The Conversation contributed the article to Space.com's expert voices.
Indranil Banik is a research fellow of astrophysics.
Using the laws of physics, we can model the motions of planets. In the early 1970s, scientists realized that this didn't work for disk galaxies, which were far from the center of the universe.
Physicists propose that an invisible substance called " dark matter" was causing the stars to speed up.
In a recent review, my colleagues and I suggest that observations across a vast range of scales are better explained in an alternative theory of gravity.
Dark matter is related to these questions.
Mond postulates that when gravity becomes very weak at the edge of the universe, it will behave differently than in the past. It is possible to explain why stars, planets and gas in the outskirts of over 150 galaxies are rotating faster than expected by just their visible mass. Mond doesn't just explain rotation curves, it also predicts them.
The philosophers of science argue that the power of prediction makes Mond superior to the standard model of the universe. According to this model, the amount of dark matter in the universe depends on how the galaxy was formed. It's not possible to predict how quickly the universe should move. Mond makes such predictions frequently and so far these have been confirmed.
We know the distribution of visible mass, but we don't know the rotation speed. It would only be possible to say that the rotation speed will be between 100 km/s and 300 km/s on the outskirts. Mond thinks that the rotation speed must be between 180 and 190 km/s.
This is consistent with both theories but Mond is preferred. The simplest solution is preferable to more complex ones and we should explain observations with as few "free parameters" as possible. Free parameters are constant numbers that must be plugged into equations to work. They aren't given by the theory, so we have to measure them. The amount of dark matter in the standard cosmological model is an example.
Theory with more free parameters is consistent with a wider range of data and makes it more complex. We used this concept when testing the standard model and Mond against various observations.
The score was between 2 and +2. A score of 2 means that a model makes a precise prediction without looking at the data. Proponents of +2 would have been able to fit almost any plausible observational result. The models were rated on how well they match the observations, with +2 indicating excellent agreement and -2 for observations that clearly show the theory is wrong. Since being able to fit anything is bad, we subtract the theoretical flexibility score from that.
Predicting a combined score of +4 in many different tests would be a good idea. A bad theory has a score between 0 and -4. These are not likely to work with the wrong physics.
The standard cosmological model scored an average of 0.25 across 32 tests, while Mond scored an average of +69. The scores for each theory are shown in figures 1 and 2 below.
There were no major problems identified for Mond, which at least plausibly agrees with all the data.
One of the most striking failures of the standard model is the fact that spiral galaxies have bright regions in their central regions. Over time, the bars change shape. Bars would slow down if they were in a halo of dark matter. Most, if not all, observed bar are fast. The standard model has very high confidence.
The models that suggested dark matter halos made a mistake because they assumed that the dark matter particles provided gravity to the matter around it, but were not affected by the normal matter. The calculations are simplified but not reflected in reality. It was clear when this was taken into account that dark matter halos don't explain their properties
Mond can explain many of the failures of the standard model in our review. Computational mistakes or limited knowledge about its failures are some of the reasons the standard cosmological model is so popular. It's possible that people's reluctance to tinker with a gravity theory that has been so successful in other areas of physics is the reason.
Mond had a huge lead over the standard model in our study. We don't think Mond is perfect, but we think it gets the big picture right.
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