On clear nights in non-light polluted areas, maybe after a tasty barbecue or out camping, one can view the majestic starry night. If the conditions are right, a foggy white band will be visible (especially in areas such as Dwejra and Miġra l-Ferħa). This band is our own galaxy, the Milky Way.

Galaxies are a composition of various gases and stellar objects, ranging from the small (asteroids) to the large (supergiants). Unfortunately, galaxies composed only of what we have observed so far would eventually be torn apart. That is, gravity alone would not be strong enough to keep these galaxies together. However, this is not what is observed as galaxies seem to be glued together.

This was further supported by research conducted by the American astronomer Vera Rubin (1928-2016) in the 1960s. By measuring the velocity of stars orbiting the centre of our galactic neighbour, Andromeda, Rubin showed that the velocities predicted by Einstein’s theory of general relativity and the velocities observed did not agree.

It was concluded that these observations could only be true if galaxies had a lot more mass in them than was previously thought. This discovery, among others, led to the introduction of what we now call dark matter.

This type of matter is estimated to contribute to 27 per cent of the universe’s density. Unfortunately, no direct evidence for this matter has yet been found. Thus, current research is trying to answer the question of whether this dark matter truly exists, and if so, what it consists of. There have been various ideas.

One hypothesis is that it is a completely new particle, called WIMPs (Weakly Interacting Massive Particles), attributing to its massive mass and weak interaction with other matter. Various experiments around the world are being carried out to detect these WIMPs. The Cryogenic Dark Matter Search (CDSM) makes use of very cold germanium and silicon crystals (at around -273.1°C), covered by a layer of aluminium and tungsten, in order to detect vibrations caused by these WIMPs. Similar experiments are being carried out by CRESST, CoGeNT and EDELWEISS. Unfortunately, this vibration is very small and can be caused by other sources.

For this reason, these experiments are carried out deep underground. There are also experiments that are looking for WIMPs indirectly, either through telescopes or through collisions in particle accelerators (LHC in CERN).

Another model is that it simply is a manifestation of gravity. It might be possible that the theory itself, general relativity, is incomplete. Thus, a more complete theory might answer this mysterious dark matter.

As the search for dark matter continues, the only thing left to do is to wait and continue to admire the Milky Way during those beautiful starry nights. After all, the galaxy will not be ripping itself apart any time soon.

Did you know!

• There is a black hole millions of times the mass of the Sun at the centre of the Milky Way galaxy.

• The sun is just one of over 100 billion stars in our galaxy, the Milky Way; and the Milky Way is just one of over 100 billion galaxies in the universe.

• Roughly 85 per cent of all stars in our galaxy are part of multiple-star systems.

• The average galaxy contains ‘only’ 40 billion stars.

For more trivia see: www.um.edu.mt/think

Sound bites

• Farthest stars in Milky Way might be ripped from another galaxy: The solar system is found roughly 26,000 light years from the Milky Way’s galactic centre. That means the light we see from the galactic core is 26,000 years old. Essentially, this is like a cosmic time machine letting us peer into the past of the universe. Astronomers at the Harvard-Smithsonian Centre for Astrophysics have been busy looking outward of the galactic core and trying to understand how some of the stars in the outer regions of the Milky Way came to be in there. Looking out to the edge of the Milky Way, some 30,000 light years they have been focusing their efforts on the 11 farthest known stars in the galaxy. By tracing back the origins of these stars they hope to understand better how the Milky Way evolved over time. The Milky Way galaxy is actually a by-produce of a series of collisions between smaller galaxies. We see their remnants in the types of star systems and the overall positions of some stars. In this case the astronomers were investigating the relationship between these stars and the smaller dwarf galaxies which orbit in the outer regions outside the Milky Way. What they found is that some of these 11 stars actually form part of a stream of stars one million light years across, connecting the galaxy to one of these dwarfs, the Sagittarius dwarf. This dwarf galaxy has collided with the Milky Way before and appears to be on collision course with us again.

https://www.sciencedaily.com/releases/2017/01/170111110810.htm

• First look at new, extremely rare galaxy: The vast majority of galaxies in the universe come in two broad types, spiral and elliptical galaxies. However, there are other types of galaxies that are much rarer. This all has to do with the way the galaxy formed. In this case, two teams working together from the University of Minnesota Duluth and the North Carolina Museum of Natural Sciences found an elliptically shaped galaxy with a ring around it, called a Hoag-type galaxy. The circular ring is composed of different gases and contributes to star formation. It is not clear why the gas would form a ring but it is thought that this may be the result of smaller dwarf galaxies being stretched out by the gravitational pull of the main elliptical galaxy.

https://www.sciencedaily.com/releases/2017/01/170104103610.htm

• To find out some more interesting science news, listen in on Radio Mocha every Monday and Friday at 1pm and on Radju Malta 2.

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