In Search of Dark Matter

by Ken Freeman, Geoff McNamara

Written for the educated non-scientist and scientist alike, the book spans a variety of scientific disciplines, from observational astronomy to particle physics. Concepts that the reader will encounter along the way are at the cutting edge of scientific research, however the themes are explained in such a way that no prior understanding of science beyond a high school education is necessary.

Genres: Science, Physics, Astronomy

174 pages, Paperback

First published January 1, 2006

Reviews

[Mohamed · Rating 4 out of 5]

There is no shortage of ideas as to what the dark matter could be. In fact, the problem is the opposite. Serious candidates have been proposed with masses ranging from 10^-5 eV = 1/8*10^-41 kg =9*10^-72 "of sun mass" (axions) up to 10^5 "of sun mass" (black holes). That's a range of masses of over 75 orders of magnitude! It should be clear that no one search technique could be used for all dark matter candidates.



Even finding a consistent categorization scheme is difficult, so we will try a few. First, as discussed above, is the baryonic vs non-baryonic distinction. The main baryonic candidates are the Massive Compact Halo Object (Macho) class of candidates. These include brown dwarf stars, jupiters, and 100 "of sun mass" black holes. Brown dwarfs are spheres of H and He with masses below 0.08 "of sun mass" , so they never begin nuclear fusion of hydrogen. Jupiters are similar but with masses near 0.001 "of sun mass". Black holes with masses near 100 "of sun mass" could be the remnants of an early generation of stars which were massive enough so that not many heavy elements were dispersed when they underwent their supernova explosions. Other, less popular, baryonic possibilities include fractal or specially placed clouds of molecular hydrogen. The non-baryonic candidates are basically elementary particles which are either not yet discovered or have non-standard properties.

Outside the baryonic/non-baryonic categories are two other possibilities which don't get much attention, but which I think should be kept in mind until the nature of the dark matter is discovered. The first is non-Newtonian gravity (MOND theory).
but watch for results from gravitational lensing which may place very stron constraints. Second, we shouldn't ignore the ``none-of-the-above" possibility which has surprised the Physics/Astronomy community several times in the past.

Among the non-baryonic candidates there are several classes of particles which are distinguished by how they came to exist in large quantity during the Early Universe, and also how they are most easily detected. The axion is mentioned as a possible solution to the strong CP problem and is in a class by itself. The largest class is the Weakly Interacting Massive Particle (Wimp) class , which consists of literally hundreds of suggested particles. The most popular of these Wimps is the neutralino from supersymmetry . Finally, if the tau and/or muon neutrinos had a mass in the 2 eV to 100 eV range, they could make up all or a portion of the dark matter (this is not the ordinary Standard Model neutrinos) .


Another important categorization scheme is the ``hot" vs ``cold" classification. A dark matter candidate is called ``hot" if it was moving at relativistic speeds at the time when galaxies could just

start to form (when the horizon first contained about 10^12 "of sun mass" ). It is called ``cold" if it was moving non-relativistically at that time. This categorization has important ramifications for structure formation, and there is a chance of determining whether the dark matter is hot or cold from studies of galaxy formation. Hot dark matter cannot cluster on galaxy scales until it has cooled to non-relativistic speeds, and so gives rise to a considerably different primordial fluctuation spectrum. Of the above candidates only the light neutrinos would be hot; all the others would be cold.

this could summary all the book in a good and more accurate manner (in the problem of the search process not how the dark matter idea starts)

[Gilda Felt · Rating 4 out of 5]

It’s been a lot of years since I took physics in high school, but the book lived up to its promise; I was able to understand all the concepts outlined and ended with a greater understanding of how we know that dark matter is there, and what it may be. That says a lot considering the short length of the book.

Unfortunately, there’s no clear answer. We know quite a bit about how the universe works, just not enough. The complete answer to the question of dark matter still basically eludes us.

[Charles · Rating 5 out of 5]

For some time, it has been known that the universe is expanding. There is a well-defined point of origin that is known as the moment of the Big Bang. As the universe exploded into existence the amount of energy and mass were fixed and the various components few away from each other.
With this fact established, the next major fact that was pursued was determining the amount of mass in the universe. If the amount was above a certain level, then the mutual gravity would cause the components to eventually slow down, reverse their direction and then come together in an event that was referred to as the Big Crunch. If the amount was less than the threshold, then the universe would continue to expand forever, eventually experiencing a heat death.
Therefore, there is a concerted effort to determine how much matter is in the universe. Initial computations of the amount of matter that was visible clearly indicated that this was a small percentage of the total matter. The mutual behavior of the components indicated that there was much more matter in the universe. This missing matter was referred to as “Dark Matter.”
This book is an excellent popular explanation of all facets of this fundamental issue in our understanding of the universe. It starts with an explanation of the fundamental issue of the ultimate fate of the universe and then moves on to the obvious lack of known matter. Following this there are simultaneous explanations of the various types of potential dark matter and the ways in which the improvements in the collection of astronomical data have made it possible to investigate whether the specific postulated forms of dark matter do in fact exist. All are done with a minimum of equations and with images that encapsulate the information.
While it is unlikely that any human will live to see the universe approach its end, it is important to learn what that end will be. This book gives a great deal of information regarding the search for this ultimate truth.

[Peter]

Dark matter must exist despite an overwhelming lack of direct evidence because all our equations are wrong without it? Scientist want us to believe in something more elusive to measurement then God because they look incompetent without it. I really thought they would have something other then a few bad equations in need of a fudge factor to explain dark matter.

[Pete · Rating 4 out of 5]

Good explanation of something we cannot see, yet we must believe exists because our experiences tell us it must. Otherwise, the universe could not exist in the manner in which see it. Seems to me that it brings religion and science closer together. To believe in dark matter takes a certain kind of faith on scientists' part.

[Gregory · Rating 4 out of 5]

I good overview of the recent history of astronomy and cosmology. It covers a great deal and does it well. There are a few subjects that it tackles that might have been hard to understand if I wasn't already familiar with them.