We reanalyze the cosmological constraints on the existence of a net universal lepton asymmetry and neutrino degeneracy .
We show that neutrinos can begin to decouple at higher temperatures than previous estimates due to several corrections which diminish the neutrino reaction rate .
For sufficiently large degeneracy , neutrino decoupling can occur before various particles annihilate and even before the QCD phase transition .
These decoupled neutrinos are therefore not heated as the particle degrees of freedom change .
The resultant ratio of the relic neutrino-to-photon temperatures after e ^ { \pm } annihilation can then be significantly reduced by more than a factor of two from that of the standard nondegenerate ratio .
This changes the expansion rate and subsequent primordial nucleosynthesis , photon decoupling , and structure formation .
In particular we analyze physically plausible lepton-asymmetric models with large \nu _ { \mu } and \nu _ { \tau } degeneracies together with a moderate \nu _ { e } degeneracy .
We show that the nucleosynthesis by itself permits very large neutrino degeneracies 0 \leq \hbox { $ \xi _ { \nu _ { \mu } } $ } , \hbox { $ \xi _ { \nu _ { \tau } } $ } \leq 40 , 0 \leq \hbox { $ \xi _ { \nu _ { e } } $ } \leq 1.4 together with large baryon densities 0.1 \leq \Omega _ { b } \hbox { $h ^ { 2 } _ { 50 } $ } \leq 1 as long as some destruction of primordial lithium has occurred .
We also show that structure formation and the power spectrum of the cosmic microwave background allows for the possibility of an \Omega = 1 , \Omega _ { \Lambda } = 0.4 , cosmological model for which there is both significant lepton asymmetry ( | \hbox { $ \xi _ { \nu _ { \mu } } $ } | = | \hbox { $ \xi _ { \nu _ { \tau } } $ } | \approx 11 ) and a relatively large baryon density ( \Omega _ { b } \hbox { $h ^ { 2 } _ { 50 } $ } \approx 0.2 ) .
Our best-fit neutrino-degenerate , high-baryon-content models are mainly distinguished by a suppression of the second peak in the microwave background power spectrum .
This is consistent with the recent high resolution data from BOOMERANG and MAXIMA-1 .