Glossary

Key terms

Every term is defined in the lesson that introduces it. The glossary is generated from the lessons, so it grows as they do.

21-centimetre line: A faint radio signal from neutral hydrogen. Stretched by expansion to wavelengths of metres, it is the only way to observe the Dark Ages directly, and nobody has yet done so convincingly. From The Dark Ages and the first light
Acoustic peaks: The characteristic sizes of the ripples, set by sound waves that ran through the early plasma. Their pattern encodes the shape and contents of the universe. From The afterglow
Airglow: Faint light emitted by molecules high in Earth's atmosphere. At the darkest sites on Earth, airglow and sunlight scattered by solar-system dust outshine all the stars and galaxies combined. From Why is the night sky dark?
Anisotropy: A difference from place to place. The afterglow's temperature varies across the sky by about one part in a hundred thousand, and those variations carry the story of the early universe. From The afterglow
Antimatter: Matter's mirror image: particles with the same mass but opposite charge. When a particle meets its antiparticle, both vanish into light. The early universe held almost equal amounts of each. From The first three minutes
Axion: A very light hypothetical particle, proposed for other reasons, that would also make a good dark matter candidate. Searches are under way. From Open questions
Background limit: The average distance a line of sight must travel before it ends on a star, of the order of a hundred thousand billion billion light-years given how sparse stars are. Light has travelled only 13.8 billion. From Why is the night sky dark?
Baryon acoustic oscillations: The imprint of the sound waves of the early universe on where galaxies sit today: a slight preference for pairs of galaxies about 150 megaparsecs apart, predicted before it was found. From How we know
Baryon-to-photon ratio: The number of matter particles for every photon of light, about six for every ten billion. It is the one number that sets the whole primordial recipe. From The first three minutes
Big Bang nucleosynthesis: The nuclear fusion of the first minutes, which turned about a quarter of ordinary matter by mass into helium and left the rest as hydrogen. From Was the Big Bang an explosion?
Big Bang nucleosynthesis: The few minutes of nuclear fusion, starting about three minutes after the Big Bang, that turned a quarter of ordinary matter by mass into helium and left the rest as hydrogen. From The first three minutes
Big Bang theory: The model in which the universe has expanded and cooled for about 13.8 billion years from an extremely hot, dense early state. It describes the expansion, not the first instant. From Was the Big Bang an explosion?
Blackbody spectrum: The exact mix of colours emitted by anything in perfect thermal balance. The afterglow's spectrum is the most perfect blackbody ever measured, which is how we know it is thermal. From The afterglow
Concordance model: Also called Lambda-CDM: the six-parameter description of the universe, with a cosmological constant and cold dark matter, that fits the afterglow, galaxy surveys, supernovae and lensing at once. From How we know
Cosmic Dawn: The period when the first stars and galaxies formed and their light began to change the universe around them, roughly 100 to 500 million years after the Big Bang. From The Dark Ages and the first light
Cosmic microwave background: The afterglow of the early universe, released about 380,000 years after the Big Bang and now cooled to 2.7 kelvin. It arrives from every direction. From Was the Big Bang an explosion?
Cosmic microwave background: The oldest light in the universe, released about 380,000 years after the Big Bang when the universe first became transparent, now stretched to microwaves at 2.7 kelvin and arriving from every direction. From The afterglow
Cosmic optical background: The visible-light part of the extragalactic background, measured directly from beyond Pluto by the New Horizons spacecraft at about 11 nanowatts per square metre per steradian. From Why is the night sky dark?
