21cm Hyperfine Transition in Neutral Hydrogen: Radio Astronomy Basics

Two major problems in cosmology are: (1) understanding what happened in the first few billionths of a second after the Big Bang (inflation), and (2) understanding when and how the first stars and galaxies formed. The period when the first stars and galaxies formed is called cosmic dawn, and the radiation from these stars gradually reionized most of the hydrogen in the universe during the epoch of reionization. Scientists are trying to detect the 21 cm hydrogen line from this period, which has been redshifted to frequencies between 55 and 110 MHz. The challenge is that the signal is extremely faint (about 100 millikelvin) compared to the galactic background (about 1 million times stronger).

Three key epochs are studied using 21 cm observations: (1) The Dark Ages, when the spin temperature of neutral hydrogen couples to the gas temperature, which cools adiabatically after decoupling from the CMB; (2) Cosmic Dawn, when the first sources form and couple the 21 cm spin temperature to the gas temperature again, with X-ray heating beginning to affect the gas; (3) Reionization, when ionization becomes very important. The differential brightness temperature (21 cm temperature minus CMB temperature) is the observable, with a scale of about 28 mK. The signal probes both the ionization fraction field and the density field, providing information about both cosmology and astrophysics. There are two types of 21 cm experiments: global detection using a single detector, and interferometric detection using arrays to measure power spectra. The EDGES experiment reported a signal much larger than expected, though this has been challenged by systematic effects related to ground plane absorption. An Indian experiment (SARAS-3) placed their instrument on a deep lake to avoid ground plane effects and ruled out the signal at 2-sigma level.

The redshifted 21cm line of neutral hydrogen serves as a powerful cosmological probe that can trace the evolution of the universe from the Cosmic Dawn (hundreds of millions of years after the Big Bang) to the late Universe, providing unique insights into dark matter distribution, dark energy, and the formation of large-scale structure; however, detecting this signal requires sophisticated foreground subtraction techniques due to the overwhelming brightness of astrophysical foregrounds at radio wavelengths.

This extensive section covers the complete foundation of 21 cm cosmology, from theoretical principles to cutting-edge experimental approaches. It begins with the three epochs of the infant universe: the Dark Ages (period after CMB decoupling until first star formation), Cosmic Dawn (birth of the first stars), and the Epoch of Reionization (when the first stars and galaxies shaped the intergalactic medium). The 21 cm line arises from neutral hydrogen's forbidden transition at 21 cm wavelength, with spin temperature coupling to different temperatures throughout cosmic history. The section details the extreme observational challenge: the cosmological signal (~millikelvin) is buried under foregrounds up to five orders of magnitude stronger. Two main observational approaches are presented: interferometers measuring spatial fluctuations via power spectra (exploiting chromatic signal vs. smooth foregrounds) and single-dish experiments measuring global sky-averaged signals. Key systematic challenges include the foreground wedge in interferometric data and antenna coupling effects. The 2018 EDGES detection of an absorption dip at cosmic dawn (~78 MHz) suggested a spin temperature-to-CMB ratio lower than standard models predict. The REACH experiment addresses this by using two simultaneous antennas to disentangle instrumental effects from cosmological signals. Future space-based missions like CosmoCube will observe from lunar orbit to access the Dark Ages period (~5 Myr after Big Bang). The section concludes with synergies between 21 cm observations and multi-wavelength probes: JWST for UV-optical observations, Chandra for X-ray observations, and gravitational wave detectors, enabling comprehensive characterization of high-redshift galaxy populations.

Neutral hydrogen serves as an ideal probe for studying the earliest epochs of cosmic history because it is the most abundant element in the universe and exists throughout the intergalactic medium. At redshifts above 6, the 21cm signal originates from neutral gas in the IGM before full reionization. The brightness temperature depends on neutral fraction, matter overdensity, CMB photon temperature, and spin/gas temperature. This makes 21cm observations uniquely sensitive to the photon budget driving reionization and the thermal state of the IGM. Current evidence suggests an overabundance of bright galaxies at high redshifts, implying either earlier reionization completion or more efficient photon production than predicted by standard models. The 21cm power spectrum encodes information about the spatial distribution of neutral hydrogen and its evolution through cosmic dawn and reionization, with different astrophysical scenarios producing distinct clustering patterns. The IGM undergoes distinct thermal phases: initially cooling adiabatically after CMB decoupling, then being heated by X-rays from the first stellar populations during cosmic dawn. This X-ray heating epoch is poorly understood observationally. HERA (Hydrogen Epoch of Reionization Array) is a compact interferometer with ~350 14-meter dishes operating 50-250 MHz (redshifts 5-25), employing drift scanning and sophisticated systematic validation to probe this cosmic era.

The Wouthuysen-Field effect describes how Lyman-alpha photons from the first ionizing sources cause indirect transitions between hydrogen spin states. Since Lyman-alpha photons have very high optical depth (due to large cross-sections), they can efficiently drive transitions from the triplet state to the singlet state via an intermediate 2p state. This process attempts to set the spin temperature back toward the kinetic temperature. Combined with X-ray heating from these same sources, this creates complex astrophysical effects that modify the standard evolution of spin temperature during the epoch of reionization.

