Dark matter is one step closer to be identified because new research from the Niels Bohr Institute in Copenhagen presents new information.
Earlier in 2007, scientists mapped the shape of the collected groupings of dark matter using Hubble technology. The universe, made up of visible celestial bodies, stars, planets and galaxies, is mostly comprised of large quantities of invisible–and as yet unidentified–dark matter.
Dark matter is invisible because it does not reflect light. There are theories as to what it might be, including that it might be made up of axions, which are hypothetical elementary particles that are predicted to have no electric charge and a very small mass and that interact minimally with ordinary visible matter.
Dark matter, first discovered in the 1970s, has mass and therefore has gravitation. This gravity can be measured according to Newton’s principal of the proportionality of mass and distance. Further, galaxies can be analyzed and weighed to derive mass. From this measurement it is clear that the greatest component of mass in galaxies is invisible dark matter.
Additionally, these galaxies, which can yield revealing measurements, gather into clusters with each cluster containing up to several thousand galaxies. Just as galaxies can collide, galaxy clusters can also collide. Astrophysicist Signe Riemer-Sorensen, a Ph. D. student at the Niels Bohr Institute, has added to the understanding of dark matter through his analysis of two clusters of galaxies that are colliding.
When galaxies collide, it is not the galaxies nor the dark matter of the galaxies that meet in the collision. What meets is the huge clouds of gas and dust which comprise about 12 percent of the mass of galaxy clusters.
These crashing gas and dust clouds in the colliding galaxies are hot and they emit x-ray that can be observed and measured. It is therefore possible to see how the clouds are actually forcibly pushed out of the galaxies while the galaxy clusters collide. Furthermore, when these hot, x-ray emitting clouds collide with each other, they become even hotter and emit even more x-ray. The result is that a large and intense front of gas is generated.
This is significant to the search for the characteristics of dark matter. Theories on dark matter suggest that it may be comprised of a new and still undetected type of particle. Among the suggestions for these new particles are types of particles that emit x-ray when they decay. The theorized axion is such a particle: as yet undetected and hypothesized to decay and emit x-ray (interestingly, the axion is also integral to theories involving extra dimensions).
Thus, in order to search for x-ray from decaying particles in dark matter, researchers need to look in places where there is an exceptionally high concentration of dark matter but no hot, x-ray emitting gas. Such a place is found in two colliding clusters of galaxies: as much as 85 percent of the mass can be dark matter and the hot gas clouds that themselves emit x-ray have been pushed out of the galaxies. That leaves available dark matter and an unobstructed search for x-ray from decaying particles can therein be conducted.
In the two colliding galaxies that Riemer-Sorensen has analyzed, whose gravitational measurements show that up to 85 percent of the mass is dark matter, no x-ray of any significance has been measured.
The result of this significantly zero measurement is that it can be concluded that if axions are the particle comprising the stuff of dark matter, then not much dark matter has aged enough to decay and emit x-ray. When insignificant x-ray is emitted from dark matter, it is possible to calculate an upper limit to how soon the hypothesized axions would begin decay and thus determine the lifetime of the investigated dark matter.
The results Riemer-Sorensen obtained indicate that, if dark matter is comprised of axions, dark matter has a very, very long lifetime. The theorized lifetime for axions is 3,000,000 billion years and the calculated age of the galactic dark matter is 13,700 billion years. The conclusion therefore is that, if axions are to be the stuff of dark matter, dark matter has a very, very long lifetime.
“The dark matter of the universe has a long lifetime,” University of Copenhagen.
