Constraints on the Variation of G from Primordial Nucleosynthesis

We study here the effect of a varying G on the evolution of the early Universe and, in particular, on primordial nucleosynthesis. This variation of G is modelled using the Brans-Dicke theory as well as a more general class of scalar-tensor theories. Modified nucleosynthesis codes are used to investigate this effect and the results obtained are used to constrain the parameters of the theories. We extend previous studies of primordial nucleosynthesis in scalar-tensor theories by including effects which can cause a slow variation of G during radiation domination and by including a late-time accelerating phase to the Universe's history. We include a brief discussion on the epoch of matter-radiation equality in Brans-Dicke theory, which is also of interest for determining the positions of the cosmic microwave background power-spectrum peaks.Comment: 10 pages, 7 figures. Published versio

BBN For Pedestrians

The simplest, `standard' model of Big Bang Nucleosynthesis (SBBN) assumes three light neutrinos (N_nu = 3) and no significant electron neutrino asymmetry, leaving only one adjustable parameter: the baryon to photon ratio eta. The primordial abundance of any one nuclide can, therefore, be used to measure the baryon abundance and the value derived from the observationally inferred primordial abundance of deuterium closely matches that from current, non-BBN data, primarily from the WMAP survey. However, using this same estimate there is a tension between the SBBN-predicted 4He and 7Li abundances and their current, observationally inferred primordial abundances, suggesting that N_nu may differ from the standard model value of three and/or that there may be a non-zero neutral lepton asymmetry (or, that systematic errors in the abundance determinations have been underestimated or overlooked). The differences are not large and the allowed ranges of the BBN parameters permitted by the data are quite small. Within these ranges, the BBN-predicted abundances of D, 3He, 4He, and 7Li are very smooth, monotonic functions of eta, N_nu, and the lepton asymmetry. It is possible to describe the dependencies of these abundances (or powers of them) upon the three parameters by simple, linear fits which, over their ranges of applicability, are accurate to a few percent or better. The fits presented here have not been maximized for their accuracy but, for their simplicity. To identify the ranges of applicability and relative accuracies, they are compared to detailed BBN calculations; their utility is illustrated with several examples. Given the tension within BBN, these fits should prove useful in facilitating studies of the viability of proposals for non-standard physics and cosmology, prior to undertaking detailed BBN calculations.Comment: Submitted to a Focus Issue on Neutrino Physics in New Journal of Physics (www.njp.org

Primordial black hole constraints in cosmologies with early matter domination

Moduli fields, a natural prediction of any supergravity and superstring-inspired supersymmetry theory, may lead to a prolonged period of matter domination in the early Universe. This can be observationally viable provided the moduli decay early enough to avoid harming nucleosynthesis. If primordial black holes form, they would be expected to do so before or during this matter dominated era. We examine the extent to which the standard primordial black hole constraints are weakened in such a cosmology. Permitted mass fractions of black holes at formation are of order $10^{-8}$, rather than the usual $10^{-20}$ or so. If the black holes form from density perturbations with a power-law spectrum, its spectral index is limited to $n \lesssim 1.3$, rather than the $n \lesssim 1.25$ obtained in the standard cosmology.Comment: 7 pages RevTeX file with four figures incorporated (uses RevTeX and epsf). Also available by e-mailing ARL, or by WWW at http://star-www.maps.susx.ac.uk/papers/infcos_papers.htm

Constraints on Cosmic Strings due to Black Holes Formed from Collapsed Cosmic String Loops

The cosmological features of primordial black holes formed from collapsed cosmic string loops are studied. Observational restrictions on a population of primordial black holes are used to restrict $f$, the fraction of cosmic string loops which collapse to form black holes, and $\mu$, the cosmic string mass-per-unit-length. Using a realistic model of cosmic strings, we find the strongest restriction on the parameters $f$ and $\mu$ is due to the energy density in $100 MeV$ photons radiated by the black holes. We also find that inert black hole remnants cannot serve as the dark matter. If earlier, crude estimates of $f$ are reliable, our results severely restrict $\mu$, and therefore limit the viability of the cosmic string large-scale structure scenario.Comment: (Plain Tex, uses tables.tex -- wrapped lines corrected), 11 pages, FERMILAB-Pub-93/137-

The Effect of Bound Dineutrons upon BBN

We have examined the effects of a bound dineutron, n2, upon big bang nucleosynthesis (BBN) as a function of its binding energy B_n2. We find a weakly bound dineutron has little impact but as B_n2 increases its presence begins to alter the flow of free nucleons to helium-4. Due to this disruption, and in the absence of changes to other binding energies or fundamental constants, BBN sets a reliable upper limit of B_n2 <~ 2.5 MeV in order to maintain the agreement with the observations of the primordial helium-4 mass fraction and D/H abundance

The fundamental constants and their variation: observational status and theoretical motivations

This article describes the various experimental bounds on the variation of the fundamental constants of nature. After a discussion on the role of fundamental constants, of their definition and link with metrology, the various constraints on the variation of the fine structure constant, the gravitational, weak and strong interactions couplings and the electron to proton mass ratio are reviewed. This review aims (1) to provide the basics of each measurement, (2) to show as clearly as possible why it constrains a given constant and (3) to point out the underlying hypotheses. Such an investigation is of importance to compare the different results, particularly in view of understanding the recent claims of the detections of a variation of the fine structure constant and of the electron to proton mass ratio in quasar absorption spectra. The theoretical models leading to the prediction of such variation are also reviewed, including Kaluza-Klein theories, string theories and other alternative theories and cosmological implications of these results are discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy