"Modified gravity" could rule out both dark matter and Planet Nine

"Modified gravity" could rule out both dark matter and Planet Nine
An artist's rendering of the hypothetical Planet Nine
An artist's rendering of the hypothetical Planet Nine
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An artist's rendering of the hypothetical Planet Nine
An artist's rendering of the hypothetical Planet Nine

Our solar system officially houses eight planets, but some scientists say there could be a ninth. And that’s not just Pluto aficionados – evidence suggests a huge undiscovered world lurks on the dark fringes out there. Now, a new study has found the outer solar system oddities could be explained by modified theories of gravity, an alternative idea to dark matter.

In the 19th century, astronomers measuring the orbit of Uranus noticed some inconsistencies between observations and predictions, and concluded that it was being influenced by the gravity of a large unseen body. Sure enough, the planet Neptune was soon discovered as a result.

In 2016 astronomers made a similar prediction: based on the bizarre orbital patterns of six icy objects in the Kuiper belt, an unknown planet with the mass of about 10 Earths could be tugging on them from the shadows. Further evidence from other objects and even the Sun’s tilt seemed to strengthen the case.

Other scientists, however, have put forward alternative explanations, including gravitational “bumper car” interactions between planets, a distant disc of rock and ice, and even a mini black hole. And now, a team has proposed an even wilder idea – modifying the theory of gravity itself.

It’s not as outrageous a claim as it might sound. While Newton’s law of universal gravitation holds up pretty well to explain the large-scale structure and motion of the universe, it does fall short in some situations. For the better part of a century, dark matter has been the favored plug to fill the holes Newton’s law leaves in our cosmological models. This mysterious substance is thought to pervade the universe, not reflecting or emitting any light and only interacting with regular matter through its strong gravity.

As widely accepted as it is, dark matter has never been directly detected despite many experiments searching for it. Some scientists suggest that instead, we may need to adjust Newton’s law of gravity. For instance, gravity’s effects may be stronger at low accelerations than Newton described, which would cancel out the need for dark matter. This model is known as Modified Newtonian dynamics (MOND), and evidence supporting it has recently been discovered in star clusters and over 150 galaxies.

In a new study, theoretical physicists have applied MOND to the dataset used in the hunt for Planet Nine. According to their model, if MOND is at work then the orbits of some objects in the outer solar system should, over millions of years, move into alignment with the Milky Way galaxy’s gravitational field. And to the team’s surprise, they found that the clustering of orbits for these objects exactly matched the observations in our solar system.

“The alignment was striking,” said Harsh Mathur, co-author of the study. “MOND is really good at explaining galactic-scale observations, but I hadn’t expected that it would have noticeable effects on the outer solar system.”

As intriguing as the study is, the team acknowledges that the dataset is rather small, and that there are still plenty of other possible explanations. In the meantime, the hunts for both dark matter and Planet Nine will likely continue.

“Regardless of the outcome, this work highlights the potential for the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems of physics,” said Katherine Brown, co-author of the study.

The research was published in The Astronomical Journal.

Source: Case Western Reserve University

Like most scientific breakthroughs, MOND is an attractive hypothesis but......when will we give it first place in the adjustment of Newton's gravitational theory?
Here's an idea I have. Gravity as we understand it attracts matter/mass towards other matter/mass. The opposite effect of the expanding universe. It might point to the reverse affect of time on matter in our universe. Each bit of matter has a center of gravity, as do the Black Holes we observe. Galaxies have a center of gravity and most appear as rotational, with spiral arms radiating away from the center. What does that mean? Let's say time in one direction, in our sense of it, the positive direction, from past to future, is a rotational force that passes from a center of gravity into our universe. Let's suppose Negative time reverses the process, but not from future to past. It only reverses the tendency for matter to head outwards towards entropy. Let's suppose the fields of energy in Space, operating at the speed of light or less, include all matter. I think that energy operating outside our universe may operate in Zero Time, and Faster Than Light, where positive and negative time are in balance, and change directions. That would result in the event horizon surrounding a black hole. It could also indicate transition points that result in gravity in all matter.
Ok... Gravity is an energy wave, just like everything else. It has a wave length of 50 billion to 5 trillion light years. It has properties of wave and "particle", although we all know there are no particles at all. The particle property is the Dark Matter. We exist in a HUGE particle of gravity. It's that particle property that limits the speed of light. The closer you get to the size of the "particle" of gravity, the more it acts as a unit, which explains the problem with the galaxies rotation problem. BTW, the wave property explains Dark Energy. Yes, our local universe is currently expanding and the rate is increasing, but it will reach equilibrium, and start back the other way, billions, if not trillions of years from now. Black Holes just recirculate everything back to the wave, and distributes at least some locally, which is why galaxies with feeding black holes are always brighter. This also suggests that gravity is actually a pushing force, rather than an attracting force... which would explain a lot too, but that's another conversation, as is, how Consciousness shapes that gravity particle property into what WE perceive as reality:) Infinite Wave Theory. It's so simple, you can't see it:)
@Karmudjun In fact, there is no need to change the gravitational theory as those new theory have at least for the moment only some effects at a galaxy size. The Einstein theory has clear effects inside the solar system. It was used to explain the movement of mercury at the time. And it has noticeable effects in space navigation. For instance GPS needs to take it into account because of the speed of the satellite.

If we discover that the MOND theory has useful applications, it could begin to be used and accepted maybe for missions in the outer solar system or future missions trying to get out of the solar system. Until that, current theories are sufficient for all known applications.
Pierre Collet
Is it worth noting that in between Newton's law and MOND, Einstein's general relativity has been explaining the precession of Mercury's orbit and why black holes are... black?
Newton's law (and its "improvements") would require gravitons to exit, but unfortunately, they are as elusive as black matter...
While MOND might explain the orbital clustering of Trans-Neptunian objects, it doesn’t necessarily account for other observations in our solar system, like the Cassini spacecraft’s measurements of Saturn’s orbit, which align more closely with predictions based on general relativity and the presence of dark matter.