Astronomy
Thermal History of the Intergalactic Medium During the Cosmic Dawn
Quick fact
During the cosmic dawn, the intergalactic medium was first cooled to just a few tens of Kelvin by the expansion of the universe, but then the first stars and black holes heated it to thousands of Kelvin, creating the conditions for the 21-centimeter radio signal that we can detect today.
Why this is interesting
You might think the universe became dark after the Big Bang, but the gas between galaxies tells a more dramatic story of heating and cooling. What made this intergalactic gas switch from a cold, opaque fog to a warm, transparent one?
Read the full explanation
Understanding Thermal History of the Intergalactic Medium During the Cosmic Dawn
Let's build a mental picture. Imagine a very hot, dense soup of particles right after the Big Bang. As the universe expands, this soup cools. At about 380,000 years after the Big Bang, the universe had cooled enough for electrons and protons to combine into neutral hydrogen atoms—this is called recombination. At that moment, the intergalactic medium (IGM) was still quite warm, but as space continued to expand, the gas cooled further, like a gas expanding in a piston. This is adiabatic cooling. For millions of years, the IGM became colder and colder, until it was colder than the cosmic microwave background (CMB) radiation that fills the universe. Then, about a few hundred million years after the Big Bang, the first stars ignited. These massive, hot stars emitted intense ultraviolet light, which began to ionize the neutral hydrogen around them. But they also produced X-rays from the remnants of supernovae and the first black holes. These X-rays penetrated the neutral gas and heated it, raising the temperature of the IGM again. This heating is crucial because it sets the stage for the 21-centimeter line: the spin temperature of neutral hydrogen, which determines how the 21-cm radiation is seen against the CMB, depends directly on the gas temperature. So the thermal history of the IGM is not just a side detail—it's what makes the cosmic dawn observable.
A deeper explanation
The thermal history of the IGM during the cosmic dawn is governed by a balance of cooling and heating mechanisms. After recombination, the IGM cools adiabatically as the universe expands, and its temperature drops as T ∝ (1+z)² in the matter-dominated era. However, this cooling is counteracted by heating from photon emission and, more importantly, from the first astrophysical sources. The first stars (Population III) are massive and short-lived, and their ultraviolet radiation ionizes nearby gas, but the supernovae they produce and the accretion onto the first black holes generate X-rays that can travel far through the neutral IGM. These X-rays heat the gas by photoionization, where a high-energy photon knocks out an electron, and the released energy becomes thermal motion. The efficiency of this heating depends on the X-ray luminosity and the spectrum. As the gas heats, it also changes the fraction of neutral hydrogen, eventually leading to reionization. The key observable is the 21-cm line from neutral hydrogen. The intensity of this line relative to the CMB is determined by the spin temperature (Ts), which characterizes the relative population of the two hyperfine levels of hydrogen. When Ts is lower than the CMB temperature, the gas absorbs CMB photons; when Ts is higher, it emits. The spin temperature is coupled to the gas kinetic temperature via collisions and Lyman-alpha coupling (Wouthuysen-Field effect). During the cosmic dawn, the gas is initially cold (Tgas < TCMB), so it absorbs, but as heating progresses, the signal flips to emission. Therefore, the observed 21-cm signal as a function of redshift provides a direct readout of the thermal history. This is why upcoming experiments like HERA and SKA aim to measure the 21-cm power spectrum—it encodes the heating and cooling of the IGM, telling us when the first stars formed and how they affected their surroundings.