Big bang nucleosynthesis

Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are two major pillars of cosmology. Standard BBN accurately predicts the primordial light element abundances ${(}^{4}\mathrm{He},$ D, ${}^{3}\mathrm{He}$ and ${}^{7}\mathrm{Li}),$ depending on one parameter, the baryon density. Light element observations are used as a baryometer. The CMB anisotropies also contain information about the content of the Universe which allows an important consistency check on the big bang model. In addition CMB observations now have sufficient accuracy to not only determine the total baryon density, but also resolve its principal constituents H and ${}^{4}\mathrm{He}.$ We present a global analysis of all recent CMB data, with special emphasis on the concordance with BBN theory and light element observations. We find ${\ensuremath{\Omega}}_{B}{h}^{2}{=0.0250}_{\ensuremath{-}0.0026}^{+0.0019}$ and ${Y}_{p}{=0.250}_{\ensuremath{-}0.014}^{+0.010}$ (fraction of baryon mass as ${}^{4}\mathrm{He})$ using CMB data alone, in agreement with ${}^{4}\mathrm{He}$ abundance observations. The determination of ${Y}_{p}$ allows us to constrain the relativistic degrees of freedom during BBN, measured through the effective number of light neutrino species, ${N}_{\ensuremath{\nu},eff}{=3.02}_{\ensuremath{-}0.79}^{+0.85},$ in accord with the standard model of particle physics. With this concordance established we show that the inclusion of standard, ${N}_{\ensuremath{\nu},eff}\ensuremath{\equiv}3,$ BBN theory priors significantly reduces the volume of parameter space. In this case, we find ${\ensuremath{\Omega}}_{B}{h}^{2}{=0.0245}_{\ensuremath{-}0.0028}^{+0.0015}$ and ${Y}_{p}{=0.2493}_{\ensuremath{-}0.0010}^{+0.0007}.$ We also find that the inclusion of deuterium abundance observations reduces the ${Y}_{p}$ and ${\ensuremath{\Omega}}_{B}{h}^{2}$ ranges by a factor of $\ensuremath{\sim}2.$ Further light element observations and CMB anisotropy experiments will refine this concordance and sharpen BBN and the CMB as tools for precision cosmology.

Big bang nucleosynthesis (BBN) describes the production of the lightest elements in the first minutes of cosmic time. We review the physics of cosmological element production, and the observations of the primordial element abundances. The comparison between theory and observation has heretofore provided our earliest probe of the universe, and given the best measure of the cosmic baryon content. However, BBN has now taken a new role in cosmology, in light of new precision measurements of the cosmic microwave background (CMB). Recent CMB anisotropy data yield a wealth of cosmological parameters; in particular, the baryon‐to‐photon ratio η = nB/nγ is measured to high precision. The confrontation between the BBN and CMB “baryometers” poses a new and stringent test of the standard cosmology; the status of this test are discussed. Moreover, it is now possible to recast the role of BBN by using the CMB to fix the baryon density and even some light element abundances. This strategy sharpens BBN into a more powerful probe of early universe physics, and of galactic nucleosynthesis processes. The impact of the CMB results on particle physics beyond the Standard Model, and on non‐standard cosmology, are illustrated. Prospects for improvement of these bounds via additional astronomical observations and nuclear experiments are discussed, as is the lingering “lithium problem.”

Probing the early universe: a review of primordial nucleosynthesis beyond the standard big bang

Primordial nucleosynthesis with CMB inputs: probing the early universe and light element astrophysics

Extrapolating the present motion of expansion of the universe backward in time, we conclude that the early universe must have been very dense. And extrapolating the (adiabatic) expansion of the cosmic background radiation backward in time, we conclude that the early universe must have been very hot. Thus, at an early time, the universe must have been very different from what it is now. There were no stars and no galaxies, but only a uniform hot plasma, consisting of free electrons and free nuclei. The chemical composition of the early universe must also have been different. The heavy elements (that is, elements other than hydrogen, deuterium, helium, and lithium) in our immediate environment were formed by nuclear reactions in the cores of stars, so these elements did not exist in the early universe. At very early times, the violent thermal collisions would have prevented the existence of any kind of nuclei, and the matter in the universe must have been in the form of free electrons, protons, and neutrons. At the earliest times, even the protons and neutrons would have been disrupted, and the universe must have contained a mix of quarks, gluons, and other elementary particles.

The most recent developments in the very light element abundance observations, especially the determination of a high (D/H) ratio ∼3 10−4 on a remote QSO line of sight, are analyzed and some of the consequences on the Big Bang nucleosynthesis (BBN) models, on the predicted baryonic density and on the galactic evolution schemes are reviewed. Should this (D/H) determination be confirmed by further observations, the simplest standard BBN model would be the most successful in these abundance predictions: the baryon density should be close to the luminous mass density Ω B ∼ (5–8)10−3 if H=100 km s−1 Mpc−1; the challenge is now to devise galactic evolution models able to account for a large D destruction and avoiding to overproduce3He and metals.

