Glossary

Every term, defined.

Short definitions for the words this site uses, in three groups: the standard picture of cosmology, information and the quantum, and the terms H-ΛCDM adds.

The standard picture

ΛCDM: Lambda cold dark matter, the standard model of cosmology. It describes a universe expanding from a hot, dense beginning, made of about 5% ordinary matter, 26% dark matter and 68% dark energy, with gravity given by Einstein's general relativity. Six numbers, fitted to data, set its details.
Ordinary matter: Everything made of atoms: stars, planets, gas, people. About 5% of the universe's contents. Physicists call it baryonic matter.
Dark matter: Mass we detect only through its gravity: it holds galaxies and clusters together but gives off no light. ΛCDM treats it as an undiscovered particle. H-ΛCDM treats it as frozen patches of the universe's information boundary, and predicts it will never turn up in a detector.
Dark energy: Whatever makes the universe's expansion speed up. In both frameworks it behaves as a cosmological constant. H-ΛCDM predicts it stays exactly constant: $w = -1$ .
Cosmological constant (Λ): The energy of empty space. In ΛCDM it's an input. In H-ΛCDM it's fixed by the vacuum of the observable universe, and its famously tiny size, $10^{-123}$ in natural units, is a count of tiles on the horizon.
Cosmic microwave background: The afterglow of the hot early universe, released about 380,000 years after the beginning. It is the most precise data set in cosmology.
Hubble constant ( $H_0$ ): How fast the universe is expanding today, in kilometres per second per megaparsec. Early-universe data give about 67; measurements of nearby galaxies give about 73.
Hubble tension: The disagreement between those two values. H-ΛCDM says nearby measurements should read slightly high, 69 to 70, by an amount it computes, and that 73 hides a measurement problem.
Megaparsec (Mpc): A unit of cosmic distance: about 3.26 million light-years.
Baryon acoustic oscillations: Sound waves in the early universe left a preferred spacing between galaxies, a standard ruler on the sky. Its length, the sound horizon, is 147.5 Mpc in ΛCDM and 150.71 Mpc in H-ΛCDM.
Horizon: A boundary beyond which signals can never reach us. The cosmic horizon bounds the observable universe.

Information and the quantum

Entropy: A measure of information: how much a system holds, or how much about it is unknown. Measured in bits or nats.
Bit and nat: Two units of information. A bit answers one yes-or-no question. A nat is the natural unit physics uses, worth about 1.44 bits.
Entanglement: A quantum link between particles: measure one and you learn about the other. A maximally entangled pair shares one ebit, $\ln 2 \approx 0.693$ nats.
Holographic principle: The information a region can hold grows with the area of its boundary, not with its volume, much as a hologram's flat film encodes a three-dimensional image. In H-ΛCDM it is derived rather than assumed.
Planck units: The natural units set jointly by gravity, quantum theory and the speed of light. The Planck length is about $1.6\times10^{-35}$ m. One Planck area is one tile of the universe's information screen.
Landauer's principle: Moving one bit of information out of its register costs at least $k_BT\ln 2$ of heat, where $T$ is the temperature. H-ΛCDM predicts the minimum rises to $k_BT$ when the bit is watched fast enough.
Quantum Zeno effect: Watching a quantum system often enough slows its change. The plasma of the early universe was watched about a billion times per expansion time.
Modular flow: The dynamics a quantum state generates by itself, a result proved by Tomita and Takesaki. In H-ΛCDM it is the universe's own clock: no outside timekeeper is needed.
Fine-structure constant (α): The strength of electromagnetism, about 1/137. H-ΛCDM computes $\alpha^{-1} = 137.032$ ; the measured value is 137.036.
Parity and chirality: Parity is mirror symmetry; chirality is handedness. The weak nuclear force breaks parity: it acts only on left-handed particles.
$E_8$: An exceptional symmetry with 248 dimensions. String theory's $E_8\times E_8$ appears in H-ΛCDM as data written on the boundary, not as something the theory derives.
Falsifiable: Able to be proven wrong by a measurement. Every H-ΛCDM prediction names the result that would end it.

H-ΛCDM's own terms

H-ΛCDM: Holographic ΛCDM, the framework this site presents. It keeps what ΛCDM gets right but starts from information on the boundary of the observable universe, and derives several numbers that ΛCDM fits. The H stands for holographic.
Simulation theory: The idea that the universe is a computation. Usually that means a computer somewhere outside it. In H-ΛCDM there is nothing outside: the simulation projects itself.
Causal diamond: The region of space and time bounded by two light cones: everything that lies in the future of the first event and in the past of the cosmic horizon. Every measurement an observer can make falls inside it.
The screen (σ): The widest surface of the causal diamond, where its two light cones meet. H-ΛCDM stores the universe's information there.
Obit: H-ΛCDM's one postulate: making one definite record costs exactly one nat of irreversibility.
Precipitation: The moment a quantum possibility becomes a definite fact, which physicists usually call decoherence or measurement. H-ΛCDM counts its cost in obits.
Processing rate (γ): How fast the universe updates its information: $\gamma = H/\ln S_H$ . Today it is $7.75\times10^{-21}$ per second.
Dead pixel: H-ΛCDM's picture of dark matter: a tile of the information screen that couldn't pay the cost of becoming definite, and froze. It still has mass; it never shines.
Little Bang: H-ΛCDM's account of a black hole that fills its information capacity. Unable to copy or compress, the region expands.