Dark Matter

Introduction

Throughout the years, scientists have been looking for the missing mass of the universe; it has yet remained an unsolved mystery. Using different methods, scientists have tried to determine the mass of the universe and surprisingly found a discrepancy suggesting that ninety percent of the mass of the universe is nowhere to be found. Then here comes the term “dark matter”, referring to this unfound matter of the universe. It is called dark because it gives off no light and matter because it has to have some mass to be able to explain the effects that they produce. There have been different perspectives about dark matter. Some scientists think that dark matter is in the form of black holes, very massive objects floating around in the universe still unseen. While there are some that believe that dark matter are subatomic particles that never or seldom interact with matter.

So how did the matter over dark matter come about? Before we will be able to tackle the issue of the theory of dark matter, when, why, and how it existed, let us first study the evolution of the different studies of the universe.

Ptolemy and the Solar System

Long time ago, scientists believed that the sun revolved around the earth, they all agreed with the scientist Ptolemy’s explanation that the earth was like a stationary globe where other seven planets revolved around it. Ptolemy’s seven planets were: the moon, the sun, Mercury, Venus, Mars, Jupiter, and Saturn. The Almagest explains Ptolemy’s work. Ptolemy’s model explained the circling motion of the planets with the stars as its background by suggesting that all the planets move in small circles as they went around the earth in their orbital path. The path that they make around the earth is called deferent while their movement about themselves is called epicycle. He also moved the planets off the center. Although this was wrong, it was helpful in predicting the position of the planets.

Copernicus and his Explanation

Nicolaus Copernicus was a Polish astronomer, and his explanation about how the solar system moves around the sun became the groundwork of modern astronomy. Furthermore, he proposed that the sun is the center of the universe. He said that if one assumed that the sun is the center of the universe, it would be simpler to describe the positions of the other planets and their movements, too. And with Galileo Galilee’s invention of the telescope, it was proven that Copernicus’ theory was valid.

Johannes Kepler and the Planetary Motions

Johannes Kepler was an assistant of Tycho Brahe. Brahe had been recording his observation from the stars in the sky and Kepler had been wise enough to examine these observations. He was able to make three laws governing the motions of the planets. Kepler’s first law of planetary motion says that the planets traveling in ellipse with the sun at one of its focus. Kepler’s second law states that if there were a straight line connecting the sun and a planet, the line would cover up the same amount of area in equal times. Kepler’s third law says that there is a relationship between the time a planet would take to complete an orbit and the cube of the planet’s average distance from the sun. This implies that the closer a planet is to the sun, the faster it moves compared to those planets that are farther from the sun. Kepler also discovered that for each planet, the time needed to complete an orbit multiplied by itself is proportional to the length of its average distance from the sun multiplied by twice itself. In equation, this means that if t is the time needed for one complete revolution and l is the average distance from the sun, then t x t is proportional to l x l x l, or in exponential form, t2is proportional to l3. This pattern that he discovered prompted him to conclude that there is a force behind the motions of the planets, and this force turned out to be gravity.

Newton and his Laws of Motion

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Newton and his Laws of Motion

With the help of Galileo and Kepler, Isaac Newton came up with his three laws of motion and his Universal Law of Gravitation. These laws are very helpful in describing the interactions between matter, force, and distance. Newton’s first law of motion states that an object at rest will stay at rest unless acted upon by an outside force. An object, too, in linear motion will continue moving unless an outside force acts on it. This law is also called the Law of Inertia. The second law states that if a force is acted upon to a moving object, the object will accelerate or change its direction in the direction of the force. In the form of an equation, this is F=ma. The third law states that for every action, there is an opposite reaction. Moreover, Newton also derived the law of universal gravitation. It says that the force of gravity is just equal to the product of two bodies multiplied by the gravitational constant divided by the square of the distance between the two objects. This is written as F= G x m1 x m2/ r2. We can apply Newton’s Law to freely falling bodies, to our solar system, and maybe even to other heavenly bodies.

In our own solar system, all the planets and the satellites follow these laws. The planets move in elliptical orbits and which is the outcome of the forward motion of the planet and the inward pull of the sun’s gravity. Without gravity, the planets would move in a linear motion just like in Newton’s first law and move on forward without stopping. This constant motion is what we call inertia, and the planets’ orbits are the balance between gravity and inertia. It makes sense to say that if the gravitational pull is greater due to its distance to the sun, then its speed and its inertia must also be greater.

For long, Newton’s Laws seemed to be sensible with almost all kinds of matter. However, Einstein and a few other scientists found some exemptions to these laws, including very small (like electrons), very massive particles like black holes and very fast (like accelerator particles). However, there are some particles that do not belong to any of these categories, and this led to the idea of the dark matter.

