Narrow Results

Recent spectroscopic observations of metal poor stars have indicated that both ⁷Li and ⁶Li have abundance plateaus as a function of the metallicity. Abundances of ⁷Li are about a factor three lower than the primordial abundance predicted by standard big-bang nucleosynthesis (SBBN), and ⁶Li abundances are ~ 1/20 of ⁷Li, whereas SBBN predicts negligible amounts of ⁶Li compared to the detected level. These discrepancies suggest that ⁶Li has another cosmological or Galactic origin. Furthermore, it appear that ⁷Li (and also ⁶Li) has been depleted from its primordial abundance by some post-BBN processes. We study the possibility that the radiative decay of long-lived particles has affected the cosmological lithium abundances in reality. We calculate the non-thermal nucleosynthesis associated with the radiative decay, and explore the allowed region of the parameters specifying the properties of long-lived particles. We also impose constraints from observations of the CMB energy spectrum. It is found that non-thermal nucleosynthesis could produces ⁶Li at the level detected in metal poor halo stars (MPHSs), when the lifetime of the unstable particles is of the order ~ 10⁸ − 10¹²s depending on their initial abundance. We conclude that a combination of two different processes could explain the lithium isotopic abundances in MPHSs. First, a non-thermal cosmological nucleosynthesis associated with the radiative decay of unstable particles; and second, about the same degree of stellar depletion of both primordial lithium isotopic abundances. If MPHSs experience ⁶Li depletion of factor much greater than ~ 3, the simple radiative decay process can not be the cause of large ⁶Li abundances in MPHSs.

Spectroscopic observations of metal poor halo stars give an indication of a possible primordial plateau of ⁶Li abundance as a function of metallicity similar to that for ⁷Li. The inferred abundance of ⁶Li is ~1000 times larger than that predicted by standard big bang nucleosynthesis (BBN) for the baryon-to-photon ratio inferred from the WMAP data, and that of ⁷Li is about 3 times smaller than the prediction. We study a possible solution to both the problems of underproduction of ⁶Li and overproduction of ⁷Li in BBN. This solution involves a hypothetical massive, negatively-charged particle that would bind to the light nuclei produced in BBN. The particle gets bound to the existing nuclei after the usual BBN, and a second epoch of nucleosynthesis can occur among nuclei bound to the particles. We numerically carry out a fully dynamical BBN calculation, simultaneously solving the recombination and ionization processes of negatively-charged particles by normal and particle-bound nuclei as well as many possible nuclear reactions among them. It is confirmed that BBN in the presence of these hypothetical particles can solve the two Li abundance problems simultaneously.

We clarify in a quantitative way the impact that distinct chemical T_c and kinetic T_k freeze-out temperatures have on the reduction of the neutrino fugacity ϒ_ν below equilibrium, i.e. ϒ_ν<1, and the increase of the neutrino temperature T_ν via partial reheating. We establish the connection between ϒ_ν and T_k via the modified reheating relation T_ν(ϒ_ν)/T_γ, where T_γ is the temperature of the background radiation. Our results demonstrate that one must introduce the chemical nonequilibrium parameter, i.e. the fugacity, ϒ_ν, as an additional standard cosmological model parameter in the evaluation of CMB fluctuations as its value allows measurement of T_k.

We report on the status of primordial nucleosynthesis in light of recent results on CMB anisotropies from WMAP experiment. Theoretical estimates for nuclei abundances, along with the corresponding uncertainties, are evaluated using a new numerical code, where all nuclear rates usually considered have been updated using the most recent available data. Moreover, additional processes neglected in previous calculations have been included. The combined analysis of CMB and primordial nucleosynthesis prediction for Deuterium gives an effective number of relativistic degrees of freedom in good agreement with the simplest scenario of three nondegenerate neutrinos. Our findings seem to point out possible systematics affecting ⁴He mass fraction measurements, or the effect of exotic physics, like a slightly degenerate relic neutrino background.

Three-body cluster-model calculations are performed for the new types of big-bang nucleosynthesis (BBN) reactions that are calalyzed by a supersymmetric (SUSY) particle stau, a scalar partner of the tau lepton. If a stau has a lifetime ≳ 10³s, it would capture a light element previously synthesized in standard BBN and form a Coulombic bound state. The bound state, an exotic atom, is expected to induce various reactions, such as (αX^-) + d → ⁶Li + X^-, in which a negatively charged stau (denoted as X^-) works as a catalyzer. Recent literature papers have claimed that some of these stau-catalyzed reactions have significantly large cross sections so that inclusion of the reactions into the BBN network calculation can change drastically abundances of some elements, giving not only a solution to the ⁶Li-⁷Li problem (calculated underproduction of ⁶Li by ~ 1000 times and overproduction of ⁷Li+⁷Be by ~ 3 times) but also a constraint on the lifetime and the primordial abundance of the elementary particle stau. However, most of these literature calculations of the reaction cross sections were made assuming too naive models or approximations that are unsuitable for those complicated low-energy nuclear reactions. We use a few-body calculational method developed by the authors, and provides precise cross sections and rates of the stau-catalyzed BBN reactions for the use in the BBN network calculation.

