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. Helium tenuous state. Specifically, Gamow asked edy of cosmic proportions, the two physicists ultimately gave up in nuclei (denoted He4) have two protons and two neutrons. Fusing Alpher to work out the nuclear reac- frustration. Alpher left academia to work for General Electric while hydrogen into helium is a battle between electromagnetism and the Triton (H3) tions that might occur during the hot, Herman moved on to General Motors Research Laboratories. strong nuclear force, two of the four forces that govern the cosmos. dense period. Within the space of a In the mid-1960s, the weight of new data finally forced the While it’s easy to push neutral neutrons together, every proton year, Alpher had worked out many acceptance of Big Bang cosmology. But, even then, Alpher’s carries a positive electric charge. Like charges repel via the elec- Neutron immense contribution was largely ignored, with credit given tromagnetic force, which gets stronger as the particles get closer. almost solely to Gamow. (It’s like trying to force the same poles of two magnets together.) Energy release In the years since, others have refined the picture Alpher and To fuse into more complex nuclei, protons must overcome this Gamow first glimpsed. Its predictions of simple-element abun- electromagnetic barrier. dances prove that we understand something about cosmic The strong nuclear force is more powerful than electromagne- HELIUM GENESIS, PART 1. Primordial origins. The secret of BBN, the secret Alpher, Gamow, and tism. But it has the odd property of kicking in only when protons 3 fusion also created tritium (H ), an Herman knew first, occurs just after the cosmos began. and neutrons get really close to each other. unstable, radioactive form of hydrogen. At a high enough density and temperature, protons whiz Some tritium nuclei captured a pro- Proton Fusion and the Big Bang around fast enough that some collisions have the energy to push ton to make normal helium (He4). Stars also make helium, so this Astronomers see galaxies rushing away from one them past the electromagnetic barrier and trigger fusion. But element is ever more common in another in today’s expanding universe. But if we because the universe is expanding and cooling, Big Bang nucleo- the universe. Ionized hydrogen could run cosmic evolution backward, everything synthesis becomes a race against time. gas clouds in other galaxies clue Energy release would draw together. The cosmos would become astronomers into helium’s abun- denser and hotter. As the clock runs backward Beat the clock dance before stars shone. toward the Big Bang, structures like galaxies melt The universe’s rapid expansion and cooling leaves only a brief win- into a thickening soup of primordial gas. Run the dow for nuclear fusion to occur. Einstein’s theory of relativity speci- clock back further, and the gas also breaks down into fies the expansion rate; nuclear physics specifies the temperature Normal helium (He4) a smooth, ultrahot sea of protons, neutrons, and other and density at each moment in cosmic history. But as the young

 Cosmology’s Greatest Discoveries ⁄⁄⁄ 2009 www.Astronomy.com  The early universe’s chemical content Neutron Proton Time after Big Bang THE AMOUNT of deute- 1 second 1 minute 5 minutes 1 hour rium peaks about 100 Greater Protons (H) seconds after the Big Bang, but much of it Neutrons becomes swept into helium nuclei. Fusion Deuteron Tritium (H3) with these helium nuclei then builds lithium and Normal 7 helium (He4) beryllium. But Be isn’t stable, and the nucleus 3 Deu 2 “Light” helium (He ) terium (H ) 7 ) Li 6 decays to Li with a half- 4 thium (Li ) Energy release e H ( ) life of 53 days. Tritium 7

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u e also decays, with a 12- Fraction of total mass li e B ( 3 Energy release H year half-life, to He . None

