An Elemental History of the Early Universe - ACS Symposium Series

Chapter 4

An Elemental History of the Early Universe

Downloaded by UNIV OF FLORIDA on November 26, 2017 | http://pubs.acs.org Publication Date (Web): October 30, 2017 | doi: 10.1021/bk-2017-1263.ch004

E. Prasad Venugopal* Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Michigan 48221-3038, United States *E-mail: [email protected].

Elemental properties play a crucial role in constraining theories of the early universe in cosmology. Attempts to explain the creation and abundances of the elements combined aspects of cosmochemistry, nuclear physics and astronomy to produce the Big Bang theory of the evolution of the universe. This chapter provides a brief overview of historical and contemporary research on elemental chemistry in cosmology.

Introduction In 1927, Georges Lemaitre, a Belgian astrophysicist and Catholic priest, published a new set of analytical solutions to Einstein’s equations of General Relativity. Lemaitre’s solutions (1), describing an expanding universe filled with matter and radiation seemed to suggest that the universe was neither static nor infinitely old. As such, Lemaitre’s work, and that of Soviet mathematician and physicist Alexander Friedmann who independently derived a similar set of solutions in 1922 (2), represented a dramatic departure from earlier cosmological models (3, 4). In solving the equations of General Relativity, Einstein was led, based on existing empirical evidence, to a universe that was spatially limited but temporally infinite and filled with a finite matter density. As is well-known, the introduction of an “ad hoc” cosmological constant into the field equations provided a repulsive force to balance the gravitational attraction of matter that otherwise predicted the collapse of the universe, a contribution that Einstein found to be “detrimental to the formal beauty of the theory” (5). The expanding universe models of Lemaitre and Friedmann did not receive much attention due to a variety of reasons, including the lack of observational evidence (6). However, that situation changed in 1929 when American astronomer, Edwin Hubble, published a study showing a linear relationship © 2017 American Chemical Society

Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


between galactic distances and the redshift of spectral lines from those galaxies (7). Through the Doppler effect, Hubble calculated the recessional velocities of galaxies from their redshifts and related them to galactic distances through Hubble’s law:

where V is the recessional velocity in the radial direction in km/s, D is the distance of the galaxy in Megaparsecs (1 Mpc = 3.26 million light-years) and H0 is called the Hubble constant. Initially Hubble calculated the value of H0 to be about 500 km/s/ Mpc. While Hubble’s observations could not identify the source of the redshift, it was soon recognized that Hubble’s Law was possible evidence for the expansion of space, as predicted by General Relativity (8). The cosmological timescale during which the expansion occurred was then easily calculated as the inverse of the and provided an approximate (model-dependent) value for Hubble constant, the age of the universe of 1-2 billion years. The scientific debates and controversies about a universe that seemed to evolve from a putative beginning began in earnest in the 1930s (6). Cosmologists were confronted with the challenge of building a finite-age universe populated by chemical elements whose relative abundances were known from terrestrial and cosmic sources. Elemental properties played a crucial role in constraining theories of the early universe that attempted to answer two fundamental questions: how were the known elements created in the universe; and, what mechanism(s) could explain the relative abundances of these elements? While the implications of these issues were far from clear in the early decades of twentieth-century cosmology, attempts to explain the creation and abundances of the elements continue to play a determinant role in viable theories of the evolution of the universe. This chapter provides a brief overview of historical and contemporary research on elemental chemistry in cosmology.

The Role of Radioactivity in Early Cosmology Henri Becquerel’s discovery of radioactivity in 1896 (9) undermined the notion of stability inherent in the Periodic Table of elements. A flurry of experimental activity followed, resulting in the discovery by Pierre Curie that radium salts emitted a constant stream of heat energy (10). Ernest Rutherford and his collaborators demonstrated that radioactive materials transmutated to other elements with the emission of radiation (11). These experiments opened new frontiers in solar-system physics and cosmology, resolving a long-standing controversy in the former while creating a new one for evolutionary models of the universe. Nineteenth-century science was witness to a sharp disagreement over the age of the Earth (12). The physicists, led by Lord Kelvin, estimated the age of the sun to be about a hundred million years based on a premise that the source of the 84



