It is invisible, impossible to identify even with the most sophisticated telescopes, yet it would constitute almost 90 percent of the total mass of the universe. It is dark matter, one of the greatest mysteries of modern cosmology: it serves to justify some astronomical observations on the mass of galaxies and galaxy clusters, it is at the basis of the most accredited theory on the formation of the first galaxies in the primordial universe, yet no one until to date he has managed to demonstrate its existence. After decades of research, one wonders whether the simplest explanation is to accept that it doesn’t exist, and start working on new cosmological hypotheses that do without it. And in fact, the latest observations from the James Webb Space Telescope go precisely in this direction.
What is dark matter used for?
The research in question was recently published in the Astrophysical Journal, and is the result of the work of an international research team which also included the Italian Federico Lelli, from the Inaf astrophysical observatory in Acetri. And it is based on observations made with the Webb telescope of the most distant, and therefore oldest, galaxies ever observed to date. Looking thus back in time, the authors of the study hoped to be able to verify some predictions of the so-called standard theory of cosmology, the Lambda-CDM (or Cold Dark Matter) model, which hypothesizes the presence of dark matter of a still unknown nature in the universe, which would have played a fundamental role in transforming the homogeneous matter that filled the universe immediately after the universe, into what we see today “concentrated” in galaxies.
“Astronomers invented dark matter to explain how we could go from a very homogeneous early universe to large galaxies with a lot of empty space around them, as we currently have,” explains Stacy McGaugh, director of the astronomy department at Western Reserve Homes. In theory, dark matter would have acted as a sort of glue in these first “moments” of the universe, forming aggregates of matter which over time, through growth, gave rise to the cosmological structure we see today. “Every large galaxy we see in the universe today was therefore predicted to have started out as one of these tiny little bits,” McGaugh points out. Looking very far away from us, and therefore at galaxies caught at the beginning of their evolution, we should therefore see small and dim galaxies, destined over time to fuel the birth of the enormous and luminous ones that we see around ours. And so, however, it wasn’t.
Galaxies already formed in the early universe
“The standard LCDM model of galaxy formation is a strictly “hierarchical” model, in which high-mass galaxies are formed thanks to the merger of many smaller proto-galaxies and/or dark matter halos,” he says on the website of Inaf Federico Lelli. “This process takes time, so massive galaxies, such as elliptical galaxies, are expected to form relatively late in the evolutionary history of the universe. In recent years, however, various observations obtained with different telescopes – Alma, Hst, Spitzer and more recently Jwst – have shown us a very different scenario: massive galaxies are already present in the primordial universe and seem to have evolved much faster than previously thought. we expected in the standard cosmological context.”
Maybe dark matter doesn’t exist
These observations, which violate what was predicted by the Lambda-CDM model, are instead perfectly in agreement with another cosmological model, known as Mond (Modified Newtonian Dynamics), which explains the birth of galaxies thanks to a modification of the gravitational laws of Newton and Einstein . “In this theory, there is no dark matter,” explains Lelli. “In the late 1990s, astrophysicist Bob Sanders used the Mond theory to predict that massive, evolved galaxies may have already been present in the early universe. This is a surprising and rather incredible prediction, in fact it was not taken seriously into consideration by most of the scientific community for a long time. Current data, however, seem to confirm Bob Sanders’ prediction from way back in 1998 (more than a quarter of a century ago!), according to which massive galaxies form over extremely short times, of the order of a few hundred million years after the Big Bang.”
It is obviously early to abandon the standard cosmological model. But if further observations confirm what has emerged in this study, it is possible that in the future we will find ourselves living in a universe without dark matter. It would mean that the James Webb Space Telescope would have helped rewrite some of the most important pages in the history of astrophysics, just three years after its launch.