As the early Universe evolved from hot and dense to cool and dilute,
it passed through a short-lived epoch when conditions of temperature
and density permitted the synthesis of astrophysically interesting
abundances of D, 3He, 4He, and 7Li. At
present, observations of
these nuclides in a variety of astrophysical sites (stars, Galactic
and extragalactic H II regions, QSOALS, etc.) have
permitted quite
precise estimates of their primordial abundances, opening a window
on early-universe cosmology and providing constraints on physics
beyond the standard models of cosmology and particle physics. The
relic abundances of D, 3He, and 7Li are nuclear
reaction rate
limited, providing good probes of the nucleon density when the
universe was less than a half hour old. This is complementary to
baryon density estimates from the CBR, whose information was encoded
some 400 kyr later, and to those provided by observations of the current
or recent universe, 13 Gyr after BBN. Of these light nuclides, D is
the baryometer of choice and current estimates of the relic abundance,
yD = 2.6 ± 0.4, suggest a baryon to photon ratio,
unchanged from BBN to the present, of
10
= 6.1 ± 0.6, in excellent agreement with
the WMAP and LSS determined value of
10
= 6.14 ± 0.25. For this choice (S = 1,
e
= 0), the less well constrained relic abundance
of 3He (y3 = 1.1 ± 0.2)
[24]
is also in agreement with its SBBN
expected value. These successes for the standard models are tempered by
the challenges posed by 4He and 7Li. The SBBN
predicted 4He mass fraction, YP = 0.248, differs
from the observationally inferred primordial
abundance adopted here, YP = 0.241 ± 0.004, by nearly
2
. However,
as discussed in Section 3.3, the
uncertainties in YP are likely dominated
by systematic, not statistical errors, so it is difficult to know if this
tension between D (and 3He and the CBR) and 4He is
cause for serious concern. In contrast to the other light nuclides, the
BBN abundance of 4He is insensitive to the nuclear reaction
rates and, hence, to the nucleon density at BBN. YP is
largely set by the neutron to proton ratio at BBN, so that
4He is an excellent probe of the weak interactions and of the
early Universe expansion rate. Perhaps the 4He challenge to
SBBN is a signal of new physics. The SBBN predicted abundance of
7Li ([Li]P = 2.65-0.06+0.05)
is nearly a factor of two higher than the observationally determined value
[37]
([Li]P = 2.37 ± 0.05). While there is some spread in the
lithium abundances inferred from the data
[38],
the largest cause for concern is that
Li is observed in the oldest stars in the Galaxy, which have had ample
time to modify their original surfaces abundances. For 7Li,
it appears likely that astrophysical uncertainties dominate at present.
Thus, while there appears to be qualitative confirmation of the standard models of cosmology and particle physics extrapolated back to the first seconds of the evolution of the Universe, precision should not be confused with accuracy. The accuracy of the presently-inferred primordial abundances of D, 3He, 4He, and 7Li remains in question and it would not be at all surprising if one or more of them changed by more than the presently-quoted errors. After all, there are only 5 (6) lines of sight where deuterium is observed in high-redshift, relatively unprocessed (low metallicity) material; 3He is only observed in the chemically processed interstellar medium of the Galaxy and the lack of variation of its abundance with metallicity or with position in the Galaxy suggest a very delicate balance between post-BBN production, destruction and survival; systematic errors and corrections to the 4He abundance inferred from observations of low metallicity, extragalactic H II regions are likely larger, maybe much larger, than the current statistical uncertainties; lithium is derived from observations of very old, very low metallicity stars (good!) in our Galaxy (bad?) and the corrections for stellar atmosphere models and, especially, for main sequence mixing-induced depletion and destruction remain large and uncertain. Much interesting, important work remains for observational and theoretical astronomers.
The current standard models receive strong support from these messengers from the early universe, confirming in broad outline our understanding of the evolution of the Universe and the particles in it, from the first seconds to the present. Any models of new physics must consider this success and avoid introducing new conflicts. Much interesting, important work remains for cosmologists and high energy theorists.
Acknowledgments
I am grateful to all those I've collaborated with over the years on the subject of this review. Discussions with J. E. Felten, K. A. Olive, E. D. Skillman, M. Tosi, D. Tytler, and J. K. Webb were especially valuable in its preparation. My research is supported by a grant (DE-FG02-91ER40690) from the US Department of Energy.