Using the WMAP determination of the baryon density, the standard model of big bang nucleosynthesis yields relatively precise predictions of the primordial light-element abundances. Currently there are two significantly different observational determinations of the primordial helium abundance, and if only statistical errors in 4He abundance determinations are considered, the discrepancies between the observational determinations and the value favored by the WMAP results are significant. Here we examine in detail some likely sources of systematic uncertainties that may resolve the differences between the two determinations. We conclude that the observational determination of the primordial helium abundance is completely limited by systematic errors and that these systematic errors have not been fully accounted for in any published observational determination of the primordial helium abundance. In principle, the observed metal-poor H II region spectra should be analyzed in a nonparametric way, such that the H II region physical conditions and the helium abundance are derived solely from the relative flux ratios of the helium and hydrogen emission lines. In practice, there are very few H II region spectra with the quality that allows this, so that most analyses depend on assumed ranges or relationships between physical parameters, resulting in parametric solutions with underestimated error bars. A representative result of our analysis yields Yp = 0.249 ± 0.009. We stress that the main result of the present work is the increase in the size of the uncertainty rather than the shift in the primordial value. Furthermore, given that most of the spectra analyzed to date do not significantly constrain the primordial helium abundance, we argue in favor of a range of allowed values of 0.232 ≤ Yp ≤ 0.258. This easily allows for concordance between measurements of the baryon-to-photon ratio (η) from WMAP, deuterium abundances, and helium abundance (although the discrepancy with lithium remains).

Four lectures on Big Bang cosmology, including microwave background radiation, Big Bang nucleosynthesis, dark matter, inflation, and baryogenesis.

The presence of light thermally coupled dark matter affects early expansion history and production of light elements during the Big Bang Nucleosynthesis. Specifically, dark matter that annihilates into Standard Model particles can modify the effective number of light species in the universe N eff, as well as the abundance of light elements created buring BBN. These quantities in turn affect the cosmic microwave background (CMB) anisotropy. We present the first joint analysis of small-scale temperature and polarization CMB anisotropy from Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT), together with Planck data and the recent primordial abundance measurements of helium and deuterium to place comprehensive bounds on the mass of light thermal-relic dark matter. We consider a range of models, including dark matter that couples to photons and Standard-Model neutrinos. We discuss the sensitivity of the inferred mass bounds on measurements of N eff, primordial element abundances and the baryon density, and quantify the sensitivity of our results to a possible existence of additional relativistic species. We find that the combination of ACT, SPT, and Planck generally leads to the most stringent mass constraint for dark matter that couples to neutrinos, improving the lower limit by 40%–80%, with respect to previous Planck analyses. On the other hand, the addition of ACT and SPT leads to a slightly weaker bound on electromagnetically coupled particles, due to a shift in the preferred values of Y p and N eff driven by the ground based experiments. In most scenarios, the combination of CMB data has a higher constraining power than the primordial abundance measurements alone, with the best results achieved when all data are combined. Combining all CMB measurements with primordial abundance measurements, we rule out masses below ∼4 MeV at 95% confidence, for all models.We show that allowing for new relativistic species can weaken the mass bounds for dark matter that couples to photons by up to an order of magnitude or more.Finally, we discuss the reach of the next generation of the CMB experiments in terms of probing the mass of the thermal relic dark matter.

Any neutral boson such as a dark photon or dark Higgs that is part of a non-standard sector of particles can mix with its standard model counterpart. When very weakly mixed with the Standard Model, these particles are produced in the early Universe via the freeze-in mechanism and subsequently decay back to standard model particles. In this work, we place constraints on such mediator decays by considering bounds from Big Bang nucleosynthesis and the cosmic microwave background radiation. We find both nucleosynthesis and CMB can constrain dark photons with a kinetic mixing parameter between log ϵ ∼ −10 to −17 for masses between 1 MeV and 100 GeV . Similarly, the dark Higgs mixing angle ϵ with the Standard Model Higgs is constrained between log ϵ ∼ −6 to −15. Dramatic improvement on the bounds from CMB spectral distortions can be achieved with proposed experiments such as PIXIE.

Cosmic strings are generic cosmological predictions of many extensions of the standard model of particle physics, such as a $U(1{)}^{\ensuremath{'}}$ symmetry-breaking phase transition in the early Universe or remnants of superstring theory. Unlike other topological defects, cosmic strings can reach a scaling regime that maintains a small fixed fraction of the total energy density of the Universe from a very early epoch until today. If present, they will oscillate and generate gravitational waves with a frequency spectrum that imprints the dominant sources of total cosmic energy density throughout the history of the Universe. We demonstrate that current and future gravitational wave detectors, such as LIGO and LISA, could be capable of measuring the frequency spectrum of gravitational waves from cosmic strings and discerning the energy composition of the Universe at times well before primordial nucleosynthesis and the cosmic microwave background where standard cosmology has yet to be tested. This work establishes a benchmark case that gravitational waves may provide an unprecedented, powerful tool for probing the evolutionary history of the very early Universe.

