Dark radiation as a probe for phase transition in the early universe
  The cosmological constant is not necessarily small in the early universe. If
a scalar field obtains a vacuum expectation value after a phase transition
(PT), a possibly large cosmological constant could present before PT. The early
cosmological constant (ECC) and the PT process may be detectable from dark
radiation (DR) today, such as in the cosmic axion background. We show that for
a broad class of DR models, the DR density and spectrum are significantly
modified by the presence of an ECC. From the density and the spectrum of the DR
today, we can deduce the temperature and the strength of the PT.