Cosmological constant: Einstein's symbol, Lambda, for an energy density of empty space that does not change with time. The simplest form dark energy could take, and so far the one that fits. From Open questions
Dark Ages: The stretch after the afterglow, before the first stars, when the universe was transparent but nothing in it shone. From The afterglow
Dark Ages: The stretch after the afterglow faded and before the first stars formed, when the universe was transparent but nothing in it shone. Roughly the first hundred million years. From The Dark Ages and the first light
Dark energy: Whatever is making the expansion of the universe speed up. It behaves like an energy belonging to empty space itself, and it makes up about two thirds of the universe's contents. From Open questions
Dark matter: Matter that neither emits nor absorbs light and reveals itself only by gravity. About five times more of it than ordinary matter. Its existence is established; its identity is not. From Open questions
Deuterium: Heavy hydrogen: a nucleus of one proton and one neutron. It is the first step of every fusion chain and the first thing the early universe had to build. From The first three minutes
Deuterium bottleneck: The delay before fusion could start. Until about three minutes, the light was intense enough to break every deuterium nucleus apart the moment it formed. From The first three minutes
Dipole: The afterglow looks slightly warmer in one direction and cooler in the opposite one because the solar system is moving through it at about 370 kilometres per second. From The afterglow
Extragalactic background light: The combined glow of every star and galaxy outside our own, summed over the whole sky: about 50 nanowatts per square metre per steradian in the optical and infrared together. From Why is the night sky dark?
Falsifiable: Capable of being shown wrong by a measurement. A claim that no observation could ever contradict is not scientific, however appealing. From How we know
Gunn–Peterson trough: A stretch of a distant quasar's spectrum blacked out by neutral hydrogen. Its appearance in quasars beyond redshift 6 marks the end of reionisation. From The Dark Ages and the first light
Horizon problem: Opposite sides of the sky show the afterglow at the same temperature to one part in a hundred thousand, though in a plain Big Bang they could never have exchanged heat. Inflation was invented largely to explain this. From Open questions
Hubble tension: The expansion rate measured from the afterglow disagrees with the rate measured directly in nearby galaxies, 67 against 73 kilometres per second per megaparsec, by more than the errors allow. From Open questions
Hubble–Lemaître law: Recession speed is proportional to distance: v = H₀d. Every observer in the universe sees the same rule. From Was the Big Bang an explosion?
Independent evidence: Measurements that rely on different physics and different instruments. The Big Bang rests on several lines that could each have failed separately. From How we know
Inflation: A hypothesised burst of extremely rapid expansion in the first fraction of a second. Well supported by observations, but not yet confirmed. From Was the Big Bang an explosion?
Inflation: A hypothesised burst of enormous expansion in the first fraction of a second. It explains why the universe is so uniform and so flat, and predicts the ripple pattern we see. Its decisive signature has not been found. From Open questions
Inverse-square law: A star twice as far away appears a quarter as bright. In the paradox, this is exactly cancelled by the four times as many stars in a shell twice as far away. From Why is the night sky dark?
Lambda-CDM: The standard model of cosmology: a cosmological constant plus cold dark matter, described by six numbers. Precise, successful, and mostly made of things we have not identified. From Open questions
Line of sight: A straight line from your eye out into space. The paradox asks what each one eventually hits. From Why is the night sky dark?
Lookback time: How long ago the light you see set out. Looking far away is looking into the past, and there is a limit to how far back there is anything to see. From Why is the night sky dark?
Metallicity: An astronomer's word for the fraction of anything heavier than helium. The first stars had a metallicity of zero. From The Dark Ages and the first light
Metric expansion: The growth of space itself, which carries galaxies apart. Galaxies do not move through space away from a centre; the distances between them increase. From Was the Big Bang an explosion?
Minihalo: A clump of dark matter about a million times the mass of the Sun, the smallest structure in which gas could gather and cool enough to make the first stars. From The Dark Ages and the first light
Modified gravity: The alternative to dark matter: perhaps gravity itself changes at large scales. It reproduces the rotation of galaxies but fails for clusters and for the afterglow. From Open questions
Molecular hydrogen cooling: The only way the first gas clouds could shed heat, since there was no carbon, oxygen or dust. It is inefficient, which kept the clouds warm and made the first stars huge. From The Dark Ages and the first light
Neutrino: A particle with almost no mass that barely interacts with anything. Neutrinos stopped interacting one second after the Big Bang and have been streaming through the universe ever since. From The first three minutes
Neutron-to-proton ratio: How many neutrons there were for every proton. It froze at about one to six around one second and fell to one to seven by the time fusion began, and it sets how much helium the universe made. From The first three minutes
Nucleon: A proton or a neutron, the two particles that make up atomic nuclei. From The first three minutes
Observable universe: The region from which light has had time to reach us. Its radius is about 46 billion light-years, larger than 13.8 billion because space expanded while the light travelled. From Was the Big Bang an explosion?
