velocities, folks looked at two different explanations. The one that would go on to be most accepted is the idea of dark matter. We can explain galaxy rotations, the orbits of galaxies in clusters, and the bending of light as it travels through space by saying there is an unobserved kind of matter that we named dark matter. Literally, we explain the odd variations in orbits by adding invisible stuff to the universe.

But dark matter wasn’t the only theory. Many researchers have also chased additional terms to add to gravity, factors that only kick in at huge distances. Called Modified Newtonian Dynamics or MOND, these theories can be made to work in all manner of situations without requiring invisible matter.

For the most part, people stopped studying MOND when it became possible to measure the distribution of invisible matter by its gravitational pull on light. But some folks kept on working on MOND, and in a new paper in The Astrophysical Journal Letters, researchers led by Kyu-Hyun Chae propose a new version of MOND that is able to explain in a consistent way both the observed motions in spiral and elliptical galaxies. This is important because these two kinds of systems have very different gravitational fields to go with their very different shapes.

If this team is right, or at least on the right path, it could indicate that the universe has once again decided to follow the rules of improv and say “Yes, and”. It could be that there is both an additional term to gravity and there is weird, pretty hard to detect stuff out there which we will call dark matter. It’s possible, maybe, that both are true.

Now, to be clear, we can explain everything we see today with just dark matter. We don’t require modified gravity to solve rotation curves; we just know gravity is missing something, and this is an interesting road to follow just in case it does allow us to find those missing pieces.

More Information

Searching for Universal Acceleration (AAS Nova)

On the Presence of a Universal Acceleration Scale in Elliptical Galaxies,” Kyu-Hyun Chae, Mariangela Bernardi, Helena Domínguez Sánchez, and Ravi K. Sheth, 2020 November 6, The Astrophysical Journal Letters

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