The Dark Ages
Background image: A supercomputer simulation of the distribution of matter in the Universe produced by cosmologists at the University of Durham. Among the nearest components of the cosmic large-scale structure are the Local Supercluster, the Great Wall, the Great Attractor, and the Shapley Concentration. In addition, observations by the Chandra X-ray Observatory have revealed part of an intergalactic web of hot gas and dark matter that is crucial in defining the cosmic landscape. The hot gas alone, which appears to lie like a fog in channels carved by rivers of gravity, is more massive than all the stars in the universe. Its detection may eventually enable astronomers to map the distribution of dark matter.
[Image courtesy of Volker Springel Millennium simulation]
Following the recombination of helium nuclei and protons with electrons to form neutral atoms, the universe became transparent. This decoupling of radiation from matter occurred when the thermal kinetic energies dropped below the binding energies of the atoms – for hydrogen, 13.6 eV. Thermal kinetic energies are of order kT, where k is Boltzmann’s constant; 13.6 eV corresponds to a temperature of about 100,000 K.
There is a certain amount of spectral structure to this early light, in particular at the 21cm band, but the dominant structure concerns small variations in energy density (and hence temperature). These follow the clumping of the matter distribution, including the distribution of dark matter, prior to decoupling. Following decoupling, with baryons and leptons in stable neutral atoms, gravitational clumping was all the more rapid. But it was more than 400,000 years before the local densities and temperatures of hydrogen and other of the light atoms produced in Big-Bang nucleosynthesis were sufficient for fusion processes to become energetically favourable, and for the first stars to be born.
The period from recombination (or decoupling of radiation from matter) is often called the dark ages, but it was hardly dark: at its conclusion, when the universe was between 400 and 800 million years old, the average temperature was about 3000K. It was as bright as surface of the sun (essentially the situation envisaged in Olber’s paradox, but for very different reasons). But the energy density of radiation in interstellar and intergalactic space subsequently played an increasingly minor role in the evolution of the universe, falling off faster with the cosmic expansion than that of the matter distribution.