Primordial Helium-4 Abundance Research Articles

Non-standard cosmic expansion histories: neutrino decoupling and primordial nucleosynthesis signatures

Cosmological scenarios with a non-standard equation of state can involve ultrastiff fluids, understood as primordial fluids for which p/ρ > 1. Their energy densities can dominate the Universe energy budget at early times, in the otherwise radiation dominated epoch. During that period the Universe undergoes a faster expansion, that has implications for any decoupling process that takes place in that era. Quintessence models or Ekpyrotic cosmologies are good examples of such scenarios. Assuming the ultrastiff state to be thermally decoupled at very early times, if ever coupled, its observational imprints are left solely in the Universe expansion rate and in the radiation energy density. We consider a complete set of ultrastiff fluids and study their signatures in the neutrino decoupling and BBN eras. Measurements of N eff alone place mild constraints on these scenarios, with forthcoming measurements from the Simons Observatory in the Chilean Atacama desert being able to test regions where still sizable effects are observable. However, when BBN data is taken into account, those regions are proven to be barely reconcilable with primordial helium-4 and deuterium abundances measurements. Our findings show that measurements of the primordial helium-4 abundance imply the tightest constraints, with measurements of primordial deuterium being — to a certain extent — competitive as well. We point out that a ∼ 60% improvement on the statistical uncertainty of the primordial helium-4 abundance measurement, will test these scenarios in the region where they can produce sizable effects. Beyond that precision the regions that are accessible degenerate with standard expectations. In that case, although potentially present, neither neutrino decoupling nor BBN observables will be sensitive probes.

Revisiting Friedmann-like cosmology with torsion: newest constraints from high-redshift observations

As one of the possible extensions of Einstein's General Theory of Relativity, it has been recently suggested that the presence of space-time torsion could solve problems of the very early and the late-time universe undergoing accelerating phases. In this paper, we use the latest observations of high-redshift data, coming from multiple measurements of quasars and baryon acoustic oscillations, to phenomenologically constrain such cosmological model in the framework of Einstein-Cartan (EC) endowed with space-time torsion. Such newly compiled quasar datasets in the cosmological analysis is crucial to this aim, since it will extend the Hubble diagram to high-redshift range in which predictions from different cosmologies can be distinguished. Our results show that out of all the candidate models, the torsion plus cosmological constant model is strongly favoured by the current high-redshift data, where torsion would be expected to yield the late-time cosmic acceleration. Specially, in the framework of Friedmann-like cosmology with torsion, the determined Hubble constant is in very good agreement with that derived from the Planck 2018 CMB results. On the other hand, our results are compatible with zero spatial curvature and there is no significant deviation from flat spatial hypersurfaces. Finally, we check the robustness of high-redshift observations by placing constraints on the torsion parameter α, which is strongly consistent with other recent works focusing on torsion effect on the primordial helium-4 abundance.

Big bang nucleosynthesis constraints on sterile neutrino and lepton asymmetry of the Universe

We consider the cosmological effects of sterile neutrinos with the masses of 150–450 MeV . The decay of sterile neutrinos changes the thermal history of the Universe and affects the energy density of radiation at recombination and the big bang nucleosynthesis (BBN) results. We derive severe constraints on the parameters of sterile neutrinos from the primordial abundances of helium-4 and deuterium. We also find that in a particular model the constraints can be considerably relaxed by assuming a large lepton asymmetry in the active neutrinos. In this case, the consistent parameters result in Neff ≃ 3.2–3.4 and can alleviate the Hubble tension.

Prospects and limitations for constraining light relics with primordial abundance measurements

The light relic density affects the thermal and expansion history of the early Universe leaving a number of observable imprints. We focus on the primordial abundances of light elements produced during the process of Big Bang nucleosynthesis which are influenced by the light relic density. Primordial abundances can be used to infer the density of light relics and thereby serve as a probe of physics beyond the Standard Model. We calculate the observational uncertainty on primordial light element abundances and associated quantities that would be required in order for these measurements to achieve sensitivity to the light relic density comparable to that anticipated from upcoming cosmic microwave background surveys. We identify the nuclear reaction rates that need to be better measured to maximize the utility of future observations. We show that improved measurements of the primordial helium-4 abundance can improve constraints on light relics, while more precise measurements of the primordial deuterium abundance are unlikely to be competitive with cosmic microwave background measurements of the light relic density.

Lorentz invariance violation in the neutrino sector: a joint analysis from big bang nucleosynthesis and the cosmic microwave background

We investigate constraints on Lorentz invariance violation in the neutrino sector from a joint analysis of big bang nucleosynthesis and the cosmic microwave background. The effect of Lorentz invariance violation during the epoch of big bang nucleosynthesis changes the predicted helium-4 abundance, which influences the power spectrum of the cosmic microwave background at the recombination epoch. In combination with the latest measurement of the primordial helium-4 abundance, the Planck 2015 data of the cosmic microwave background anisotropies give a strong constraint on the deformation parameter since adding the primordial helium measurement breaks the degeneracy between the deformation parameter and the physical dark matter density.

