Could Dark Matter Be Brown Dwarfs?   The trouble with assuming that dark matter is the hulks of dead stars is that, at least in our neighborhood, stars seem to be the best part of half a dozen light years apart, and if dark matter consists of relatively heavy things, the total number of them wouldn't have to be very many times greater than the number of visible stars, and so they would still be scattered pretty thinly around the Galaxy, certainly several light years apart. At those large distances, even white dwarfs are difficult to see, without knowing what to look for, and neutron stars and black holes are completely unobservable. But suppose that the dark matter is not in the form of relatively heavy things, but much, much less massive objects, such as brown dwarfs, the "stillborn" stars that result when a cloud of gas has too little mass to become a Main Sequence star. Such objects are less than a tenth the mass of the Sun, and in many cases, may be considerably less massive than that, so if their total mass was ten times the mass of the visible stars, there would have to be an extremely large number of them -- at least a few hundred times as many, in fact.   At first thought, the idea that there would have to be such huge numbers of brown dwarfs for them to be the dark matter makes the whole proposal seem absurd, but there is a happy side effect of this result, because if they do exist in such large numbers, they must be literally all over the place. The Galaxy would have to be practically crawling with the things, so that they were no more than a few light months apart, instead of light years, as normal stars seem to be.   Now, even at a distance of a few light months, brown dwarfs would be hard to observe, because they are too cool to give off any visible light worth speaking of, and even with infrared telescopes would be very, very faint. But the technology to observe them does exist, and in fact has existed for the best part of 40 years, so if the dark matter were brown dwarfs, then, as soon as the idea was proposed, it should be possible to start finding a few, and then some more, and finally, such huge numbers that it would be obvious that this is the correct answer. This makes the idea much more attractive, since, as pointed out above, a good subject for study should be something that has a good chance of being proven right, or wrong, sometime SOON, and the theory of brown dwarfs as dark matter was, as a result, quite eagerly accepted, when first proposed. For quite a while, magazine articles about dark matter would confidently proclaim that most of the mass of the Galaxy, and of the Universe, was made up of brown dwarfs, and "any day now," brown dwarfs would be found to exist in such large numbers that the idea would be known to be correct.   Unfortunately, despite nearly four decades of looking for them, brown dwarfs don't seem to exist in anywhere near the numbers required to explain dark matter. In fact, for most of that time, they didn't seem to exist at all. At any given time in the last 40 years, there were a few, to a few dozen, "candidates" for brown dwarfs, objects whose observable properties suggested that they might be brown dwarfs, but were not yet known to be brown dwarfs, for sure. In every case, however, it was soon shown that the observed objects were not brown dwarfs, at least until very recently. As technology has improved, it has finally, within the last few years, become possible to observe brown dwarfs, and we now suspect that they might be as common as normal stars, or at least, not too many times less common.   The trouble is, since brown dwarfs are much less massive than normal stars, to explain the dark matter, they can't be that few in number. They would have to be far more abundant than they seem to be, and as a result, although they do contribute a very small additional amount to the "observed" mass of the Galaxy, they don't in any way help explain dark matter. Could Dark Matter Be Neutrinos?   Once it turned out that brown dwarfs wouldn't work, people working on the problem of dark matter turned their thoughts in another direction. Perhaps, instead of looking for a lot of relatively low mass objects, they should be looking for an incredible number of almost massless particles. Again, this sounds a little far-fetched, but there is actually a very good reason for hoping that this might be the answer -- the properties of the neutrino.   Neutrinos are massless, or nearly massless, particles which move through space at the speed of light, or very nearly the speed of light. They are known to exist in incredibly large numbers, as they are continually created inside stars, especially during the death-throes of massive stars, and were probably made in very large numbers during the Big Bang, in the Cosmic Fireball which began the Universe as we know it. As it turns out, it is extremely difficult to observe neutrinos, because they are practically "ghost" particles, capable of going through stars, or planets, as if they didn't even exist. Every second, it is estimated that several trillions of neutrinos pass through every square inch of the Earth's surface (and, therefore, your body, as well), but they don't notice that the Earth (or you) exists, and just go right through it, at least under normal circumstances.   Trying to observe such particles is obviously very difficult, but it can be done, using the right tools. You can't really look at all of them, but you can stop an occasional one, and from the theories which describe the physics of neutrinos, calculate how many there ought to be that we aren't managing to stop. This is, in fact, how we get the estimate of trillions per square inch per second, just above. Neutrino "telescopes" occasionally observe the effects of a neutrino interacting with material inside the "telescope," and by extrapolating to how many neutrinos didn't interact, we can estimate the total numbers.   The only trouble with this idea is that when their existence was first proposed, in the 1930's, as a way of explaining a troublesome physics experiment, neutrinos were thought to be massless. In the experiment, energy and momentum didn't seem to add up right, after the experiment, whereas mass did. So it was proposed that some kind of massless particle was carrying energy and momentum away, and the neutrino was "invented." A number of years later, particles having exactly the properties of the neutrino were observed, and were presumed to be the predicted particle. But if neutrinos are massless, then even in tens of trillions per square inch per second, scattered throughout the Galaxy, their mass-energy would be far, far less than the mass of known material, and would not explain the dark matter. So why was it proposed that neutrinos might be the dark matter?   As you have read in the book, neutrinos are created in the thermonuclear fusion reactions which power the Sun and other stars. Now that neutrino "telescopes" can observe neutrinos, it ought to be possible to observe the neutrinos coming from the Sun, and verify that they are indeed being created inside the Sun. But experiments to try to do just that have always shown that the number of neutrinos coming from the Sun is less than expected, in fact, at first, far less than expected.   To a certain extent, the fact that the number of neutrinos coming out of the Sun is less than expected could be due to errors in the theory of how the Sun works. We can't actually see what is happening inside the Sun, as there is far too much stuff in the way to see through, and in fact that is partly why people wanted to observe neutrinos in the first place. It is so hard for the light which is now being created in the center of the Sun to get out, that the light which is now leaving the surface of the Sun was actually created more than a million years ago, and in the long trip that it took from the center to the surface, it bounced back and forth, from one subatomic particle to another, trillions of trillions of times before finally reaching the surface, and during all those collisions, the light is completely changed, so that when it leaves the surface, it only tells us what conditions are like at the surface, and whatever happened to it during its long journey is completely "forgotten". For the neutrinos, however, the Sun might as well not exist, so when they arrive on the Earth, they are presumably completely unchanged from when they were created, only 500 seconds earlier. As a result, if we could properly observe them, we would gain valuable knowledge of the conditions in the center of the Sun.   Since the number of neutrinos coming from the Sun is lower than expected, a number of minor changes have been made in theoretical calculations of the Sun's structure, but there is a limit to how much those calculations can be changed, without making the results look quite different from what the Sun is known to be like. Most of the difference between the original estimates of how many neutrinos are coming from the Sun, and the observed numbers, can be explained by presuming that the core of the Sun is a little cooler than we originally thought, and a little denser, but there is still an intractable error of about 1/3 of the predicted amount, which can only be gotten rid of by changing the theory so much that it gives completely unreasonable estimates of what the Sun, and other stars, should look like.   This remaining error is sometimes referred to as the Solar neutrino "problem". For a long time, it was thought that the solution to the problem might lie in some astronomical factor not yet thought of, but in recent years, the opinion has been growing that the answer may lie, not in astronomy, but in the physics of the neutrino.