Pedagoguery

The universe has gone through many stages in its existence, but few are more mysterious than the dark age. Unlike when we are talking about human history, the dark age of the universe is meant quite literally. Very little light was around during that time. It all starts with the recombination.

Before the recombination, matter was a plasma – that is it consisted of free electrons, photons, protons, and a few other things like helium nuclei and exotic particles. Through a process called Thompson scattering, the photons interacted with the electrons and protons, keeping everything in equilibrium, meaning that the radiation temperature was the same as the kinetic temperature of the matter. However, due to the expansion of the universe, both temperatures were dropping, and when the universe was about 400,000 years old, a major transition occurred. The temperature dropped below 3000 Kelvins, which meant that the photons were no longer energetic enough to prevent the electrons and protons from combining into neutral hydrogen. Once that occurred, Thompson scattering was no longer effective, and the radiation and matter “decoupled”. The radiation became what we see now as the microwave background radiation, while the matter became the galaxies we see all around us.

You would expect that as the universe continued to expand, the neutral hydrogen gas would simply get colder and would stay neutral. But that is not the case. Most of the free hydrogen we observe is ionized – stripped of its electrons. Now, in the grand scheme of things, it does not take much energy to ionize hydrogen. To ionize a kilogram of hydrogen, it takes only one millionth of the energy released when a kilogram of hydrogen undergoes nuclear fusion, and only one ten millionth the energy released when one kilogram of hydrogen falls into a black hole. We do know that by the time the universe was one billion years old, the gas had been completely re-ionized. The question is what happened to it in the meantime.

During the dark ages, there were three distinct reservoirs of energy. You had the energy of the background radiation, the kinetic energy of the gas, and a third source that is unavailable in a plasma. All particles have a quantum property called spin. In the case of protons and electrons, the spins actually act like tiny magnets. In a hydrogen atom, the spins of the proton and the electron can either be aligned to point in the same direction, or in opposite directions, which is termed “antialigned”. The antialigned configuration has the lower energy, and if an atom flips from aligned to antialigned, it emits a photon with a wavelength of 21 centimeters. Thus, this third reservoir is termed spin energy. All three reservoirs have distinct temperature, and the relationships between them underwent a complicated dance during the dark ages. Early on, all three were in synch with each other. This is due to the fact that there were still a few free electrons around. These acted as mediators between the background radiation and the kinetic energy reservoirs. Collisions between atoms served to transfer energy between the kinetic and spin reservoirs. And finally, absorption and emission of 21-cm photons served to link the background radiation and spin reservoirs. However, by 10 million years after the big bang, the background radiation had become too dilute to effectively interact with the residual free electrons.

During this next stage, the kinetic and spin temperatures dropped well below the background radiation temperature, while keeping in synch with each other. The gas was a net absorber of 21-cm radiation, but not enough to keep it up with the background radiation. By 100 million years after the big bang, expansion had diluted the gas density to the point where collisions were no longer common enough to equalize the kinetic and spin temperatures, so the kinetic temperature started to fall below the other two. Without the drag of the kinetic reservoir, the spin energy started absorbing energy from the background radiation, and eventually achieved equilibrium, becoming neither a net absorber nor a net emitter of radiation.

The final transition occurred as the first stars and black holes lit up, re-ionizing the gas. During this time, the kinetic and spin temperatures started to climb, and energy was pumped into them by the high energy radiation put out by the stars and black holes. Eventually, they reached equilibrium with each other, at a level far above the background radiation.

Given these scenarios, and the fact that the 21-cm line is red shifted more the farther back you go, astronomers can use this to probe the condition of the universe at various stages of the early universe, and potentially determine whether stars or black holes were the dominant factor in re-ionizing the universe.

Next time, the creation of the elements.