Dark Matter – Cosmic Glue?

Ever since mankind was able to get beyond his basic needs for survival and move to higher levels of thinking, he has always questioned the unknown. His basic powers of observation have answered many of the fundamental questions that were explained away by the presence of a God. The stars are no exception and have not escaped the observations of Man. The question, “ Is there more?’, is alive and well.

While researching topics for this paper, I came across a series of lectures and articles by Professor Kim Griest of the University of California at San Diego. His presentation style and simple manner of explanation allows for the general layman a basic understanding of the complex topic of dark matter. Is dark matter real and if so, what is its purpose for the universe?

When man began his observation of the stars, these were only points of light suspended in the night sky. They all moved across the night sky through the seasons. All except one, the North Star. They used the position of the stars to help them navigate across unknown oceans and return home. As time went on, Man became more curious about these points of light in the sky. In order for all of this to make sense to them, they placed the Earth as the center of the Universe.

It was not until men like Copernicus and Galileo said that the Earth was not the center of the Universe and that it revolved around the sun. Soon new speculations about the night sky began a much more intense observation of the stars. This, of course, was not without persecution of these ideas by the religious side. With the invention of the telescope, Man was now able to get a little closer to the stars. These telescopes could only see an occasional planet and the sun, but other stars were still only points of light.

As telescopes began to evolve, other artifacts began to be apparent. One such artifact was the Nebulae or cloud like smudges. For a while smudges were thought to be composed of gas until much bigger telescopes were developed that could give us a much closer look. It turns out that many of these smudges were clusters of stars. If we look at our own Milky Way Galaxy, we can see these smudges very prominently with our naked eye on very clear, dark nights. Yet, as we examine the smudges of our own galaxy, we can see that indeed they are clusters of millions of stars. The better the telescopes, the farther we could see and find that smudges beyond our own galaxy were galaxies themselves.

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Unfortunately, all new discoveries were limited by visual observation. This limitation was based on the fact that if it is to be seen, we must have light. Knowing the speed of light, we were able to calculate luminosity of stars and that of whole galaxies. This allowed us to determine distances far greater than anyone could imagine. But any discovery of new information was limited only by our technology. Now we are able to determine whether stars were traveling closer towards us or moving farther away. By the use of Doppler techniques were we able to find if a star was coming towards us or in fact, moving away from us. The shorter the wavelength, the more compressed the wave of light would be thus moving it closer to the blue spectrum of light. The longer the wavelength, the more it would move towards the red spectrum of light, thus the star was moving away. Today, we use more advanced techniques that allow us to “see” into the electromagnetic spectrum that we can not see with our own eyes moving us into radio signals, microwave, infrared, ultra violet, x-ray and even gamma rays. We are also able, through the use of spectrometry, to determine the composition of many stars with Hydrogen being the main fuel of a star. Regardless of what we used, we were still looking at light as our main source of information.

Since stars and star systems could be found in specific regions of the sky, it was thought that the Universe was static. In 1910, Albert Einstein said that the Universe was expanding and contracting. (Ronald W. Clark; Einstein – The Life and Times: 1971) Einstein’s General Theory of Relativity was the first theory to be applied to the whole Universe. Yet, Einstein himself thought that his theory was wrong. His theory depended upon 2 numbers: Hubble’s constant which addressed the expansion rate and the Omega which involved the density of the amount of the mass in the Universe. (Kim Griest, “Dark Matter and the Fate of the Universe”, Feb. 22, 2000) Whatever he did with these numbers, the Universe was either expanding or contracting. This was not good enough for him so he added a third number, the cosmological constant. By using this number, he could then balance the other numbers thus making the Universe static.

But Hubble showed that the Universe was expanding causing Einstein to call his cosmological constant his “greatest blunder”. We have found that there is expansion going on, but the galaxies and stars are not moving away from each other and are fixed in positions relative to each other. So astronomers began looking not at the stars and galaxies but at the darkness of space in between them. If this is so, then there must be something that makes up this space. The best way to describe this is the Raisin Bread Theory which states that the raisins in the dough are all located in positions relative to each other and as the bread is baked, it is the bread that is expanding with the raisins maintaining their positions within the bread. So space now was thought to have substance.

Another observation that supports this theory is the fact that galaxies are rotating at tremendous speeds and even with such speeds, the galaxies remain intact instead of being flung outwards into space. By measuring the total luminosity of the galaxy, we can estimate somewhat accurately,

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the total number of stars in that galaxy. What we find is that there is not nearly enough mass to keep the galaxy intact. There are not enough stars for the total gravity to keep the stars bound in their rotations around the galaxy because of the speed of their rotation. So in order for there to be enough mass, it must be coming from something else at least by a factor of 10 or more. (Kim Griest, Feb.22, 2000). This “something else” then forms a “halo” that surrounds the entire galaxy. This “something else” is what is completely dominating the gravity. It is suspected that it is matter that we can not see, hence the term “dark matter”. Illustration 1 is an artist rendition of what a galaxy may really look like if we could see this dark matter.


Illustration 1
(Zebu.uoregon.ed)

So when trying to calculate the total mass, we find that the mass of the disk is not enough as shown in the curve labeled “disk” below. If we add the mass of the gas in the galaxy, we find that plus the mass of the stars is still not enough. Therefore, there is another curve that is produced when we calculate a new curve with the proper masses to sustain that rotation. (See rotational curve graphs below.) This graph is based on a well studied galaxy know as NGC 3198.

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So the race is on to find out what this “stuff” is, how much there is and what is its basic form. Two basic particles that are being researched are MACHOs (Massive Astrophysical Compact Halo Objects) and WIMPS (Weakly Interacting Massive Particles). WIMPS seem to be attracting the most attention. These particles are so small that they can pass through mass with no interaction. They can slip through the space of atoms.

The problem is that with all the research and millions of dollars invested, we are yet to find a particle that fits the picture that astronomers and physicists are looking for. The main culprit is the background noise or radiation coming from space itself. We are still looking for a signal that can be labeled to the illusive particle. So great laboratory facilities with massive instruments to collect these particles have been built deep into the Earth to eliminate the background noise. Even so, the particle remains a mystery. So the question remains. Is dark matter real? The math says it is. Theories that galaxies would otherwise fling their contents into space say it is. So what is it?

At the risk of sounding foolish, as most novices to this subject matter are, I venture to use the old saying, “can’t see the forest for the trees.” I would love to see dark matter discovered and solidly identified. But is it possible that dark matter is not a particle at all but a “soup of particles” and what we are doing is trying to identify tomatoe soup by trying to isolate a single sugar? This is like trying to identify water in an ocean by attempting to isolate a hydrogen neutron. There isn’t one! The blame for this failure is the background noise one gets. Is it possible that all of the background noise put together is in fact the dark matter we have been searching for. If so, it becomes the forest that we can’t see.

Regardless, the subject of dark matter is a facinating one as it seems to be the glue that holds everything together. It has stretching ability which accounts for the fixed relative position of all the observable objects in space. If that is so, then it should reach a point where it can not stretch any further and begin to contract again towards “the big crunch” and if this is true, then the universe really does expand and contract. Maybe Einstein was not really wrong after all when he first said so.

Bibliography

Clark, Ronald W.; Einstein The Life and Times; World Publishing Corp.; 1971, 1984; pg 266-271

Griest, Kim Professor; University of California at San Diego; “Dark Matter and the Fate of the Universe”; February 22, 2000

Griest, Kim Professor; University of California at San Diego; “Mystery of Empty Space”; March 7, 2010