An extraordinary discovered has recently revolutionized Particle Physics and Cosmology. The understanding of the Universe had proceeded rather linearly from the beginning of the 20th century, when Hubble had discovered the expansion of the Universe. But when, in the 1970s, scientists tried to put together the many pieces of the cosmic puzzle (like the abundance of light chemical elements in the universe, the motion of stars in galaxies, and the velocity dispersion of galaxies in clusters) to come up with a consistent cosmological model, these pieces just didn’t seem to fit. To complete the puzzle, the existence of a new form of matter, dark matter, had to be postulated. It solves in a single stroke many problems in astrophysics and cosmology.
In an article that appeared on October 1, 1974, in the prestigious Astrophysical Journal, renowned Princeton cosmologists including Prof. Jim Peebles – who will participate in this Symposium – described the paradigm shift that was taking place with these shocking words:
“There are reasons, increasing in number and quality, to believe that the masses of ordinary galaxies may have been underestimated by a factor 10 or more.”
A factor of ten! All of a sudden, the familiar galaxies that had been observed and studied for decades, the ordinary systems of stars and gas whose structure was believed to be well understood, had become uncomfortably big, too massive, and, overall, bizarre. Nobody knew anymore what they, or at that point anything else in the universe, were actually made of. The standard picture of a galaxy, thought to be a simple disk of stars rotating along with their planets around a common center, and immersed in a sea of dilute gas, had suddenly become inaccurate and misleading.
Gas and stars were just a small part of a much larger, more massive halo of invisible matter. The implications of this paradigm shift are staggering, and so deep that we have just started exploring them. The most important, perhaps, is that the existence of stars, black holes, supernovae, planets, and the Earth itself, in short, everything we know, is possible thanks to a sort of “cosmic scaffolding” made up of dark matter. Take dark matter away from a galaxy, and its stars and planets would break loose like bullets in the intergalactic space. This also means that we, as human beings, are not made of the same stuff that most the universe is composed of: we are special, in a way we could never have suspected.
From modifications of gravity to new particles, and from faint stars to mirror worlds, the list of solutions proposed to the dark matter puzzle is very, very long, and new ideas continue to crop up today. Fortunately, we are about to get some answers: by the end of this decade we will witness a revolution in this field of research, for either we will find dark matter particles, therefore opening up an entirely new branch of science, or we will inevitably witness the decline of the leading theories, forcing us to revisit our description of the Universe.
[Text adapted from the book "Behind the Scenes of the Universe", by G. Bertone]
[Image: Large scale projection through the Illustris volume at z=0, centered on the most massive cluster, 15 Mpc/h deep. Shows dark matter density overlaid with the gas velocity field. Credit: Illustris Collaboration]