This is not a highly technical treatment of dark matter and dark energy. There are many Kos readers who know a lot about these topics. I hope they add helpful comments. This diary is for the Kos readers who feel they don't know much about them. They want a little more information. When astronomers finally do report they know the nature of dark matter and dark energy, the story will have more significance.
More below the squiggle.
|What Can Be Seen and Detected?|
The historic view below (click for a much bigger image) is actually a composite of images taken by Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). It took 400 orbits to make the observations. The Hubble telescope's Advanced Camera for Surveys' wide-field camera snapped 800 exposures, which equals two exposures per orbit. The exposures were taken over four months, from Sept. 24, 2003 to Jan. 16, 2004. The 800 exposures amounted to about 1 million seconds or 11.3 days of viewing time. The average exposure time was 21 minutes. The observations were taken in visible to near-infrared light. Astronomers compare the Ultra Deep Field view to looking through an eight-foot-long soda straw. Astronomers would need about 50 Ultra Deep Fields to cover the entire Moon. Hubble's keen vision (0.085 arc seconds.) is equivalent to standing at the U.S. Capitol and seeing the date on a quarter a mile away at the Washington monument. More details here.
The Hubble Ultra Deep Field (HUDF) field contains an estimated 10,000 galaxies. In ground-based images, the patch of sky in which the galaxies reside (just one-tenth the diameter of the full Moon) is largely empty. Located in the constellation Fornax, the region is below the constellation Orion.
The final ACS image, assembled by Anton Koekemoer of the Space Telescope Science Institute, is studded with a wide range of galaxies of various sizes, shapes, and colors. In vibrant contrast to the image's rich harvest of classic spiral and elliptical galaxies, there is a zoo of oddball galaxies littering the field. Some look like toothpicks; others like links on a bracelet. A few appear to be interacting. Their strange shapes are a far cry from the majestic spiral and elliptical galaxies we see today. These oddball galaxies chronicle a period when the universe was more chaotic. Order and structure were just beginning to emerge.
What we are seeing in these images is made of the the matter we can detect with cameras, our eyes, telescopes, etc. This matter makes up less than 5% of the universe.
|The Case for Dark Matter|
In the 1970s, Vera Rubin made careful observations of galaxy rotation rates. Rubin and Kent Ford began making Doppler observations of the orbital speeds in spiral galaxies. Their results were quite surprising.
The stars far from the centers of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer in. This was odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit. It followed that there had to be a tremendous amount of unseen matter in the outer regions of galaxies where the visible stars are relatively few. Rubin and Ford went on to study some sixty spiral galaxies and always found the same thing. “What you see in a spiral galaxy,” Rubin concluded, “is not what you get.”
"In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That's probably a good number for the ratio of our ignorance-to-knowledge. We're out of kindergarten, but only in about third grade." —Vera Rubin
Her calculations showed that galaxies must contain about ten times as much “dark” mass as can be seen in visible stars. Ninety percent of the mass in galaxies is invisible and unidentified.
Additional studies of clusters, or groupings, of galaxies support this conclusion.
Here is a link to a helpful article by Martin Rees, Astronomer Royal and Professor of Cosmology and Astrophysics at the University of Cambridge on the subject of Dark Matter.
|The Case for Dark Energy|
Evidence for dark energy comes from supernova studies in the 1990s. The Hubble Space Telescope and detector developments allowed astronomers to study supernovae at much farther distances than before. Type 1a supernovae have a very uniform behavior when their light output is studied. They brighten quickly, then more gradually decrease in brightness over time. This behavior allows astronomers to use them as a sort of standard brightness candle. By observing how bright it appears and comparing to how bright 1a supernovae are at closer known distances, one can calculate the distance to the farther one.
Hubble's Law of the Expanding Universe tell us that farther things are moving away from us at faster speeds due to the Big Bang. The supernova studies in the 1990s showed that the far distant points in space are actually farther than Hubble's Law predicts. Space is expanding at an increasing, or accelerating rate.
About 72% of the Universe is in a form of mass and energy capable of creating that acceleration. It is not known what it is.
Here is a link to a recent excellent diary by palantir on the awarding of the 2011 Nobel Prize in Physics for this supernova work.
Here is a link to a helpful article by John D. Barrow, cosmologist and Professor of Mathematical Sciences at the University of Cambridge on the subject of Dark Energy.
|Time Line of the Universe|
I hope this brief summary of Dark Matter and Dark Energy is helpful. Much is being learned by the astronomy community about these topics with more research. And, there are hopes that the experiments at the CERN Large Hadron Collider will yield new insights. Some day it is hoped we will finally know what makes up the other 95% of the universe.
|The Fabric of the Cosmos on PBS NOVA|
"The Fabric of the Cosmos," a four-hour series based on the book by renowned physicist and author Brian Greene, takes us to the frontiers of physics to see how scientists are piecing together the most complete picture yet of space, time, and the universe. The first episode was Nov. 2 at 9 Eastern. Episodes 2, 3, and 4 are scheduled for Nov. 9, 16, and 23. Episodes can be viewed online after they air.