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Normal u i of the beryllium or tritium l 4 Lithium (Li l helium (He ) y 3 r formed during BBN sur- HELIUM GENESIS, PART 2. “Light” helium (He ) also formed e B vives today. in the Big Bang’s opening act. Stars convert deuterium to Helium (He3) He3 , but, beyond this, little is known. Some argue the Lesser actions of stellar furnaces result in little net He3 production 100 billion K 10 billion K 1 billion K 100 million K or destruction. If this is true, the total amount of deuterium Temperature (kelvins) universe ages, each temperature and He3 remains approximately constant. and density regime allows only cer- tain particles to exist and certain kinds of reactions between those particles. Fusion can’t start until protons and neutrons — collectively, nucleons — form. A millionth of a second after the Big grinds to a halt. More complex elements must await the first stars Bang, when the temperature is a mere 2 trillion K, the universe has — several hundred million years in the future. cooled enough that quarks can coalesce into protons and neutrons. Scientists must follow all possible reactions, their pace, Triton About 1 second after the Big Bang, the ratio of neutrons to and all their products. Most importantly, physicists protons becomes fixed, and fusion reactions can begin. But this must perform these calculations in a cosmic back- window of opportunity lasts only 3 minutes. After this time, the ground of continually changing temperature and cosmos will have expanded and cooled so much, it won’t support density. It is a tremendous task. But when the fusion reactions at all. smoke clears, BBN predicts exactly how much As BBN begins, protons outnumber neutrons 7 to 1. The hydrogen, helium, deuterium, and other Energy release difference emerges because neutrons are slightly light elements exist in the cosmos. heavier than protons, and this mass difference Lithium nucleus (Li7) allows a neutron to decay spontaneously into a From H to us proton, electron, and a ghostly particle called While stellar nucleosynthe- CONSTRUCTING LITHIUM. The heftiest survivor of Big a neutrino. Left to its own devices, a lone FUSION sis could not match the Bang nucleosynthesis is lithium (Li7). Some stars pro- neutron will, on average, decay into a pro- reactions begin observation that helium duce the element, others destroy it. Astronomers study ton and an electron in just 15 minutes. makes up one-quarter of its abundance in the atmospheres of stars in our gal- about 1 second the cosmos’ mass, Big Bang axy’s halo to infer its original value. Observations and Save the neutron nucleosynthesis nails it right out stellar models suggest these stars contain about half of the lithium available before stars began to shine. Fusion saved the neutrons. They collided after the Big Bang of the gate. BBN’s main prediction is with the abundant protons and fused and last only the copious early production of He4. This together as a deuteron — the simplest result ends up being remarkably insensi- compound nucleus. A deuteron, a nucleus 3 minutes. tive to details in the calculation. Barring of deuterium (denoted H2), is a second stable major changes to the basic scenario, BBN isotope of hydrogen. always leads to helium production close to the and neutrons — with high accuracy. Using precise measurements stand what they are. In this way, BBN not only has provided proof Deuteron formation can’t start up in earnest observed amount. Fundamentally, all that really of light-element abundances in regions as diverse as stars and that a Big Bang must have occurred, but it also gives us strong evi- until about 100 seconds after the Big Bang. Once it matters is that a Big Bang occurred. intergalactic clouds, astrophysicists now can claim that the den- dence that we have much to learn about our universe. does, it triggers a cascade of reactions that leads to nuclei with 2 Helium abundance isn’t all that sensitive to conditions in the sity of normal matter in the cosmos is only around 2 percent Much has changed from Alpher and Gamow’s first calculations protons and 2 neutrons — helium. For example, a deuteron may early universe, but deuterium is another story. The denser the early of the value needed to halt the universe’s expansion in the future. 60 years ago to the current era of precision cosmology. Now, a collide with a neutron to make tritium (H3), which then collides universe was at the beginning of the fusion era, the more likely it is Most astronomers and cosmologists believe the universe’s total wealth of high-quality data lets scientists test competing cos- with a proton to make normal helium (He4). Or the deuteron that all the deuterium would end up in helium nuclei. That some density (the sum of all kinds of matter and energy) exactly equals mological models. But while astronomers have firm reasons for could collide with a proton to make a nucleus of light helium deuterium remains — even 0.01 percent relative to hydrogen — this critical density. believing in the reality of the Big Bang, they don’t need to rely on (He3), which then collides with a neutron to make He4. tells physicists something about the young universe. Because BBN predicts such a tiny fraction for stuff like us, the cutting-edge physics to do so. Big Bang nucleosynthesis shows us Other reactions create a small amount of lithium and beryllium. This and other trace elements let physicists determine the rest of the universe must be composed of dark matter and dark that a brief period of well-understood physics has consequences But that’s as far down the periodic table as we can go before fusion universe’s baryonic density — a measure of matter like protons energy — “dark” in the sense that astronomers don’t yet under- that trickle down 13.7 billion years to the universe we observe.

 Cosmology’s Greatest Discoveries ⁄⁄⁄ 2009 www.Astronomy.com 