Cosmic neutrino background

The cosmic neutrino background (CNB or C $ν$ B) is the universe's background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos.

The C $ν$ B is a relic of the Big Bang; while the cosmic microwave background radiation (CMB) dates from when the universe was 379,000 years old, the C $ν$ B decoupled (separated) from matter when the universe was just one second old. It is estimated that today, the C $ν$ B has a temperature of roughly 1.95 K.

As neutrinos rarely interact with matter, these neutrinos still exist today. They have a very low energy, around 10⁻⁴ to 10⁻⁶ eV. Even high energy neutrinos are notoriously difficult to detect, and the C $ν$ B has energies around 10¹⁰ times smaller, so the C $ν$ B may not be directly observed in detail for many years, if at all. However, Big Bang cosmology makes many predictions about the C $ν$ B, and there is very strong indirect evidence that the C $ν$ B exists.

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