New debate on dark matter

New debate on dark matter

New debate on dark matter

Recent observations of galaxy clusters reveal a discrepancy with the dark matter hypothesis, leading to a resurgence of debate about the actual existence of this elusive ingredient in the universe.

Dark matter in crisis

Most of us astronomers tend to talk about dark matter as if it were an ingredient of the universe of unknown nature, formed by some kind of exotic particles yet to be discovered , but whose existence is beyond doubt.

Indeed, dark matter halos explain very well the rapid motions of stars in peripheral regions of galaxies and also the relative motions of galaxies in large clusters.

However, opinions about dark matter are not completely unanimous and there are scientists who seriously question its existence. Among other things, the properties that are assigned to the hypothetical matter are not able to explain some observations, such as, for example, the spatial distribution of the small satellite galaxies of the Milky Way or Andromeda.

To name just one, the Czech astronomer Pavel Kroupa has spent more than a decade at the Max-Planck Institute (Bonn) criticizing what he considers to be a widely accepted dogma without the precise scientific rigor and even maintains a website dedicated to it. which he calls the dark matter crisis.

Lost galaxies

Another problem that dark matter encountered a decade ago concerned the number of satellite galaxies that a galaxy like the Milky Way should have. This is what was called ” the problem of lost satellite galaxies “. Computer simulations to reproduce the evolution of these types of galaxies surrounded by large halos of dark matter, indicated that there should be many more satellite galaxies than were observed.

A reasonable agreement between theory and observations has recently been reached thanks to two factors. On the one hand, observations with more powerful telescopes have revealed smaller and smaller galaxies . On the other hand, the models have been made more precise by including all the possible effects of ordinary matter that could suppress satellite galaxies: supernova explosions, jets ejected by black holes, etc.

Rectifying Newton

Among the alternatives to the dark matter hypothesis, perhaps the one with the most adherents, is the so-called “Modified Newtonian Dynamics”, or simply MOND (for its acronym in English).

According to this hypothesis, the motions of stars in galaxies and those of galaxies in clusters could well be explained by a modification of Newton's second law . However, before modifying such sacrosanct laws, the evidence should be overwhelming, something that at the moment is not. And that is why the MOND hypothesis has a very minor following among physicists and astrophysicists.

The subhalos: a new crisis

Now that the problem of lost satellite galaxies seemed to have found a remedy, a team of astronomers coordinated by Massimo Meneghetti (working at the Bologna Observatory, Italy, and Caltech, USA) has uncovered another discrepancy between the dark matter models. and observations.

This time the problem concerns large clusters of galaxies. Models predict that the distribution of dark matter in these superstructures cannot be very homogeneous. The halos that surround the galaxies and the dark material in the clusters must form irregular clouds , which have been called subhalos. The simulations come to predict the approximate number of subhalos that must exist in a given cluster.

In order to count the actual number of dark subhalos (which, of course, are not visible), Meneghetti and his team have examined very precise images of various clusters that were taken by the Hubble space telescope and by ESO's large VLT telescopes. in the Chilean Atacama Desert. To count the subhalos, the astronomers looked at the amplification and distortion effects of light due to gravitational lensing. Indeed, the dark matter in each subhalo creates small arcs or multiple images, thus betraying its presence.

The result of these observations is that the number of observed subhales is ten times higher than predicted by the cosmological simulations. In a way, the problem is now the opposite of lost satellites. At that time, satellite galaxies were missing from the observations, whereas now subhalos are 'left over' .

At the moment there is no explanation for this discrepancy posed by a new jug of cold water on dark matter. More and more precise observations will have to be made, but critics of the hypothesis are already sharpening their arguments. And MOND supporters raise their voices for cosmological simulations to be performed by embedding MOND in the models. The debate on dark matter is thus taking on new momentum .

The results of Meneghetti and colleagues have been published just a few days ago in the prestigious journal Science. The manuscript can be consulted here.

Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.

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