Primordial nucleosynthesis provides a probe of the Universe during its early evolution. Given the progress exploring the constituents, structure, and recent evolution of the Universe, it is timely to review the status of Big Bang Nucleosynthesis (BBN) and to confront its predictions, and the constraints which emerge from them, with those derived from independent observations of the Universe at much later epochs in its evolution. Following an overview of the key physics controlling element synthesis in the early Universe, the predictions of the standard models of cosmology and particle physics (SBBN) are presented, along with those from some non-standard models. The observational data used to infer the primordial abundances are described, with an emphasis on the distinction between precision and accuracy. These relic abundances are compared with predictions, testing the internal consistency of BBN and enabling a comparison of the BBN constraints with those derived from the WMAP Cosmic Background Radiation data. Emerging from these comparisons is a successful standard model along with constraints on (or hints of) physics beyond the standard models of particle physics and of cosmology.

There is a significant discrepancy between the current theoretical prediction of the cosmological lithium abundance, mostly produced as ⁷Be during the Big Bang, and its observationally inferred value. We investigate whether the resonant enhancement of ⁷ Be burning reactions may alleviate this discrepancy. We identify one narrow nuclear level in ⁹B, E_5/2⁺ ≃ 16.7 MeV that is not sufficiently studied experimentally, and being just ~ 200 keV above the ⁷Be+d threshold, may lead to the resonant enhancement of ⁷Be(d, γ)⁹B and ⁷Be(d, p)αα reactions. We determine the relationship between the domain of resonant energies E_r and the deuterium separation width Γ_d that results in the significant depletion of the cosmological lithium abundance and find that (E_r, Γ_d)≃(170-220, 10-40) keV can eliminate the current discrepancy. Such a large width at this resonant energy can be only achieved if the interaction radius for the deuterium entrance channel is very large, a₂₇ ≥ 10 fm. New nuclear experimental and theoretical work is needed to clarify the role this resonance plays on the BBN prediction of the lithium abundance. Alternatively, the most liberal interpretation of the allowed parameters of 16.7 MeV resonance can significantly increase the errors in predicted lithium abundance: [⁷Li/H]_BBN = (2.5-6) × 10^-10.

Primordial nucleosynthesis, or big bang nucleosynthesis (BBN), is one of the three evidences for the big bang model, together with the expansion of the universe and the cosmic microwave background. There is a good global agreement over a range of nine orders of magnitude between abundances of ⁴He, D, ³He and ⁷Li deduced from observations, and calculated in primordial nucleosynthesis. However, there remains a yet-unexplained discrepancy of a factor $\approx 3$, between the calculated and observed lithium primordial abundances, that has not been reduced, neither by recent nuclear physics experiments, nor by new observations. The precision in deuterium observations in cosmological clouds has recently improved dramatically, so that nuclear cross-sections involved in deuterium BBN needs to be known with similar precision. We will briefly discuss nuclear aspects related to the BBN of Li and D, BBN with nonstandard neutron sources, and finally, improved sensitivity studies using a Monte Carlo method that can be used in other sites of nucleosynthesis.

We review important reactions in the Big Bang Nucleosynthesis (BBN) model involving a long-lived negatively charged massive particle, $X^{-}$, which is much heavier than nucleons. This model can explain the observed ⁷Li abundances of metal-poor stars, and predicts a primordial ⁹Be abundance that is larger than the standard BBN prediction. In the BBN epoch, nuclei recombine with the $X^{-}$ particle. Because of the heavy $X^{-}$ mass, the atomic size of bound states $A_X$ is as small as the nuclear size. The nonresonant recombination rates are then dominated by the $D$-wave $\to 2P$ transition for ⁷Li and ${}^{7,9}$Be. The ⁷Be destruction occurs via a recombination with the $X^{-}$ followed by a proton capture, and the primordial ⁷Li abundance is reduced. Also, the ⁹Be production occurs via the recombination of ⁷Li and $X^{-}$ followed by deuteron capture. The initial abundance and the lifetime of the $X^{-}$ particles are constrained from a BBN reaction network calculation. We derived parameter region for the ⁷Li reduction allowed in supersymmetric or Kaluza–Klein (KK) models. We find that either the selectron, smuon, KK electron or KK muon could be candidates for the $X^{-}$ with $m_X\sim {𝒪}(1)$ TeV, while the stau and KK tau cannot.

