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Space & Astronomy

New theory of gravity could remove need for dark matter

July 11, 2022 · Comment icon 14 comments

What if dark matter doesn't actually exist ? Image Credit: CC BY 4.0 ESO / S. Brunier
What if we could explain the movement of stars and galaxies without relying on dark matter to fill in the gaps?
Indranil Banik - a postdoctoral research fellow of astrophysics at the University of St Andrews - takes a look at an alternative theory of gravity known as Milgromian dynamics.

We can model the motions of planets in the Solar System quite accurately using Newton's laws of physics. But in the early 1970s, scientists noticed that this didn't work for disc galaxies - stars at their outer edges, far from the gravitational force of all the matter at their centre - were moving much faster than Newton's theory predicted.

This made physicists propose that an invisible substance called "dark matter" was providing extra gravitational pull, causing the stars to speed up - a theory that's become hugely popular. However, in a recent review my colleagues and I suggest that observations across a vast range of scales are much better explained in an alternative theory of gravity proposed by Israeli physicist Mordehai Milgrom in 1982 called Milgromian dynamics or Mond - requiring no invisible matter.

Mond's main postulate is that when gravity becomes very weak, as occurs at the edge of galaxies, it starts behaving differently from Newtonian physics. In this way, it is possible to explain why stars, planets and gas in the outskirts of over 150 galaxies rotate faster than expected based on just their visible mass. But Mond doesn't merely explain such rotation curves, in many cases, it predicts them.

Philosophers of science have argued that this power of prediction makes Mond superior to the standard cosmological model, which proposes there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have a highly uncertain amount of dark matter that depends on details of how the galaxy formed - which we don't always know. This makes it impossible to predict how quickly galaxies should rotate. But such predictions are routinely made with Mond, and so far these have been confirmed.

Imagine that we know the distribution of visible mass in a galaxy but do not yet know its rotation speed. In the standard cosmological model, it would only be possible to say with some confidence that the rotation speed will come out between 100km/s and 300km/s on the outskirts. Mond makes a more definite prediction that the rotation speed must be in the range 180-190km/s.

If observations later reveal a rotation speed of 188km/s, then this is consistent with both theories - but clearly, Mond is preferred. This is a modern version of Occam's razor - that the simplest solution is preferable to more complex ones, in this case that we should explain observations with as few "free parameters" as possible. Free parameters are constants - certain numbers that we must plug into equations to make them work. But they are not given by the theory itself - there's no reason they should have any particular value - so we have to measure them observationally. An example is the gravitation constant, G, in Newton's gravity theory or the amount of dark matter in galaxies within the standard cosmological model.

We introduced a concept known as "theoretical flexibility" to capture the underlying idea of Occam's razor that a theory with more free parameters is consistent with a wider range of data - making it more complex. In our review, we used this concept when testing the standard cosmological model and Mond against various astronomical observations, such as the rotation of galaxies and the motions within galaxy clusters.

Each time, we gave a theoretical flexibility score between -2 and +2. A score of -2 indicates that a model makes a clear, precise prediction without peeking at the data. Conversely, +2 implies "anything goes" - theorists would have been able to fit almost any plausible observational result (because there are so many free parameters). We also rated how well each model matches the observations, with +2 indicating excellent agreement and -2 reserved for observations that clearly show the theory is wrong. We then subtract the theoretical flexibility score from that for the agreement with observations, since matching the data well is good - but being able to fit anything is bad.

A good theory would make clear predictions which are later confirmed, ideally getting a combined score of +4 in many different tests (+2 -(-2) = +4). A bad theory would get a score between 0 and -4 (-2 -(+2)= -4). Precise predictions would fail in this case - these are unlikely to work with the wrong physics.
We found an average score for the standard cosmological model of -0.25 across 32 tests, while Mond achieved an average of +1.69 across 29 tests.

It is immediately apparent that no major problems were identified for Mond, which at least plausibly agrees with all the data (notice that the bottom two rows denoting falsifications are blank in figure 2).

The problems with dark matter

One of the most striking failures of the standard cosmological model relates to "galaxy bars" - rod-shaped bright regions made of stars - that spiral galaxies often have in their central regions (see lead image). The bars rotate over time. If galaxies were embedded in massive halos of dark matter, their bars would slow down. However, most, if not all, observed galaxy bars are fast. This falsifies the standard cosmological model with very high confidence.

