Astrophysics, Cosmology & Relativity — 60 Terms Defined

Accretion Disk Astrophysics, Cosmology & Relativity Watts or Solar Luminosity A rotating disk structure formed by infalling diffuse matter orbiting a massive central body like a white dwarf, neutron star, or black hole. L_acc = η M_dot c² (Efficiency η ~ 0.1 to 0.42)

Astronomical Unit (AU) Astrophysics, Cosmology & Relativity Astronomical Units (AU) An astronomical unit of length defined as exactly 149,597,870,700 meters, representing the mean distance from Earth to the Sun. 1 AU = 149,597,870,700 m ≈ 1.496 × 10¹¹ m

Big Bang Astrophysics, Cosmology & Relativity Cosmological Epoch (Years) The prevailing cosmological model describing the universe as originating from an extremely hot, dense state ~13.8 billion years ago and expanding ever since. T_universe ≈ 13.787 ± 0.020 billion years

Binary Star System Astrophysics, Cosmology & Relativity AU (Semi-major axis) & Years (Period) A stellar system consisting of two gravitationally bound stars orbiting around their common center of mass. P² = (4 π² / (G (M₁ + M₂))) a³ (Kepler's 3rd Law)

Black Hole Astrophysics, Cosmology & Relativity Solar Masses (M_☉) or Kilograms A region of spacetime where gravitational acceleration is so intense that nothing, including light, can escape beyond its event horizon. R_s = 2 G M / c²

Brown Dwarf Astrophysics, Cosmology & Relativity Jupiter Masses (M_J) or M_☉ A substellar object intermediate in mass between giant gas planets and low-mass stars, incapable of sustaining stable core hydrogen fusion. 13 M_J ≤ M < 80 M_J (~0.013 to 0.08 M_☉)

Chandrasekhar Limit Astrophysics, Cosmology & Relativity Solar Masses (M_☉) The maximum mass that a stable white dwarf star can support against gravitational collapse via electron degeneracy pressure. M_Ch ≈ 1.4 M_☉ (1.44 Solar Masses)

Cosmic Inflation Astrophysics, Cosmology & Relativity Ultra-early Epoch (t ~ 10⁻³⁶ s) A cosmological theory proposing a brief phase of exponential spatial expansion in the early universe (~10⁻³⁶ to 10⁻³² seconds after the Big Bang). a(t) = a₀ e^(H t) (Exponential scale factor growth)

Cosmic Microwave Background (CMB) Astrophysics, Cosmology & Relativity Kelvin (2.725 K Blackbody) The electromagnetic thermal radiation leftover from the recombination epoch of the early universe (~380,000 years after the Big Bang). T_CMB = 2.72548 ± 0.00057 K, λ_peak ≈ 1.06 mm

Cosmic Strings Astrophysics, Cosmology & Relativity 1D Topological Defect Hypothetical 1D subatomic-width topological line defects that may have formed during GUT phase transitions in the early universe. Linear Mass Density G μ / c² ~ 10⁻⁶

Cosmological Redshift Astrophysics, Cosmology & Relativity Dimensionless Quantity (z) The stretching of photon wavelengths to longer, redder values as light travels across the expanding space of the universe. z = (λ_observed - λ_emitted) / λ_emitted, 1 + z = a(t_obs) / a(t_emit)

Dark Energy Astrophysics, Cosmology & Relativity Cosmological Constant Λ (J/m³) An unknown form of energy with negative pressure that permeates all space and drives the accelerated expansion of the universe. Ω_Λ ≈ 0.68, w = P / ρ ≈ -1

Dark Matter Astrophysics, Cosmology & Relativity M_☉ or kg (Cosmological Density) A non-baryonic form of matter that does not absorb, reflect, or emit electromagnetic radiation, detectable solely through its gravitational effects. Ω_dm ≈ 0.27 (27% of total cosmic mass-energy)

Dark Matter Halo Astrophysics, Cosmology & Relativity kpc (Halo Radius) & M_☉ (Halo Mass) An extended, nearly spherical region of dark matter enveloping a galaxy or galaxy cluster that extends far beyond the visible stellar disk. NFW Density Profile: ρ(r) = ρ₀ / ((r/r_s) (1 + r/r_s)²)

Drake Equation Astrophysics, Cosmology & Relativity Number of Civilizations N A probabilistic equation formulated by Frank Drake in 1961 to estimate the number N of active, communicative extraterrestrial civilizations in the Milky Way. N = R* · f_p · n_e · f_l · f_i · f_c · L

Einstein Field Equations Astrophysics, Cosmology & Relativity Non-linear 4D Tensor Equations The set of 10 non-linear partial differential equations in general relativity relating spacetime curvature (Ricci tensor R_μν) to stress-energy-momentum T_μν. R_μν - ½ R g_μν + Λ g_μν = (8 π G / c⁴) T_μν

Equivalence Principle Astrophysics, Cosmology & Relativity Fundamental Relativistic Principle The foundational postulate of general relativity asserting that the physical effects of a uniform gravitational field are locally indistinguishable from uniform mechanical acceleration. m_inertial = m_gravitational

Event Horizon Astrophysics, Cosmology & Relativity km or m (Boundary Radius) The boundary surrounding a black hole beyond which escape velocity exceeds the speed of light, preventing any matter or signal from escaping. R_s = 2 G M / c²

Exoplanet Astrophysics, Cosmology & Relativity Earth Masses (M_⊕) or Jupiter Masses (M_J) A planet situated outside the Solar System that orbits a star, stellar remnant, or brown dwarf. Transit Depth ΔF / F = (R_planet / R_star)²

Fermi Paradox Astrophysics, Cosmology & Relativity Astronomical Paradox The apparent contradiction between high probabilistic estimates for the existence of extraterrestrial civilizations and the complete lack of physical evidence or contact. Enrico Fermi: 'Where is everybody?'

Galaxy Astrophysics, Cosmology & Relativity Solar Masses (M_☉) and Kiloparsecs (kpc) A massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. M_galaxy = v² r / G (includes Dark Matter)

Galaxy Cluster Astrophysics, Cosmology & Relativity Megaparsecs (Mpc) & M_☉ A gravitationally bound structure containing hundreds to thousands of individual galaxies bound together by gravity. M_total ~ 10¹⁴–10¹⁵ M_☉ (80% Dark Matter, 15% Intracluster Gas, 5% Stars)

General Relativity Astrophysics, Cosmology & Relativity Geometric Field Theory Albert Einstein's 1915 geometric theory of gravitation describing gravity not as an attractive force, but as the curvature of 4D spacetime produced by mass and energy. G_μν + Λ g_μν = (8 π G / c⁴) T_μν

Geodesic Astrophysics, Cosmology & Relativity Spacetime Path Trajectory The path of shortest (or extreme) spacetime interval between two points in curved spacetime, representing the free-fall trajectory of particles under gravity. (d²x^μ / dτ²) + Γ^μ_αβ (dx^α / dτ) (dx^β / dτ) = 0