Axions and saxions from the primordial supersymmetric plasma and extra radiation signatures

We calculate the rate for thermal production of axions and saxions via scattering of quarks, gluons, squarks, and gluinos in the primordial supersymmetric plasma. Systematic field theoretical methods such as hard thermal loop resummation are applied to obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling. We calculate the thermally produced yield and the decoupling temperature for both axions and saxions. For the generic case in which saxion decays into axions are possible, the emitted axions can constitute extra radiation already prior to big bang nucleosynthesis and well thereafter. We update associated limits imposed by recent studies of the primordial helium-4 abundance and by precision cosmology of the cosmic microwave background and large scale structure. We show that the trend towards extra radiation seen in those studies can be explained by late decays of thermal saxions into axions and that upcoming Planck results will probe supersymmetric axion models with unprecedented sensitivity.

Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis

We analyze the influence of sterile neutrinos with the masses in the MeV range on the primordial abundances of Helium-4 and Deuterium. We solve explicitly the Boltzmann equations for all particle species, taking into account neutrino flavour oscillations and demonstrate that the abundances are sensitive mostly to the sterile neutrino lifetime and only weakly to the way the active-sterile mixing is distributed between flavours. The decay of these particles also perturbs the spectra of (decoupled) neutrinos and heats photons, changing the ratio of neutrino to photon energy density, that can be interpreted as extra neutrino species at the recombination epoch. We derive upper bounds on the lifetime of sterile neutrinos based on both astrophysical and cosmological measurements of Helium-4 and Deuterium. We also demonstrate that the recent results of Izotov & Thuan [1], who find 2σ higher than predicted by the standard primordial nucleosynthesis value of Helium-4 abundance, are consistent with the presence in the plasma of sterile neutrinos with the lifetime 0.01–2 seconds.

Big bang nucleosynthesis with a stiff fluid

Models that lead to a cosmological stiff fluid component, with a density $\rho_S$ that scales as $a^{-6}$, where $a$ is the scale factor, have been proposed recently in a variety of contexts. We calculate numerically the effect of such a stiff fluid on the primordial element abundances. Because the stiff fluid energy density decreases with the scale factor more rapidly than radiation, it produces a relatively larger change in the primordial helium-4 abundance than in the other element abundances, relative to the changes produced by an additional radiation component. We show that the helium-4 abundance varies linearly with the density of the stiff fluid at a fixed fiducial temperature. Taking $\rho_{S10}$ and $\rho_{R10}$ to be the stiff fluid energy density and the standard density in relativistic particles, respectively, at $T = 10$ MeV, we find that the change in the primordial helium abundance is well-fit by $\Delta Y_p = 0.00024(\rho_{S10}/\rho_{R10})$. The changes in the helium-4 abundance produced by additional radiation or by a stiff fluid are identical when these two components have equal density at a "pivot temperature", $T_*$, where we find $T_* = 0.55$ MeV. Current estimates of the primordial $^4$He abundance give the constraint on a stiff fluid energy density of $\rho_{S10}/\rho_{R10} < 30$.

Extended quintessence and the primordial helium abundance

In extended quintessence models, a scalar field which couples to the curvature scalar R provides most of the energy density of the universe. We point out that such models can also lead naturally to a decrease in the primordial abundance of helium-4, relieving the tension which currently exists between the primordial helium-4 abundance inferred from observations and the amount predicted by standard big bang nucleosynthesis corresponding to the observed deuterium abundance. Using negative power-law potentials for the quintessence field, we determine the range of model parameters which can lead to an interesting reduction in the helium-4 abundance, and we show that it overlaps with the region allowed by other constraints on extended quintessence models.

Predicting Big Bang Deuterium

We present new upper and lower bounds to the primordial abundances of deuterium and helium-3 based on observational data from the solar system and the interstellar medium. Independent of any model for the primordial production of the elements we find (at the 95\% C.L.): $1.5 \times 10^{-5} \le (D/H)_P \le 10.0 \times 10^{-5}$ and $(^3He/H)_P \le 2.6\times 10^{-5}$. When combined with the predictions of standard big bang nucleosynthesis, these constraints lead to a 95\% C.L. bound on the primordial abundance of deuterium: $(D/H)_{best} = (3.5^{+2.7}_{-1.8})\times 10^{-5}$. Measurements of deuterium absorption in the spectra of high redshift QSOs will directly test this prediction. The implications of this prediction for the primordial abundances of helium-4 and lithium-7 are discussed, as well as those for the universal density of baryons.

Helium formation in a class of radiative cosmological models

Helium formation is studied in a class of exact radiation-filled cosmological models in which the space-time geometry is represented by the Friedmann-Robertson-Walker metric and the source of the gravitational field is a non-co-moving imperfect fluid. Using the methods described by Barrow (1984) we find it possible to calculate the predicted helium-4 abundance in the class of radiative models analytically in terms of the results in the corresponding standard model. It is found that different amounts of helium-4 are produced at different space-time locations in the models. It is also found that while there is less helium-4 produced than in the standard model, the models give rise to predictions for the primordial helium-4 abundance within the observed range. In view of current estimates for the pre-galactic helium abundance and the predictions obtained from the standard model, models in which less helium is produced are of particular interest. The techniques employed in the calculation are discussed, and the assumptions and approximations that have been made are described in some detail.