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Big Bang Forgedthe First Elements Nuclear reactions in the universe’s first minutes How the made the lightest elements. How it happened laid the groundwork Big Bang for everything that followed. ⁄⁄⁄ BY ADAM FRANK forged the fi rst elements THE BIRTH OF DEUTERIUM. The most fragile of oments after the Big Bang, as the universe while sulfur is yellow and powdery — because the atoms Neutron the light elements, deuterium (H2), formed in the quickly expanded from an unimaginably making each element differ. It’s hard to believe scientists universe’s first minutes when protons and neutrons dense, impossibly hot state, something won- were still vigorously debating the reality of atoms even 100 Proton stuck together. All astrophysical processes destroy derful happened. Over the course of the first years ago. But once researchers confirmed the reality of the this nucleus, so its abundance has been declining 3 minutes, the first elements were born. atom in the early decades of the previous century, they began since the Big Bang. Deuterium’s absorption feature MEvery instant of every day, evidence that the universe probing its internal structure. in the spectra of quasars helps astronomers pin began in a cosmic fireball stares us in the face. Proof that the Every atom, they found, contains a central nucleus com- down its original abundance. The fusion reactions illustrated here involve photon emission; other, faster universe was once hot and dense resides in the very atoms posed of one or more protons, which carry a positive electric reactions also were present. from which the stars, planets, and we ourselves are built. charge. Hydrogen, the simplest and most abundant element, Energy release ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY Big Bang nucleosynthesis — BBN, for short — is the field has a single proton in its nucleus. It’s the number of protons of astrophysics linking the observed abundances of the chemi- in a nucleus that distinguishes one element from another. cal elements to theoretical predictions based on the Big Bang. The nucleus also may contain another particle, called a Along with the universal redshift of galaxies and the cosmic neutron. It’s slightly heavier than the proton and lacks an microwave background, BBN is one of the great pillars on electrical charge. The number of neutrons in a nucleus is which modern cosmology stands. what distinguishes one variation of a single element — called BBN is a remarkable mix of precise astronomical observa- an isotope — from another. tion and exacting physical theory. Using only the abundances A third kind of particle, the negatively charged electron, of the lightest elements, hydrogen and helium, BBN spins out orbits each nucleus at a great distance. Compared to protons a detailed picture of our cosmic beginnings. It is a remarkable and neutrons, electrons weigh next to nothing. The discovery tale and a grand triumph of science’s power and precision. of atomic, and then nuclear, structure answered questions Most amazing of all, the events that drive this story, with con- about the nature of matter that had haunted philosophers sequences stretching across space and time, unfolded in little and scientists for 2,000 years. Three quarks constitute neutrons and protons more than the span of a typical TV commercial break. Until the 1930s, physicists could not explain elemental abundances. Why is it so much easier to find hydrogen atoms Elemental origins than, say, iron atoms? And good luck finding a lutetium atom. We recognize more than 116 distinct chemical elements today. Hydrogen is vastly more abundant than iron, which is vastly Each appears different to us — copper is metallic and shiny, more abundant than lutetium. Why? Deuteron In 1937, German-American physicist Hans Bethe (1906– Adam Frank is an astrophysicist at the University of Rochester in New York 2005) was returning by train to his Ithaca, New York, home and a member of Astronomy’s Editorial Advisory Board. following a nuclear physics conference in Washington. It just © 2011 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com www.Astronomy.com might have been the most productive train ride in history: By using Stellar nucleosynthesis predicted a cosmos with too little NUCLEAR SPEAK the time to explore equations for the newly developing science of helium. Observations show that helium makes up about 24 percent BIG BANG NUCLEON nuclear physics, Bethe discovered the secrets of stellar fusion. Tak- of the universe’s normal matter. Everything heavier accounts for The event that spawned A proton or neutron. ing into account the high temperatures and densities inferred by less than 2 percent of the total, and all the rest is hydrogen. For space, time, and the astronomers to exist at the centers of stars, Bethe showed how years, astronomers were left scratching their heads at this glaring expanding universe. NUCLEOSYNTHESIS simple elements can be squeezed together to form more complex