March 2012 -  Dark Matter

For those of us who grew up in cities, the neighborhoods were the centers of our  world. This is where we learned to ride bikes, played Zorro with our friends and  if we were fortunate, found our true love or our best friend.

What  else resides in neighborhoods (not necessarily earthly)? If you guessed Stars and Galaxies, you are correct. The Galactic Neighborhoods are named: Local Group, Cloud, Cluster, Super Cluster and Wall.
 
Our Milky Way neighborhood is a Local Group located on the out skirts of the Virgo Super Cluster. Yes, we are in the farm region of the Virgo Galaxies. There are about 30 Galaxies in our Local Group, Andromeda being the largest Spiral Galaxy and the Milky Way is the second largest Spiral Galaxy. The other Galaxies are small potatoes (irregular and elliptical) that hang around the two big Galaxies.

What keeps our neighborhood a tight knit community is Gravity. It’s also the glue that is counteracting the forces of Dark Energy and keeping our group of Galaxies together. But more than just togetherness, Gravity has sent the Milky Way Galaxy on a collision course with the Andromeda Galaxy, at an approach velocity rate of about 180 miles per sec. In more than a billion years these two big Galaxies will merge to form a Giant Galaxy of more than 200 Billion stars.
 
Astronomer Vera Rubin in the 1970’s was studying the properties of Galaxies including Andromeda. She noticed something strange. The stars orbiting at the furthest point from the center of the Galaxy were orbiting at about the same rate as the stars closest to the center. The laws of gravity dictate that stars closest to the center orbit at faster velocity than the stars further away. Vera Rubin also noted that these far away stars traveling at great velocities did not escape the gravitational pull of the Galaxy Center

A good example that illustrates this quandary would be discovering that in our Solar system, Jupiter was orbiting the Sun at the same velocity as the planet Mercury. In reality this is not the case because Mercury takes 88 earth days to go around the Sun and the planet Jupiter takes 12 earth years. The Sun’s Gravity gets weaker by distance. But if Jupiter’s velocity was the same as Mercury’s, we would have to conclude there is unknown source of gravity propelling Jupiter towards that velocity and restrain it from escaping our solar system.

Before Vera Rubin’s discovery, Astronomers were wrestling with another mystery:  gravitational lensing. For those of us who wear glasses or contact lenses, the lens in our glasses bend and focus light to improve our vision. Light traveling through Space is likewise bent by Gravity from nearby Galaxies and Stars. Astronomers do not have a problem with this, but they do have a problem when that light is bent and there is no source near by or explanation.
 
The Stars, Galaxies, planets and people are made from Atomic Matter Protons, Neutrons, Electrons..etc). One of the characteristics of Matter is that it interacts with other Matter. If your automobile is trying to occupy the same space as another automobile, the automobiles/Matter are interacting. The Phenomena in Space that is bending light, speeding up the velocity of Stars and keeping Galaxies from flying apart is Matter because it creates Gravity but it is not Atomic Matter because it does not interact with Atomic Matter (as far as we know today).
 
Astronomers have labeled this Phenomenon as Dark Matter. It’s been calculated to comprise about 25 percent of our Universe.

Unlike our lack of knowledge about Dark Energy, we have in place experiments that will discover the nature of Dark Matter.

Last year the Space Shuttle Endeavour delivered and installed the Alpha Magnetic
Spectrometer (AMS) on the International Space Station.
 
“The Alpha Magnetic spectrometer (AMS) is a state-of-the-art particle physics detector designed to operate as an external module on the International Space Station. It will use the unique environment of space to study the universe and its origin by searching for antimatter, dark matter while performing precision measurements of cosmic rays composition and flux. The AMS observations will help answering fundamental questions, such as "What makes up the universe's invisible mass?" or "What did happen to the primordial antimatter?"”

Both Fermi Lab and CERN are working on Dark Matter experiments.
 
My source: Sean Carroll   California Institute of Technology