29:50 Modern Astronomy
Fall 1999
Lecture 28 ...November 1, 1999
Large Scale Structure in the Universe

Tentative plan for Thursday night, 7PM for observing
Watch the skies! Watch the skies!
SC1 chart of eastern skies right now.

Last time discussed supermassive black holes as sources of power for radio galaxies and quasars. Evidence for this
(1) We can see a black hole with a mass of in the center of the Milky Way.
(2) We see many other black holes in other large galaxies; example was M106 with .

There are some striking examples in nearby giant elliptical galaxies:
(1) Picture of M87
(2) Picture of Jet of M87
(3) Picture of Accretion Disk in M87
(4) Picture of Accretion Disk in NGC4261

Conclusion: It is virtually certain that the accretion disks surrounding supermassive black holes power the radio galaxies and quasars we see. It seems highly likely that every large galaxy like the Milky Way has at least a several million solar mass black hole at its center. Think of this when you look at Andromeda and M33. It is even conceivable that these black holes formed the ``nucleation center'' around which the galaxy formed.
Birth of a Quasar

Large Scale Structures in the Universe

(3) ``We are now at the point, when thinking about the universe as a whole, of considering galaxies like atoms in a gas.''

The question at the moment is whether galaxies are distributed uniformly, like atoms in a gas, or whether they are clumped into even bigger entities, clusters of galaxies or even galaxies of galaxies.

Clusters of Galaxies certainly exist. The Virgo Cluster is the nearest of them. The Abell catalog of Virgo-like clusters, which appeared in 1958, contained 2700 clusters of galaxies.
Map of Abell Clusters.
(4) We can repeat the question as to whether the clusters are uniformly distributed or are organized into clumps.
Illustration of uniform and clumped distribution of points.
Poster with galaxies.
To answer this, you need measurements of distances to galaxies. You can do that if you measure the distance to a whole bunch of galaxies. There are many programs to measure the distances to very many galaxies. At the present, we have such data on 11000 galaxies in the northern celestial hemisphere, and 12000 in the southern. The results are extremely interesting.
Figure 25.27 from book.
Figure 25.29
Space locations of 9325 galaxies
These surveys show the existence of vast Voids in space, which sometimes are 100 Megaparsecs on a side. At the present it is not known if these are completely empty of all matter, or simply are places where galaxies have not formed.

(9) ``More data are needed''. To better determine the nature of these voids, we have to have more galaxies, covering a much bigger volume of space. At the present time a project called the Sloan Digital Sky Survey is underway with a dedicated large telescope. During the 5 year project, the goal is to measure the distances to one million galaxies and 100000 Quasars. This will give us a much clearer view of these voids.

Cosmology

Up until now we have discussed all the objects in the universe. We will end by discussing the nature of the universe itself. This subject is called Cosmology. Cosmology deals with questions such as how hold the universe is, what it will be like in the remote future, and what conditions were like in the beginning. Traditionally, one thinks of this as more in the realm of origin legends, religion, etc rather than in the sphere of hard-nosed physical science. The development which led to this as a discipline within astronomy came about in the 1920's, due to a discovery by Edwin Hubble.

At one level the ability of extragalactic astronomy to say something about the origins of the universe can be described very simply. As we have discussed in the last several lectures, we can detect extragalactic objects at vast distances from the Earth. It takes even light vast intervals of time to traverse such distances. For example, the Virgo cluster of galaxies is about 50 million light years away, so we are seeing these galaxies as they were 50 million years ago. We are seeing 50 million years back in the history of the universe.

Now it turns out 50 million years back is not all the long on the timescale of galaxies, but as we say, the Virgo cluster is the closest of the major clusters of galaxies, so we can do much better. The most distant of the ``Abell clusters'' are 4 billion light years away, and we can easily see objects more distant still. The most distant objects in the ``Hubble Deep Field'' are of order 10 billion light years or more away. In this case we are seeing much further back in time, and over times which are long, even for galaxies.

If there were a beginning to the universe, we should be able to see far enough that there would be no more objects. We would see an end to things. As I have said, we can see evidence for exactly this in the redshift distribution of quasasrs. So by looking out to ever greater distances in the universe, we can address this question as to whether the universe had a beginning. As you know from the comments I have dropped all semester, there is a positive answer to this question.

