DARK MATTER by Anmol Subba

DARK MATTER

FOR PEOPLE IN HURRY

                                    by Anmol Subba(Dept. of Physics)

Those who are students of science or those who watch documentaries on cosmos have surely heard about DARK MATTER. But how many of you exactly know what dark matter really is?

WHAT IS DARK MATTER?

     But first I want to ask you a question: 

                    What is in our universe?

Those who study science may say matter or energy. But they are just partially right, as in our universe, we find that the matter and energy that we can observe composite of only 5% of what actually is in our universe. This simply means that you, your family, Earth , Solar system, billions of galaxies make only just a tiny bit of the actual universe. So naturally now u may question: What about the remaining 95%? or more curiously why do scientist think observable matter and energy composite of only 5%?

    

Here are few reasons:

1.GALAXY CLUSTERS 

    Throughout space, astrophysical objects of all sizes swirl and orbit: planets revolve around our sun, stars orbit around our galactic center, and individual galaxies in groups whiz around themselves. To keep these objects tightly bound together, the gravitational pull felt by an object must be strong enough to balance the energy it has due to its motion. A fast moving object with more kinetic energy is harder to keep gravitationally bound.  In 1933, Fritz Zwicky  was studying the nearest very large cluster of galaxies to us in space: the Coma cluster .
He used the virial theorem, an equation which relates the average kinetic energy of a system to its total potential energy, to infer the gravitational mass of the cluster. He then compared that to the mass inferred from the bright, luminous matter (stars and gas) in the galaxies. You’d expect those two numbers — gravitational mass and mass due to luminous matter — to match, wouldn’t you? But instead, he found that the mass from the luminous matter was not enough to keep the cluster bound, and was several times smaller than the inferred gravitational mass. Assuming that the luminous matter constituted all of the mass in each galaxy, they should have been flying apart! He thus coined the term “dark matter” for the material that must therefore be present, quietly holding the galaxy cluster tightly together.

2.GALACTIC ROTATION CURVE

Similar evidence was observed within galaxies themselves. From standard Newtonian dynamics, we expect the velocity of stars to fall as you move from the near the center of mass of a galaxy to its outer edges. But when studying the Andromeda galaxy in the 1960s, Vera Rubin and Kent Ford found something very different: the velocity of stars remained approximately constant, regardless of how far they were from the galactic center.This and many future observations of the velocities of stars in spiral galaxies hinted that the mass of the galaxy must not be entirely defined by the objects we could see with our telescopes, which Rubin and Ford presented at an American Astronomical Society meeting in 1975. If instead a large fraction of the galaxy’s mass resided in a diffuse dark matter ‘halo’ that extended well beyond the edges of the luminous matter, the observed galactic rotation curves could be explained.

I hope these two out of many reasons proves that Dark Matter should exist which are invisible to us. So now moving into big question what exactly is Dark Matter? To be honest no one knows for sure. But what we know for sure is that They are invisible to us. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.

WHAT IS DARK MATTER MADE UP OF?

The familiar material of the universe, known as baryonic matter, is composed of protons, neutrons and electrons. Dark matter may be made of baryonic or non-baryonic matter. To hold the elements of the universe together, dark matter must make up approximately 80 percent of its matter. Again, we dont know for sure what Dark Matter are really made up of.In simple language scientist think Dark Matter is made of up whole new types of particles that we havent seen or detected. In more complex way the answer would be:

Most scientists think that dark matter is composed of non-baryonic matter. The lead candidate, WIMPS (weakly interacting massive particles), have ten to a hundred times the mass of a proton, but their weak interactions with "normal" matter make them difficult to detect. Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the foremost candidate, though they have yet to be spotted. 

Sterile neutrinos are another candidate. Neutrinos are particles that don't make up regular matter. A river of neutrinos streams from the sun, but because they rarely interact with normal matter, they pass through the Earth and its inhabitants. There are three known types of neutrinos; a fourth, the sterile neutrino, is proposed as a dark matter candidate. The sterile neutrino would only interact with regular matter through gravity.

A third possibility exists — that the laws of gravity that have thus far successfully described the motion of objects within the solar system require revision.