Philosophical Society of Washington

Minutes of the 2088th Meeting

Speaker: Richard Mushotzky, Astrophysicist, NASA Goddard Space Flight Center
Topic: “Clusters of Galaxies as Cosmological Probes”

President Agger called the 2088th meeting to order at 8:15 p.m. on March 20, 1998. The Recording Secretary read the minutes of the 2087th meeting and they were approved.

The speaker for the 2088th meeting was Richard Mushotzky, an astrophysicist at the NASA Goddard Space Flight Center. The title of his talk was, “Clusters of Galaxies as Cosmological Probes.”

Astronomers are trying to understand the origin and evolution of the universe for us. To do this they address cosmological parameters which include traditional standards such as the Hubble constant, HO, which sets the scale of the expansion rate of the universe, and thus, its age and size. Another fundamental parameter is the mass density of the universe, which is a measure of how much gravitational interacting material actually exists. Other parameters of interest include, how much of the matter in the universe is “normal” and how much of it is “dark?” Also, how did the structures of the universe originate and grow? This is determined by measuring the amplitude of the fluctuations in the universe — or measuring its inhomogeneity. Clusters in the universe aid in this work.

Based on the visual, clusters are many galaxies in a small volume. Based on X-rays, clusters are entities with a huge amount of very hot gas in the same volume. Further, clusters are the largest bound objects in the universe. Since clusters retain an imprint of how they were formed, studies of them have profound implications for the origin and evolution of structure in the universe. Consequently, they provide a fair sample of the universe as a whole and act as a reasonable universe-laboratory. The results of this modeling show that 200f the material in the universe is normal matter and the rest is dark matter. This establishes the dominance of dark matter as 5 times that of normal matter. Mr. Mushotzky pointed out that there is no dark matter in our solar system but there is dark matter in our galaxy. The cluster modeling also indicates that our universe is not closed.

Clusters are very large objects. To give you an idea of the relative size of a cluster, they have an angular size of about the diameter of a full moon and are about a billion light years away. They are primarily X-ray objects. One of the surprises of the last 20 years is the realization that most of the “normal” material in clusters is in hot, X-ray emitting gas. The mass of cluster gas exceeds that in individual galaxies by a ratio of 5:1. The recent advances in X-ray astronomy allow the measurement of the mass of the cluster and the determination of the total amount of heavy elements in the hot cluster gas from X-ray imaging spectroscopy. Most of the “normal material” in the hot, X-ray emitting gas is enriched in iron, oxygen and silicon.

The force of gravity in the clusters is so strong it bends light such that they operate as gravitational lenses, as predicted by the theory of general relativity. A gravitational lens makes a very distorted and brightened image of the background objects — often creating giant arcs. The size, shape and number of arcs is a measure of the magnitude and shape of the gravitational field in the cluster and can thus be used to measure the cluster mass.

Mr. Mushotzky then closed his presentation and kindly answered questions from the floor. President Agger thereupon thanked Mr. Mushotzky for the society, announced the next meeting and made the usual parking announcement. She then adjourned the 2088th meeting at 9:44 p.m.


Respectfully submitted,
Bill Spargo
Recording Secretary

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