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Register a new userCosmological Solutions to the Lithium Problem
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The abundance of primordial lithium is derived from the observed spectroscopy of metal-poor stars in the galactic halo. However, the observationally inferred abundance remains at about a factor of three below the abundance predicted by standard big bang nucleosynthesis (BBN). The resolution of this dilemma can be either astrophysical (stars destroy lithium after BBN), nuclear (reactions destroy lithium during BBN), or cosmological, i.e. new physics beyond the standard BBN is responsible for destroying lithium. Here, we overview a variety of possible cosmological solutions, and their shortcomings. On the one hand, we examine the possibility of physical processes that modify the velocity distribution of particles from the usually assumed Maxwell-Boltzmann statistics. A physical justification for this is an inhomogeneous spatial distribution of domains of primordial magnetic field strength as a means to reduce the primordial lithium abundance. Another possibility is that scattering with the mildly relativistic electrons in the background plasma alters the baryon distribution to one resembling a Fermi-Dirac distribution. We show that neither of these possibilities can adequately resolve the lithium problem. A number of alternate hybrid models are discussed including a mix of neutrino degeneracy, unified dark matter, axion cooling, and the presence of decaying and/or charged supersymmetric particles.
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Big Bang nucleosynthesis (BBN) theory predicts the abundances of the light elements D, $^3$He, $^4$He and $^7$Li produced in the early universe. The primordial abundances of D and $^4$He inferred from observational data are in good agreement with predictions, however, the BBN theory overestimates the primordial $^7$Li abundance by about a factor of three. This is the so-called cosmological lithium problem. Solutions to this problem using conventional astrophysics and nuclear physics have not been successful over the past few decades, probably indicating the presence of new physics during the era of BBN. We have investigated the impact on BBN predictions of adopting a generalized distribution to describe the velocities of nucleons in the framework of Tsallis non-extensive statistics. This generalized velocity distribution is characterized by a parameter $q$, and reduces to the usually assumed Maxwell-Boltzmann distribution for $q$ = 1. We find excellent agreement between predicted and observed primordial abundances of D, $^4$He and $^7$Li for $1.069leq q leq 1.082$, suggesting a possible new solution to the cosmological lithium problem.
In this paper we investigate possible solutions to the coincidence problem in flat phantom dark energy models with a constant dark energy equation of state and quintessence models with a linear scalar field potential. These models are representative of a broader class of cosmological scenarios in which the universe has a finite lifetime. We show that, in the absence of anthropic constraints, including a prior probability for the models inversely proportional to the total lifetime of the universe excludes models very close to the $Lambda {rm CDM}$ model. This relates a cosmological solution to the coincidence problem with a dynamical dark energy component having an equation of state parameter not too close to -1 at the present time. We further show, that anthropic constraints, if they are sufficiently stringent, may solve the coincidence problem without the need for dynamical dark energy.
In the primordial Big Bang nucleosynthesis (BBN), only the lightest nuclides (D, $^3$He, $^4$He, and $^7$Li) were synthesized in appreciable quantities, and these relics provide us a unique window on the early universe. Currently, BBN simulations give acceptable agreement between theoretical and observed abundances of D and $^4$He, but it is still difficult to reconcile the predicted $^7$Li abundance with the observation for the Galactic halo stars. The BBN model overestimates the primordial $^7$Li abundance by about a factor of three, so called the cosmological lithium problem, a long-lasting pending issue in BBN. Great efforts have been paid in the past decades, however, the conventional nuclear physics seems unable to resolve such problem. It is well-known that the classical Maxwell-Boltzmann (MB) velocity distribution has been usually assumed for nuclei in the Big-Bang plasma. In this work, we have thoroughly investigated the impact of non-extensive Tsallis statistics (deviating from the MB) on thermonuclear reaction rates involved in standard models of BBN. It shows that the predicted primordial abundances of D, $^4$He, and $^7$Li agree very well with those observed ones by introducing a non-extensive parameter $q$. It is discovered that the velocities of nuclei in a hot Big-Bang plasma indeed violate the classical Maxwell-Boltzmann (MB) distribution in a very small deviation of about 6.3--8.2%. Thus, we have for the first time found a new solution to the cosmological lithium problem without introducing any mysterious theories. Furthermore, the implications of non-extensive statistics in other exotic high-temperature and density astrophysical environments should be explored, which might offer new insight into the nucleosynthesis of heavy elements.
The possibility that the so-called lithium problem, i.e. the disagreement between the theoretical abundance predicted for primordial $^7$Li assuming standard nucleosynthesis and the value inferred from astrophysical measurements, can be solved through a non-thermal BBN mechanism has been investigated by several authors. In particular, it has been shown that the decay of a MeV-mass particle, like, e.g., a sterile neutrino, decaying after BBN not only solves the lithium problem, but also satisfies cosmological and laboratory bounds, making such a scenario worth to be investigated in further detail. In this paper, we constrain the parameters of the model with the combination of current data, including Planck 2015 measurements of temperature and polarization anisotropies of the CMB, FIRAS limits on spectral distortions, astrophysical measurements of primordial abundances and laboratory constraints. We find that a sterile neutrino with mass $M_S=4.35_{-0.17}^{+0.13},MeV$ (at $95%$ c.l.), a decay time $tau_S=1.8_{-1.3}^{+2.5}cdot 10^5,s$ (at $95%$ c.l.) and an initial density $bar{n}_S/bar{n}_{cmb}=1.7_{-0.6}^{+3.5}cdot 10^{-4}$ (at $95%$ c.l.) in units of the number density of CMB photons, perfectly accounts for the difference between predicted and observed $^7$Li primordial abundance. This model also predicts an increase of the effective number of relativistic degrees of freedom at the time of CMB decoupling $Delta N_{eff}^{cmb}equiv N_{eff}^{cmb}-3.046=0.34_{-0.14}^{+0.16}$ at $95%$ c.l.. The required abundance of sterile neutrinos is incompatible with the standard thermal history of the Universe, but could be realized in a low reheating temperature scenario. We provide forecasts for future experiments finding that the combination of measurements from the COrE+ and PIXIE missions will allow to significantly reduce the permitted region for the sterile lifetime and density.
The standard theory of electromagnetic cascades onto a photon background predicts a quasi-universal shape for the resulting non-thermal photon spectrum. This has been applied to very disparate fields, including non-thermal big bang nucleosynthesis (BBN). However, once the energy of the injected photons falls below the pair-production threshold the spectral shape is very different, a fact that has been overlooked in past literature. This loophole may have important phenomenological consequences, since it generically alters the BBN bounds on non-thermal relics: for instance it allows to re-open the possibility of purely electromagnetic solutions to the so-called cosmological lithium problem, which were thought to be excluded by other cosmological constraints. We show this with a proof-of-principle example and a simple particle physics model, compared with previous literature.
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