sun’s heat was solely due to gravitational contraction. Kelvin followed up with a calculation that placed a similar upper-limit on the age of the earth by treating it as a sphere that began in a primordial heated state and gradually cooled to its present state. This timescale was far shorter than that required by Charles Lyell’s theories of geology as well as the theory of biological evolution championed by Darwin and his followers. The dispute was resolved by the discovery of radioactivity in terrestrial elements, such as uranium, contradicting Kelvin’s premise that no other sources of heat were present in the earth’s interior. In subsequent years, radioactive dating of the earth’s crust provided more consistent estimates of the age of the earth, consistently placing it at a few billion years (13). Presenting his view that the paradox had been resolved, Rutherford concluded: “The discovery of the radio-active elements, which in their disintegration liberate enormous amounts of energy, thus increases the possible limit of the duration of life on this planet, and allows the time claimed by the geologist and biologist for the process of evolution” (quoted in (12)). The recalculated age of the earth however, posed a serious problem for cosmologists, who realized that even small changes in the density of matter or radiation in the equations of general relativity predicted a universe that was not only in conflict with the Hubble time, but was younger than the earth and sun. The independent determinations of the Hubble value and the half-lives of radioactive elements meant that this was not a problem that was easily resolved. Writing in 1952, cosmologist Hermann Bondi wrote: “[F]or more than fifteen years, all work in cosmology was affected and indeed oppressed by the short value of [the age of the universe] T (1.8 x 109 years) so confidently claimed to have been established observationally. The time-scale difficulty, as the discrepancy between T and the ages of the Earth and stars was called, of cosmological theories, and the effects of the removal of this influence, have not yet been worked out fully (14).” Nevertheless, the phenomenon of radioactivity caught the attention of cosmologists, and some prominent chemists, as a mechanism for creating the known elements in the universe. Though Rutherford was referring to the ages of the earth and sun, his 1904 suggestion that “it is natural to ask what part radio-active substances play in cosmical physics” (15) foresaw the emergence of the field of cosmochemistry. Lemaitre’s model of 1931 suggested that the universe began as a “unique” and “unstable” atom that subsequently divided into “smaller and smaller atoms by a kind of super-radioactive process” (6). Others, including Walther Nernst, Gilbert Lewis and James Jeans, suggested similar mechanisms for the production of elements in stars and the early universe based on the disintegration of heavy elements into lighter one, but in the absence of a viable theory of the nucleus, these ideas remained highly speculative. The situation changed in the late twenties when multiple physicists, including the nuclear theorist George Gamow, provided a theoretical explanation of 85 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

radioactive alpha-decay through the quantum-tunneling effect in which an alpha-particle was emitted by an unstable nucleus despite being forbidden by classical energy conservation principles. Soon, it was recognized that a reverse nuclear process could be responsible for synthesizing the heavier elements from lighter ones (16), a notion that was solidified by the discoveries of the neutron and deuterium in 1932. The advent of the field of nuclear astrophysics was instrumental in the post-war period in constructing theories of primordial and stellar nucleosynthesis.


Primordial Nucleosynthesis and Stellar Thermodynamics Questions about elemental constituents and energy production in stars had long interested both cosmologists and chemists, a project that began in the early nineteenth century when Joseph von Fraunhofer produced a detailed solar absorption spectrum. The advent of improved equipment and experimental techniques in the early twentieth century significantly aided the study of stellar spectra. In 1925, Cecilia Payne completed a detailed study of the spectral characteristics of the sun as part of her dissertation (17) for a Ph.D. in Astronomy. Payne demonstrated that hydrogen and helium were the dominant components in the solar atmosphere, and by extension stellar interiors and the universe. Further observations of stellar spectra confirmed her results. In 1937, geochemist Victor Goldschmidt (18) published the results of a decade-long study in which he presented detailed tables of data on terrestrial and cosmic abundances of most elements then known to exist. Goldschmidt’s study was a major contribution to the growing field of cosmochemistry (19, 20). Attempts to reproduce the relative abundances of matter and radiation in the universe were initially based on the idea of modeling the universe as a system in thermodynamic equilibrium. Among the earliest theorists to apply this concept were the chemists Harold Urey, Charles Bradley and Richard Tolman. In a 1931 paper (21), Urey and Bradley calculated the abundances of terrestrial isotopes on a hypothesis of “equilibrium by a number of transmutation reactions” and found little agreement with data. Tolman, a mathematical physicist and physical chemist at Caltech, combined special relativity and thermodynamics to calculate “the possible formation of helium out of hydrogen in accordance with a quasi-chemical reaction” and the “possible transformation of matter into radiation.” Disappointingly, Tolman’s calculations led to either a helium-filled universe or one virtually devoid of matter (22). Nuclear physics had flourished in the nineteen thirties into a mature field based on the pioneering work of theorists such as Hans Bethe and experimentalists such as Enrico Fermi who bombarded various elements with neutrons to induce radioactivity by neutron capture. The impetus to apply the newly developed quantum theory of nuclear reactions to explain cosmic elemental abundances received a boost with the publication of a paper by Cornell University nuclear physicist, Hans Bethe, in 1939. Using previous estimates of stellar temperatures, Bethe identified two sources of energy production in stars (23), believed to be hot, dense gases in equilibrium. These processes, the proton-proton chain for 86 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


