If the terms dark matter and dark energy confuse you, this post is for you. These terms are often found in astronomy and physics news stories. No doubt, many wonder what scientists mean when they use these expressions. I hope to clarify them enough so you can feel like you have a better basic understanding of what in the world they are saying.
This is not a highly technical treatment of dark matter and dark energy. This post is for the readers who feel they don’t know much about them and want a little more information. When astronomers finally do report that they know the nature of dark matter and dark energy, their story might have more significance to the rest of us.
What Matter Can Normally Be Seen and Detected?
The Universe is a big place and getting bigger. You have seen it with your own eyes on a clear night with good seeing conditions away from light pollution. You have seen it in the millions of wonderful images from ground based telescopes and those in space. Most people are very familiar with the Hubble Space Telescope images. You can browse for the rest of your life at this one site alone. Here is a magnificent image of a portion of the Carina Nebula.
A key question for astronomers is whether the universe appears to be the same at this very early time as it did when the cosmos was only 1 or 2 billion years old. The image below (click it for a much bigger version) is actually a composite of many 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 ACS camera snapped 800 exposures, 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 area of 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 image contains an estimated 10,000 galaxies. From images on Earth, this part of the sky looks empty. The region is below the constellation Orion. This image is literally a look back in time.
Assembled by Anton Koekemoer of the Space Telescope Science Institute, the image is littered with galaxies of various sizes, shapes, and colors. They contrast with the classic spiral and elliptical galaxy shapes we see today. The strange shapes of these early galaxies tell of a period when the universe was young and more chaotic. Order and structure were just beginning to emerge.
The stuff we are seeing in these images of the early universe is made of the same kinds of matter we can detect with cameras, our eyes, and telescopes today. The striking fact about this matter is that it makes up only about 4% of the universe that we now infer is present.
The Case for Dark Matter
Galaxies, and larger groups of galaxies, rotate about their center of mass. The mass making up galaxies, and groups of them, gravitationally pulls inward allowing the outermost parts to rotate. They appear to rotate too fast. But, they don’t fly apart. Some invisible matter with mass is present pulling strongly and allowing the rapid rotation seen. This invisible dark matter has been a mystery for decades.
The evidence for matter which we cannot see is simple if you consider the physics which infers its presence. Galaxies are known to rotate. If you are on a rotating merry-go-round, you need to lean inward as you move in order to create a small centrally directed force on your body. This inward force keeps you from going in a straight line tangentially off the edge of the ride. The faster the ride, the greater the force needed. In the galactic merry-go-round, this force is the pull of gravity. The central bulge of the galaxy is very massive. It attracts the smaller massed stars that make up the disc keeping them in rotation around the galaxy. 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.
About 26% of the matter in the universe is of a form that is invisible to all know detection methods. It is dark.
Since the 1980s, scientists have actively conducted experiments designed to detect dark matter. None of the detectors have yielded positive results. This article from January 2014 Scientific American sums up the attempts from the 15 detectors worldwide. In general, the prospects look discouraging for finding out what dark matter is made of. New ideas and theories will need to be devised.
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
Observation since the 1990s show the universe is expanding more rapidly as time passes. It contradicts the traditional belief that it was expanding but slowing down. It was believed that some day it would stop expanding and begin to fall inward, contracting to a Big Crunch. That does not seem to be the case. Some form of invisible dark energy is causing this expansion.
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 predictable uniform behavior allows astronomers to use them as a sort of standard brightness candle. By observing how bright very distant supernovae appear, and comparing to how bright supernovae are at closer known distances, one can calculate the distance to the farther ones. It is the same concept as judging how away far car headlights are by how bright they appear.
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. It has proven extremely valuable at knowing distances to objects far out in space. The supernova studies in the 1990s showed that the farthest distant points in space are actually farther than Hubble’s Law predicts. Space is expanding at an increasing, or accelerating rate due to some unknown invisible energetic source.
About 70% of the Universe is in a form of energy capable of creating that acceleration. It is not known what it is.
You might prefer to watch this 12 minute video from KQED-TV about the discovery of dark energy. It is very good.
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
Below is a graphic of the growth and change of the universe over 13.7 billion years from the Wilkinson Microwave Anisotropy Probe (WMAP) mission. The far left depicts the earliest moment we can study, what most people call the Big Bang. A period of “inflation” produced a burst of exponential growth in the universe. The vertical scale represents the size of the universe. Time is on the horizontal scale. For the six or seven billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravitational force.
But, notice how the size of the cone is flaring outward as time progresses. In the more recent billions of years, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The graphic shows this effect as the shape grows increasingly wider at the right end. This is the accelerating expansion due to dark energy discovered from supernova studies in the 1990s.
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 96% of the universe. For now, there are no definitive answers.