The universe is the grandest merger story that there is. Complete with mysterious origins, forces of light and darkness, and chemistry complex enough to make the chemical conglomerate BASF blush, the trip from the first moments after the Big Bang to the formation of the first stars is a story of coming together at length scales spanning many orders of magnitude. To piece together this story, scientists have turned to the skies, but also to the laboratory to simulate some of the most extreme environments in the history our universe. The resulting narrative is full of surprises. Not least among these, is how nearly it didn’t happen—and wouldn’t have, without the roles played by some unlikely heroes. Two of the most important, at least when it comes to the formation of stars, which produced the heavier elements necessary for life to emerge, are a bit surprising: dark matter and molecular hydrogen. Details aside, here is their story.
Dark Matter
The Big Bang created matter through processes we still do not fully understand. Most of it—around 84 percent by mass—was a form of matter that does not interact with or emit light. Called dark matter, it appears to interact only gravitationally. The remaining 16 percent, dubbed baryonic or ordinary matter, makes up the everyday universe that we call home. Ordinary matter interacts not only gravitationally but also electromagnetically, by emitting and absorbing photons (sometimes called radiation by the cognoscenti and known as light in the vernacular).
As the universe expanded and cooled, some of the energy from the Big Bang converted into ordinary matter: electrons, neutrons, and protons (the latter are equivalent to ionized hydrogen atoms). Today, protons and neutrons comfortably rest together in the nuclei of atoms. But in the seconds after the Big Bang, any protons and neutrons that fused to form heavier atomic nuclei were rapidly blown apart by high-energy photons called gamma rays. The residual thermal radiation field of the Big Bang provided plenty of those. It was too hot to cook. But things got better a few seconds later, when the radiation temperature dropped to about a trillion degrees Kelvin—still quite a bit hotter than the 300 Kelvin room temperature to which we are accustomed, but a world of difference for matter in the early universe.
The intensity of the residual heat from the Big Bang made the early universe too smooth for gas clouds to form.
Heavier nuclei could now survive the gamma-ray bombardment. Primordial nucleosynthesis kicked in, enabling nuclear forces to bind protons and neutrons together, until the expansion of the universe made it too cold for these fusion reactions to continue. In these 20 minutes, the universe was populated with atoms. The resulting elemental composition of the universe weighed in at roughly 76 percent hydrogen, 24 percent helium, and trace amounts of lithium—all ionized, since it was too hot for electrons to stably orbit these nuclei. And that was it, until the first stars formed and began to forge all the other elements of the periodic table.
Before these stars could form, however, newly-formed hydrogen and helium atoms needed to gather together to make dense clouds. These clouds would have been produced when slightly denser regions of the universe gravitationally attracted matter from their surroundings. The question is, was the early universe clumpy enough for this to have happened?
To answer the question, we can look to the modern-day night sky. In it, we see a faint glow of microwave radiation that has an even fainter pattern in it. This so-called cosmic microwave background structure dates back to 377,000 years after the Big Bang, a mere fraction of the universe’s current age of 13.8 billion years, and analogous to less than a day in the 81-year life expectancy for a woman living today in the United States.
At that time, the universe had just cooled to about 3,000 Kelvin. Free electrons started to be captured into orbit around protons, forming neutral hydrogen atoms. Photons from the flash of the Big Bang, whose progress had been impeded by their scattering off of unbound electrons, could now finally stream throughout the cosmos, essentially free. These photons continue to permeate the universe today, at a frigid temperature of only 2.7 Kelvin, and constitute the cosmic microwave background that we have measured using an array of ground-based, balloon-born, and satellite telescopes.
These sky maps suggested something surprising: The intensity of the residual heat from the Big Bang made the early universe too smooth for gas clouds to form.
Enter dark matter. Because it does not interact directly with light, it was unaffected by the same radiation that smoothed out ordinary matter. Therefore it was left with a relatively high degree of clumpiness. It, rather than regular matter, initiated the formation of the stars and galaxies that make up the modern structure of the universe. Regions of space with an above-average density of dark matter gravitationally attracted matter from regions with lower densities. Halos of dark matter formed and merged with other halos, bringing ordinary matter along for the ride.
Molecular Hydrogen
Once the universe went neutral, gas began to form into clouds. As ordinary matter accelerated into the gravitational wells of dark matter, gravitational potential energy converted into kinetic energy, creating a hot gas of fast-moving particles with high kinetic energies embedded within halos of dark matter. Starting from temperatures around 1,000 Kelvin, these gas clouds eventually gave birth to the first stars when the universe was roughly half a billion years old (about four years into the lifespan of the typical U.S. woman).
For a star to form, a gas cloud needs to reach a certain density; but if its constituent molecules are too hot, zipping around in every direction, this density may be unreachable. The first step toward making star-forming clouds was for gas atoms to slow down by radiating their kinetic energy out of the cloud and into the larger universe, which by this time had cooled to below 100 Kelvin.
But they can’t cool themselves: As atoms collide like billiard balls, they exchange kinetic energy. But the total kinetic energy of the gas remains unchanged. They needed a catalyst to cool off.
Chemists have named the first reaction associative detachment, a name fit for a psychiatric condition out of the DSM-V.
This catalyst was molecular hydrogen (two hydrogen atoms bound together by sharing their electrons). Hot particles colliding with this dumbbell-shaped molecule transferred some of their own energy to the molecule, causing it to rotate. Eventually these excited hydrogen molecules would relax back to their lowest-energy (or ground) state by emitting a photon that escaped from the cloud, carrying the energy out into the universe.
To make molecular hydrogen, the atomic gas clouds needed to do some chemistry. It might be surprising to hear that any chemistry was going on at all, given that the entire universe had just three elements. The most sophisticated chemical models of early gas clouds, however, include nearly 500 possible reactions. Fortunately, to understand molecular hydrogen formation, we need concern ourselves with only two key processes.
Chemists have named the first reaction associative detachment, a name fit for a psychiatric condition out of the DSM-V for which a clinician might prescribe some primordial lithium. Initially, most of the hydrogen in a gas cloud was in neutral atomic form, with the positive charge of a single proton cancelled out by the negative charge of a single orbiting electron. However, a small fraction of its atoms captured two electrons, creating a negatively charged hydrogen ion. These neutral hydrogen atoms and charged hydrogen ions “associated” with each other, causing the extra electron to detach and leaving behind neutral molecular hydrogen. In chemical notation, this can be represented as H + H- → H + e-. Associative detachment converted only about 0.01 percent of atomic hydrogen to molecules, but that small fraction allowed the clouds to begin to cool and become denser.
When the cloud had become sufficiently cool and dense, a second chemical reaction began. It is called three-body association, and written as H + H + H → H + H. This ménage-à-trois begins with three separate hydrogen atoms, and ends with two of them coupled and the third one left out in the cold. Three-body association converted essentially all of the cloud’s remaining atomic hydrogen into molecular hydrogen. Once all of the hydrogen was fully molecular, the cloud cooled to the point where its gas could condense enough to form a star.
Stars
From the formation of a dense cloud to the ignition of fusion at the heart of a star is a process whose complexity far exceeds what came before it. In fact, even the most sophisticated computer simulations available have yet to reach the point where the object becomes stellar in size, and fusion begins. Simulating most of the 200-million-year process is relatively easy, requiring only about 12 hours using high speed, parallel processing computer power. The problem lies in the final 10,000 years. As the density of the gas goes up, the structure of the cloud changes more and more rapidly. So, whereas for early times one needs only to calculate how the cloud changes every 100,000 years or so, for the final 10,000 years one must calculate the change every few days. This dramatic increase in the required number of calculation translates into more than a year of non-stop computer time on today’s fastest machines. Running simulations for the full range of possible starting conditions in these primordial clouds exceeds what can be achieved in a human lifetime. As a result, we still do not know the mass distribution for the first generation of stars. Since the mass of a star determines what elements it forges in its core, this hinders our ability to follow the pathway by which the universe began to synthesize the elements needed for life. Those of us who cannot wait to know the answer are counting on yet another hero: Moore’s Law.
Daniel Wolf Savin is a contrabass-playing astrophysicist at Columbia University.