stars with lower core temperatures, and the carbon-nitrogen chain for hotter stars, were responsible for producing He4 (identified as α-particles in his paper) from hydrogen. He found the agreement with observational data from stellar spectroscopy to be “excellent”. However, Bethe made a categorical claim in his paper that “no elements heavier than He4 can be built up in ordinary stars” because both processes that he considered led to the production of He4. The non-existence of stable nuclei at atomic weights of five and eight precluded the building up of heavier elements through proton capture by helium. Bethe considered other possible reactions to bypass this elemental roadblock. In particular, he tested the possibility that the C12 isotope could be “formed directly in a collision between 3 α-particles”. Production of the C12 isotope could have led to heavier nuclei of nitrogen and oxygen through collisions with protons. Bethe also found that reactions involving the carbon-nitrogen-oxygen group were able to successfully account for the energy production in stars. However, his calculations showed that the 3α C12 process was highly temperature-dependent and required temperatures of about a billion degrees to “make it as probable” as the proton-proton collisions. Ultimately, Bethe concluded that “there is no way in which nuclei heavier than helium can be produced permanently in the interior of stars under present conditions.” The advent of World War II produced a relative lull in the application of nuclear chemistry to cosmology and stellar astrophysics. But, Bethe’s work had revived interest in the notion of a superdense, superhot early universe that was capable of synthesizing heavier elements. Attempts were made to combine known nuclear reaction rates with the principle of thermal equilibrium (the equivalent of the law of mass action applied to nuclei) to accurately calculate elemental abundances. Most attempts failed to reproduce Goldschmidt’s data and led to various disagreements about the hypothesis of a hot, dense universe. But, theorists such as George Gamow, who had studied under Friedmann in the Soviet Union, continued to apply their knowledge of nuclear physics to explaining the origin of the chemical elements. In 1948, Gamow published a paper with Ralph Alpher and Hans Bethe that was generally considered to be the first significant exposition of a hot Big Bang cosmology (24). Their paper, titled “The Origin of Chemical Elements” was predicated on the two key principles of thermodynamic equilibrium and neutron capture applied to a hot, dense, expanding primordial “neutron gas.”. However, they quickly recognized that: “various nuclear species must have originated not as the result of an equilibrium corresponding to a certain temperature and density, but rather as a consequence of a continuous building-up process arrested by a rapid expansion and cooling of the primordial matter (24).” They postulated that protons formed from neutron decay from the reaction n → p + e- would first form deuterium by capturing a neutron, following which “subsequent neutron captures resulted in the building up of heavier and heavier nuclei.” With assumptions about the time-dependence of matter density, Alpher, who completed his dissertation on this topic, was able to demonstrate 87



