Abstract
The products of primordial nucleosynthesis and the cosmic microwave
background (CMB) photons are relics from the early evolution of the Universe
whose observations probe the standard model of cosmology and provide windows on
new physics beyond the standard models of cosmology and of particle physics. In
the standard, hot big bang cosmology, long before any stars have formed a
significant fraction (~25%) of the baryonic mass in the Universe should be in
the form of helium-4 nuclei. Since current 4He observations are restricted to
low redshift regions where stellar nucleosynthesis has occurred, observations
of high redshift, prestellar 4He would constitute a fundamental test of the
hot, big bang cosmology. At recombination, long after big bang nucleosynthesis
(BBN) has ended, the temperature anisotropy spectrum imprinted on the CMB
depends on the 4He abundance through its connection to the electron density and
the effect of the electron density on Silk damping. Since the relic abundance
of 4He is relatively insensitive to the universal density of baryons, but is
sensitive to a non-standard, early Universe expansion rate, the primordial mass
fraction of 4He, Yp, offers a test of the consistency of the standard models of
BBN and the CMB and, provides constraints on non-standard physics. Here, the
WMAP seven year data (supplemented by other CMB experiments), which lead to an
indirect determination of Yp at high redshift, are compared to the BBN
predictions and to the independent, direct observations of 4He in low redshift,
extragalactic HII regions. At present, given the very large uncertainties in
the CMB-determined primordial 4He abundance (as well as for the helium
abundances inferred from H II region observations), any differences between the
BBN predictions and the CMB observations are small, at a level < 1.5 sigma.
Users
Please
log in to take part in the discussion (add own reviews or comments).