For three decades, the big bang theory has dominated cosmology. According to this theory, the Universe began expanding in a titanic explosion between 10 and 15 billion years ago. We observe this expansion in the recession of most other galaxies from ours, and we see the afterglow of the explosion itself in the microwave background radiation that permeates the Universe.
The standard theory of the big bang says that in the minutes following the explosion the Universe was fairly smooth and homogeneous. Recently, however, a new big bang theory has emerged, postulating that matter was spread very unevenly in the early Universe, with regions of high density and low density. These inhomogeneities of matter would not have imprinted themselves on the microwave background, which is smooth, because particles of matter were then only a trace component in a Universe that was filled with photons. But such inhomogeneities may have altered the abundance of the lightest elements in ways that astronomers can detect today.
According to the standard theory, the big bang created significant quantities of only five nuclei, all lightweight. These nuclei formed from nuclear reactions through a process astronomers call nucleosynthesis. The principal products of this primordial process were hydrogen-1 (which consists of just one proton) and helium-4 (which consists of two protons and two neutrons), and today most of the Universe is still hydrogen-1 and helium-4. The big bang also produced a little deuterium (an isotope of hydrogen, which has one proton and one neutron), another helium isotope, helium-3 (two protons and one neutron), and lithium-7 (three protons and four neutrons).
The big bang theory predicts how much of each of these elements were created . Hydrogen-1 made up roughly 76 per cent of the mass of the early Universe and helium-4, about 24 per cent, whereas the numbers of deuterium and helium-3 nuclei were thousands of times less and the number of lithium-7 nuclei billions of times less than the number of hydrogen-1 nuclei. Astronomers measure these abundances in objects that reveal the composition of the early Universe, such as the oldest stars in our Milky Way.
This agreement vindicates the standard big bang theory. Yet scientists have reason to consider alternatives. In 1984, Ed Witten, a physicist at Princeton University, speculated that before the Universe was one second old, it might have broken into regions of matter with high density and low density. In 1985, James Applegate of Columbia University and Craig Hogan of the University of Arizona explored how such inhomogeneities would affect primordial nucleosynthesis. Since then, scientists have studied the consequences in more detail.
The differences can be drastic. In the standard big bang, protons outnumber neutrons everywhere. In an inhomogeneous big bang, the same is also true - at first. Protons, neutrons, and electrons are concentrated in the high-density regions, and all try to diffuse into the low-density regions. But only the neutrons succeed. The protons cannot, because they have positive charge and get trapped in the high-density regions by electrons, which have negative charge. The neutrons, however, lack charge, escape the high-density regions, and fill the low-density regions. Thus, two different regimes emerge: proton-rich high-density regions and neutron-rich low-density regions.
Nuclear reactions in the high-density regions mimic those predicted by the standard big bang because protons drive the nucleosynthesis. In the low-density regions, however, neutrons dominate. Under the right conditions, these low-density regions produce elements heavier than hydrogen, helium and lithium, which have atomic numbers of 1, 2 and 3. Small amounts of beryllium and boron, with atomic numbers of 4 and 5, may arise, as may even heavier elements .
In 1989, Richard Boyd of Ohio State University and Toshitaka Kajino of Tokyo Metropolitan University reported that an inhomogeneous Universe could produce measurable amounts of beryllium and boron. Boyd and Kajino included a nuclear reaction that other scientists had neglected. In this reaction, two neutron-rich isotopes meet to form beryllium. Both neutron-rich isotopes should be common in the neutron-rich low-density regions. Hydrogen-3 (one proton and two neutrons) hits lithium-7 (three protons and four neutrons) to create beryllium-9 (four protons and five neutrons) and a neutron. Beryllium-9 is the only stable, non-radioactive isotope of beryllium. If it formed during the big bang, it could survive until the present. By including this beryllium-producing reaction, Boyd and Kajino also found that even a homogeneous early Universe would produce a small quantity of beryllium, yielding a beryllium to hydrogen ratio of 1.5 x 10
Beryllium and boron are rare. The Sun, for example, has a beryllium to hydrogen ratio of only 1.4 x 10
Because a homogeneous big bang cannot produce detectable amounts of beryllium and boron, and an inhomogeneous big bang may, the presence of beryllium and boron could distinguish between the two theories. But the beryllium and boron in the Sun tell us nothing useful, because the Sun was born billions of years after the big bang and does not contain pure material from the early Universe. To investigate the composition of the early Universe, we must turn instead to the oldest stars in the Galaxy. These stars harbour material that emerged from the big bang, so beryllium and boron in old stars could signify an inhomogeneous big bang.
Astronomers recognise old stars in our Galaxy because the stars have much less iron than the Sun. According to the standard theory, the big bang produced no iron, which has atomic number 26. Instead, stars create iron when they explode. Over time, as more and more stars have exploded, the Galaxy has grown more and more iron-rich. Old stars, born when the Galaxy had little iron, have little iron themselves.
In 1988, astronomers reported detecting beryllium in three old stars with between one-tenth to one-twentieth the Sun's abundance of iron. The beryllium to hydrogen ratio in these stars is between 1 x 10
Old though these stars are, they may not be old enough to probe the early Universe. In 1991, astronomers announced they had discovered beryllium in an even older and more iron-poor star. Gerard Gilmore of Cambridge University, Bengt Edvardsson of the Astronomical Observatory in Uppsala, Sweden, and Poul Nissen of Aarhus University in Denmark observed HD 140283, a star with only one-four-hundredth the iron abundance of the Sun. They measured the beryllium to hydrogen ratio of HD 140283 to be 1.6 x 10
These results are tantalising but not definitive, because even in such an old star the beryllium may have come not from the big bang but from spallation. To distinguish between the two possibilities, astronomers must find boron. Inhomogeneous big bang models usually produce more beryllium than boron, whereas spallation produces more boron than beryllium. (The Sun, for example, has more boron than beryllium.) So, if iron-poor stars have more beryllium than boron, these elements probably formed in an inhomogeneous big bang. But no one has yet discovered boron in an iron-poor star. Boron is difficult to detect by the standard methods of spectroscopy because its characteristic lines of absorption lie in the ultraviolet part of the spectrum, which the Earth's atmosphere blocks. To detect the +element requires a satellite, such as the Hubble Space Telescope.
Inhomogeneous big bang models can yield more than just beryllium and boron, however. They can also produce even heavier elements, such as carbon, nitrogen, oxygen and iron. And that may solve a mystery that has long puzzled astronomers. If the Universe began with only hydrogen, helium and lithium, then the first stars that arose in the Galaxy should consist only of these three elements. For decades, astronomers have searched for such stars but have never found any. If the early Universe was inhomogeneous, then the big bang may have created heavy elements, explaining why all stars in the Galaxy have been found to contain heavy elements.
An inhomogeneous Universe has another major effect: it could change astronomers' estimates of how much mass the Universe has. If the Universe has a low density of mass, the Universe will expand forever. If the Universe has a high density of mass, the gravitational pull of the mass will someday halt the expansion of the Universe and cause the Universe to collapse. The dividing line between these two outcomes is called the critical density. To denote the density of the Universe, which controls the destiny of the Universe, astronomers employ the last letter of the Greek alphabet,
Using the standard theory of the big bang, we can estimate
Unfortunately, another factor also affects the predictions - the Hubble constant, which is unknown. The Hubble constant takes its name from American astronomer Edwin Hubble, who in the 1920s discovered that the Universe was expanding. It quantifies the fact that distant galaxies recede from Earth faster than nearby galaxies. Most astronomers believe that the Hubble constant lies between 50 and 100 kilometres per second per megaparsec. (One megaparsec is 3.26 million light years.) For example, if the Hubble constant is 75, a galaxy that is 1 megaparsec farther than another recedes from us at 75 kilometres per second faster than the other. The bigger the Hubble constant, the faster the Universe expands and the greater the density of the Universe must be to halt the expansion. So, the density given by the abundance of light element will imply a lower value of
Despite the huge uncertainty in the Hubble constant,the abundances of the light elements tightly constrain the density of the Universe. In 1991, a team led by Terry Walker of the Harvard-Smithsonian Center for Astrophysics published the latest analysis. The scientists find that if the Hubble constant is 100,
Most astronomers, however, believe that
And some cosmologists think it is even higher. In particular, inflationary cosmology, which postulates that the Universe expanded rapidly when it was a fraction of a second old, predicts that
However, even astronomers who hate inflation may need non-baryonic matter, because
Despite its appeal, the inhomogeneous big bang has problems. In the standard homogeneous model, the light element abundances depend on just one parameter other than the Hubble constant:
Most astronomers, therefore, will need more evidence before they junk the standard big bang. Beryllium and boron will be crucial elements, but so will others, such as carbon. Nowadays, most carbon is carbon-12 (six protons and six neutrons), because stars create carbon-12 by fusing three helium-4 nuclei together. But the neutron-rich low-density regions of an inhomogeneous big bang could have created a lot of carbon-13, which has one more neutron than carbon-12. So the material that emerged from an inhomogeneous big bang might have a higher carbon-13 to carbon-12 ratio than Earth does.
To see whether such nuclei formed in the big bang, we need not look to the edge of the Universe. Instead, the best objects to study are right here in the Milky Way Galaxy, where the composition of the oldest stars may reveal how the entire Universe began.
Ken Croswell is an astronomer in Berkeley, California.
Further reading Workshop on Primordial Nucleosynthesis, edited by William J. Thompson, Bruce W. Carney and Hugon J. Karwowski, World Scientific, 1990.
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1: PRIMORDIAL ALCHEMY AND THE DENSITY OF THE UNIVERSE
The standard big bang theory explains the radically different abundances of five light nuclei observed in old stars and galaxies lacking in iron: hydrogen-1, helium-4, deuterium, helium-3, and lithium-7. Forged in the fiery aftermath of the big bang, these elements reveal the density of the entire Universe,
Pretend the early Universe is an epic play, in which the various nuclei are actors. Two of the five actors - hydrogen-1 and helium-4 - will emerge from the play as the victors, splitting between them more than 99.9 per cent of the mass of the Universe. Hydrogen-1 is the simplest nucleus, with one proton and no neutrons. Helium-4 is the most tightly bound light nucleus. It has two protons and two neutrons.
Playing secondary, but nonetheless vital, roles will be deuterium, helium-3 and lithium-7. Deuterium, or hydrogen-2, contains one proton and one neutron. Helium-3 is the lighter of the two stable isotopes of helium. It has two protons but only one neutron. Lithium-7 will be the heaviest stable actor, with three protons and four neutrons. Finally, two species will make fleeting appearances, tritium (hydrogen-3) and beryllium-7. Both are radioactive, so any that are formed during the big bang will not last.
Let the play begin. The scene: the expanding Universe, one second after the big bang. The temperature is 10 billion degrees. Photons are everywhere, vastly outnumbering protons and neutrons. Protons themselves outnumber neutrons, by about a factor of five. It turns out that the denser the Universe, the higher the neutron to proton ratio. Because nearly all neutrons will get incorporated into helium-4 (recall that hydrogen-1 has no neutrons), the greater the neutron-to-proton ratio, the more helium-4 will emerge. Thus, the denser the Universe and the higher
The Universe expands, the clock reads 10 seconds, and the temperature falls to 3 billion degrees. But not much happens. Protons and neutrons smash into each other, trying to form nuclei of deuterium (hydrogen-2),
p + n
but because the Universe is so hot, high-energy photons tear the deuterium apart. Without deuterium, nucleosynthesis cannot proceed.
Finally, about 100 seconds after the big bang, the temperature falls to 1 billion degrees and high-energy photons diminish, allowing deuterium to survive. Protons and neutrons and even other deuterons quickly convert deuterium into hydrogen-3 and helium-3:
H
H
H
H
Hydrogen-3 and helium-3 do not last long. They get turned into ultrastable helium-4:
H
H
H
H
H
The denser the Universe, the more collisions there are and the faster these reactions convert deuterium and helium-3 into helium-4. So, the denser the Universe and the higher
Nuclei with masses of 5 or 8 are not stable, so species heavier than helium-4 do not form easily. Nevertheless, a little lithium-7 arises when helium-4 meets tritium,
H
but it is destroyed by protons,
L
The denser the Universe, the more protons destroy lithium-7, and the less lithium-7 will emerge. However, at still higher densities, a new actor - beryllium-7 - debuts:
H
Beryllium-7 is unstable. It captures an electron and decays into lithium-7:
B
All nucleosynthesis ceases about 1000 seconds after the big bang, when the Universe becomes too cool for nuclear reactions. But the half-life of beryllium-7 is 53 days, so beryllium-7 decays long after nucleosynthesis stops. Therefore, the lithium-7 created from beryllium-7 survives, because protons no longer have enough energy to tear the lithium-7 apart. The denser the Universe, the more beryllium-7 forms, so the more lithium-7 forms. As a result, lithium-7 is more complicated than the other species: the abundance of lithium-7 first decreases with increasing
The abundances of deuterium, helium-3, helium-4 and lithium-7 relative to hydrogen-1 therefore all depend in different ways on
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2: BREWING HEAVY ELEMENTS IN A BUMPY BIG BANG
An inhomogeneous big bang excites astronomers in part because it may explain why the Galaxy has no stars made purely of hydrogen, helium and lithium. A homogeneous big bang produces only these three light elements, so the first stars born should contain no heavy elements. Yet astronomers have never found a star without heavy elements.
In a homogeneous big bang, protons outnumber neutrons, and the heaviest element created is lithium-7. If a proton hits lithium-7,
L
the lithium-7 disintegrates into two helium-4 nuclei, and the party is over: no heavy elements form. In contrast, in an inhomogeneous big bang, the low-density regions may build elements heavier than lithium, because the low-density regions abound with neutrons rather than protons. As a result, lithium-7 can get hit by a neutron, creating lithium-8:
L
Like all nuclei with mass 8, lithium-8 is radioactive. It has a half-life of just 0.8 second. If, before it decays, lithium-8 meets helium-4,
L
then boron-11 is created, and boron-11 is stable. Boron-11 can also form from beryllium-9, which itself arises when lithium-7 meets the neutron-heavy isotope hydrogen-3:
L
B
Even if a homogeneous big bang created boron-11, it would not do any good, because a proton would split the boron-11 into three helium-4 nuclei,
B
and again the party is over. In contrast, in the low-density regions of an inhomo-geneous big bang, the boron-11 is more likely to meet a neutron, which creates a heavier isotope of boron:
B
Boron-12 is radioactive. It emits an electron and decays into carbon:
B
Additional hits by neutrons create heavier isotopes of carbon:
C
C
C
the last of which decays into nitrogen-15:
C
Further nuclear reactions create still heavier elements. If the material emerging from the big bang harboured a trace of these heavy elements as well as hydrogen, helium and lithium, then the first stars that formed should contain a small amount of heavy elements, in perfect agreement with observations.
