cosmic web
Definition: Large galaxy redshift surveys (the CfA survey in the 1980s, the Sloan Digital Sky Survey since 2000) revealed that galaxies are not scattered randomly through space but trace an enormous filamentary "cosmic web" of walls, filaments, and voids hundreds of millions of light-years across — structure that must trace the gravitational scaffolding of dark matter assembled since the early universe.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: If dark energy is well-described by a cosmological constant, its effect on the expansion rate becomes increasingly important at late cosmic times as matter density dilutes with the growing volume of space, exactly the pattern needed to explain why the acceleration was not detected until measurements reached sufficiently distant (early-epoch) supernovae.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
galaxy filament
Definition: Large galaxy redshift surveys (the CfA survey in the 1980s, the Sloan Digital Sky Survey since 2000) revealed that galaxies are not scattered randomly through space but trace an enormous filamentary "cosmic web" of walls, filaments, and voids hundreds of millions of light-years across — structure that must trace the gravitational scaffolding of dark matter assembled since the early universe.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: The Sloan Great Wall, a filament of galaxies over a billion light-years long, is one of the largest known coherent structures in the observable universe, far larger than any single galaxy or galaxy cluster, yet its existence is a natural prediction of gravitational structure growth from small initial density fluctuations amplified over billions of years.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
void
Definition: Large galaxy redshift surveys (the CfA survey in the 1980s, the Sloan Digital Sky Survey since 2000) revealed that galaxies are not scattered randomly through space but trace an enormous filamentary "cosmic web" of walls, filaments, and voids hundreds of millions of light-years across — structure that must trace the gravitational scaffolding of dark matter assembled since the early universe.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: On cosmological scales, the total energy budget of the universe is dominated not by ordinary matter or even dark matter, but by dark energy, an unexplained form of energy (possibly a property of space itself) causing the universe’s expansion to accelerate rather than decelerate.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
galaxy cluster
Definition: Gravitational lensing (the bending of background light by foreground mass, predicted by general relativity) directly maps the total mass distribution in galaxy clusters, independent of whether that mass is visible, and consistently reveals far more mass than visible light accounts for.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: The Sloan Great Wall, a filament of galaxies over a billion light-years long, is one of the largest known coherent structures in the observable universe, far larger than any single galaxy or galaxy cluster, yet its existence is a natural prediction of gravitational structure growth from small initial density fluctuations amplified over billions of years.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
gravitational lensing
Definition: Gravitational lensing (the bending of background light by foreground mass, predicted by general relativity) directly maps the total mass distribution in galaxy clusters, independent of whether that mass is visible, and consistently reveals far more mass than visible light accounts for.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: Gravitational lensing maps of merging clusters (such as the Bullet Cluster) show that the lensing mass (inferred from background image distortion) is spatially offset from the hot X-ray-emitting gas (the dominant visible baryonic mass), directly demonstrating that most cluster mass is not the visible hot gas and behaves differently during the merger — strong, largely model-independent evidence against dark matter simply being unseen ordinary matter.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
dark energy
Definition: On cosmological scales, the total energy budget of the universe is dominated not by ordinary matter or even dark matter, but by dark energy, an unexplained form of energy (possibly a property of space itself) causing the universe’s expansion to accelerate rather than decelerate.
Why it matters here: Large-scale structure, cluster dynamics, and gravitational lensing extend the case for dark matter from individual galaxies to the largest cosmic scales, while independent supernova distance measurements reveal a second, distinct dark component: dark energy, driving accelerating cosmic expansion.
Relationship: Distant Type Ia supernovae (Lecture 08’s "standard candles") appear systematically fainter than expected for a universe whose expansion is slowing down under gravity alone, implying the expansion is instead accelerating (Riess, Perlmutter, and Schmidt, Nobel Prize 2011) — attributed to dark energy.
Boundary or common confusion: Conflating dark matter and dark energy. Dark matter clumps gravitationally and explains structure formation and excess gravitating mass; dark energy is smoothly distributed and drives accelerating expansion — they solve different observational puzzles and are supported by largely independent evidence.
Equation connection: \( \Omega_{\rm baryon} \approx 0.05, \qquad \Omega_{\rm dark\ matter} \approx 0.25, \qquad \Omega_{\rm dark\ energy} \approx 0.70 \)