agreement with Goldschmidt’s data on the logarithmically decreasing abundances of the elements. Subsequent papers also recognized that the early universe was radiation-dominated with a blackbody spectrum that they calculated to have a current temperature of about 5 K. The incipient big bang model of the universe was born. The αβγ-paper, as it was popularly called, was successful in explaining the hydrogen and helium abundances through primordial processes. But, its failure to account for the mass gaps at atomic weights five and eight remained a serious flaw in the theory. In addition, the theory effectively posited that elemental abundances would remain static over time, an assumption that contradicted Bethe’s calculations showing the chemical evolution of hydrogen in stellar nucleosynthesis. Since the latter was well-founded on known nuclear reactions and spectroscopic data, it undermined the claims of big bang protagonists. These uncertainties and contradictions essentially remained until the appearance of a seminal paper on nucleosynthesis in the year 1957. Fred Hoyle, the eminent British physicist and astronomer, was no fan of the big bang theory. An opponent of the idea that the universe had a beginning, Hoyle, along with his collaborators, proposed an alternative model of the universe (25, 26) in the same year as the αβγ-paper. The steady-state theory posited that the universe was infinitely old and large, static and unchanging. To accommodate the effects of expansion, matter was continually created, primarily in the form of hydrogen. The production of heavier elements was posited to occur solely in stars. After more than a decade of work on stellar nucleosynthesis, Hoyle coauthored a landmark paper with fellow astrophysicists Margaret Burbidge, Geoffrey Burbidge, and William Fowler, that substantially resolved the vexing question of the origin of the elements. The 1957 paper (27), titled Synthesis of the elements in stars (but known more as the B2FH paper after the initials of the authors), began by challenging existing theories on the primordial formation of elements. Including the big bang model, they noted that none of the existing theories could explain the element abundance curves originally published by Goldschmidt (18) and later improved by Hans Seuss and Harold Urey (28). On the contrary, they proposed that “stars are the seat of origin of the elements” related to “the known fact that nuclear transformations are currently taking place inside stars.” The B2FH paper provided a detailed and comprehensive description of stellar processes that produced elements heavier than hydrogen. They found that the temperatures and densities in stellar cores were sufficiently high to produce “progressive conversion of light nuclei into heavier ones as the temperature rises.” Remarkably, they found that the 3α-process that Bethe had earlier discounted was a significant reaction in the buildup of heavier elements from He4. In 1953, Hoyle had predicted the existence of an new, and subsequently discovered, resonant state of the C12-nucleus as a necessity for carbon synthesis in stars. The B2FH authors found that the cross-section for C12-production was dramatically increased due to the presence of this resonance, circumventing the roadblocks at atomic weights five and eight. The paper also gave a plausible explanation of the uneven cosmic distribution of these elements through the ejection of material from stars, including in supernovae explosions. 88


The publication of the B2FH-paper in 1957 and the discovery of the cosmic microwave background radiation in 1965 (29) established Big Bang cosmology as the dominant theory of the creation and evolution of the universe. In the intervening decades, the main contours of element production and processes in the theory had been mapped out. Increasingly precise elemental observations have also produced stringent tests of the predictions of Big Bang cosmology, which the theory has passed with flying colors. A detailed and eminently readable account of the Big Bang can be found in Joseph Silk’s book, The Big Bang (30).


Elemental Abundances and Modern Cosmology A few seconds after the big bang, the universe was dominated by radiation accompanied by a much smaller fraction of ordinary matter, including protons, neutrons, and electrons. The ratio of baryons (protons and neutrons) to photons that was established in the early universe, believed to remain constant over time, is the only free parameter in the standard big bang model. Historically, observed primordial abundances were used to place limits on this value. However, in recent years, it has been independently measured with increasing precision from detailed maps of the cosmic microwave background radiation (CMBR), resulting in a currently accepted value of η = (6.1 ± 0.2) × 10−10 (31). At the onset of nucleosynthesis, the heavier and slow-moving neutrons and protons were in thermal equilibrium, constantly creating and destroying each other through decay reactions with electrons and positrons. However, there was roughly one neutron for every six protons, a consequence of the proton’s lower mass and the temperature of the universe at that time. As the universe cooled and expanded, this interconversion ceased and the ratio “froze out” at 1:7 due to the decay of a small fraction of the remaining neutrons into protons, electrons and anti-neutrinos, with a half-life of about 10 minutes. This ratio, well-established from the standard model in particle physics, is an additional input into theoretical calculations of light element abundances in the standard big bang model. The primordial synthesis of light nuclei was the result of a competition between temperature-dependent nuclear reaction rates and a temperature-lowering expansion of the universe (32). The first element to be created was deuterium (“heavy water”), the result of the capture of a neutron by a proton. Much of the produced deuterium was destroyed, either through disintegration by high-energy radiation or through conversion into tritium, helium-3 and finally helium-4 by neutron absorption. A small fraction, roughly one for every hundred thousand hydrogen nuclei, that survived can be detected today. Since deuterium would not survive production in denser and hotter stellar interiors, any detected abundance must have been produced in the early universe. The calculation is very sensitive to the value of η, with larger values dictating a smaller deuterium abundance due to an increased cross-section for neutron absorption. Thus, deuterium poses a significant test and constraint on big bang theory. The series of neutron capture reactions that began with the deuterium nucleus resulted in the production of helium-4 with its high binding energy. Consequently, ordinary helium is the second most abundant element in the universe, with about 89 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


