Topics in Cosmology and Particle Physics

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

The early hot, dense, expanding Universe was a primordial reactor in which the light nuclides D, 3He, 4He and 7Li were synthesized in astrophysically interesting abundances. The challenge to the standard hot big bang model (Big Bang Nucleosynthesis ≡ BBN) is the comparison between the observed and predicted abundances, the latter which depend only on the universal abundance of nucleons. The current status of observations is reviewed and the inferred primordial abundances are used to confront BBN. This comparison suggests consistency for BBN for a narrow range in the nucleon abundance but, looming on the horizon are some potential crises which will be outlined.

During the past two decades, cosmologists turned to particle physics in order to explore the physics of the very early Universe. The main link between the physics of the smallest and largest structures in the Universe is the idea of spontaneous symmetry breaking, familiar from condensed matter physics. Implementing this mechanism into cosmology leads to the interesting possibility that phase transitions related to the breaking of symmetries in high energy particle physics took place during the early history of the Universe. These cosmological phase transitions may help us understand many of the challenges faced by the standard hot Big Bang model of cosmology, while offering a unique window into the very early Universe and the physics of high energy particle interactions.

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.

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

With some hubris, we would argue that the standard Big Bang model is the best model of the observable cosmos that has yet been devised. Does it have problems? Yes! Might theoretical and observational developments in the future bear on the model’s correctness at some level of detail? Very likely! Could any such developments rule out the Big Bang model as a descriptor? Perhaps. It is not at all impossible that something new in the way of theory or observation will send cosmologists back to the drawing board. Moreover, the Big Bang model did not appear out of nowhere, unopposed. Other attempts to understand the cosmos were being put forward at the same time, and their successes and failures certainly influenced the Big Bang model. There are still valiant efforts by some cosmologists to come up with alternatives, including in particular an argument that the correct model is a modified Steady State model. This quasi-Steady State model was presented, as we have already mentioned, in 1999 by G. Burbidge, F. Hoyle, and J. Narlikar, in Nature and in Physics Today, and a strong refutation was published in the same issue of Physics Today, by Andre Albrecht, otherwise known for his work on inflationary models.

The cosmic connections between physics on the very largest and very smallest scales are reviewed with an emphasis on the symbiotic relation between elementary particle physics and cosmology. After a review of the early Universe as a cosmic accelerator, various cosmological and astrophysical constraints on models of particle physics are outlined. To illustrate this approach to particle physics via cosmology, reference is made to several areas of current research: baryon non-conservation and baryon asymmetry; free quarks, heavy hadrons and other exotic relics; primordial nucleosynthesis and neutrino masses.

<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.

Big bang nucleosynthesis

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.”

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.

During the Big Bang, ${}^{6}{\rm Li}$ was synthesized via the ${}^{2}{\rm H}(\alpha,\gamma){}^{6}{\rm Li}$ reaction. After almost 25 years of the failed attempts to measure the ${}^{2}{\rm H}(\alpha,\gamma){}^{6}{\rm Li}$ reaction in the lab at the Big Bang energies, just recently the LUNA collaboration presented the first successful measurements at two different Big Bang energies [M. Anders {\it et al.}, Phys. Rev. Lett. {\bf 113}, 042501 (2014)]. In this paper we will discuss how to improve the accuracy of the direct experiment. To this end the photon's angular distribution is calculated in the potential model. It contains contributions from electric dipole and quadrupole transitions and their interference, which dramatically changes the photon's angular distribution. The calculated distributions at different Big Bang energies have a single peak at $\sim 50^{\circ}$. These calculations provide the best kinematic conditions to measure the ${}^{2}{\rm H}(\alpha,\gamma){}^{6}{\rm Li}$ reaction. The expressions for the total cross section and astrophysical factor are also derived by integrating the differential cross section over the photon's solid angle. The LUNA data are in excellent agreement with our calculations using a potential approach combined with a well established asymptotic normalization coefficient for ${}^{6}{\rm Li} \to \alpha +d$. Comparisons of the available experimental data for the $S_{24}$ astrophysical factor and different calculations are presented. The Big Bang lithium isotopic ratio ${}^{6}{\rm Li}/^{7}{\rm Li} = (1.5 \pm 0.3)\times 10^{-5}$ following from the LUNA data and the present analysis are discussed in the context of the disagreement between the observational data and the standard Big Bang model, which constitutes the second Lithium problem.

Supporters of the standard Big Bang theory point to the abundances of light elements, predicted by Big Bang Nucleosynthesis (BBN) as one of the main observational supports of the theory. However, current data no longer confirm BBN. Instead, measurements of the abundances of He3, He4, and D clearly contradict BBN at more than a 3σ level, eliminating a key support of the Big Bang.

In recent years there has been considerable growth in research in so-called 'interface' areas where nuclear physics, particle physics and astrophysics meet; sharing common goals and language, as well as instrumentation techniques. Examples include: Probing the standard model by measuring elements of the CKM matrix The use of colliders and detectors with access to the highest energies to search for the underlying structure of matter and energy Fundamental interactions at the nuclear and particle level required to understand processes from solar burning to supernovae Neutrino physics; including oscillations, mass and interactions probing the standard solar model Hadronic physics dealing with the interactions, binding and structure of the strongly interacting particles under various conditions The increased use of 4 detectors in low-energy nuclear physics to select specific processes of interest. These are only a few examples of what has become a major meeting place for much of the exciting physics of the early 21st Century. In today's science, the spectrum of particles, the nucleus as well as the large-scale components of the universe, are our laboratories for testing and understanding nature.These exciting areas of cross-over require much further research and exploration. The Editorial Board and staff of Journal of Physics G: Nuclear and Particle Physics wish to strengthen their support for physicists working in such areas of intersection. To make our coverage of these interconnections more explicit, we have redrafted our scope statement as follows:Theoretical and experimental topics in the physics of elementary particles and fields, intermediate-energy physics and nuclear physics. The particle astrophysics section includes all aspects of experimental and theoretical research into cosmic rays, nuclear and particle astrophysics, gamma ray astronomy, neutrino astrophysics and dark matter. We also welcome articles from all areas of interface between these fields.We would like to invite all physicists working on these topics to publish their work in the pages of J.Phys.G. Naturally we shall continue to welcome outstanding research in our core nuclear and particle physics areas.Since its launch in 1975, J.Phys.G has had a broad scope covering all aspects of the physics of elementary particles, nuclear physics, and nuclear and particle astrophysics. We have always felt this provides the journal's authors and readers with a unique overview of these increasingly interconnected subjects.When you publish in J.Phys.G you'll benefit from fast and impartial peer review, with an average receipt-to-first decision time for papers of 58 days. We strive to provide outstanding author service with online article submission, tracking of manuscripts and electronic refereeing. Publishing with us provides your paper with wide international visibility in print and online at www.iop.org/journals/jphysg. As well as being accessible to our subscribers, each new paper is freely available to all for 30 days on our Website

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