The Big Bang Nucleosynthesis theory accurately reproduces the abundances of light elements in the universes, except for the ⁷Li abundance. The calculated ⁷Li abundance with the baryon-to-photon ratio fixed by the observations of the cosmic microwave background (CMB) is inconsistent with the observed lithium abundances on the surface of metal-poor halo stars, and this problem is called “⁷Li problem”. Previous studies proposed to resolve this ⁷Li problem include photon cooling (possibly via the Bose–Einstein condensation of a scalar particle), the decay of a long-lived $X$ particle (possibly the next-to-lightest supersymmetric particle), or an energy density of a primordial magnetic field (PMF). We review and analyze the results of these solutions both separately and in concert, and the constraint on the $X$ particles and the PMF parameters from observed light-element abundances with a likelihood analysis. We can discover parameter ranges of the $X$ particles which can solve the ⁷Li problem and constrain the energy density of the PMF.

Big Bang Nucleosynthesis (BBN) explores the first few minutes of nuclei formation during the Big Bang. We present updated 2$\sigma$ for the abundances of the four primary light nuclides — D, ³He, ⁴He, and ⁷Li — in BBN. A modified standard BBN code was used in a Monte Carlo analysis of the nucleosynthesis uncertainties as a function of the baryon-to-photon ratio. Reaction rates were updated to those of NACRE, REACLIB, and $R$-Matrix calculations. The results were then used to derive a new constraint on the effective number of neutrinos.

The light elements and their isotopes were produced during standard big bang nucleosynthesis (SBBN) during the first minutes after the creation of the universe. Comparing the calculated abundances of these light species with observed abundances, it appears that all species match very well except for lithium (⁷Li) which is overproduced by the SBBN. This discrepancy is rather challenging for several reasons to be considered on astrophysical and on nuclear physics ground, or by invoking nonstandard assumptions which are the focus of this paper. In particular, we consider a variation of the chemical potentials of the neutrinos and their temperature. In addition, we investigated the effect of dark matter on ⁷Li production. We argue that including nonstandard assumptions can lead to a significant reduction of the ⁷Li abundance compared to that of SBBN. This aspect of lithium production in the early universe may help to resolve the outstanding cosmological lithium problem.

The effects of introducing a small amount of nonthermal distribution (NTD) of elements in big bang nucleosynthesis (BBN) are studied by allowing a fraction of the NTD to be time-dependent so that it contributes only during a certain period of the BBN evolution. The fraction is modeled as a Gaussian-shaped function of $log(T)$, where $T$ is the temperature of the cosmos, and thus the function is specified by three parameters; the central temporal position, the width and the magnitude. The change in the average nuclear reaction rates due to the presence of the NTD is assumed to be proportional to the Maxwellian reaction rates but with temperature $T_{\mathrm{\text{NTD}}}\equiv \zeta T$, $\zeta$ being another parameter of our model. By scanning a wide four-dimensional parametric space at about half a million points, we have found about 130 points with ${\chi }^2<1$, at which the predicted primordial abundances of light elements are consistent with the observations. The magnitude parameter ${𝜀}_0$ of these points turns out to be scattered over a very wide range from ${𝜀}_0\sim 10^{-19}$ to $\sim 10^{-1}$, and the $\zeta$-parameter is found to be strongly correlated with the magnitude parameter ${𝜀}_0$. The temperature region with $0.3\times 10^9\text{K}\lesssim T\lesssim 0.4\times 10^9\text{K}$ or the temporal region $t\simeq 10^3$s seems to play a central role in lowering ${\chi }^2$.

The astronomical dark matter is an essential component of the Universe and yet its nature is still unresolved. It could be made of neutral and massive elementary particles which are their own antimatter partners. These dark matter species undergo mutual annihilations whose effects are briefly reviewed in this article. Dark matter annihilation plays a key role at early times as it sets the relic abundance of the particles once they have decoupled from the primordial plasma. A weak annihilation cross section naturally leads to a cosmological abundance in agreement with observations. Dark matter species subsequently annihilate — or decay — during Big Bang nucleosynthesis and could play havoc with the light element abundances unless they offer a possible solution to the ⁷Li problem. They could also reionize the intergalactic medium after recombination and leave visible imprints in the cosmic microwave background. But one of the most exciting aspects of the question lies in the possibility to indirectly detect the dark matter species through the rare antimatter particles — antiprotons, positrons and antideuterons — which they produce as they currently annihilate inside the galactic halo. Finally, the effects of dark matter annihilation on stars is discussed.

