Cosmology: Neutrinos as the Only Final Dark Matter

Abstract

Even though neutrinos and antineutrinos are everywhere in the Universe, their critical importance might be overlooked, especially because that at least one species of neutrinos has the mass 0.058 eV, far larger than the cosmic thermalization temperature 1.9^\circ K. The non-zero mass makes neutrinos participate the galaxy formation from the very beginning, in view of the process of clustering. Unlike the cosmic microwave background (CMB), the cosmic background neutrinos (CB\nu) cannot be uniform. Thus, we wish to examine the questions such as: Is there some new source for neutrinos or antineutrinos, that might be detectible experimentally? Is there some new interaction of neutrinos with the visible world, that may be of numerical importance at, e.g., the ultra high energies (\ge 10^{13} eV)? One major conclusion is that, on the basis of the Standard Model, neutrinos would eventually become the {\it only} dark-matter species left in our Universe. Our Cosmos is limited in energy for various particles, electrons or photons without threshold, while protons or neutrinos having the following hurdles to overcome in reaching extreme energies such as 10^{18} eV. In an electron-rich medium, the threshold is 10^{15} eV for an ultra high energy (UHECR) proton, due to p + e^- \to n + \nu_e. On the other hand, the cosmic background neutrinos would cut off UHECR neutrinos of greater than 10^{13} eV if at least one kind of neutrinos has the mass 0.05 eV (as suggested by the experimental value of 0.058 eV), due to \nu +{\bar \nu}_{CB} \to e^- + e^+; this, plus the clustering due to mass, gives us some hope that this effect might be detectible.