Table of Contents
“Invisible matter which construct the universe: DARK MATTER”
The actual density of the luminous matter in the universe is just a few percent of percent. Adding in the mass equivalent of the radiation in the universe increases the density only a little. But is luminous matter—the stars and galaxies we see in the sky-the only matter in the universe ?Apparently not.
Very strong evidence indicates that a large amount of dark matter is also present; so much, in fact, that at least 90 percent of all matter in the universe is nonluminous. For instance, the rotation speeds of the outer star stars in spiral galaxies are unexpectedly high, which suggest that a spherical halo of invisible matter must surround each galaxy.
Similarly, the motions of individual galaxies in clusters of them imply gravitational fields about 10 times more powerful than the visible matter of the galaxies provides. Still other observations support the idea of a preponderance of dark matter in the universe. What can the dark matter be?
The most obvious candidate is ordinary matter in various established forms, ranging from planet like lumps too small to support the fusion reactions that would make them stars, through burnt-out dwarf stars, to black holes. The snag here is that, in the required numbers, such objects would certainly have been detected already. Another possibility rooted in what we already know is the sea of neutrinos (over 100 million per cubic meter) that pervades space. Neutrinos appear to have mass, but very little, nowhere near enough to account for all the dark matter.
Indeed, if neutrinos were responsible for the dark matter, the universe could not have evolved to what it is today, galaxies, for example, would have to be much younger than they are. So neutrinos, too, may be part of the answer, but only part.
WIMPs and Axions
There is no shortage of other possibilities, all classed as cold dark matter, “Cold”. It’s means that the particles involved are relatively slow-moving, different, say, neutrinos, which constitute hot dark matter. Two main kinds of cold dark matter have been proposed, WIMPs and axions. WIMPs (weakly interacting massive particles) are hypothetical leftovers from the early moments of the universe.
An example is the photino, one of the particles predicted by the supersymmetry approach to elementary particles. The photino is supposed to be stable and to have a mass of between 10 and 109 GeV/c?, much more than the proton mass of 0.938 GeV/c?. Axions are weakly interacting bosons associated with a field introduced to solve a major difficulty in the Standard Model. WIMPs and axions are being sought experimentally, thus far without success.
The dark matter and dark matter needed to account for the motions of stars in galaxies and of galax-galaxies in galactic clusters brings the total density of the universe up to about 0.1pc. There may be still more dark matter.
In 1980 the American physicist Alan Guth proposed that, 10^-35 s after the Big Bang, the universe underwent an extremely rapid expansion triggered by the separation of the single unified interaction into the strong and electroweak interactions. During the expansion the universe blew up from smaller than a proton to about a grapefruit in size in 10^-30 s .
The inflationary universe automatically takes care of a number of previously troublesome problems in the Big Bang picture, and its basic concept is widely accepted. One of Guth’s conclusion was that the density of matter in the universe must be exactly the critical density Pc. If the inflationary scenario is correct, then, the universe is not only perfectly flat but as much as perhaps 99 percent, not merely 90 percent, of the matter in it is dark matter.
Finding the nature of the dark matter is clearly one of the most fundamental of all outstanding scientific problems.
Evidence of dark matter and properties of Dark Matter
The modern evidence for Dark Matter dates back to the early years of the last century. However, it is interesting to note that astronomical data had already faced a Dark Matter crisis before: in the 1840s, it was found that the data of the orbits of the planets in the Solar System was not consistent with the mass observed.
In particular, Uranus had an anomalous orbit. Le Verrier predicted a new source of matter that had not been detected before in the form of a new planet. He even predicted its orbit. This was discovered on the 31st of August of 1846. On the 23rd of September of 1846, Neptune was discovered. The Solar System had also an anomaly in the orbit of Mercury.
Le Verrier predicted an inner planet (Vulcan), and some observers claimed detection. But we know, this was not a real detection. Indeed, the orbit of Mercury is anomalous in Newtonian dynamics because of the need to include General Relativity corrections.
Hence, it was the theory of gravity that was going to fail. There is something that is important about these two examples, which is that Newtonian gravity which was not being accurate in two situations with two very different gravitational potentials (φN ∼ GM/r) and that the solution to one of the problem couldn’t fix the other one. For instance, General Relativity corrects Newtonian dynamics in situations where φN
corrections may be relevant.
The latter are very small for the outer planets. In the case of Dark Matter, we’ll also find a multi-scale phenomenology, that can be explained by a single hypothesis.