Events that preceded the possible formation of WATER and AMIDE from the 4 early elements.
Primordial Nucleosynthesis
Baryons (protons and neutrons) are the present day representation of quark-based matter. The density of baryons in the universe has recently been accurately determined by data on the cosmic microwave background radiation (CMB). The fraction of all mass and energy in the universe represented by baryons is only about 4-5%, but it is what the visible stars and ordinary chemical materials are made of. At about 10 milliseconds into the Big Bang quarks combined into protons and neutrons. It appears that the resulting mix did not have large scale non-uniformity (i.e. patches of more neutrons or more protons in different places). Up to about 100 seconds after the Big Bang, nuclear reactions of protons and neutrons went some way toward the production of heavier elements than hydrogen. The process was abruptly cut short by continued evolution of the universe toward lower density and temperature. The best current calculational estimates for the end product distribution of this Big Bang Nucleosynthesis (BBN) use the CMB baryon density as a constraining parameter, and are given by Cyburt [1] as:
Protons (H atom nuclei) +
4He (mass fraction) YP = 0.2485±0.0005
D/H (relative abundance of deuterons/protons) = 2.55×10-5
3He/H (relative abundance)= 1.0×10-5
7Li/H (relative abundance) = 4.3×10-10
In the Cyburt paper the nuclear cross-sections for all the reaction rates are reviewed in detail, and the calculated BBN results are compared with observations, with generally good results. A useful earlier diagrammatic listing of the nuclear reaction rates is given in [2], but the calculational results were less accurate in [2] because the CMB data on baryon density was not then available.
Later in the expansion of the universe (at 400,000 years) the free electrons re-combined with the above nuclei to produce neutral atoms of hydrogen, deuterium, 3He, 4He and 7Li. The re-combination radiation from this event was the origin of the CMB seen today.
The First Stars
For most of the next 100 million years there were no stars or other galactic objects. The hydrogen and helium that comprised almost all of the baryonic matter became less dense and cooled further as the universe expanded. However, the seeds of non-uniformity existed in the matter of the universe, which in addition to 4-5% baryonic matter also included about 23% “dark matter” that was initially known to exist by its effect on the motion of stars within galaxies – the visible matter in many galaxies appeared to be insufficiently massive to keep the outer stars gravitationally bound to the galaxy. The cosmic microwave background (CMB) radiation carries a faint imprint of initial non-uniformities in the universe and it is suggested that quantum fluctuations in the dark matter density, introduced prior to the quark era, led ultimately to these CMB imprints. Although the composition of the dark matter is still unknown, its dominant mass relative to that of the baryons gave it the dominant role in the further development of these fluctuations into higher density “lumps” that attracted baryonic matter into localities where it could form into the first stars [3]. The dark matter is thought to collapse into “halos” at these locations, and yet these halos do not continue to collapse because the dark matter has pressure to resist the pull of gravity. On the other hand, baryonic matter has less pressure and continues to accrete within the dark matter halos in numerous lumps of 100 to 1,000 solar masses. At the approximate time of 70 million years after the big bang [3] the first of these lumps undergoes an accelerating collapse that is completed within 1 million years, and ignites into a star that has thermonuclear fusion as its energy source.
These first stars, formed from essentially nothing but hydrogen and helium and referred to as belonging to Population III, have a predictable series of nuclear fusion reactions, described for example in [4]. In some ranges of formation mass the constituent material immediately collapses into a black hole without further nucleosynthesis. The range of masses between 63 and 130 solar masses lies between two black hole limits and appears to be an important range for nucleosynthesis. Especially toward the lower end of this range, the end product of nucleosynthesis is a stellar core comprising mostly oxygen, with some carbon and very small amounts of higher mass species. The life of these massive stars is rather short, possibly only a few million years. The end of helium burning leads to collapse during a brief period of “pair production”, followed by explosive oxygen burning reactions that blow the star apart, or at least cause it to shed much of its mass. It seems certain that most of the “PopIII” stars exploded releasing huge quantities of oxygen into the young Universe. Lesser amounts of carbon and much lesser amounts of nitrogen were also released.
