Lesson Objectives:- Dark matter and dark energy
- Dark matter in the Milky Way and other galaxies
- Dark matter in galaxy clusters
- Does dark matter really exist?
By observing gravitational effects on things such as stars and clouds of gas, scientists have discovered that there must be far more matter than what we see. For example, studying the rotation of the Milky Way galaxy seems to indicate that most of the galaxy's mass is in its halo, even though most of the stars and clouds of gas are in the disk and bulge.
This mysterious, invisible mass that emits no detectable radiation is called dark matter. It gives off no light, and does not seem to interact with other matter and energy in any way except through gravity.
Dark energy is a mysterious, unseen force that appears to be countering the effect of gravity on very large scales, causing the expansion of the universe to accelerate instead of decelerate with time.
Note that dark matter and dark energy have never been directly observed, but are thought to exist based on their apparent influence on the observed motions of stars and gas clouds, and the expansion of the universe.
There are several key pieces of evidence that lead scientists to believe that dark matter is the dominant source of gravity in our universe.
The first looks at distribution of mass in our Milky Way galaxy. As we have learned previously, looking at a star's orbit around the galaxy -- its distance from the center and its orbital speed -- allows us to calculate the amount of mass within that star's orbit. By looking at the orbits of different stars at different distances from the center of our galaxy, we get an idea of how mass is distributed in the Milky Way.
Based on how bright the central bulge of the galaxy is, we would assume that to be where most of the mass is concentrated. In that case, as stars get farther away from the bulge, their orbital speeds should get slower, just like Uranus orbits the Sun more slowly than Earth, which is slower than Mercury.
The reality, however, is different. Orbital speeds remain high no matter how far you get from the Milky Way's center, suggesting that most of the mass in the galaxy is distributed throughout the spherical halo that surrounds the disk of our galaxy. Since there is no detectable radiation coming from all this mass, we call it dark matter.
Analysis of orbital speeds and distances in other galaxies show the same pattern -- orbital speeds of stars remain fairly constant as we look farther from the galaxy's center, leading us to conclude that most of the mass must lie beyond the luminous, star-filled parts of the galaxy.
In general, studies of the mass distribution in our galaxy and in other galaxies indicates that the mass of the dark matter is over ten times greater than the mass of the galaxy's stars.
Looking at galaxy clusters, where multiple galaxies are orbiting the center of a cluster, reveals another source of evidence for the existence of dark matter.
There are three ways in which scientists look for dark matter in galaxy clusters. The first is by looking at the orbits of galaxies in the cluster. Just like in individual galaxies, the mass of the stars in the cluster does not seem to account for the total mass of the cluster.
The second method is to look at the hot gas that fills the space between a cluster's galaxies, sometimes called the intracluster medium. This hot gas can help scientists to calculate the mass of a cluster because its temperature depends on the cluster's total mass. Studies based on this method have shown similar results to looking at the orbits of galaxies, with dark matter seeming to make up forty times more mass than the combined mass of the stars in the cluster's galaxies.
Finally, scientists look for gravitational lensing, which is where massive objects can bend light beams. By looking at how strongly light paths are distorted, they can tell how massive an object is. When a galaxy lies far behind a cluster, the presence of gravitational lensing can help us determine how massive the cluster is.
The idea that dark matter exists is based on its observed gravitational effects on matter we can see. The only way dark matter does not exist is if some of our assumptions about how gravity works are incorrect. Most scientists do not think that is likely as Newton's laws of motion and gravity have been proven time and time again in many different scenarios.
What might dark matter be made of? The two basic possibilities are that it could be made of ordinary matter, also called baryonic matter, or it could be made of exotic matter, or nonbaryonic matter. Ordinary matter refers to matter made up of protons, neutrons, and electrons, but in a form too dark for us to detect with current technology. Based on the Big Bang Theory, however, calculations seem to indicate that not enough ordinary matter could have formed to account for all of the dark matter scientists believe they have found.
That leaves the possibility that dark matter is made of exotic particles that we have yet to discover. These hypothetical sub-atomic particles are called weakly interacting massive particles, or wimps, and may make up the majority of dark matter. Unfortunately, studies using dark matter detectors and particle accelerators have yet to confirm the existence of wimps.