A computer simulation managed for the first time to explain how it is possible for galaxies virtually free of dark matter to appear in the Universe, without having to change the standard cosmological model, our best theory to encompass the evolution of the cosmos.
The work was first authored by Jorge Moreno, from Caltech (USA), and was published in Nature Astronomy. It is undoubtedly a source of great relief among enthusiasts of the roundest version ever formulated of how the Universe became what it is over the last 13.8 billion years.
The so-called standard model postulates, in addition to the conventional matter that forms everything we see, the existence of a certain amount of “cold” dark matter – particles that we don’t know what they are, but we have excellent reason to believe that they exist, since they emanate gravity. – and dark energy, in the form of Einstein’s cosmological constant – something like an energy present in the vacuum itself that, on the largest scales, acts against gravity, accelerating the expansion of the Universe.
Based on these premises, researchers perform cosmological computer simulations, contrasting what the theory predicts for the general appearance of the Universe from the Big Bang to today with what we can observe with the telescope. Overall, things fall into place very well. But a recent finding was making cosmologists stand on end. Actually two. Two galaxies, found in 2018 and 2019 by the team of astronomer Pieter Van Dokkun of Yale University, cataloged as DF2 and DF4, appeared to have almost no dark matter.
This was in stark contrast to the standard model, where galaxies are born from large cradles of dark matter, and to all the simulations done so far, which seemed to indicate such an occurrence as an impossibility. And then a cockroach began to happen in the community: would the model be punctured?
So Moreno and his colleagues ran their simulation of galaxy evolution, using an unusually high resolution for such experiments, and found that, yes, the Standard Model could produce galaxies free of dark matter. This apparently happens when smaller galaxies make close encounters with a bigger sister. There the dark matter, because it circulates more easily, ends up separated from the mass of gas and stars. This, by the way, must be the case for DF2 and DF4, which are satellites of the galaxy NGC 1052.
In addition to saving our better understanding of the Universe from an embarrassing failure, the new simulation makes a prediction: that 30% of large central galaxies with 100 billion suns or more have at least one satellite galaxy with 100 million 1 billion suns that are deficient in dark matter. That is, the ball is now back in the court of observational astronomers, who need to find more galaxies like DF2 and DF4 to confirm a new claim emanating from the model.
This column is published on Mondays, in Folha Corrida.
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