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Dark Matter: The Existential Manifestation

 

“Whenever anything went wrong for me at work someone would say, ‘Have you ever thought of a career in which you paint? …’” Dr. Rubin, a lady physicist proudly told to her high school physics teachers in the hallway who replied bluntly, “You’ll do right as long as you stay away from science”. Years later she recalled, “It takes an enormous amount of self-esteem to listen to thing like that and not be dominated.” 

 

Introduction

One of the greatest achievements of 20th century science was the determination of chemical elements of the universe. With only a little one hundred elements, scientists could explain trillions upon trillions of possible forms of matter from DNA to animals to exploding stars. The familiar elements- (like C, O2 , Fe etc. ) that make up earth were same as the elements that make up distant stars and planets. Analyzing the light from different stars physicist found precisely the same familiar elements found in our own backyards, no more, no less. By the late twentieth century and avalanche of new data has confirmed that over 90% of universe is made up of invisible form of matter, called Dark Matter.

 

Why dark matter?

The scientist who first suspected that there was something wrong about conception of universe was Fritz Zwicky. In 1930, he was studying the coma cluster of galaxies which were about 300 light years away from earth (1 light year is the distance travelled by light in a year) and was puzzled by the fact that they were revolving so fast that they should be unstable. To confirm his observations, he had to calculate the mass of a galaxy. Of course, no one actually puts a star on a scale. Newton’s laws are sufficient to determine the mass of a galaxy. Once we know what time it takes for various revolutions, we can then determine the total mass of galaxy using Newton’s laws of motion. Zwicky calculated the mass necessary to bind this cluster of galaxies by analyzing the rate at which they orbit each other. He found that this mass was twenty times the mass required to bind the clusters of galaxies. He postulated that there should be some form of mysterious “dunkle matarie” or simply dark matter, whose gravitational pull held this cluster together.

Zwicky was led to postulate the existence of dark matter because of his unshakable belief towards Newton’s laws. (This was not the first time that scientists predicted the presence of unseen objects in faith of Newton laws. The planets Neptune, Pluto in fact were discovered because the orbit of closer planets such as Saturn, deviated from Newton’s laws.) Zwicky’s results were largely ignored. Astronomical community, over the years in 1970, however obtained the same “missing mass problem” in their result while calculating the velocity of hydrogen gas that surround a galaxy but dismissed them as error in calculation. In 1973 two physicists Ostriker and Pebbles showed that the standard picture of our galaxy (based on our solar system) deviated from their result obtained after calculating the velocities of distant stars. They concluded their study by showing that a galaxy can become stable if it is surrounded by massive invisible matter that holds galaxy together.

But after decades of skepticism, in 1981, Vera Rubin and her colleagues carefully and persistently analyzed hundreds of galaxies and obtained the same result as obtained by Fritz Zwicky. Vera Rubin, who was made to focus on more “ladylike” career of painting became the star icon for the verification of dark matter.

 

What next?

In 1986, Bodhan Paczynski of Princeton University realized that if the star light from distant stars travelled through the clump of dark matter, the dark matter might bend the light and act as a magnifying lens making stars appear much brighter. In 1994, two groups independently reported such a stellar brightening. Since then other group of astronomers have joined in, hoping to find more evidences of such a stellar brightening.

 

What is dark matter composed of?

When the existence of dark matter is no longer in dispute, its composition is a matter of controversy. First there is the “hot dark matter” thought, which tells us that dark matter is made up of neutrino which is difficult to detect. If the electron neutrino for example is found to have a tiny mass there is chance that it may make up the “missing mass problem”. Electron neutrino was found to have mass which was equal to one millionth of mass of an electron. However, this result is not finally accepted by physics community.

Secondly there is “cold dark matter” thought which tells us that dark matter is made up of heavier, slow moving subparticles. These particles have been given strange names such as “axions”, “WIMPs”, “MACHOs”. Rather its easy to remain unanswered than studying these weird particles. But, perhaps the most promising candidate for WIMPs (Weakly Interacting Massive Particles) are the superparticles or “sparticles” for short. According to supersymmetry, every particle must have a super partner with different “spin” called “superparticles”.

 

What is “spin” and what are “sparticles”?

All objects have “spin” or angular momentum. Everything from distant galaxies (which may take millions of years for one rotation) to subatomic particles (which can rotate millions of time in a second). It is simply a quantum version of angular momentum. However, in quantum world, the spin of particles does not occur in random amount. Just like light, which occurs in discrete bundles called photons, sub-atomic particles can only spin with certain discrete angular momentum.

Particles are the most fundamental unit of an atom. They are divided into two categories in quantum mechanics: “Fermions” and “Bosons”.

As an example of fermions look at your body, the electron and proton that make up your body all are fermions. Everything you see around you are made up of fermions, even this sheet of paper in which you are reading my article is made up of fermions. Fermions have half integral spin.

As an example of boson think of gravity that prevents you from being spun into outer space because gravity is due to exchange of a boson called “graviton”. Or think of light itself or the electromagnetic interactions due to exchange of “photon”. Or strong interactions between proton and neutron due to exchange of “meson” or weak interactions due to exchange of “W-particle” and “Z-particle”. These all are bosons which have integral spin: 0, 1, 2, … and so on. Bosons are named after an Indian physicist called Satyendra Bose.

Fermion           Spin Bosons Spin
Electron 1/2 Π meson 0
 Quarks 1/2 W-boson 1
Neutron 1/2 Photon 1
Proton 1/2 Graviton 2

 

The spin of particle is quantized and measured in terms of reduced Planck’s constant ‘ħ’ (i.e. h/2(pie)), a very small number.

Fermions are further divided into quarks and leptons. Quark is a family of tiny particles that make up particles like proton, neutron. They are: “Up”, Down”, “Charm”, “Strange”, “Top”, and “Bottom”. Lepton is a family of “electron”, “electron-neutrino”, “muon”, “muon neutrino”, “tau”, and “tau-neutrino”.

Superparticles are the super partners of above-mentioned quantum particles. For an example, super-partner of spin 1 photon is “photino”, super-partner of gluon (particle that holds quarks together) is “gluino”. The superparticles of fermions are “sfermions” which consists of “squarks” and “sleptons”. The main criticism of sparticles is that we have never seen them in laboratory. However, “stop quark” and “gluino” were recently discovered at CERN. If dark matter turns out to be composed of sparticles, belief in superstring theory would receive an enormous boost.

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