Craig J. Copi

Big bang nucleosynthesis and the baryon density of the universe

For almost 30 years, the predictions of big-bang nucleosynthesis have been used to test the big-bang model to within a fraction of a second of the bang. The agreement between the predicted and observed abundances of deuterium, helium-3, helium-4, and lithium-7 confirms the standard cosmology model and allows accurate determination of the baryon density, between 1.7 x 10(-31) and 4.1 x 10(-31) grams per cubic centimeter (corresponding to about 1 to 15 percent of the critical density). This measurement of the density of ordinary matter is pivotal to the establishment of two dark-matter problems: (i) most of the baryons are dark, and (ii) if the total mass density is greater than about 15 percent of the critical density, as many determinations indicate, the bulk of the dark matter must be non-baryonic, composed of elementary particles left from the earliest moments.

Assessing Big-Bang nucleosynthesis

Systematic uncertainties in the light-element abundances and their evolution complicate a rigorous statistical assessment. However, using Bayesian methods we show that the following statement is robust: The predicted and measured abundances are consistent with 95% credibility only if the baryon-to-photon ratio is between 2{times}10{sup {minus}10} and 6.5{times}10{sup {minus}10} and the number of light neutrino species is less than 3.9. Our analysis suggests that the {sup 4}He abundance may have been systematically underestimated. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

On the Significance of Population II6Li Abundances

We study the significance of 6Li abundances measured in metal-poor halo stars. We explore possible depletion factors for 6Li, defined as the ratio of the protostellar to the observed abundance, in the three stars where it has been detected; to this end, we assume that 6Li/H scales as 16O/H throughout the Galactic evolution. This assumption is motivated by the recent observations of a similar scaling for 9Be and by the fact that, apart from α - α fusion creation of 6Li, both elements are only created in p, α - C, N, O spallation reactions. We examine possible uncertainties attached to the observations and to the modeling of 6Li Galactic evolution; notably, we include a recent evaluation of the primordial production of 6Li. The depletion factor D6 in the hottest turnoff star HD 84937 is constrained to D6 ≤ 4; this implies that at least one star on the Spite plateau has not depleted its primordial 7Li by more than a factor 4, even by extreme dilution. This constraint is in fact stronger when one takes all constraints into account; indeed, no current stellar model is able to reproduce the abundances of 7Li as a function of metallicity and effective temperature, and yet allow D7 ≥ 2, while keeping D6 ≤ 4. Therefore,7Li should not be depleted by more than a factor 2 on the Spite plateau. If direct nuclear burning was the depletion mechanism, then 7Li would be depleted by less than 2%. Moreover, all three 6Li observations are in excellent agreement with all standard expectations: big bang nucleosynthesis with 2 < η10 < 6.5, and standard stellar isochrones of 6Li survival in metal-poor stars. We also discuss possible deviations from our assumption of a scaling of 6Li/H with 16O/H because of α - α creation of 6Li in various sites.

Big Bang Nucleosynthesis and a New Approach to Galactic Chemical Evolution

Big bang production of deuterium is the best indicator of the baryon density; however, only the present abundance of D is known (and only locally) and its chemical evolution is intertwined with that of 3He. Because Galactic abundances are spatially heterogeneous, mean chemical-evolution models are not well suited for extrapolating the pre-solar D and 3He abundances to their primeval values. We introduce a new approach which explicitly addresses heterogeneity by statistically tracing the history of the pre-solar material back to its primeval beginning. We show that the decade-old concordance interval η ≈ (2-8) × 10-10 based on D and 3He is well founded.

A Stochastic Approach to Chemical Evolution

Observations of elemental abundances in the Galaxy have repeatedly shown an intrinsic scatter as a function of time and metallicity. The standard approach to chemical evolution does not attempt to address this scatter in abundances since only the mean evolution is followed. In this work, the scatter is addressed via a stochastic approach to solving chemical evolution models. Three simple chemical evolution scenarios are studied using this stochastic approach: a closed box model, an infall model, and an outflow model. These models are solved for the solar neighborhood in a Monte Carlo fashion. The evolutionary history of one particular region is determined randomly based on the star formation rate and the initial mass function. Following the evolution in an ensemble of such regions leads to the predicted spread in abundances expected, based solely on different evolutionary histories of otherwise identical regions. In this work, 13 isotopes are followed, including the light elements, the CNO elements, a few α-elements, and iron. It is found that the predicted spread in abundances for a 105 M☉ region is in good agreement with observations for the α-elements. For CN, the agreement is not as good, perhaps indicating the need for more physics input for low-mass stellar evolution. Similarly for the light elements, the predicted scatter is quite small, which is in contradiction to the observations of 3He in H II regions. The models are tuned for the solar neighborhood so that good agreement with H II regions is not expected. This has important implications for low-mass stellar evolution and on using chemical evolution to determine the primordial light-element abundances in order to test big bang nucleosynthesis.

Big-bang nucleosynthesis limit to the number of neutrino species

Concern about the systematic uncertainty in the {sup 4}He abundance as well as the chemical evolution of {sup 3}He leads us to reexamine this important limit. It is shown that with conservative assumptions no more than the equivalent four massless neutrino species are allowed. Even with the most extreme estimates of the astrophysical uncertainties a meaningful limit still exists, less than five massless neutrino species, and illustrates the robustness of this argument. We show that a definitive measurement of the deuterium abundance in high-redshift hydrogen clouds can sharpen the limit. {copyright} {ital 1997} {ital The American Physical Society}

Cosmological Implications of the First Measurement of the Local ISM Abundance of 3He

Deuterium plays a crucial role in testing big bang nucleosynthesis. However, its chemical evolution is intertwined with that of 3He. Gloeckler & Geisss new measurement of the 3He abundance and the Hubble Space Telescope measurement of D, both in the local ISM today, can be compared to the presolar nebula abundances of D and 3He. Within the uncertainties, the sum of D +3He relative to hydrogen is unchanged. This indicates that over the past 4.5 Gyr there has been at most modest stellar production of 3He, in contradiction with stellar modeling, or modest stellar destruction of 3He, in contradiction with efficient solar spoons. The constancy of D +3He alleviates some of the cosmic tension between the big bang 4He abundance and those of D and 3He.

Large-scale baryon isocurvature inhomogeneities

Big bang nucleosynthesis constraints on baryon isocurvature perturbations are determined. A simple model ignoring the effects of the scale of the perturbations is first reviewed. This model is then extended to test the claim that large amplitude perturbations will collapse, forming compact objects and preventing their baryons from contributing to the observed baryon density. It is found that baryon isocurvature perturbations are constrained to provide only a slight increase in the density of baryons in the universe over the standard homogeneous model. In particular it is found that models which rely on power laws and the random phase approximation for the power spectrum are incompatible with big bang nucleosynthesis unless an {em ad hoc}, small scale cutoff is included.

Latest nucleosynthesis constraints on the baryon density of the universe

Big-bang nucleosynthesis plays a crucial role in constraining big-bang cosmology. Although the uncertainties in the observations of the light elements are governed by systematic effects, firm bounds on the density of baryons in the Universe can be set. On-going observations of deuterium in high-redshift quasar absorption systems will provide tight constraints on the density of baryons.