Is it possible that dark-matter is composed of a large number of neutrinos from the big bang?

Question

They seem to have all the properties of dark-matter (massive, with no electromagnetic interaction). Could it be that many of the neutrinos produced since the big bang have formed massive neutrino clusters that act as dark-matter?

Answer 1

Short answer: dark matter appears to be cold, but for any reasonable neutrino mass and dark matter temperature, neutrinos would be relativistic.

Answer 2

There are a couple of reasons why the three types of known neutrino (and their antiparticles) cannot be the missing dark matter.

We now know roughly what the masses of neutrinos are. The sum for all 3 is about 0.3eV and the heaviest one must be bigger than about 0.04eV.

If you work out how many are produced in the big bang, and you assume they are non-relativistic (see below) it turns out that their density today (about 56 cm$^{-3}$ for each of the six species) is about a tenth that of the luminous matter in the universe and about 0.3% of the density required to produce a flat universe and nowhere near the 25% required of any dark matter candidate.

The neutrinos decoupled from the rest of the matter shortly after the big bang when the universe was at temperatures of $10^9$ K, and they have now cooled to just below 2 K. Thus current background neutrinos have $kT/mc^2\sim 10^{-3}$ and are not highly relativistic, but still to fast to be trapped by individual galaxies. However, what is important is that when large scale structures were beginning to form, when the universe was at $10^6$ K, the neutrinos were very relativistic (hot dark matter). Such particles would act to erase structure and the universe would not be the way we see it today.

Thus even if you found a way to make neutrinos clump together (to overcome the second objection), there are not enough of them to overcome the first objection.

Answer 3

Not really, there are many models in which dark matter is assumed to be relativistic, and hence warm. For example here there are discussed some features in the cases where dark matter is composed from sterile neutrinos or light gravitinos. Also, dark matter can be a multi component field. One answer with high positive probability would be that dark matter, if it exist and the General Relativity in the shape as we know today is valid, would not be cold, but non-relativistic, i.e. warm.

Answer 4

Neutrinos were not just created in the Big Bang but ceased interacting with baryonic matter after one second when the temperature of the universe was approximately 10 billion kelvins, or 1 MeV. Because neutrinos have mass the force of gravity restricts them to galaxies. All main sequence stars produce neutrinos from the fusion process and if they don't interact with baryonic matter there is a good chance most of them are still around. Lisa Randall hypothesised a disc of 'dark matter' running right through the Milky Way. A 180-trillion-mile 'disc of death' was how the Sunday Times reported it in a review of her book on Dark Matter and the Dinosaurs. It is theorised that the Earth gets a gravitational 'nudge' every 35 million years as it passes through an alignment with this disc of dark matter. This parallels thinking on a dilemma posed by Jupiter's moon Io. Because Io is so far from the sun it should have cooled down by now but there appear to be volcanoes on the surface. This has been linked to the fact that Io is regularly stretched 100 metres by gravity alignment with Jupiter's other moons. Friction is thought to be the cause of the volcanoes on Io, which effectively means they may be fuelled by gravity alone as a power source. Lisa Randall's disc of dark matter is probably all the neutrinos in the galaxy obeying the speed limit. Because they cannot acquire energy to reverse time they are stuck in a galaxy until the black hole at its centre does something about it.