16 December 2016 News

Mini-spiral galaxies may transform how dark matter is perceived

An image of spiral galaxy Messier 74 - could diminutive versions of these objects change the way scientists think about dark matter? Image: NASA
An image of spiral galaxy Messier 74 - could diminutive versions of these objects change the way scientists think about dark matter? Image: NASA

Could mini-spiral galaxies shed light on the nature of dark matter? Yes, according to studies by researchers at the International School for Advanced Studies (SISSA) in Trieste, Italy, who are suggesting that these objects may serve as "portals" to a completely new form of physics which can explain phenomena like matter and dark energy.

Mini-spiral galaxies are as the name suggests - spiral galaxies that look similar to our own Milky Way, but are ten thousand times smaller. By studying these scaled-down objects from a statistical point of view, Professor Paolo Salucci of SISSA, and Ekaterina Karukes, who recently earned her PhD at the same institution, have been able to effectively erase the "individual" variability of each object, thus revealing the general characteristics of the galaxies.

"We studied 36 galaxies, which was a sufficient number for statistical study. By doing this, we found a link between the structure of ordinary, or luminous matter like stars, dust and gas, with dark matter,” said Salucci. "Most dark matter, according to the most credible hypotheses, would be non-baryonic or WIMP. It would not interact with ordinary matter except through gravitational force," continues Karukes. "Our observations, however, disagree with this notion."

Non-baryonic matter are particles such as electrons or neutrinos, which are classed as leptons. Baryonic matter on the other hand is made up of particles found in an atomic nuclei, such as protons and neutrons. WIMPS stands for Weakly Interacting Massive Particles and these hypothetical particles are thought to include neutrino-like particles such as photinos, higgsinos or sneutrinos, and new heavy neutrinos.

Nonetheless, despite making up approximately 27 percent of the matter in the Universe, a consolidated consensus of what dark matter actually is, no-one can yet agree upon. However, this might be about to change as Salucci and Karukes show that in the objects they observed, the structure of dark matter mimics visible matter in its own way. "If, for a given mass, the luminous matter in a galaxy is closely compacted, so is the dark matter. Similarly, if the former is more widespread than in other galaxies, so is the latter,” explains Salucci. "It is a very strong effect that cannot be explained trivially using the Standard Model of particles."

The Standard Model explains how the basic building blocks of matter interact through three fundamental forces, the strong force, the weak force, and the electromagnetic force. Although this model describes the micro world very well, it falls short of explaining gravitational force, as fitting gravity comfortably into this framework has proved to be a difficult challenge for physicists. The standard model does also not explain what the nature of dark matter might be or what happened to antimatter after the big bang.

"From our observations, the phenomenon, and thus the necessity, is incredibly obvious. At the same time, this can be a starting point for exploring this new kind of physics," continues Salucci. "Even in the largest spiral galaxies we find effects similar to the ones we observed, but they are signals that we can try to explain using the framework of the Standard Model through astrophysical processes within galaxies. With mini-spirals, however, there is no simple explanation. These 36 items are the tip of the iceberg of a phenomenon that we will probably find everywhere and that will help us discover what we cannot yet see. "

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