one helium nucleus produced in the big bang for every twelve hydrogen nuclei. Over 99%, by weight, of ordinary matter in the universe is made up of hydrogen (about 75%) and helium (about 24%). Though erroneously attributed to stellar interiors, Hans Bethe’s admonition in 1939 that ““no elements heavier than He4 can be built up” became a defining feature of the standard big bang theory. The absence of stable nuclides at atomic mass numbers of 5 and 8 effectively ended primordial nucleosynthesis within minutes after the big bang. There was no way that a helium nucleus could capture a proton or another helium to produce a heavier element. The temperatures and densities in the early universe also did not allow for the alternative nuclear processes suggested by stellar nucleosynthesis. Calculations do predict a small fraction (~10-5) of helium-3 that avoided neutron capture, as well as trace amounts of lithium-7 (fraction~10-5) from the capture of a helium-4 nucleus by tritium. In the past fifty years, one of the significant challenges facing modern astronomy has been spectroscopic observations and determinations of light-element abundances in the universe to test the predictions of the big bang and competing models. Primordial abundance observations are complicated by the ongoing chemical evolution of elements in the universe, particularly in stars, galaxies and the interstellar medium. Spectroscopic signatures of individual atoms, or the molecules they constitute, are typically obtained from many cosmic sources considered to either be good indicators of original abundances, or for which the processes of chemical evolution are well understood (33). In the latter case, the abundances of elements such as oxygen and nitrogen are good indicators of the extent of chemical evolution that has occurred in these sources. Astronomers typically refer to the “metallicity” of gas clouds, stars and galaxies to indicate the fraction of their mass present in elements beyond hydrogen and helium. Consequently, sources with low or zero metallicity are better indicators of primordial element abundances. The presence of deuterium is inferred from the absorption lines of some of the oldest objects in the universe, quasars. Quasars are believed to be the nuclei of active galaxies formed more than ten billion years ago. The absorption of quasar light by highly redshifted hydrogen clouds produces a distinct signature of any deuterium present. Current data suggests that the deuterium to hydrogen ratio is about 27 × 10−8 (33). He4 abundances are typically measured from the emission spectra of highly ionized interstellar gas clouds with low metallicity and show an abundance ratio of 0.24 ± 0.006 relative to hydrogen. The abundance of He3 is more elusive to measure. The spectral signature from He3 mimics that of He4 to a large extent making it difficult to distinguish between the two. Despite this, measurements from ionized gas in our galaxy infer a ratio of about (1.1 ± 0.2) × 10−5 or about one He3 for every 100,000 hydrogen nuclei (32). Data on Li7 are obtained from old, low-mass, low-metallicity stars found in the halos of galaxies. The presumption is that older stars were formed earlier in the history of the universe and contain primordial abundances of elements like Li7 in their outer non-reactive layers. However, theoretical assumptions built into these measurements have produced large uncertainties in the Li7-H ratio of (1.81 − 2.65) ×10−10 (33). 90



Figure 1 shows the comparison between the predictions of big bang nucleosynthesis and observations. As mentioned earlier, the baryon-photon ratio (η) plotted along the horizontal logarithmic-scale axis is an independent parameter into theoretical calculations. The vertical strip indicates the currently accepted value of η = (6.1 ± 0.2) × 10−10 (31). The relative abundances of the elements are plotted along the vertical axis, also a logarithmic scale. The horizontal lines indicate observational values. Overall, it is clear the agreement between data and theory is rather impressive for all expect Li7, which, as noted earlier, has higher uncertainties embedded in the observed value. Extensions or alternatives to the standard big bang model are subject to the strong constraints on elemental abundances reflected in the figure.

