What might the dark matter be?

There has been quite a lot of speculation about what dark matter really is. Or what is constitutes of.

We know that dark matter can be anything that is non-luminous. Such as planets. The earth is not particularly massive, but planets like Jupiter are rather big. Still there is a problem. First, we assume that planets only form around stars. And if that is true, then even the biggest planets are dwarfed by stars near them. That means, they contribute only a few percent of their mass. Another Omega = 0.005 or so, which clearly isn’t enough.

A bigger problem comes from the Big Bang Nucleosynthesis (BBN). At the birth of the universe – as we understand it at this time – when the big bang occurred, the universe was an extremely hot soup of all sorts of particles. And then it grew and got colder and the ‘ordinary matter’ particles such as Neutrons, Protons and Electrons started to cool enough to form atoms of the material in the universe as we see them today. This is predominantly Hydrogen and Helium.

BBN is a great story. It not only conveys that Hydrogen and Helium are by far the most abundant elements in the universe, but also gets them quite accurately in terms of their proportions. But, there is a catch.

The amounts of each element that forms, depends very much on the amount of ordinary atom-forming matter; what are known as Baryons. BBN predicts all the right amount for the current universe only if the original amount of baryonic matter was about Omega=0.1

The amount of baryonic matter is larger than the amount of visible matter, hence there is some dark ordinary matter, like planets and burned out stars. But there is not enough to explain rotation curves and cluster velocities.

Dim Stars – Jupiter, Brown Dwarfs, White Dwarfs

Other ‘ordinary matter’ candidates include stars that are not that massive enough to start burning and become luminous. These are called ‘Brown Dwarfs’, or even Jupiter. If Jupiter was ten times heavier, it would have started burning as a small star. These possibilities suffer the same problem from BBN as planets. There just isn’t enough baryons to make a difference.

But it is always possible that BBN is wrong. And there are many looking into that too. Still its success has been rather high and people would rather concentrate on other possible solutions.

Exotic Matter

Even though the term exotic is used, Exotic matter does not have to be exotic at all. It is just matter that isn’t electrons, protons or neutrons. Lots of particles other then these are known to exist. And some theories do use them to solve the dark matter problem.

Neutrinos

Neutrinos are particles that are known to exist. They are thought to be massless, but there have been some recent evidence that they may have a small mass. There are so many neutrinos in the universe. That even a small mass would be important for dark matter, a mass of 93eV, or one five-thousandth the mass of an electron, would make Omega=1

WIMPS

Most other exotic matter falls into the category of WIMPS, or Weakly, Interacting Massive Particles. These are, as a class, heavy particles that only interact weakly with other matter. There are many possible such particles. They do have some exotic names, such as, neutralinos, axions and the like.

Changes to Gravity

One possibility – might seem quite far fetched – is that we don’t really understand gravity particularly well. We only know of gravity and its laws on individual objects and its effects on other object. What we do not know is if gravity can collectively act as a single force. Or even how gravity works on large size galaxies. This seems odd, but must not be discounted.