This paper reports on the measurements of the ²H(p,γ)³He reaction cross section performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) facility in the Gran Sasso Laboratory (LNGS) and at the 3 MV Tandetron accelerator at HZDR Dresden Rossendorf. Overall, the cross section has been measured in the energy range relevant for protostars and the Sun (2.5–22 keV), for Big Bang Nucleosynthesis (BBN) (50–400 keV) and even at much higher energies (400–800 keV). After a general introduction on the astrophysical and cosmological relevance of ²H(p,γ)³He reaction cross section, the experimental setups used for each measurement campaign are described, focusing in particular on the importance of going underground. The results obtained so far and the perspectives of the still on-going measurements are also discussed.

To understand the evolution of the universe it is important to understand Big Bang Nucleosythesis (BBN) and its reactions, including the ²H(p,γ)³He reaction. The baryon density and the number of neutrino species are two cosmological parameters to describe the universe. These parameters inform the calculated primordial abundances of the elements by way of BBN cross sections. The cross section of the ²H(p,γ)³He reaction has the highest uncertainty among BBN reactions, it is necessary to perform further investigations on this reaction. The aim of the present work is to introduce one of the experimental approaches adopted by the LUNA (Laboratory for Underground Nuclear Physics) collaboration, whose goal is to measure the ²H(p,γ)³He reaction cross section in the energy range 30 < E_c.m. [keV] < 300 with not yet reached precision.

The standard ΛCDM cosmological model now seems to face some puzzles. One of the most serious problems is the so-called Hubble tension; the values of the Hubble constant H₀ obtained by local measurements look inconsistent with that inferred from Cosmic Microwave Background (CMB). Although introducing extra energy components such as the extra radiation or Early Dark Energy appears to be promising, such extra components could alter the abundance of light elements synthesized by Big Bang Nucleosynthesis (BBN). We perform a Monte Carlo simulation to evaluate the effect of those extra component scenarios to solve the Hubble tension on the BBN prediction.

The ⁶Li abundance observed in metal-poor halo stars exhibits a possible plateau similar to that for ⁷Li. The observed abundance of ⁶Li is a factor of 10³ larger and that of ⁷Li is a factor of 3 lower than the abundances predicted in the standard big bang when the baryon-to-photon ratio is fixed by WMAP, so that some mechanism would have produced ⁶Li and deplete ⁷Li. We show that both of these abundance anomalies can be explained by the existence of a long-lived massive, negatively-charged leptonic particle during nucleosynthesis. Such particles would capture onto the synthesized nuclei thereby reducing the reaction Coulomb barriers and opening new transfer reaction possibilities, and catalyzing a second round of big bang nucleosynthesis. We perform a fully dynamical calculation of big bang nucleosynthesis taking account of the recombination and many possible nuclear reactions and confirmed that this solution to both of the Li problems can be achieved with or without the additional effects of stellar destruction.

We derive new constraints on the neutron lifetime based on the recent Planck 2015 observations of temperature and polarization anisotropies of the CMB. Under the assumption of standard Big Bang Nucleosynthesis, we show that Planck data constrain the neutron lifetime to τ_n = (907 ± 69) [s] at 68% c.l.. Moreover, by including the direct measurements of primordial Helium abundance of Aver et al. (2015) and Izotov et al. (2014), we show that cosmological data provide the stringent constraints τ_n = (875±19) [s] and τ_n = (921±11) [s] respectively. The latter appears to be in tension with neutron lifetime value quoted by the Particle Data Group (τ_n = (880.3 ± 1.1) [s]). Future CMB surveys as COrE+, in combination with a weak lensing survey as EUCLID, could constrain the neutron lifetime up to a ~ 6 s precision.

This chapter presents an overview of cosmological framework that is necessary to perform cosmological simulations. First, we start with a brief history of cosmological studies of the Universe, such as the discovery of Hubble’s law and cosmic microwave background radiation which constitute the major observational evidence of expanding Big Bang cosmology. Second, we present the basics of General Relativity theory and Friedmann models that describe the expanding universe. Under this theoretical framework, we introduce various cosmological parameters and current best-fit Λ cold dark matter (CDM) model. Third, we discuss the history and development of computational cosmology which was achieved concurrently with the evolution of supercomputers.