Dark Matter and Dark Energy

A couple weeks ago, Neil deGrasse Tyson concluded the first (and probably only) season of the rebooted Cosmos with a discussion of the 'Dark Matter' and 'Dark Energy' that exists in our universe (unfortunately Hulu took it down cough cough, but it starts around 13:10 in Episode 13). I was particularly pleased with this episode because it brought back memories of a research paper I wrote in middle school on the same topic. I figured I'd republish it here for my own entertainment. Boy, did I love exclamation marks!

I wrote this on April 13th, 2007 for my 8th grade Science class.

Dark Matter And Dark Energy

By Andrew Braren
Section A - Science
Section E - Language Arts

The average 10 year-old child would be able to define gravity as the force that pulls things down to Earth. They’d also be able to tell you that energy is a force that makes objects move, but ask that same child why dark matter pulls things towards empty space and why dark energy is expanding the universe, and you would most likely get a blank stare of confusion. At this time, the same face is shared between all scientists trying to contemplate the mysteries behind this darkness. And therefore the question still remains; “What is dark matter? What is dark energy? And why should we care?”

The contemplation of dark energy started way back in the 1920’s. The great genius Albert Einstein looked at the heavens and came up with the term “Cosmological Constant”. This meant that even the emptiness of space held a static charge and could therefore be affected by gravity’s pull. At this time, almost nothing was known about the boundaries of our universe, and Einstein’s theory was the one that made the most sense to the public. However, after Edwin Hubble’s telescope was made, scientists noticed that the universe was expanding as galaxies were being dispersed throughout the new reaches. This cosmic expansion was confirmed in 1929.

(Above is an artist’s interpretation of a supernova. As said below, these could be used to measure the rate of cosmic expansion)

The theory of an expanding universe was much harder to grasp since there was no explainable force that was pushing the boundary further. The mystery remained since nobody had a way of tracking this expansion, until almost 50 years later in 1977 when Saul Perlmutter came up with an idea. He knew that the universe was expanding, and since it was applying a force it must also be affected by the pull of gravity. So, he predicted that the universe was slowly decelerating its expansion. He tested this theory by using type 1a supernovae as a “standard candle.” By observing one of these explosions and measuring its distance from earth he believed it was possible to track whether or not the cosmic expansion was slowing or speeding up.

Adam Reiss also believed in this theory and the two cosmologists worked against each other to find the solution first. Between the two of them, more than 50 supernovae were recorded! However, they noticed that the explosions were dimmer than they had expected, meaning that the expansion was accelerating. The two refused to believe this, and re-checked their results. Adam re-crunched his numbers while Saul looked for dust that could have possibly dimmed the explosions.

However, in 1998, they agreed and announced to the public that the universe’s expansion was, in fact, accelerating. A university student Michael Turner coined the term, “Dark Energy” in the same year, as a placeholder for the mysterious force pushing out the universe.

(About 10 billion years ago, cosmic expansion slowed. Currently, the expansion is accelerating!)

The discovery of dark matter began in 1963 when Bell Laboratories found a microwave signal coming from every direction of the heavens. They believed that these signals might be the echo of the Big Bang that theorists thought began our universe. At the time, people believed that all mass came from that single astonishingly dense, pea-sized object that detonated billions of years ago. But in the 1970’s, cosmologist Vera Rubin noticed that the mass in each galaxy didn’t make sense! She found that galaxies such as our own Milky Way should have torn apart at the rate that they were spinning. The gravity that its mass was producing wouldn’t be enough to keep that rate of rotation, which sent scientists to believe that there was “missing mass” somewhere in our universe.

This “Dark Matter” as it was called could not emit light or have an electric charge since it couldn’t be seen through X-ray or infrared. One theory scientists had was that dark matter might be made of small particles that could not be seen. But, this theory was disproven in the 1980’s by David Schramm who proved that dark matter could not be baryonic (made of matter, emits light) in any way. During the same time period, scientists reluctantly accepted the existence of dark matter since its affects on galaxies could not be explained without it.

Today, scientists have gathered numerous facts that help us understand dark energy and dark matter. We now know that dark energy makes up about 74% of our universe, and causes our universe to expand. While we can’t exactly say where it came from, we know that it existed even before the Big Bang that occurred billions of years ago. The current theory behind the expansion of our universe pictures an epic battle between all the physical matter of the universe concentrated into a ball, and a large concentration of dark energy as its membrane. The enormous gravitational force pulled the dark energy towards the center of the universe while the dark energy tried to work against the gravity and move outward. Then, once the Big Bang occurred, the matter spread out in all directions and dark energy got the upper hand of the battle. As the universe expanded, galaxies and planets began to form and gravity slowly became more concentrated and powerful. Because of this, the expansion of the universe slowed quite a bit for a prolonged time. But, as proven by Perlmutter and Reiss, the universe’s expansion is now accelerating faster than ever! This is why many scientists such as Perlmutter were shocked to find that the expansion was accelerating! Since the gravity in our universe is getting stronger, shouldn’t dark energy be slowing down? This question may soon cause humans to re-think the way we look at physics, namely, gravity. If gravity is actually what we believe it to be, the universe shouldn’t be accelerating. So, does gravity change its effects after a certain distance? Does it really only affect physical matter only? These are questions that we’ve neglected for years and that we may have to come back to in order to understand dark energy.