Figure 1. Comparison of primordial element abundances predicted by the big bang against observations. Reproduced with permission from reference (32). Copyright 2006 Albert Einstein Institute. 91 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Conclusion While our solar neighborhood is dominated by radiation and ordinary matter, the elements of the periodic table constitute less than 5% of the matter density in the known universe. The remaining 95% is postulated to exist in the form of dark matter and dark energy (30), the composition, distribution and impact of which remain among the many open questions in cosmology today. Yet, even as cosmology grapples with new ideas into the nature of the universe, viable theories will always be those that can successfully incorporate its elemental history.


References 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Lemaitre, G. Ann. Soc. Sci. Brux. 1927, A47, 49–56. Friedmann, A. Z. Phys. 1922, 10, 377–386. Einstein, A.; Minkowski, H.; Weyl, H. In The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity; Lorentz, H. A., Ed.; Dover: New York, 1952; pp 175−188. DeSitter, W. Mon. Not. R. Astron. Soc. 1917, 78, 3–28. Einstein, A.; Davis, F. A. The Principle of Relativity; Dover: New York, 1923. Kragh, H. S. Cosmology and controversy: The historical development of two theories of the universe; Princeton University Press: Princeton, NJ, 1996. Hubble, E. P. Proc. Natl. Acad. Sci. U.S.A. 1929, 15, 168–173. DeSitter, W. Proc. Natl. Acad. Sci. U.S.A. 1930, 16, 474–488. Becquerel, H. C. R. Acad. Sci. 1896, 122, 420–421. Curie, P.; Laborde, A. C. R. Acad. Sci. 1903, 136, 673–675. Rutherford, E.; Soddy, F. Philos. Mag. 1903, 6, 576–91. Burchfield, J. D. Lord Kelvin and the Age of the Earth; University of Chicago Press: Chicago, IL, 2014. Russell, H. N. Proc. R. Soc. London, Ser. A 1921, 99, 84–86. Bondi, H. Cosmology; Dover: New York, 1952. Rutherford, E. The Collected Papers of Lord Rutherford of Nelson; Routledge: New York, 2014; Vol. 1, pp 641−649. Atkinson, R. D’E.; Houtermans, F. G. Nature 1929, 123, 567–568. Wayman, P. A. Astron. Geophys. 2002, 43 (1), 1.27–1.29. Goldschmidt, V. M. J. Chem. Soc. 1937, 0, 655–673. Cowley, C. R. An Introduction to Cosmochemistry; Cambridge University Press: New York, 1995. Kragh, H. S. Ann. Sci. 2000, 57, 353–368. Urey, H. C.; Bradley, C. A., Jr. Phys. Rev. 1931, 38, 718–724. Tolman, R. C. Relativity, Thermodynamics and Cosmology; Oxford University Press: New York, 1934. Bethe, H. A. Phys. Rev. 1939, 55, 434–456. Alpher, R. A.; Bethe, H.; Gamow, G. Phys. Rev. 1948, 73, 803–804. Hoyle, F. Mon. Not. R. Astron. Soc. 1948, 108, 372–382. Bondi, H.; Gold, T. Mon. Not. R. Astron. Soc. 1948, 108, 252–270. 92



27. Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. Rev. Mod. Phys. 1957, 29, 547–650. 28. Suess, H. E.; Urey, H. C. Rev. Mod. Phys. 1956, 28, 53–74. 29. Penzias, A. A.; Wilson, R. W. Astrophys. J. 1965, 142, 419–421. 30. Silk, J. The Big Bang; W. H. Freeman and Company: New York, 2001. 31. NASA/GSFC. Cosmological Parameters Table. https://lambda.gsfc.nasa. gov/product/map/dr5/parameters.cfm (accessed May 25, 2017). 32. Weiss, A. Elements of the past: Big Bang Nucleosynthesis and observation. http://www.einstein-online.info/spotlights/BBN_obs (accessed March 8, 2017). 33. Signore, M.; Puy, D. Eur. Phys. J. C 2009, 59, 117–172.

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An Elemental History of the Early Universe - ACS Symposium Series

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