The discovery which led to the realization that the universe had a beginning is Hubble's Law, which I have discussed as the only way of determining distances to distant galaxies.

The precise value of the Hubble constant is currently believed to be about 70 kilometers per second per Megaparsec.
Hubble's Law has a number of major consequences.
First It shows that the universe is expanding, or getting bigger with time. Every second, the distance between us and every other galaxy is getting bigger, so the universe is expanding. An analog of this is shown in Figure 23-35 of the textbook, which shows the same thing applied to raisins in a loaf of bread.
Transparency with raisin bread analog.

The Second major consequence of Hubble's Law is the physical significance of Hubble's constant. It is nothing other than the reciprocal of the time since the expansion started. Stated somewhat differently, we now see all galaxies moving away from us. If we ran the film backwards, we would see the galaxies coming nearer and nearer, until eventually everything was on top of everything else. The time from this infinitely compact state to the present is the reciprocal of the Hubble constant. Actually, it turns out that the reciprocal of the Hubble constant actually overestimates the time since this universal expansion began. Nonetheless, the true age is quite close to that given by the Hubble constant.

The fact that the Hubble constant is related to the age of the universe can be illustrated in a very simple, and equivalent way. Let's imagine we are standing on a desert lake bed and we see cars driving away from us in all directions. Furthermore (in analogy with the true Hubble's constant) we see that the further away they are the faster they are moving; the cars one mile away are driving at 60 miles per hour, those two miles away are driving at 120 miles per hour, and those three miles away are driving at 180 miles per hour.

It is clear from this observation that in the past, cars were much closer to you (and to each other) than they are now. You can then calculate how long ago the cars were at the point where you are now standing. In the case of the cars at one mile, you known that an object driving 60 miles per hour is going at a mile a minute. Thus is would have been ``here'' one minute ago. The car two miles away is receding at twice the speed, but has twice the distance to travel. It turns out it would have been at your position one minute ago as well. And so on. You would conclude that there was an infinite concentration of cars one minute ago.

You would be able to deduce the time since this concentration from the value of the ``Hubble'' constant. If the ``Hubble constant'' were half as large a value, i.e. 30 miles per hour per mile, the time since the expansion of the cars would be twice as long.

Similar calculations pertain to the cosmic case. If km/sec/Mpc, the time since the expansion began is 18 billion years. In the case of km/sec/Mpc, the corresponding value is 10 billion years. Again remember that the true value for the age of the universe will be somewhat smaller than this.

These are interesting numbers. First, it shows that the universe as a whole is not vastly older than the solar system with an age of 4.5 billion years. Second, the answer ``makes sense'' in that it is in the range of the oldest objects we know of in astronomy.
Question for audience: what are the oldest objects we have discussed in class?
The third point is that it is something of a tight fit. In the case of the reciprocal of the Hubble constant, the upper limit to the age of the universe, is 10 billion years. There are objects which we believe are older than that. Obviously the universe must be older than the oldest discrete objects, so either the Hubble constant must be lowered, or we have made some mistake in determining the ages of these ``oldest objects''.

Will the expansion go on forever? A natural question to ask is if the universe is expanding now, will it always be so, or will there be a time in the remote future that the expansion comes to a stop, and the universe begin contracting. If this is the case, in the remote future you would see galaxies approaching us. This can be expressed in the form of a graph.
Plot of l(t). Figures 26-12 and 26-13
Think of the y axis as the average distance between galaxies. Hubble's Law says that l is increasing with time. If the expansion has always occurred at the same rate, we have a straight line. This is referred to as an empty universe. Another possibility is that l will continue to increase without bound in the future; if this is the correct description, you could come back at any time in the future and still see the galaxies moving apart. This is referred to as an open universe. Another possibility is that of a closed universe, in which l will reach a maximum at some point, followed by a contraction.

Between the open and closed cases is one in which the mean distance between galaxies would increase and reach a maximum value at .

Obviously only one of these mathematical models can describe the true universe, so the questions arise as to what determines the true model, and which one we live in.

For the next couple of weeks I will discuss what kind of mathematical model we use to describe the universe as a whole, what influences the future history of the universe, what things were like in the very beginning (the ``Big Bang''), and what observational evidence we have for all of this.

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