The Universe and the Discovery of Dark Matter

Our solar system is only a part of a galaxy called the Milky Way. A galaxy is made up of billions of stars, gas, and dust. And way beyond our own galaxy are other galaxies, billions of them. They are all in motion, and they make up our great universe.

All galaxies are held together by gravity. The force of gravity should be strongest at the center where most of the matter is concentrated, there in the pack of heavy stars. Our Milky Way is a spiral galaxy; it has a center where stars are packed closely together, and a somewhat flat disk that spreads out-still made up of stars and dust.

In 1932, a scientist named Jan Oort claimed that not all of the motion of the galaxies can be explained so easily. He found out about this when he was trying to measure the gravitational pull of the Milky Way’s disk on its nearby stars. To his surprise, the mass that he got was twice the mass of the stars and other heavenly bodies that he could see.

Still in the 1930s, a scientist by the name of Fritz Zwicky found another puzzle. While he was studying the movement of galaxies as they moved about in clusters, he noticed that they were moving faster than their gravity could handle. So he inferred that some force with gravitational pull is holding the galaxies together. Zwicky called this mass the missing mass where today it is called dark matter. It is termed dark because it doesn’t reflect any light and matter, because it has gravitational pull and that it has mass, although nobody can see it.

In the later part of the century, in the 1930s specifically, a scientist named Vera Rubin opened the ways for further investigation about dark matter. Vera Rubin wanted to investigate on the movement of the stars, and how do their spiral galaxies turning.

She studied the Andromeda Galaxy, which is also a spiral galaxy (like the Milky Way galaxy) and its neighboring stars. It has a bulge at the center, and has that flat disk around it too, that is made up stars and gas and dust. As we go farther to the disk, gravity should decrease since matter is not in compact there. This means that the stars should move slower than those stars near the center of the galaxy. However, Rubin found a very startling discovery. As the scientists predicted, the stars far away from the center of Andromeda galaxy should be moving rather slowly, yet they were moving at about the same speed as the stars near the central bulge.

Rubin and her colleagues continued to observe more galaxies, and they found out the same observation even with our own galaxy, the Milky Way. They continued to observe 200 galaxies in all, big and small galaxies, yet, the orbital velocities of the stars from the bulge or the center did not increase, it remained constant; there are even times when it increased.

So then scientists believed that invisible matter really exists. Something must explain the anomalous behavior of the galaxies, that something must have gravitational pull, which holds the galaxies together. Scientists strongly believe that in the emptiness of the galaxies, there must exist some very massive invisible matter. It cannot be detected using light of any amount of wavelength. This mysterious matter must be responsible for the gravitational force that keeps the distant stars from moving as fast as the other stars. It keeps these distant stars from falling off into the deepest ends of the universe, and holds the galaxies close together. Today, the mystery of the dark matter remains unsolved, it now seems that this universe is not what we seemed to know it is. There is still so much about the universe that has to be known and explained. Some 90 to 98 percent of the universe is believed to be dark matter (Hagan, 2003).

The Critical Search for Dark Matter

How do scientists look for dark matter when in fact it is invisible? There are two categories by which this dark matter has been named: MACHOs (Massive Astrophysical Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). Their names help us remember which is which. MACHOs are those that are the big and strong dark matter and these are made up of ordinary matter called baryonic matter. On the contrary, WIMPs are made up of non-baryonic matter and they are weak subatomic matter. Astronomers look for MACHOs while particle physicists are trying to look for WIMPs.

MACHOs

Massive Compact Halo Objects are non-light giving objects that are believed to make up the halos around galaxies. Brown dwarfs and black holes are thought to be part of MACHOs. Brown dwarfs were explained by the formation of stars while black holes are explained in Einstein’s general theory of relativity.

Brown dwarfs are made of hydrogen, the same composition as our sun, but brown dwarfs are normally so much smaller. Stars are formed due to a collapsed mass of hydrogen by its own gravity, and with the extreme pressure, it initiates a nuclear reaction, it then gives off light and energy. Brown dwarfs are not real stars; they are different because of their low mass and they do not have enough force of gravity to make them light up when they form. They are just an buildup of hydrogen gas held close by gravity. They do give off some amount of heat and light.

Black holes are too much accumulation of matter. It is matter that collapsed under its own gravity into a very small area. Black holes are very dense so that anything that passes through it cannot escape from it. Its gravitational field is too much so that even light cannot escape from it. Some stars will circle it, revolving around it, just like the motion of the planets around the sun or the motion of satellites. They are called black because they do not emit any amount of light.