<p indent=0mm>Firstly proposed in 1946 by George Gamow, the hot big bang theory is now the most widely accepted cosmological model of the Universe, where the Universe expanded from a very high-density state dominated by radiation. The theory has been vindicated by the observation of the cosmic microwave background, our emerging knowledge of the large-scale structure of the Universe, and the rough consistency between calculations and observations of primordial abundances of the lightest elements in nature: Hydrogen, helium, and lithium. Primordial big-bang nucleosynthesis (BBN) began when the Universe was <sc>3 minutes</sc> old and ended less than half an hour later when nuclear reactions were quenched by the low temperature (about <sc>10⁹ K,</sc> i.e., ~1 GK, or <italic>kT</italic><sc>~100 keV)</sc> and density conditions in the expanding Universe. Only the lightest nuclides (²H, ³He, ⁴He, and ⁷Li) were synthesized in appreciable quantities through BBN, and these relics provide us a unique window on the early Universe. Constrained by the baryon density determined by the Wilkinson Microwave Anisotropy Probe (WMAP) and PLANCK satellites, the primordial abundances of ²H (referred to as D hereafter) and ⁴He inferred from observational data are in good general agreement with predictions; however, the standard BBN theory overestimates the primordial ⁷Li abundance by about a factor of three to four. This significant deviation is the so-called “cosmological lithium problem.” Over about two decades, many attempts to resolve this discrepancy using conventional nuclear physics have been unsuccessful yet. The possible solutions to this unresolved problem have been suggested, which can be mainly categorized into three groups, according to which part of the preceding analysis is called into question: (1) Astrophysical solutions revise the measured primordial lithium abundance; (2) solutions beyond the standard model invoke new particle physics or nonstandard cosmological physics; (3) nuclear physics solutions alter the reaction flow into and out of mass-7. This paper systematically reviews three major aspects of this long-pending cosmological lithium problem. It includes the scientific background, motivation, possible solutions and attempts made in the past. Here, we mainly focus on the nuclear physics solutions, which might solve this problem in an elegant way. In this review, the goals of different research directions are summarized and evaluated, and the promising direction is guided for future investigations.

The constraints on the present baryon density from primordial nucleosynthesis in universes with interacting radiation and matter are investigated. For illustration, a class of exact cosmological models is studied in which two separate, interacting fluids act as the source of the gravitational field, a radiative perfect fluid modelling the cosmic microwave background and a second perfect fluid modelling the observed material content of the Universe. Althought the two fluid models under consideration are found to predict primordial element abundances similar to those predicted in the standard model (and consequently in general accord with observed values), the upper limit on the present baryon density inferred from the observed abundances of the light elements is found to be greater than that in the standard model due to the different evolution of the baryon density in the models. From this result, and using the fact that the upper limit on Ω B (the ratio of the present value of the baryon density to the value of the critical density) is further weakened in inhomogeneous cosmological models, it is found that unlike the situation in the standard model, cosmologies with Ω B ∼1 are permitted without violating the constraints of nucleosynthesis, thereby allowing the possibility that the Universe could be closed by baryonic matter alone.

Big Bang Nucleosynthesis (BBN) theory describes the formation of light isotopes such as D, 3 He, 4 He, 6 Li and 7 Li in the first minutes of cosmic time. Their abundance only depends on the baryon density, on particle physics and on nuclear astrophysics, through the competition between the universal expansion rate and the yields of the relevant nuclear reactions. As the expansion rate increases with the number of neutrino families (and any other relativistic species), the comparison between computed and observed abundances of light isotopes allows to constrain the number of neutrinos species, provided that the knowledge of the relevant nuclear processes is accurate enough. Starting from the present uncertainty of the relevant parameters (i.e. baryon density, observed abundance of isotopes and BBN nuclear cross sections), it will be shown that a renewed study of several nuclear reactions, possibly with existing or proposed underground accelerator facilities, is essential to improve the accuracy of computed abundances of light isotopes, providing the BBN theory a powerful probe of particle physics beyond the standard model. In particular, it will be shown that the accurate measurement of the D(p, ) 3 He reaction at BBN energies (40-400 keV), is of primary importance to constrain the number of active neutrinos and/or the lepton degeneracy in the neutrino sector.

The plateau of 7 Li abundance as a function of the iron abundance by spec- troscopic observations of metal-poor halo stars (MPHSs) indicates its primordial origin. The observed abundance levels are about a factor of three smaller than the primordial 7 Li abundance predicted in the standard Big Bang Nucleosynthesis (BBN) model. This discrepancy might originate from exotic particle and nuclear processes operating in BBN epoch. Some particle models include heavy (m 1 GeV) long-lived colored particles which would be confined inside exotic heavy hadrons, i.e., strongly interacting massive particles (SIMPs). We have found reactions which destroy 7 Be and 7 Li during BBN in the scenario of BBN catalyzed by a long-lived sub-strongly interacting massive particle (sub-SIMP, X). The reactions are non radiative X captures of 7 Be and 7 Li which can be operative if the X particle interacts with nuclei strongly enough to drive 7 Be destruction but not strongly enough to form a bound state with 4 He of relative angular momentum L = 1. We suggest that 7 Li problem can be solved as a result of a new process beyond the standard model through which the observable signature was left on the primordial Li abundance.