12.5 Dark Matter and Dark Energy

Dark matter and dark energy are cosmic mysteries that shape our universe. These invisible forces make up 95% of the cosmos, influencing galaxy formation, cosmic structure, and the universe's expansion.

Scientists use observations like galactic rotation curves and gravitational lensing to study dark matter and energy. Understanding these phenomena is crucial for explaining the universe's past, present, and future evolution.

Dark Matter and Dark Energy

Definitions and Cosmic Composition

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• Dark matter hypothetical form of matter exerts gravitational effects on visible matter without interacting with electromagnetic radiation
• Dark energy hypothetical form of energy permeates all space drives accelerating expansion of the universe
• Observable universe composition ~27% dark matter, ~68% dark energy, ~5% ordinary matter (protons, neutrons, electrons)
• Dark matter provides gravitational scaffolding for visible matter to coalesce crucial for galaxy formation and structure
• Dark energy counteracts gravity's attractive force on cosmic scales accelerates universe expansion
• Lambda-CDM model current standard model of cosmology incorporates dark matter and dark energy as fundamental components

Roles in Cosmic Structure

• Dark matter forms cosmic web large-scale structure of filaments and voids guides distribution of visible matter
• Enhanced gravitational attraction between galaxies due to dark matter leads to formation of galaxy clusters and superclusters
• Dark energy's repulsive effect counteracts matter's gravitational pull causes accelerating expansion of space over time
• Balance between dark matter attraction and dark energy repulsion determines universe's ultimate fate (Big Freeze, Big Rip)
• Hierarchical model of galaxy formation relies on dark matter providing initial density fluctuations to seed structure growth
• Relative densities of dark matter and dark energy influence space-time geometry and overall universe curvature

Evidence for Dark Matter and Dark Energy

Galactic and Cluster Observations

• Galactic rotation curves reveal faster-than-expected galaxy rotation based on visible mass indicates presence of dark matter
• Gravitational lensing observations show galaxy clusters contain more mass than visible matter alone can account for
• Cosmic microwave background radiation temperature fluctuations provide evidence for dark matter in early universe
• Type Ia supernovae observations demonstrate distant galaxies moving away at accelerating rate supports existence of dark energy
• Large-scale structure formation and galaxy cluster dynamics require dark matter to explain observed patterns and velocities
• Bullet Cluster collision of two galaxy clusters provides direct empirical evidence for dark matter through gravitational lensing effects

Cosmological Implications

• Dark matter enhances structure formation in the early universe explains observed distribution of galaxies and clusters
• Cosmic microwave background power spectrum matches predictions of models including dark matter and dark energy
• Baryon acoustic oscillations in large-scale structure surveys consistent with presence of dark matter and dark energy
• Weak lensing surveys map dark matter distribution across large areas of sky confirm its role in cosmic web formation
• Integrated Sachs-Wolfe effect detection provides additional evidence for dark energy's influence on cosmic expansion
• Big Bang nucleosynthesis predictions for light element abundances constrain amount of baryonic matter in universe support need for non-baryonic dark matter

Effects on the Universe

Structural Impact

• Dark matter forms gravitational wells trap ordinary matter lead to galaxy and star formation
• Filamentary structure of cosmic web shaped by dark matter distribution guides flow of baryonic matter
• Galaxy rotation stabilized by dark matter halos prevents rapid disintegration of spiral arms
• Gravitational lensing effects enhanced by dark matter concentrations allow observation of distant galaxies and quasars
• Dark matter bridges between galaxies in clusters facilitate mergers and interactions shape galactic evolution
• Dwarf galaxies abundance and distribution around larger galaxies explained by dark matter substructure

Evolutionary Consequences

• Universe expansion history influenced by changing balance between dark matter attraction and dark energy repulsion
• Structure growth rate in universe affected by dark energy slows down as expansion accelerates
• Galaxy cluster formation and evolution modulated by interplay between dark matter concentration and dark energy expansion
• Cosmic voids grow larger over time as dark energy pushes matter away from underdense regions
• Future of cosmic structures (galaxies, clusters) determined by long-term dominance of dark energy
• Potential for "Big Rip" scenario if dark energy strength increases over time could tear apart all bound structures

Theories of Dark Matter and Dark Energy

Dark Matter Candidates

• Weakly Interacting Massive Particles (WIMPs) leading dark matter candidate predicted by supersymmetry theories in particle physics
• Axions hypothetical particles proposed to solve strong CP problem in quantum chromodynamics potential explanation for dark matter
• Sterile neutrinos hypothetical particles related to standard neutrinos could account for dark matter properties
• Primordial black holes formed in early universe proposed as alternative to particle dark matter
• Self-interacting dark matter models attempt to explain observed galaxy core densities and cluster dynamics
• Fuzzy dark matter ultra-light boson particles could explain some small-scale structure observations

Dark Energy Models

• Cosmological constant simplest dark energy model represents energy density of vacuum originally proposed by Einstein
• Quintessence models propose dynamic scalar field as alternative to cosmological constant explain dark energy
• Phantom energy hypothetical form of dark energy with negative kinetic energy could lead to Big Rip scenario
• Modified gravity theories (MOND) attempt to explain galactic dynamics without invoking dark matter
• Chameleon fields propose dark energy properties vary depending on local matter density
• Quantum field theory in curved spacetime and holographic principle explored to reconcile dark energy with quantum mechanics