The more we study galaxies, the more credible an alternative to the existence of dark matter particles becomes while the latter still elude Earth’s ever more efficient detectors for its hunt. Mond’s theory, which modifies the laws of gravitation, has thus just scored new points with the open star clusters of the Milky Way studied by the Gaia mission.
The standard cosmological model with dark matter and dark energy, of which one of the main pioneers was the Nobel Prize in Physics James Peebles, has been spectacularly verified by the analyzes of the observations of fossil radiation made by the Planck satellite. There are other predictions of this model that are well supported, as philosopher of science Karl Popper would say, by other observations. But not everything is perfect and a small number of anomalies, such as the one discovered a few years ago with the divergent estimates of the Hubble-Lemaître constant which reflects the accelerated expansion of the observable cosmos, suggests that new physics may be 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 hypothetical dark matter particles with a modification of Newton’s laws of celestial mechanics, and finally with a modification of the laws of the theory theory of gravitation proposed by Einstein just over a century ago.
Indeed, 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 from the modified Newtonian theory of dynamics.
The theory has met with growing success in recent years in the world of stellar dynamics in galaxies and may be about to be strongly supported by the ongoing James-Webb observations of galaxies observed between 250 and 500 million years only after the big Bang.
In 2016, Stacy McGaugh gave this lecture on why researchers like him turned to the Mond theory proposed in the early 1980s by Israeli physicist Mordehai Milgrom. Stacy McGaugh was a strong believer in the existence of dark matter particles early in her career and therefore is backwards, seeing the contradictions between the predictions of dark matter theory in the world of galaxies and, conversely, the successes encountered naturally by the general framework dictated by Milgrom by changing 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 get a fairly accurate French translation, you click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Auto-translate”. Choose “French”. © TEDx
Clusters of about 100-1,000 stars
Yet today, an international team of astronomers led in particular by Pavel Kroupa of the University of Bonn in Germany, Tereza Jerabkova, now at the European Southern Observatory also in Germany, has just published a paper on Monthly Notices of the Royal Astronomical Society (available in free access on arXiv) in which the researchers argue that Mond’s theory best describes the observations of ESA’s Gaia mission, particularly regarding some properties of open clusters in the Milky Way.
We recall what Futura had already explained about these clusters. These are concentrations of stars (from 100 to 1,000), located in the disk of the Milky Way. We knew more than a thousand of them but there were still many more to discover. One of the most famous open clusters is the Pleiades (M45). Unlike globular clusters, such as 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 disk of our Galaxy, clouds that gravitationally collapsed as they fragmented to give these stars.
Our Sun was born in one of these open clusters. But, because many of them are weakly gravitationally bound, the stars quickly disperse in the Milky Way, so that half of the clusters are less than 200 million years old. Some take longer to dissipate, and less than 1% are estimated to 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 stars in open clusters can be properly defined and studied. However, a theory of gravitation is found to predict the deformation of open star clusters under the tidal forces of the Milky Way’s disk.
Numerical simulations confirmed by the Gaia mission
Newton’s theory therefore predicts that there must be tidal tails in front of and behind an open cluster, elongated according to the cluster’s orbit – tails in which the stars of the cluster move as they accumulate during its history before its collapse. “final” “dissolution” In the Galaxy.
Newtonian dynamics predicts that on average there should be about as many stars in the posterior tail as in the anterior case and that, again 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 stellar populations in tidal tails. This asymmetry can be evaluated and it is found that many more stars tend to leave clusters via the anterior tail than via the posterior tail.
According to Kroupa, Jerabkova and their colleagues, these predictions are fully verified by analyzes of the data collected by Gaia, analyzes performed using a new mathematical technique for identifying stars belonging to specific open clusters, initially developed by Tereza Jerabkova.
As a bonus, Mond also predicts significantly shorter lifetimes for open clusters than Newton’s theory, consistent with observations that continue to confuse astronomers.
The researchers remain cautious about pointing out some limitations of their work and admit that we still have to wait for consolidated results. We can still recall that certain dark matter models also lead to predictions that are those of Mond’s 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, on average a few hundred thousand stars, which formed in the first billion year 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 but it is likely that they are ten to twenty times as many. Their sizes range from a few tens to several hundred light-years, and they are found in elliptical orbits all around our Galaxy, distributed isotropically around the central bulge and in the halo.
It is thanks to 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 since 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 the neutral atomic hydrogen allowed Oort and his colleagues to partially lift the veil on the structure of the Milky Way. It appeared to possess spiral arms like certain spiral galaxies discovered and identified as “island-universes” similar to ours since the days of Hubble.