Another problem is that the original models that suggested galaxies have dark matter halos made a big mistake - they assumed that the dark matter particles provided gravity to the matter around it, but were not affected by the gravitational pull of the normal matter. This simplified the calculations, but it doesn't reflect reality. When this was taken into account in subsequent simulations it was clear that dark matter halos around galaxies do not reliably explain their properties.

There are many other failures of the standard cosmological model that we investigated in our review, with Mond often able to naturally explain the observations. The reason the standard cosmological model is nevertheless so popular could be down to computational mistakes or limited knowledge about its failures, some of which were discovered quite recently. It could also be due to people's reluctance to tweak a gravity theory that has been so successful in many other areas of physics.

The huge lead of Mond over the standard cosmological model in our study led us to conclude that Mond is strongly favoured by the available observations. While we do not claim that Mond is perfect, we still think it gets the big picture correct - galaxies really do lack dark matter.

Indranil Banik, Postdoctoral Research Fellow of Astrophysics, University of St Andrews

This article is republished from The Conversation under a Creative Commons license.

Read the original article. The Conversation

Source: The Conversation | Comments (14)

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Recent comments on this story
Comment icon #5 Posted by zep73 2 years ago
I very much appreciate your contribution, my friend, but dark matter fails the razor test on several parameters. First off it adds a huge quantity of undetectable matter, that can only be indirectly observed through intricate hypothesised methods, and second of all it demands too many wildcard factors to incorporate in Relativity and predict outcomes. Mond is just an updated math solution, that works excellently with Relativity, and easily predicts outcomes. It slips below the razor with no issues. Just because a hypothesis is consensus favored, it doesn't mean it's right. Once the consensus w... [More]
Comment icon #6 Posted by Manwon Lender 2 years ago
First let me thank you for your kind words I always try to help! Well I am not certain it actually does and neither did Einstein or the Current Scientific Community so I am unwilling to discard it yet. While I am certainly not an expert in this area, I have done some serious reading on the subject and below  what I have discovered and what I believe, if you have something that’s more accurate please send me a link! Dark matter as the Bose–Einstein condensation in loop quantum cosmology published 2016: Continuous Bose–E... [More]
Comment icon #7 Posted by Autistocrates 2 years ago
"Dark Matter" sounds prejudicial against those having cosmopolitan origins. Perhaps, they should refer to it as "Matter of which emits no rays of illumination". That way, nobody's feelings should get hurt
Comment icon #8 Posted by Nutrition Fact 2 years ago
Comment icon #9 Posted by Silver Surfer 2 years ago
Not to be confused with Doesn't matter.
Comment icon #10 Posted by zep73 2 years ago
It's on my to-do list. (Got enough on it to last 100 years...) The community certainly isn't happy about Newtonian solutions in astrophysics, so the skepticism is big. They keep pointing to the Bullet Cluster. But what if they're interpreting the Bullet Cluster wrongly?  If a solution is perfect in any other way, except for one single observation, maybe the observation needs to be re-examined? Well, enough of this. The community has discarded this, so Banik's attempt to solve gravity with Mond has failed.
Comment icon #11 Posted by L.A.T.1961 2 years ago
I consider MOND to be, what is sometimes describes as, an elegant solution. Unfortunately it needs to be more than elegant to do the job.  Which is a shame.  I notice Indranil Banik and others have also looked at explaining why some measurements seems to suggest MOND does not work and why that might be the wrong conclusion. "We present hydrodynamical star-forming simulations in the Milgromian dynamics (MOND) framework of a gas-rich disc galaxy with properties similar to AGC 114905, which has recently been argued to have a rotation curve (RC) that is inconsistent with the MOND prediction." h... [More]
Comment icon #12 Posted by Cookie Monster 2 years ago
It took years to detect the first neutrinos due to their incredibly small size meaning they would rarely hit an atom. One possibility is there are other particles like neutrinos, but they are so small a collision with an atom is extremely rare. Meaning they would be almost impossible to detect. We dont need WIMPs, we just need huge quantities of particles really small, way smaller than the electron, way smaller than the neutrino.
Comment icon #13 Posted by Hyperionxvii 2 years ago
It's just the space between the pixels, this proves the simulation theory, which will now be a law.
Comment icon #14 Posted by joc 2 years ago
So, I have a rhetorical question...what are Time, Distance and Velocity relative to? They are all relative to static points created by the human mind.  In reality, there are no static points because everything is always constantly in motion, constantly changing. I think all of our calculations therefore will always be somewhat skewed because everything is measured from static points that don't actually exist.   thank you

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