[(As said below, our universe can branch off into one of three different futures. Above is a diagram showing possible fates)](http://www.nasa.gov/missions/deepspace/f_dark-energy.html )

This mysterious force is ultimately the decider of our universe’s fate. We may either experience “Indefinite Cosmic Expansion” where the universe continues to expand forever, “The Big Crunch” where dark energy suddenly reverses and pushes the universe back into one dense ball, or “The Big Rip” where dark energy becomes unequal in different areas and causes our galaxies to be ripped and pulled apart in different directions. (Observe diagram above) Einstein predicts that we won’t have to worry about the unfortunate latter two until about 30 billions years from now.

Dark matter is slightly more understood but its existence is not. The invisible mass makes up about 22% of the universe, which leaves only 4% to the physical mass. Like dark energy, it doesn’t emit light or form stars, however, it is affected by gravity and even has its own gravitational pull! Although we can’t see it, we can tell that it’s there by looking at its effects on nearby galaxies and constellations. Imagine our world was always covered by thick fog, we could tell that the moon was above our atmosphere by observing its effects on the ocean’s tide! In the same way , we can measure how much dark matter is within an area. Scientists have predicted that our universe wouldn’t even exist without this missing mass!

After measuring certain areas of our sky, we’ve noticed that dark matter provides a framework for the matter in our universe. Very light gasses seem to collect near areas with the lowest concentration of dark matter. Solid matter seems to collect near large areas of dark matter, and also seem to have created a lot of their stars earlier in their lives. However, gas galaxies are steadily creating stars even today. While dark matter is also non-baryonic like dark energy, scientists do have some ideas of what it can be made of.

Two of the smallest known particles called the axion and neutrino fit a description similar to dark matter. While they are made of physical matter, they cannot be seen through X-ray or infrared and they also cannot clump together to form stars and galaxies. However, capturing one of these particles is nearly impossible with even the latest detection and capturing mechanisms. Scientists today are looking to the LHC (Large Hedron Collider) to hopefully create one of these fundamental particles by recreating a miniature big bang. Vera Rubin says that she’s been through many situations such as this only to be disappointed by another mystery to unroll. The LHC goes online in 2010, and if it fails, many cosmologists are planning on throwing their hands up and giving the win to Mother Nature.

Even now, scientists along with the public are mind-boggled by these mysterious anomalies. However, there is enough proof to show that dark matter and energy exist.

As said before, Perlmutter and Reiss were able to use type 1a supernovae as a standard candle to measure how fast the universe was accelerating. The reason dark energy has to exist is because the expansion wouldn’t be possible in any way without a force to cause its acceleration. Perlmutter believes that the next step in dark energy research would be to launch a space telescope to eliminate the variable of dust in the atmosphere fogging space images. But, finding the funding needed to launch it is almost as hard as solving the mystery itself.

Dark matter’s existence is a little bit easier to prove. We can show it exists by observing how it affects galaxy and star movement. Another way not mentioned before is by observing how it bends light! During the 1930’s Einstein predicted that the gravity from dark matter could bend light rays and act like a lens. Because of this, the images of galaxies that we get from telescopes such as the Hubble become distorted. With this knowledge, the Hubble Cosmic Evolution Survey (COSMOS) is examining one area of the sky 9x the size of the moon. Along with COSMOS, the Subaru telescope atop Hawaii’s Mona Kea is looking at the colors of stars and galaxies to measure how far away they are. The more red they see in an image, the further it is from earth and the farther back in time we can see in the galaxy’s past since light takes time to travel! This way, we can see how dark matter spreads and affects new areas. With these two telescopes working together, cosmologists can create a 3D map of dark matter areas in our universe! With all this evidence, dark matter can be confirmed as an existing piece of our universe.

Despite our extensive research, there are still many things we don’t comprehend about dark matter and dark energy. If gravity is getting stronger, why is our universe still expanding? And how come some galaxies lie in low dark matter areas, while some dark matter areas have no matter within them? These questions just may be too big for us humans to comprehend. In fact, some things about our universe humans can never even learn to comprehend, as strange as that sounds. For now, the next step would be to re-examine what gravity truly is, and if we can’t find that, we may just have to accept that us humans can’t understand everything. Maybe a future Einstein can open up these mysteries, but for now, they remain dark.