Dark Matter: Basics of MACHOS and WIMPS

In the 1930s, an astronomer Fritz Zwicky noticed that in one of the clusters of Galaxies, many individual galaxies were moving around so fast there must have been a tremendous amount of gravitational pull toward the centre of the cluster. If it were not so, the galaxies would literally fling out of the cluster. But the mount of matter that was required to produce that much gravity exceeded the matter observed in these Galaxies. That implied that there was indeed an extra matter than what is observed. This invisible extra matter was called Dark Matter.

Taking the theory forward, in 1970s, two astronomers showed that the stars in Andromeda Galaxy were moving so fast that for the stars to stay in the galaxy there had to be a tremendous amount of matter surrounding and enveloping the entire galaxy like a giant cocoon. The interesting thing is that while it did not emit any electromagnetic radiation, and consequently not visible to telescopes, it exerted only gravity. After further researches, it was confirmed that Dark matter does exist and is an important constituent of matter around galaxies, in clusters of galaxies, and throughout the universe. Not only that, it is now estimated that around 80 percent of the matter in the universe is dark matter!

The direct observational evidence of the existence of dark matter came from careful radio astronomy–supported studies of the rotation rates of individual galaxies, including Milky Way. Studying there rotational behaviour, the astronomers discovered that most galaxies appear to be surrounded by a giant or galactic halo containing matter capable of exerting gravitational influence but not emitting any observable radiation. Further, it was also indicated that majority of a galaxy’s mass lays in this very large halo, which is around 10 times the diameter of the visible galaxy. For example, our own Milky Way galaxy contains about 100 billion stars and it is thought to have been surrounded by a dark matter halo that probably extends out to about 750,000 light-years. The mass of this dark matter halo appears to be about 10 times greater than the estimated mass of all the visible stars in our galaxy.

But nobody has a real idea of what dark matter is and what these galactic halos are made of, except some educated guesswork. On this basis, two schools of thought emerged. One school supported the idea of MACHOS or Massive Compact Halo Objects and another school advocated WIMPs or Weekly Interacting Massive Particles.

MACHOS

Machos are made of ordinary matter particles called Baryons. To understand Baryons, we take an example of atom. Rutherford, the fater of Nuclear Physics had proved log ago that an atom is not made of uniform material. Each atom has a nucleus having protons and neutrons and surrounding elections. When we go to subatomic particles level, we find that the neutrons and protons are also not composed of uniform material. Rather, a Proton is composed of three much smaller charged particles called Quarks. Similarly, the neutron is also composed of three quarks. Each quark has its own anti-quark. Quarks and / or Antiquarks combine to form composite particles. These composite particles are called hadrons. This means that every composite particle made up of quarks and Antiquarks are called Hadrons.
Thus, protons and neutrons are a kind of Hadrons.

There are two families of Hadrons, viz. Baryons and Mesons. Baryons are made up of three quarks, while mesons are made up of one quark and one antiquark. Protons and Neutrons are made up of 3 quarks and thus are placed in Baryons family of Hadrons.

The advocates of the MACHOS say that the unobserved material in the dark matter is made of heavy particles of the Baryon family viz. Neutrons and Protons. They give examples of the black holes or neutron stars as well as brown dwarfs to support their theory. They present the brown dwarf, neutron stars and black holes as MACHO candidates.

But, currently, Machos are not accepted as explanation to Dark Matter. The current researchers say that the MACHOs can only account for a very small percentage of dark matter and most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects.

WIMPs

The second school of thought says that the dark matter consists primarily of exotic particles that they collectively refer to as WIMPs. These exotic particles represent a hypothetical form of matter called nonbaryonic matter—matter that does not contain baryons (protons or neutrons).

Then there are some other scientists, who suggest that the true nature of dark matter may not require an “all or nothing” characterization. To them, it seems that dark matter might exist in several forms, including MACHOs in the inner regions of a dark matter galactic halo as well as swarms of WIMPs farther out in the galactic halo.

Current Status

Despite of all this, Dark Matter has been a mystery that has haunted astronomers for nearly 80 years now. Currently, the physicists are convinced that dark matter cannot be made of ordinary quarks, electrons and other standard particles. It has to be something else, and what fits the bill best is a type of particle that responds to only two of the four basic forces of nature—gravity and the weak nuclear force, to be specific. The other two are the strong force and electromagnetism.

As of now, the entire focus on Dark Matter research is on WIMPs, the hypothetical exotic particles. Currently, WIMPs stand there where Higgs Boson stood before it was finally found via Large Hadron Collider, in 2012. The LHC has also turned its attention to finding WIMPS, among other things. The LHC scientists are trying to detect it in their experiments.