Olbers' paradox: The puzzle that in an infinite, unchanging universe full of stars, every line of sight would end on a star and the whole sky would be as bright as the Sun's surface. Named after Heinrich Olbers, who discussed it in 1823, though Kepler saw it two centuries earlier. From Why is the night sky dark?
Polarisation: A preferred orientation of the light's vibration, imprinted on the afterglow as it scattered for the last time. The E-mode pattern has been measured; the B-mode pattern that inflation would leave has not. From The afterglow
Population III stars: The first generation of stars, made only of hydrogen and helium. Predicted to be far more massive, hotter and shorter-lived than stars today. None has been confirmed by observation yet. From The Dark Ages and the first light
Prediction: A statement about a measurement not yet made. The strongest evidence for a theory is a prediction made before the data existed and confirmed afterwards. From How we know
Primordial abundances: The proportions of hydrogen, helium, deuterium and lithium the universe had before any star existed. Measured in gas that stars have never polluted. From The first three minutes
Quark: One of the fundamental particles that make up protons and neutrons, three at a time. Quarks moved freely only in the first ten microseconds; they have been locked inside protons and neutrons ever since. From The first three minutes
Recombination: The moment, at about 3,000 kelvin, when electrons bound to nuclei to make neutral atoms and the universe became transparent to light. From Was the Big Bang an explosion?
Recombination: The period, ending about 380,000 years after the Big Bang, when electrons bound to nuclei to make the first neutral atoms. The name is a historical accident: the electrons had never been combined before. From The afterglow
Redshift: The stretching of light to longer, redder wavelengths. Cosmological redshift is caused by space expanding while the light travels. From Was the Big Bang an explosion?
Redshift as a clock: Astronomers label early epochs by redshift, z. Redshift 1,090 is the afterglow; 20 to 30 is the first stars; 14 is the earliest confirmed galaxy; 6 is the end of reionisation. From The Dark Ages and the first light
Reionisation: The period when ultraviolet light from the first stars and galaxies stripped the electrons back off the hydrogen between them, ending about a billion years after the Big Bang. From The Dark Ages and the first light
Scientific theory: Not a guess but the opposite: a framework that has made predictions which could have failed, and survived them. Gravity, evolution and the Big Bang are theories in this sense. From How we know
Singularity: The point at which the equations of general relativity predict infinite density and stop working. It marks the limit of the theory, not a known physical event. From Was the Big Bang an explosion?
Sound horizon: How far a sound wave could travel in the plasma before the afterglow was released: about 145 megaparsecs today. It is the ruler that sets the size of the largest ripples. From The afterglow
Standard candle: An object of known brightness, such as a type Ia supernova, whose apparent brightness gives its distance. From How we know
Steady-state theory: The Big Bang's serious rival from 1948 to the mid-1960s: an expanding universe with no beginning, kept at constant density by the continuous creation of new matter. From How we know
Surface brightness: How bright a surface looks per unit area on the sky. Distance does not change it: the Sun's disc would look just as bright per patch from twice as far, only smaller. That is why a sky covered in stellar surfaces would be as bright as the Sun. From Why is the night sky dark?
Surface of last scattering: The shell around us from which the afterglow's photons last bounced off an electron before travelling freely. It is not a physical surface; it is the distance at which we see the universe as it was 380,000 years in. From The afterglow
Surface-brightness dimming: In an expanding universe a source at redshift z looks dimmer by a factor of (1+z) to the fourth power. It makes the night sky about 40 percent darker than it would otherwise be, but it is not the main reason the sky is dark. From Why is the night sky dark?
Time dilation: Clocks in the distant universe appear to run slow by a factor of (1+z). Distant supernovae brighten and fade more slowly by exactly that factor. From How we know
Tired light: The idea that redshift is caused by light losing energy on its journey rather than by expansion. It predicts no time dilation and fails the supernova and surface-brightness tests. From How we know
WIMP: Weakly interacting massive particle, the long-favoured candidate for dark matter. Decades of increasingly sensitive searches have not found one. From Open questions