Physics
Radio Astronomy in Studying the Epoch of Reionization
Quick fact
Using the 21-cm line of neutral hydrogen, radio astronomers can map the distribution of gas during the Epoch of Reionization—but the signal is so faint that it is only one-hundred-thousandth of the noise from our own galaxy, requiring extreme precision and years of integration.
Why this is interesting
How can we 'see' the first stars that switched on the universe's lights, when the universe was a fog of hydrogen gas? Radio telescopes can probe this ancient epoch by catching a whisper of the universe's first light.
Read the full explanation
Understanding Radio Astronomy in Studying the Epoch of Reionization
Think of the early universe as a dark, murky cloud of neutral hydrogen. After the Big Bang, the universe cooled, and electrons bound to protons to form hydrogen atoms. This 'dark ages' lasted until the first stars and galaxies ignited. Their intense ultraviolet radiation began to split these neutral hydrogen atoms into protons and electrons—a process called ionization. The Epoch of Reionization is that transformative period, lasting roughly from 380,000 to 1 billion years after the Big Bang, when the universe went from neutral to ionized. But we can't see this directly with ordinary telescopes. However, neutral hydrogen emits a special radio signal: the 21-cm line. When an electron in a hydrogen atom spontaneously flips its spin, it emits a photon with a wavelength of 21 cm. The more neutral hydrogen there is, the stronger (or absorption) this signal. Crucially, because the universe is expanding, that 21-cm light is stretched as it travels to us, meaning it arrives at longer wavelengths—radio wavelengths. By tuning our radio telescopes to different frequencies, we can 'tune' to different cosmic eras. Detecting these faint signals reveals the distribution and abundance of neutral hydrogen, showing us where and when reionization happened. Radio telescopes on Earth, particularly arrays like LOFAR or the upcoming SKA, are designed to detect this signal. They observe low-frequency radio waves (around 50-200 MHz). The challenge is that these signals are incredibly weak, and they are swamped by other radio sources, like our own galaxy. Yet, by carefully subtracting the foreground and improving sensitivity, astronomers build a picture of the cosmic reionization—a key milestone in cosmic history.
A deeper explanation
The 21-cm line arises from a quantum transition in neutral hydrogen: the hyperfine splitting of the ground state due to the interaction between the magnetic moments of the proton and the electron. When the electron's spin flips from being parallel to antiparallel with the proton, a photon with energy 5.87 µeV is emitted, corresponding to a wavelength of 21.1 cm. Because hydrogen is the most abundant element, this line is a powerful probe. During the Epoch of Reionization, the spin temperature of the gas is coupled to the gas temperature (through collisions and Lyman-alpha radiation) or to the cosmic microwave background. If the gas is hotter than the CMB, we see the 21-cm line in emission; if cooler, in absorption. By measuring the brightness temperature of this line as a function of frequency, we can reconstruct the ionization state and temperature of the intergalactic medium. The crucial technique is tomography: observed redshift maps frequency to distance, allowing us to create 3D maps of the hydrogen distribution. But the signal is extremely faint—typically tens of millikelvin against a foreground of thousands of kelvin. To overcome this, radio astronomers use interferometers: arrays of many small antennas that synthesize a huge collecting area and provide precise angular resolution. They also employ spectral fitting to separate the foregrounds, which have relatively smooth frequency spectra, from the 21-cm signal, which has rapid frequency fluctuations. Advanced calibration and statistical tools, like power spectra, are used to extract the signal from the noise. The importance of studying the EoR is twofold: it tells us about the first stars and galaxies—the sources that ionized the cosmos—and it constrains cosmological models and the formation of large-scale structure. Future experiments, like the Square Kilometre Array, will push sensitivity to detect the signal in detail, opening a new window into the universe's first billion years.