Starlight and intergalactic hydrogen
Begin with ultraviolet light leaving a galaxy. Count ionizations and recombinations, follow an expanding volume, then read the radio signal left by neutral hydrogen. Work the examples as each new quantity appears.
L1 · Concept
How starlight changes intergalactic hydrogen
Ultraviolet photons escaping young galaxies ionize intergalactic hydrogen and must also replace ions lost through recombination.
L2 · Chapter
What does half ionized mean?
A half-ionized parcel and a volume split into neutral and fully ionized regions can share an average but recombine at different rates.
L2 · Chapter
Supplying photons to an expanding volume
Escaped photons grow the ionized volume while recombination consumes part of the supply; a comoving box keeps the hydrogen count fixed.
L3 · Chapter
Where the photons go when ionized bubbles meet
Overlapping regions contain one copy of the gas. Follow excess supply into neutral volume, then distinguish volume conservation from bubble shape.
L3 · Chapter
Reading ionized bubbles through neutral hydrogen
Derive the ideal high-spin-temperature 21 cm signal, then use simple waves to distinguish mean brightness, spatial power and Fourier phase.
L3 · Method
Recompute a fixed-source ionization history
Read the original cell field, check its photon balance, and use independent quadrature and explicit Fourier projections to examine the same history.
L4 · Paper guide
Reading HERA's 21 cm upper limits
Read the markers, errors and windows in HERA's power spectrum, recompute two published table rows, and connect an upper limit to early-universe physics.