The huge photon release (X-rays and ultraviolet light) of these first stellar explosions was enough to partially re-ionize the local galactic and even intergalactic medium, which modified the molecular cooling rate and allowed smaller masses to condense into stars also based on purely hydrogen and helium input, but ranging around 10 solar masses [5]. Evidence from the absorption of light from quasars and from analysis of the cosmic microwave background radiation [6] pinpoints this period of re-ionization as lying between redshifts of about 20 down to 7, corresponding to 100m to 800m years after the big bang [7], with a likely most complete ionization at redshift 11 corresponding to 450 million years [6]. The total mass incorporated into these smaller stars, labelled Pop II.5, exceeded the total mass in Pop III stars by ten times [5], and the main nucleosynthetic product was again oxygen, with minor carbon and nitrogen contributions.
Theory and observation suggest, therefore, that by 100 to 500 million years into the life of the Universe the constituent elements of water and the amino acids had been released in large quantities by the first generations of stars. These large mass stars of Pop III and Pop II.5 types had burned rapidly and exploded releasing mainly oxygen but also carbon and nitrogen into the galaxies that they belonged to. Well before 1Gyr, the “metal” content (elements higher than helium) of these galaxies rose to a level that is conjectured to be 10-4 times the solar metallicity, when a new smaller type of star, comparable in size to the sun, was able to form in great profusion, referred to as Pop II stars. Conditions during the reionization era between 100 million and 800 million years were not sympathetic to complex chemistry, as there was strong radiation and an ionizing chemical environment that broke any complex molecules that formed into small radical constituents. The “metals” released by PopIII and PopII.5 stars were spread throughout the integalactic medium well before 1Gyr into the life of the Universe [8], but it is in the more dense galactic regions after 800 million years that the next part of the story belongs – the first stable complex chemistry of hydrogen, oxygen, carbon and nitrogen.
Traces of oxygen and carbon within the first galaxies were able to induce smaller condensation fragments than hydrogen alone, via radiation on ground state hyperfine transitions (for carbon it is the singly ionized ground state). A smaller generation of stars and proto-planetary accretion discs was then able to form [9]. Particularly in the accretion discs, the dominant chemical was water, more so than in our own solar system. Carbon and nitrogen bearing molecules such as ammonia and hydrogen cyanide, together with water could form amides.
[1] R. H. Cyburt “Primordial Nucleosynthesis for the new cosmology: Determining uncertainties and examining concordance”, Phys. Rev. D70, 023505 (2004) 25pages.
[2] C. J. Copi, D. N. Schramm and M. S. Turner, “Big-Bang Nucleosynthesis and the Baryon Density of the Universe”, Science 267, 192-199 (1995).
[3] V. Bromm, P. S. Coppi and R. B. Larson, “The formation of the first stars I: The primordial star-forming cloud”, The Astrophysical Journal 564, 23-51 (2002).
[4] A. Heger and S. E. Woosley, “The nucleosynthetic signature of population III”, The Astrophysical Journal 567, 532-543 (2002).
[5] T. H. Greif and V. Bromm, “Two populations of metal-free stars in the early Universe”, Mon. Not. R. Astron. Soc. 373, 128-138 (2006).
[6] J. Dunkley et al. “Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data”, submitted to Astrophysical J. Supp. Series (2008).
[7] R. Barkana and A. Loeb, “Detecting reionization in the star formation histories of high-redshift galaxies”, Mon. Not. R. Astron. Soc. 371, 395-400 (2006).
[8] F. Calura and F. Matteucci, “Cosmic evolution of metal densities: the enrichment of the intergalactic medium”, Mon. Not. R. Astron. Soc. 369, 465-478 (2006).
6-30-08 Malcolm McGeoch