What happened with COBE. COBE is a satellite whose purpose was to measure the microwave background from above the atmosphere. When I was a kid, there were two competing theories about the origin of the universe: the "Steady State" theory, which was designed to save the idea of a universe that had been here forever; and the "Big Bang" theory, which was based on the fact that observations of the galaxies showed they were moving away from the earth in a fashion consistent with the idea that the universe was expanding from a singularity, or single point it had occupied in the distant past.
At the time, the Big Bang seemed to be favored by religious people, since it was consistent with the notion that things began sometime, like the Bible said, rather than having been here forever. Steady state was favored by the "atheists", and people who prefered the idea that the age of the universe was infinite, that it has always been around. Ironically, now that it has won the controversy, the Big Bang is poo-pooed by the religionists. This is, presumably, because measurements lead to the conclusion that the expansion of the universe has been going on for nearly 14 billion years, and that is a lot longer than some religionists have been able to handle.
Anway, the following discussion tries to explain the profound significance of the picture that is at the first of this post. The Steady State theory said that the universe has alway been there. I learned about the theory from a book by Fred Hoyle, an astronomer who advocated it. At the time (this was in the late 1950s) it was also known that the universe was expanding. The expansion was a problem for an infinitely old universe, since if it had been expanding for an infinitely long time, then everything should be infinitely far apart, and humans shouldn't be able to see anything in the sky -- it should be dark. The explanation for this was that the expansion was occurring, but matter was being spontaneously created to fill in the space that was left by the expansion. If this sounds weird, it was, but the rate that matter would have to be created was so small that its creation couldn't be measured by the technology of the time (it probably couldn't be measured with today's technology either, for that matter).
The Big Bang theory, on the other hand, said that there was an event in which the universe began, and that event was actually the greatest explosion that has ever been seen (with accurate measurements the consensus now is that this event occurred about 13.7 billion years ago, give or take a few minutes). In other words, God said, "Let there be light," and he wasn't kidding.
Hoyle preferred the Steady State theory, and he seemed to do so because the Big Bang was consistent with the idea of a creator. Although I loved the book -- so much in fact that nearly 50 years later I credit it with influencing me the most in my choice of career -- I preferred the Big Bang for the same reason. The question was, how do you decide which theory is correct (or for that matter if they were both wrong? There's no rule that says when people have come up with two ideas to explain something, that at least one of the ideas is right. They could both be wrong, no one is has been smart enough to imagine yet what the right answer is).
The decision between the theories went something like this -- If you look at the physics of any explosion you find that radiation (heat, light) is produced. For the Big Bang, this heat and light expands to fill the universe that is created with the explosion. As the heat and light of the Big Bang explosion expand, physicists also know that the wave lengths of the radiation get longer and longer, because the universe that was created at the explosion is expanding. But while it continues to expand, it turns out this radiation -- a form of energy -- should never go away or disappear. Its wavelengths just get longer and longer (longer and longer wavelengths correspond to cooler and cooler temperatures). This radiation, it was then known in the 1950s and 60s, should still be around 13.7 billion years later.
On the other hand, if the universe had always been here as the Steady State postulated, then there was no reason for this radiation to exist. Therefore, if it could be detected then that would be strong evidence in favor of the Big Bang. Otherwise, scientists were inclined to go with the Steady State theory.
Furthermore, the radiation should have a non-uniform distribution according to wavelength ... that is, there should be more energy at some wavelengths than at others. Specifiically there should be a lot at very long wavelengths and not much at all at much shorter wavelengths. The shape of this distribution, or spectrum, is also well known, and is predicted very precisely by an equation which is derived in undergraduate physics courses. I learned the derivation the first time in a junior level course at Baylor University in 1966.
It's often called a "black body spectrum" because it is the distribution of radiation that it emitted from an object purely by virtue of the objects temperature. Everything that has a temperature -- and everything in the universe has a temperature, there is nothing at absolute zero although some things are close -- emits radiation. The radiation that is emitted has a spectrum or distribution of wavelengths, and this distribution depends on the temperature of the object. The lower the temperature, the longer most of the wavelengths are that are emitted. Scientists knew that the universe must be billions of years old (they could figure this out by seeing how fast the universe was expanding and how far away things were, and caculating how long it must have taken at that speed for the universe to get as big as it was), so they were able to estimate from what they already knew about "black body" radiation about where in the range of possible wavelengths the spectrum of radiation left over from the big bang ought to lie.
The problem was, if the universe was as old as they thought, the wavelengths of this radiation would now be very long (early on when the universe was hot, the wavelengths were very short, but now when it is larger and cold, the wavelengths should be very long). A lot of these long wavelengths turn out to be absorbed by the atmosphere, although some get through. The first detection of them was by accident, even. Later detectors were sent into the atmosphere with balloons to get better measurements, but the bottom line was that while the radiation was detected at certain wavelengths, and most scientists believed that it was there, and hence that the Big Bang was right, nevertheless, the interference of the atmosphere prevented them from seeing the whole, complete spectrum that they new must be there.
So, when we started sending satellites into orbit, one of the things that became possible was putting a satellite into orbit above the atmosphere to more precisely measure the complete spectrum. That is what COBE is ... the COsmic Background Explorer. Its purpose was to measure the spectrum of the black body radiastion left over from the birth of the universe.
The picture is a graph of those first results. I'm a physicist myself, but when it comes to cosmology I'm much more of an interested (and awed) bystander. If cosmology were baseball, I wouldn't be on a team; but I'd be a fan who watches a lot of games and has a pretty good grasp of the stats of at least some of the players. So it turns out that when I saw the picture above for the first time, I was amazed. I don't remember what I did exactly, other than point it out to others who could appreciate it, but in my mind it was awesome. COBE measured some thirty or more points all along the range of the spectrum. The solid curve is the graph of the fairly simple equation generated by the theory that I studied in college. The points cannot be seen because they are covered/obscured by the curve. That is, the measurement was very precise (the "error bars" are less than the thickness of the curve that is drawn on the graph), and the points almost exactly fit the theory. What variation they have from the theory again is so small that they are within the thickness of the line on the graph.
For someone who has experience in the comparision of theory to measurement, the degree of agreement is astounding, stunning. When one considers the greater meaning and significance of the measurement, it can be a very emotional experience. The link at the first of this post is a narrative which contains a description of the response of a scientific audience that was one of the earliest to see this spectrum 25 years ago. A lot of work has been done since then. For example, scientists have already spent years studying the slight variations of the data from the theory -- that is, those variations that are within the thickness of the line that is in the graph -- because these variations contain information about why the universe looks today the way it does. But the simple, beautiful spectrum still marks a defining moment.
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