Which particles are the most abundant with mass in the universe?

Which particles are the most abundant with mass in the universe?

• A. Neutrinos
• B. Protons
• C. Photons
• D. Electrons

Answer: (A)

The main idea here is that when we talk about particles that have mass in the universe, relic neutrinos win in sheer numbers. After the Big Bang, neutrinos decoupled from matter very early and have been free-streaming through space ever since. This creates a cosmic neutrino background with a huge number density, on the order of a few hundred neutrinos per cubic centimeter for all flavors combined. Photons, while more numerous, don’t count because they are massless. Protons and electrons do carry mass, but their overall numbers per unit volume are much smaller in the vast, mostly empty space between galaxies compared with the dense, uniform background of relic neutrinos. So, among particles with mass, neutrinos are the most abundant simply because there are so many of them in the universe, even though each individual neutrino is incredibly light.

The main idea here is that when we talk about particles that have mass in the universe, relic neutrinos win in sheer numbers. After the Big Bang, neutrinos decoupled from matter very early and have been free-streaming through space ever since. This creates a cosmic neutrino background with a huge number density, on the order of a few hundred neutrinos per cubic centimeter for all flavors combined. Photons, while more numerous, don’t count because they are massless. Protons and electrons do carry mass, but their overall numbers per unit volume are much smaller in the vast, mostly empty space between galaxies compared with the dense, uniform background of relic neutrinos. So, among particles with mass, neutrinos are the most abundant simply because there are so many of them in the universe, even though each individual neutrino is incredibly light.