= Solution
A large <neutrino degeneracy parameter> raises the relativistic energy density and therefore the expansion rate. <Big Bang nucleosynthesis> tests this through primordial light-element abundances; an electron-neutrino chemical potential also shifts neutron--proton chemical equilibrium directly. The <Cosmic microwave background power spectrum> tests the changed expansion rate, matter-radiation equality, sound horizon, damping scale, and neutrino anisotropic stress. Finally, <large-scale structure of the universe> and the matter power spectrum test the altered equality scale and <neutrino free streaming>. These observables distinguish a degenerate-neutrino cosmology from the standard thermal relic scenario in complementary epochs.
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