The Mond Theory Explains Open Star Clusters Better Than Newton’s Gravitation

The more we study the galaxies, the more an alternative to the existence of dark matter particles becomes credible while the latter still elude on Earth the increasingly efficient detectors for its hunting. The Mond theory, which modifies the laws of gravitation, has thus just scored new points with the open clusters of stars in the Milky Way studied by the Gaia mission.

The standard cosmological model with dark matter and dark energy, one of the main pioneers of which was the Nobel Prize in Physics James Peebles, has been spectacularly verified by analyzes of observations of fossil radiation made by the Planck satellite. There are other predictions of this model that are well supported, as the philosopher of science Karl Popper would have said, by other observations. But all is not perfect and a small number of anomalies, such as the one discovered for a few years with the divergent estimates of the Hubble-Lemaître constant which reflects the accelerated expansion of the observable cosmos, suggests that new physics is perhaps at work, explaining these anomalies.

Without questioning the Big Bang theory in its main lines, it might be necessary, for example, to replace the effects of the hypothetical particles of dark matter by a modification of Newton’s laws of celestial mechanics, and finally by a modification of the laws of the relativistic theory of gravitation proposed by Einstein a little over a century ago now.

In fact, in the early 1980s, the Israeli physicist Mordehai Milgrom had proposed the theory which was baptized with the name of Mond, an acronym for Modified newtonian dynamics in English, which can be translated into French by theory of modified Newtonian dynamics.

The theory has met with increasing success in recent years as far as the world of star dynamics in galaxies is concerned and it may be about to be strongly supported by ongoing observations from the James- Webb concerning galaxies observed between 250 and 500 million years only after the Big Bang.


In 2016, Stacy McGaugh gave this lecture on the reasons that led researchers like him to turn to the Mond theory proposed in the early 1980s by Israeli physicist Mordehai Milgrom. Stacy McGaugh was a strong proponent of the existence of dark matter particles early in her career and so it is backwards, seeing contradictions between predictions from dark matter theory in the world of galaxies and, on the contrary, the successes naturally encountered by the general framework laid down by Milgrom modifying Newton’s laws of celestial mechanics, which Stacy McGaugh, like Planck when he discovered quantum mechanics, resigned himself to admitting that it was necessary to change the fundamental laws of gravitation and not introduce new particles into astrophysics. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © TEDx

Clusters of around 100 to 1,000 stars

Still, today, an international team of astronomers led in particular by Pavel Kroupa from the University of Bonn in Germany, Tereza Jerabkovanow at the European Southern Observatory also in Germany, has just published an article in Monthly Notices of the Royal Astronomical Society (available in free access on arXiv) where the researchers argue that the Mond theory better describes observations from the ESA’s Gaia mission in particular regarding certain properties of open star clusters in the Milky Way.

Let us recall what Futura had already explained about these clusters. These are concentrations of stars (from 100 to 1,000), located in the disc of the Milky Way. We knew more than a thousand of them but there were still many others to discover. One of the most famous open clusters is that of the Pleiades (M45). Unlike globular clusters, like Messier 9, which are more than ten billion years old, these clusters contain young stars. They were born in gigantic molecular clouds located in the disc of our Galaxy, clouds which collapsed gravitationally while fragmenting to give these stars.

Our Sun was born in one of these open clusters. But, since many of them are weakly gravitationally bound, the stars disperse rapidly in the Milky Way, so that half of the clusters are less than 200 million years old. Some take longer to dissipate and it is estimated that less than 1% of them survive for two billion years.

Gaia is a mission that makes precision measurements of the positions and velocities of stars in the Milky Way so that it can properly define and study stars in open clusters. However, it turns out that a theory of gravitation makes predictions about the deformation of open clusters of stars under the effect of the tidal forces of the disc of the Milky Way.

Numerical simulations confirmed by the Gaia mission

Newton’s theory thus predicts that there must be some kind of tidal tails in front of and behind an open cluster, elongated according to the orbit of the cluster – tails in which the stars of the cluster move. accumulate during its history before its final “dissolution” in the Galaxy.

Newtonian dynamics predicts that on average there should be roughly as many stars in the back tail as in the front case and that, still on average, there is a definite time for a cluster to dissipate.

However, and this is where things get interesting, Mond predicts an asymmetry for star populations in tidal tails. This asymmetry can be assessed and it is found that many more stars tend to leave the clusters through the front tail than through the back tail.

According to Kroupa, Jerabkova and their colleagues, these predictions are fully verified by analyzes of data collected by Gaia, analyzes carried out using a new mathematical technique to identify stars belonging to specific open clusters, initially developed by Tereza Jerabkova.

As a bonus, Mond also predicts a significantly shorter lifetime for open clusters than Newton’s theory, which ties in with observations that continue to baffle astronomers.

The researchers remain cautious in pointing out certain limitations of their work and concede that we still have to wait to have firmly established results. We can still remember that certain models of dark matter also lead to predictions which are those of the Mond theory without changing the fundamental laws of gravitation. These are the so-called “fuzzy” dark matter models, fuzzy dark matter or FDM in English.

Did you know ?

Globular clusters, which should not be confused with open clusters, are very dense spheroidal concentrations, a few hundred thousand stars on average, which formed in the first billion years of history. of the observable universe. One of the most famous is the Hercules cluster (M13), to which the famous Arecibo message was sent in 1974.

In the case of the Milky Way, we know more than 150 of them but it is probable that there are ten to twenty times more. Their sizes are between several tens and several hundreds of light-years and they are located in elliptical orbits all around our Galaxy, isotropically distributed around the central bulge and in the halo.

It is thanks to the globular clusters that the astronomer Harlow Shapley was able to determine the size of the Milky Way and the position occupied by the Sun. It took him four years of work from 1914 to achieve his goal. However, the exact structure of our Galaxy, which was known to be a disk, remained undetermined at the time. Fortunately, a few decades later, the discovery of the famous line 21 cm from neutral atomic hydrogen allowed Oort and his colleagues to partially lift the veil on the structure of the Milky Way. It appeared that it possessed spiral arms like certain spiral galaxies discovered and identified as “universe-islands” similar to ours since the work of Hubble.

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