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X-raying Dark Matter

by Louise Good

Colliding galaxies

Two colliding galaxy clusters. In this image, dark matter is seen as blue, and ordinary matter, mostly in the form of hot gas, as pink. The separation between the pink and the blue provides direct evidence for the existence of dark matter. IfA's Harald Ebeling discovered this cluster. Credit: X-ray (NASA/CXC/Stanford/S.Allen); Optical/Lensing (NASA/STScI/UC Santa Barbara/M.Bradac).

Galaxy clusters are the most massive objects in the Universe and therefore can give us clues to important properties of the Universe, said IfA astronomer Pat Henry, who gave the Frontiers of Astronomy Community Lecture, "X-raying Dark Matter" at the Manoa Valley Theatre on September 30.

Modern scientists are fairly certain that about 14 billion years ago the Universe was very hot and dense, and that it has been expanding and cooling ever since. As the Universe cooled and expanded, slightly overdense sites grew by gravitationally attracting surrounding matter, so that over billions of years, stars, galaxies, and clusters of galaxies formed.

In the 1930s, Fritz Zwicky and Sinclair Smith measured the speed of galaxies in the Virgo and Coma Clusters, and determined that they must have 100 times more mass than the galaxies provided, because otherwise the galaxies in these clusters would fly apart. Zwicky and Smith had discovered dark matter, the unseen material (because it gives off no light) that is believed to be the majority of matter in the Universe.

In the 1970s, scientists began using observations of X-rays to learn about the Universe. In 1979, Henry and his colleagues reported on "the first extensive detection of X-rays emission" from very distant clusters of galaxies. This has led to some important discoveries by Henry and many other scientists.

They found that hot gas (hydrogen and helium at a hundred million degrees) pervading galaxy clusters is much more massive than the galaxies themselves. When clusters collide, most of this normal matter (the hot gas) separates from the dark matter because when the hot gas from one cluster crashes into that of another, they slow each other down by their "wind resistance." But the collision does not slow down the dark matter from the two clusters. It passes right through. Astronomers say that dark matter does not "interact strongly" with itself.

Another way we can know that dark matter exists is to see the effect of its mass, which bends light. This is called gravitational lensing. When a galaxy happens to be behind and precisely aligned with a cluster in the sky, the gravitational field of the cluster (strongly enhanced by its dark matter) distorts the image of the more distant galaxy. The amount of distortion indicates the amount of the cluster's mass.

The growth rate of clusters depends on the amount of matter and energy in the Universe because they stop growing once they have attracted everything within their gravitational reach. Since it would take billions of years to measure the growth of a cluster, scientists use the fact that light (including X-rays) takes time to travel from distant objects to Earth to study clusters of different ages. If a cluster is very far away, say a distance of a billion light-years, we are seeing it as it was a billion years ago. So Henry and his colleagues took X-ray pictures of clusters at a range of distances representing a range of times since their X-rays left the clusters and then measured the growth rate of the clusters' mass. They saw that the mass of the clusters was growing over time. This scientific result, achieved in the early 1990s, was greeted by great skepticism that has gradually lessened over the years as more and better data have been obtained. These data imply a Universe that is 75 percent dark energy, 21 percent dark matter, and 4 percent normal matter. Because of the amount of dark energy, the Universe will expand forever.

Henry stated, "The nature of dark matter and dark energy is the most fundamental problem in cosmology, and some say, in all of physics." He also emphasized that because we have no fundamental theory that explains what dark matter and dark energy are, today's scientists may be very wrong about these two concepts, just as ancient and medieval astronomers were wrong about Earth being the center of the solar system.