Light isotopes of hydrogen and helium formed minutes after the Big Bang. The study of one of these primordial isotopes, helium-3, has now been proposed as a useful strategy for constraining the physics of the standard cosmological model.
The accepted theory of cosmology, known as the standard cosmological model, invokes the existence of a hot early Universe about 13.7 billion years ago. At that time, matter (elementary particles) and radiation (photons) coexisted as an essentially amorphous plasma from which nuclei, atoms, stars and galaxies progressively formed. The observation of 'relics' from that period, and their comparison with theoretical predictions, allowed the standard model to be established, and helps scientists to probe the physics of the Universe and to determine the values of its fundamental properties. Writing in The Astrophysical Journal Letters, Cooke1 suggests that observations of the abundance of one such relic, the rare helium isotope 3He, might provide information about the number of low-mass particle species in the Universe, thus constraining the standard model of nuclear and particle physics.
The hot early Universe left two types of major relic: the faint glow of microwave photons known as the cosmic microwave background (CMB), which is almost the same in all directions of the sky; and the light elements hydrogen and helium. These elements consist of the abundant isotopes 1H and 4He, and the rare ones, 2H (deuterium, also abbreviated to D) and 3He. All of these isotopes were produced by a process called Big Bang nucleosynthesis (BBN), through nuclear reactions between protons and neutrons during the first few minutes of the hot early Universe.
According to theory, the presently observed properties of the cosmological relics depend on the physics of the early Universe. For instance, the abundances of primordial deuterium and 3He depend sensitively on the density of normal (baryonic) matter at that time: the higher the density, the less deuterium and 3He are produced, because they are more frequently destroyed by primordial nuclear reactions. Similarly, the morphology of the ripples detected in the CMB depends strongly on the cosmic baryonic density.
The primordial abundance of 4He is more sensitive to the expansion rate of the early Universe than to its bayonic density. That rate depends, in turn, on the number density of photons and other relativistic particles, including electrons, positrons and three flavours of neutrino in the standard model of particle physics. The sum total of all those species is usually parameterized by an 'effective number of neutrino species', Neff. In the standard model, Neff is 3.046, but its value can be different in non-standard models that predict the formation of new particle species.
Theoretical predictions of BBN have improved considerably over the years, and all of the relevant nuclear reaction rates have been measured in the laboratory2. But comparison of these predictions with observations requires the primordial abundances of the light nuclei to be reliably established, which is difficult to do. After more than 13 billion years of cosmic evolution, the abundances of all elements in the Universe have been altered by the workings of stars: those of 1H and deuterium are reduced compared with primordial abundances, because stars 'burn' these isotopes in nuclear reactions, whereas the abundances of all other isotopes have steadily increased because they are produced by stars. Regions of the Universe that have evolved very little must therefore be sought if primordial abundances are to be established.
In the case of deuterium, which is the most sensitive chemical probe of baryonic density, observations are made in remote gas clouds more than 10 billion light years (about 3 billion parsecs) away, and therefore more than 10 billion years old. The low content of 'metals' (defined by astronomers as elements heavier than helium) in such clouds ensures that their composition is barely affected by stellar activity. The observed isotopic ratio of deuterium to 1H shows little variation around the average observed value3, and points to a baryonic density of 4.5% of the critical cosmic density (the density value that determines whether the Universe is open — expanding forever — or closed). This is in excellent agreement with the value determined from the latest CMB observations by the European Space Agency's Planck mission4.
4He is conventionally used as a probe of Neff. The abundance of this isotope is measured through the intensity of its emission lines in the gas spectra of nearby galaxies that have low metal content, but those measurements are affected by systematic uncertainties. Even worse, the latest analyses point to a primordial 4He abundance that seems to be significantly higher than the one suggested by the Planck mission's CMB study5,6.
To resolve this problem and to reduce the uncertainties, Cooke proposes that the usually neglected primordial isotope 3He should be included in the analyses. According to BBN theory, the ratio of the primordial abundance of 3He to that of 4He depends on both Neff and the cosmic baryonic density, in a way that is opposite to the dependence of the ratio of deuterium to 1H; that is, 3He:4He decreases with Neff, whereas D:1H increases. So, by combining analyses of both the hydrogen and helium isotopic ratios, the value of Neff can be constrained better than by using either the abundance of 4He or the D:1H ratio alone (Fig. 1).
Implementing this idea is far from trivial, however, on both observational and theoretical grounds. First, uncertainties in nuclear-reaction rates will have to be further reduced to make 3He a useful probe for precision cosmology. Second, unlike deuterium, which is always destroyed by stars, 3He is produced by low-mass stars but destroyed by higher-mass ones, to a poorly known extent. This makes it difficult to determine its primordial abundance unambiguously, even by looking in low-metallicity environments.
Moreover, 3He is 10,000 times less abundant than 4He, and so its weak emission line will be hard to identify in the background of the much brighter 4He line — especially if the latter is broadened by rapid thermal or turbulent motions of the emitting gas. A statistically significant detection of 3He would require a high signal-to-noise ratio, of more than 500. This will be obtainable only using the next generation of telescopes, which will have mirrors 30 metres or more in diameter.
Nevertheless, Cooke's suggestion is of great interest, because the standard cosmological model should be checked as accurately as possible with every available method, in view of its prominent role in modern physics. In particular, Cooke's strategy should allow potential departures from the standard model to be probed in a complementary way to existing strategies.Footnote 1
Cooke, R. Astrophys. J. 812, L12 (2015).
Cyburt, R. H., Fields, B. D., Olive, K. A. & Yeh, T.-H. Preprint at http://arxiv.org/abs/1505.01076 (2015).
Cooke, R. J., Pettini, M., Jorgenson, R. A., Murphy, M. T. & Steidel, C. C. Astrophys. J. 781, 31 (2014).
Planck Collaboration. Preprint at http://arxiv.org/abs/1502.01589 (2015).
Aver, E., Olive, K. A. & Skillman, E. D. J. Cosmol. Astropart. Phys. 7, 011 (2015).
Izotov, Y. I., Thuan, T. X. & Guseva, N. G. Mon. Not. R. Astron. Soc. 445, 778 (2014).
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