Glossary

Every jargon term used across the site — defined in one plain sentence and one technical sentence. Hover any dotted-underline word elsewhere on the site to jump back here.

Boltzmann code: PlainSoftware that evolves cosmological perturbations forward in time — e.g. CAMB, CLASS.; TechnicalNumerical linear-perturbation solver (CAMB, CLASS) integrating the coupled Einstein-Boltzmann hierarchy for CMB and P(k) predictions.
brane: PlainA sheet-like slice of a higher-dimensional space; our 3D universe is treated as one such sheet.; TechnicalA codimension-1 hypersurface in a higher-dimensional bulk on which Standard Model fields are localised.
Brick 4: PlainThe part of the theory that picks the special scale k_c. Currently fitted, not yet derived.; TechnicalScale-selection component of the seven-brick framework — phenomenological (◐ 45%). Target of Stage 3 first-principles derivation.
bulk: PlainThe higher-dimensional space surrounding the brane. Gravity can reach into it; ordinary matter cannot.; TechnicalThe full 5D spacetime outside the 3-brane. Only gravity and bulk scalars (e.g. the radion) propagate here.
CMB-S4: PlainA next-generation ground-based CMB experiment that will sharpen small-scale tests.; TechnicalStage-IV ground-based CMB survey; will constrain damping-tail suppression to sub-percent, testing the model's 1–3% prediction.
DESI / Euclid: PlainMassive galaxy surveys mapping how structure grew — key tests for this model.; TechnicalDESI (spectroscopic BAO/RSD) and Euclid (weak lensing + spectra) will constrain σ₈(z) and P(k) at percent level through 2029.
G_eff: PlainThe effective strength of gravity. During the leakage epoch it dips to ~75% of the normal value.; TechnicalEffective Newton's constant felt by matter perturbations; model gives G_eff/G_N ≈ 0.75 during the leakage epoch (β₂ ≈ 3.33).
k_c (small-scale cutoff): PlainThe scale below which structure formation is suppressed — the model predicts a soft edge at k_c ≈ 0.75 h/Mpc.; TechnicalPhenomenological exponential cutoff in P(k), imprinted by the leakage event; current best fit k_c ≈ 0.75 h/Mpc.
leakage epoch: PlainA brief early-universe window (z ≈ 50,000 → 1,000) when the mechanism was active.; TechnicalRedshift interval during which the radion excursion suppresses G_eff and imprints the k_c cutoff; ends before recombination.
Lyman-α forest: PlainA pattern of absorption dips in quasar light that maps hydrogen clumping on small cosmic scales.; TechnicalNeutral-hydrogen absorption in quasar spectra used to reconstruct the 1D matter power spectrum at k ≈ 0.1–10 h/Mpc.
phenomenological: PlainFitted to what we see, not yet derived from a deeper theory. Self-consistent but incomplete.; TechnicalA relation motivated by observations and internal consistency, awaiting first-principles derivation. Distinct from an ad-hoc fit.
radion: PlainA hypothetical scalar field that sets the size of an extra spatial dimension — think of it as a dial.; TechnicalScalar modulus controlling the radius of the compact 5th dimension in a 5D braneworld setup; couples to matter via T^μ_μ.
S₈ tension: PlainA mismatch: the late universe looks less lumpy than the early universe (via ΛCDM) predicts.; TechnicalPersistent ~2–3σ offset between weak-lensing S₈ ≈ 0.76–0.79 (KiDS, DES, HSC) and Planck-inferred S₈ ≈ 0.83 under ΛCDM.
WDM / FDM: PlainWarm and fuzzy dark matter — rival ideas that also suppress small-scale structure by changing the dark-matter particle.; TechnicalWarm dark matter (keV thermal relic) and fuzzy dark matter (ultralight scalar, m ~ 10⁻²² eV); produce similar P(k) cutoffs via different physics.
ΛCDM: PlainThe standard cosmological model: dark energy (Λ) plus cold dark matter. Fits most data very well.; TechnicalSix-parameter concordance model: flat FLRW, cold dark matter, cosmological constant, adiabatic Gaussian initial conditions.
σ₈: PlainHow lumpy matter is on 8 Mpc/h scales today. Lower means smoother.; TechnicalRMS of linear matter density fluctuations in spheres of radius 8 h⁻¹ Mpc. Model target σ₈ ≈ 0.76 ± 0.03.

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