(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

For previous posts in this series, see here.

In the study of our universe so far, one fact becomes resoundingly clear. Humans occupy a tiny volume of the universe. All our scientific theories have been discovered using data that has been generated within that volume. What gives us the confidence that these same laws can be applied to distant regions as well? One answer is that we have no choice but to make that assumption. Another is that when do make such an extrapolation we get a reasonably satisfactory understanding of the behavior of distant stars and galaxies, thus justifying our decision.

But perhaps the most important reason is the Hubble result discussed earlier, that every distant galaxy is moving away from us with a speed that is proportional to the distance from us. This could only happen if either the Earth occupied a privileged place in the universe or if the universe was such that there is no such privileged place at all and every point in the universe is equivalent. The former option has been abandoned ever since the Copernican revolution. Since the location of the Earth is no different from any other point, the laws we discover here must be the same laws that apply everywhere.

This leads to what is called the Cosmological Principle, the idea that the universe is homogenous (i.e., is the same irrespective of which point in the universe we may happen to find ourselves in) and isotropic (i.e., looks the same irrespective of which direction in the sky we choose to look). But it is not assumed that the density of the universe is a constant in time, which distinguishes it from the Perfect Cosmological Principle that led to the Steady State theory. In fact, the Big Bang theory explicitly argues that the universe is continuously expanding and getting less dense as it does so.

Of course, the homogeneity and isotropy of the universe is true only on a large enough scale. On small scales, we see all kinds of non-uniformities. After all, most of space is empty with just a few pockets of dense matter consisting of stars, planets, and galaxies. For example, there is no planet like Earth anywhere near us, and when we look out at the night sky, the direction that contains the plane of our local galaxy (the Milky Way) looks very different from what we see when we look in other directions.

Furthermore, even on a large scale, the universe cannot be perfectly homogenous and isotropic because that would not have allowed for the matter that existed at the time of the Big Bang to eventually separate into the clumps that eventually led to stars and galaxies. In order to explain star formation, cosmological theories predict that the early universe must have had slight inhomogeneities and that there should be visible traces of this history. If we look out into the universe and measure its temperature in all directions, there should be very slight variations in temperature, of the order of one part in a hundred thousand. The Cosmic Microwave Background Explorer (COBE) satellite was launched in 1989 to investigate this and its results released in 1992 found just this variation, further supporting the Big Bang theory. In the image below, the changes in color show the minute temperature variation of the cosmic microwave background radiation, which corresponds to the density variation.

Although the Big Bang cosmological theory has been very successful, along the way some problems have arisen that have led to interesting developments. One problem was with the motion of stars on the outer edges of rotating spiral galaxies. If we apply established theories of gravity and assume that all the mass in the universe is what we can ‘see’ (i.e., matter we are already familiar with and can be observed by our detectors because they emit electromagnetic radiation), then we can calculate the speeds those stars should have. But the pattern of speeds that were observed does not agree with those predictions. The problem can be solved if we assume that there exists matter that we cannot see, i.e., matter that is outside the detection range of our detectors, although it still exerts gravitational forces since it has mass. For this reason, this new form of matter has been given the name ‘dark matter’.

This so-called ‘dark matter’ has still not been directly detected but fairly strong circumstantial evidence has convinced most physicists that it should exist and that there is a lot it around. The amount of dark matter present is currently estimated to be about five times the visible matter that we know about and can see. Of course, if it is the dominant form of matter in the universe, then it becomes vital that we learn more about it and major efforts are underway to try and detect it. The difficulty with this endeavor, of course, is that while this dark matter may consist of things that we are familiar with (such as dust grains, nuclei, and small rocks), it is also quite possible that this matter consists of entities unlike anything that we have encountered before. So we are in a very real sense searching in the dark, not really knowing what we are looking for, how we should look, and how we will know if we have detected it. All we really know is that there seems to be a hell of a lot of it.

But that is just the kind of puzzle that scientists relish and major efforts are currently underway to solve it.

Next: If the dark matter puzzle isn’t enough to keep scientists busy, we now have dark energy.

POST SCRIPT: Honoring death wishes

From That Mitchell and Webb Look.