How to Detect MACHOs

It is a very difficult task for astronomers to be able to detect those objects that give off very little or no light at all. However, as for the moment, there have been many inventions regarding telescopes of greater power. This helps the astronomers detect and see these MACHOs. Using the Hubble telescope, astronomers are able to detect brown dwarfs. However, images from the Hubble telescope did not find many brown dwarfs contrary to what they expected. Another way of predicting the presence of a black hole is through the observation of circling stars. When astronomers notice that stars are circling around something but then couldn’t see anything, then that must be a black hole. Scientists, a group of American and Japanese scientists, announced that there is convincing evidence as to the existence of black holes. This was led by Dr. Makoto Miyosi and Dr. James Moran. However, all black holes and brown dwarfs could not suffice to the alleged missing mass of the universe.

WIMPs

Weakly Interactive Massive Particles are thought to be smaller than atoms. They are thought to be made up of non-baryonic matter but usually interacts with baryonic matter gravitationally, and that they pass right through ordinary matter. Because each WIMP is said to be very small, then there must be millions of them passing through all kinds of matter right here and right now. The big problem in detecting and finding matter is that they rarely interact with ordinary matter.

How to Detect WIMPs

Detecting WIMPs relies from the theory that WIMPs will interact with ordinary matter. Since WIMPs just pass through matter, there is a possibility that a rare interaction between a WIMP and a particle of a solid might happen. There have been two ongoing experiments regarding the detection of WIMPs. One is led by Dr. Bernard Sadoulet and Walter Stockwell of the Center for Particle Astrophysics. They are using a crystal that they cooled at the temperature of almost zero, so that its particles will be restricted from moving and when a WIMP will hit the crystal, what will register is in the interaction of the WIMP with the crystal form of heat. Another one of similar study is the AMANDA project. AMANDA or the Antartica Muon and Neutrino Detector Array project is done in Antartica, and is in collaboration of the University of Chicago, Princeton University, and AT&T. It is also partially funded by the Nat’l. Science Foundation. Instead of using a crystal, they are using the Antarctic ice sheet itself as a detector.

Evidences of Dark Matter

Many evidences for dark matter come from studies regarding large-scale structure such as the galaxies and the galaxy clusters. Galaxies show that it is made up mostly of dark matter. There is a galaxy cluster called the Abel 2029, which is composed of thousands of galaxies formed and surrounded by a giant cloud of hot gases, and an estimated amount of dark matter equal to more than a trillion of suns. Located at the center of the cluster is an elliptical galaxy that is thought to be due to the combination of smaller galaxies.

Lately, there have been reports of a newly discovered galaxy that was thought to be made of almost wholly dark matter. It was named VirgoH121 and is found 50 million light years away from the Virgo cluster. However, VirgoH121 does not have any visible stars. Scientists estimate that this includes roughly 1000 times as much dark matter as hydrogen. An also, it has a total mass of one-tenth of the mass of the Milky Way Galaxy. Moreover, the Milky Way is believed to be made of 10 times as much dark matter than ordinary matter (Dark Matter, 2005).

Why Should We Study Dark Matter

The knowledge of dark matter has two major roles; one is giving us evidence in knowing the origin of the universe. Theories say that dark matter must have clumped together and made the lumps, which we see as our galaxies now. Second, it will give us the idea of how the universe will end.

The Universe and Dark Matter

Since the discovery of dark matter, we are now getting confused as to how the universe was really formed and how will this end.

The Big Bang Theory was born in the 1950s and it says that the universe began with a giant explosion. The Big Bang theory is the new model of the origin of the universe. It further says that all the things in the universe before was a giant clump of matter. After the giant explosion, or the Big Bang, the matter was distributed evenly in all directions. Then, attracted by gravity, the matter started to clump and form what we see now as the planets and the stars. However questions like how did the clumping begin and what started it started to arise. Some scientists take the WIMPs as an answer. They must have triggered the clumping, since they only interact with baryonic matter gravitationally.

There are three theories regarding the situation of the end of the universe. First, if the universe is closed, the universe will be pulled back to a single compact mass. Second is if the universe is open, it will continue expanding forever. And lastly, if the universe is flat, it will not have enough mass to stop its expansion neither will it have enough to pull itself back in. It is said that the critical density of a flat universe is 1. Without dark matter, the critical density of our universe will be somewhat between 0.1 and 0.01, and that would be an open universe. If there is too much dark matter, then we must have a closed universe. With the right amount of dark matter, we will stay as a flat universe. Therefore, this means that the amount of dark matter will determine what will be the end of our universe (Miller,1995).

References:

“Dark Matter”.2005. 6 May 2005 <http://en.wikipedia.org>

Hagan, David. “Dark Matter Study Guide.”2003. Science Museum of Virginia. 6 May 2005.

<www.smv.org/pubs/Dark%20Mat%20Study%20Guide%20DOC.pdf >

Miller, Chris. “Cosmic Hide and Seek: The Search for the Missing Mass.”1995.6 May 2005

<http://www.eclipse.net>