The Wouthuysen-Field effect describes how Lyman-alpha photons couple hydrogen's spin temperature to its kinetic temperature. When hydrogen atoms absorb low-energy Lyman-alpha photons, they transition between spin states, causing the spin temperature to follow the Lyman-alpha photon profile. After the Big Bang, dark matter cools faster than radiation (gamma = 5/3 vs. 4/3), while gas initially remains coupled to the CMB temperature. As the universe expands, the gas temperature drops faster than the CMB temperature, creating a temperature difference that affects the 21cm signal observed from neutral hydrogen.

The Wouthuysen-Field effect couples the spin temperature to the gas temperature through resonant scattering of Lyman Alpha photons. When a hydrogen atom in the ground state absorbs a Lyman Alpha photon, it can be excited to one of the two P levels, then de-excite to the other hyperfine level, inducing a spin flip. This process, combined with collisional coupling, establishes the spin temperature. The gas temperature itself is set by X-ray heating from sources like X-ray binaries in star-forming galaxies, with X-rays having very long mean free paths that allow them to heat gas far from their sources.

At early times, high densities enable collisional coupling that keeps spin temperature equal to kinetic temperature (which tracks CMB temperature). As the universe expands and densities decrease, collisional coupling fails. Without collisions, the spin temperature tends toward the CMB temperature. However, spontaneous absorption and re-emission of Lyman-alpha photons provides an alternative coupling mechanism. Since the IGM is optically thick to Lyman-alpha photons (scattering ~1 million times before redshifting out), this process couples spin temperature to kinetic temperature even when collisions are ineffective. When the first stars form and flood the universe with UV photons, these photons cascade through Lyman-alpha and couple the spin temperature to the gas temperature, allowing measurement of gas temperature through 21cm observations.

The spin temperature T_s determines whether the 21 cm signal appears in absorption or emission. In early universe with high density, collisions efficiently set T_s = T_CMB, producing no net signal. After first galaxies form, collision rates drop, and the Wouthuysen-Field mechanism becomes dominant: hydrogen atoms absorb Lyman-alpha photons and cascade through excited states to populate the hyperfine triplet state. Since the universe is optically thick to Lyman-alpha, this couples T_s to the kinetic gas temperature. If gas is colder than CMB (~100 K), absorption occurs; if hotter, emission occurs.

Two complementary observational strategies probe the 21 cm signal. Global experiments use single antennas to measure total sky signal, being cheap but fundamentally limited by having only one measurement, preventing statistical discrimination between cosmic signal and foregrounds. The EDGES experiment claimed detection of a large dip at z=15-20, expected because heating is 10x more efficient than ionization. Interferometers address this by measuring spatial structure, enabling separation of foregrounds (smooth in frequency) from cosmic signal in the E-window where foregrounds are minimized. First-generation interferometers (MWA, LOFAR, GMRT, HERA) face foreground contamination 3-4 orders of magnitude stronger than the cosmic signal, requiring sophisticated analysis techniques.

HERA (Hydrogen Epoch of Reionization Array) represents cutting-edge engineering for detecting 21 cm signals from cosmic dawn. Located in South Africa's Karoo desert, this radio interferometer uses 14-meter wire-mesh dishes arranged in a regular array, operating between 50-225 MHz. The design exploits the fact that at these frequencies, the mesh appears as a smooth mirror. Despite community efforts, no definitive detection has occurred yet, with upper limits at ~100-200 mK compared to theoretical predictions of ~few mK. Major challenges include Milky Way foreground contamination and radio frequency interference requiring sophisticated data reduction pipelines. The telescope stares at zenith, with Earth rotation providing sky coverage, targeting redshifts from z=6 to z=25 covering cosmic dawn and reionization.

Radio telescopes evolved from single-dish designs to interferometric arrays to overcome hardware limitations. Single-dish telescopes like the Green Bank Telescope collect light like buckets, with resolution proportional to diameter. However, building extremely large single dishes is impractical due to structural limitations and cost. Interferometry solves this by combining signals from multiple smaller antennas spread apart, with resolution depending on baseline distance rather than individual size. Modern 21 cm telescopes use arrays of dipole antennas instead of dishes, offering advantages in manufacturability and scalability. These arrays cannot be steered like dishes but continuously monitor the entire sky. The signal weakness requires enormous collecting areas, pushing the boundaries of engineering and computation. Two complementary detection strategies exist: global instruments measure average signal strength across the sky, while interferometers create detailed spatial maps. Major experiments include EDGES in Western Australia and SARAS in India, which aim to validate or refute detections of the dark age signal while carefully accounting for instrumental systematics.

Radio interferometry is a technique used to observe 21cm signals by combining signals from multiple antennas spread across a large area, achieving the resolution of a much larger single antenna. The SKA (Square Kilometre Array) will use approximately 130,000 antennas, making it the most powerful radio telescope ever constructed. The SKA will observe 21cm signals and has multiple scientific goals including studying cosmic magnetism, dark matter, dark energy, and potentially detecting signals from extraterrestrial civilizations. Initial observations are expected to begin around 2027, with full completion in the 2030s. The lunar far side is proposed as an ideal location for radio astronomy because it is shielded from Earth's radio interference and the ionosphere's blocking effects. Several countries are developing plans for lunar radio observatories. The 2030s will see major advances in radio astronomy through the SKA, while the 2040s-2050s may bring lunar-based radio observatories, representing a major transformation in how humanity observes the universe.