Neutrinos as Dark Matter

I've discussed neutrinos before, way back in June, but we can also look at neutrinos as a form of dark matter. As previously mentioned, neutrinos are highly non-interactive as it would take a block of lead an eighth of a light-year wide to stop just one neutrino. However, they are abundant as they are a product of the fusion going on in the cores of stars.
We also know that neutrinos come in six different flavors, depending on the associated lepton (electron, positron, muon, anti-muon, tau particle, and anti-tau particle) and that when they interact, they can change flavors.
But remember, that neutrinos have virtually no mass, though they are not massless. Since there are so many stars and have been billions of stars in our own galaxy creating neutrinos every time fusion occurs, there must be a lot of neutrinos existing in our galaxy, as well as other galaxies. A lot of something that has little mass ends up having a huge mass. Therefore neutrinos are probably another component of dark matter.
Again, the hard part to confirm that neutrinos make up dark matter is (1) that they are nearly massless, so any gravity they impart is small, and (2) they virtually do not react with matter. So although there is a lot of neutrinos out there, they are hard to detect.

Neutrinos

 

In the last post, we discussed how energy is created in the nucleus of the Sun via the proton-proton chain. One of the remnants of converting four hydrogen nuclei into one helium atom is a particle called an electron neutrino.

 

Why is an electron neutrino weird? Well, there are a few reasons.

 

1. Neutrinos are not exactly massless, but very near so. It is believed that neutrinos could be part of what is known as dark matter. If one can get enough neutrinos, they can make a substantial dent in the amount of dark matter theorized to exist.
2. They are highly non-reactive. Right now, there are approximately 65 billion neutrinos are passing through every square centimeter of your body right now. But don’t worry, they don’t like you. On average, it would take a block of lead one-eighth of a light-year* wide to stop one neutrino.

*one light-year is the distance light travels in one year. It is approximately 5.9 trillion miles, or 25 million times the distance from the Earth to the Moon

1. There is a way to count neutrinos, but it takes a lot of water. Here is a link to a neutrino detector in Ontario, Sudbury Neutrino Observatory
2. There a three types, or flavors, of neutrinos. These three neutrino flavors are associated with the three leptons; electrons, muons, and tau particles. There are also the anti-neutrinos associated with the antileptons; positrons (anti-electrons), anti-muons, and tau anti-particles.
3. Neutrinos can change flavors if they interact with matter. Neutrino detectors only count a third of that expected by the fusion of hydrogen into helium. This was known as the Solar Neutrino Problem until it was discovered that neutrinos can change flavor.