failure in the midst of a spectacular success. Processes in stars and the ones, a process that releases energy. In fact, the answer had already been found and forgot- DEUTERON early universe that create In a single stroke, Bethe showed how the fusion ten. Hiding in their journals was a paper that could A hydrogen nucleus (pro- new atomic nuclei from of elements fuels the stars, that stellar cores are solve the light-element puzzle. But accepting the ton) bound to a neutron; existing protons and alchemical furnaces transmuting one kind of solution it offered meant opening a door to a nucleus of deuterium. neutrons. matter into another. Bethe’s success con- HIDING IN the dawn of time. FUSION TRITON vinced physicists and astronomers that the astronomers’ journals handiwork of stars could explain all the Beyond steady state The merger of protons and A hydrogen nucleus neutrons to form atomic (proton) bound to two elements and their abundances. In 1948, Ralph Alpher (1921–2007), was a paper that could ARCHIVES VISUAL SEGRÈ EMILIO AIP ARCHIVES VISUAL SEGRÈ EMILIO AIP nuclei, accompanied by neutrons; a nucleus of They were both right and wrong. a wiry, young graduate from George a characteristic energy tritium. In 1957, British astronomers Geoffrey solve the puzzle, but its Washington University, wrote a doctoral GEORGE GAMOW, a Russian- RALPH ALPHER and Gamow American scientist and a found that observed abun- release. The fusion of and Margaret Burbidge, American thesis that began, for the first time, at the solution meant opening pioneer in nuclear physics, dances of light elements, hydrogen into helium astronomer Willy Fowler, and British beginning. Under the tutelage of George a door to the dawn suggested the universe like hydrogen and helium, powers the Sun. astrophysicist Fred Hoyle published a Gamow (1904–1968), a Russian-refugee originated from a hot sea are a consequence of a hot, monumental work that put the theory of of time. physicist known as much for his heavy of radiation and particles. expanding early universe. stellar nucleosynthesis on firm ground. Often drinking as for his genius, Alpher set out known as B2FH, the paper refined earlier studies to describe nuclear physics in the realm of an of the crucial aspects with meticulous attention to mathematical subatomic particles. At this point, the universe has a temperature into a single coherent picture that accounted for the infant expanding universe. detail. It was a triumphant physics tour de force — but one the of about 100 billion kelvins. A teaspoon of cosmic matter weighs observed abundances of elements — almost. It’s difficult to imagine now how bold, how radical scientific establishment promptly forgot. While Alpher’s first cal- more than 100,000 tons. While the astronomers could nail down elements like carbon, this endeavor was. In 1948, few scientists were thinking about culations contained some missteps, they got the fundamentals of This is where BBN begins. By going back only to about 0.01 oxygen, and iron, their model couldn’t get the simplest elements cosmology, and those who were had locked themselves into the Big Bang nucleosynthesis correct. second after the beginning, physicists limit themselves to a tem- right. The theory predicted so-called steady-state model. Steady-state cosmology held that, Alpher, along with collaborator Robert Herman, spent the next perature and density domain they can work with comfortably. hydrogen and helium even with expansion, the universe never changed its appearance few years refining his models and examining the implications of a More than 60 years of particle accelerator experiments validate proportions that were or its condition. The cosmos had always looked — and always cooling and expanding universe. The team even predicted the pres- their understanding. Running the clock forward from 0.01 second, completely different would look — just as it does now. ence and temperature of a cosmic microwave background from the BBN describes the universe’s next 3 minutes in astonishing detail. from what astron- Gamow and Alpher were beyond the leading edge. The origin redshifted light released when the universe had cooled enough that From the chaos of those first moments, fusion physics leaves an omers observe. of the elements had always been a cherished problem to Gamow. electrons could combine with nuclei to form atoms. unalterable imprint on the universe. To choreograph this dance, Deuteron He asked Alpher to imagine what might happen in a Alpher said he and Herman expended “a hell of a lot of energy” BBN requires two critical components — an understanding of universe that started out small, hot, and dense giving talks to convince astronomers that the results deserved a fusion processes and the physical conditions in the young cosmos. and expanded to its present enlarged, cold, serious look. But their work received little attention, and, in a trag- A hydrogen nucleus (denoted H) is a single proton.
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