12.5 Dark Matter and Dark Energy
4 min read•august 16, 2024
and 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 and 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
- current standard model of cosmology incorporates dark matter and dark energy as fundamental components
Roles in Cosmic Structure
- Dark matter forms 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 (, )
- 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
- temperature fluctuations provide evidence for dark matter in early universe
- 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
- in large-scale structure surveys consistent with presence of dark matter and dark energy
- map dark matter distribution across large areas of sky confirm its role in cosmic web formation
- 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
- leading dark matter candidate predicted by supersymmetry theories in particle physics
- hypothetical particles proposed to solve strong CP problem in quantum chromodynamics potential explanation for dark matter
- hypothetical particles related to standard neutrinos could account for dark matter properties
- formed in early universe proposed as alternative to particle dark matter
- models attempt to explain observed galaxy core densities and cluster dynamics
- ultra-light boson particles could explain some small-scale structure observations
Dark Energy Models
- simplest dark energy model represents energy density of vacuum originally proposed by Einstein
- propose dynamic scalar field as alternative to cosmological constant explain dark 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
- 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
Key Terms to Review (30)
Accelerating universe: An accelerating universe refers to the observation that the expansion of the universe is not only continuing but is also speeding up over time. This phenomenon is primarily attributed to dark energy, a mysterious force that counteracts the gravitational attraction of matter, leading to an accelerated rate of expansion as the universe ages.
Albert Einstein: Albert Einstein was a theoretical physicist known for developing the theory of relativity, which revolutionized our understanding of space, time, and gravity. His work laid the foundation for many modern physics concepts, influencing various areas including the behavior of light, atomic structure, and the nature of the universe itself.
Axions: Axions are hypothetical elementary particles proposed as a solution to the strong CP problem in quantum chromodynamics, and they are also considered a candidate for dark matter. These particles are predicted to be extremely light and weakly interacting, making them difficult to detect directly. Their existence could help explain various phenomena in astrophysics and cosmology, especially in relation to the mysterious components of dark matter and dark energy in the universe.
Baryon acoustic oscillations: Baryon acoustic oscillations refer to the regular, periodic fluctuations in the density of visible baryonic matter (normal matter) in the universe caused by sound waves in the hot plasma of the early universe. These oscillations played a crucial role in the formation of the large-scale structure of the universe and are imprinted in the cosmic microwave background radiation as well as in the distribution of galaxies. Understanding these oscillations helps shed light on fundamental aspects of cosmology, including the nature of dark matter and dark energy.
Big freeze: The big freeze is a theoretical scenario for the ultimate fate of the universe, where it continues to expand indefinitely until stars burn out, galaxies drift apart, and the universe becomes cold and dark. In this scenario, as cosmic expansion accelerates due to dark energy, the universe approaches a state of maximum entropy, leading to a lifeless, stagnant expanse with little to no activity.
Big rip: The big rip is a hypothetical cosmological event in which the universe's accelerated expansion eventually tears apart all matter, from galaxies to individual atoms. This concept is closely tied to the effects of dark energy, which is believed to be driving the accelerated expansion of the universe, and raises questions about the ultimate fate of cosmic structures and the fundamental nature of spacetime.
Chameleon Fields: Chameleon fields are hypothetical scalar fields that can vary their properties depending on the local environment, particularly in the context of gravity and dark energy. These fields are proposed to help explain the accelerated expansion of the universe by interacting differently with matter and energy at different scales. Their dynamic nature allows them to potentially mimic the effects of dark energy while being less detectable under certain conditions.
Computer simulations: Computer simulations are virtual models that use computational algorithms to replicate complex physical systems and phenomena. They help scientists and researchers visualize, predict, and analyze behaviors of systems that might be difficult or impossible to study directly, especially in fields like astrophysics where dark matter and dark energy play crucial roles in the universe's structure and evolution.
Cosmic microwave background radiation: Cosmic microwave background radiation (CMB) is the afterglow of the Big Bang, representing a uniform field of microwave radiation that fills the universe and is a critical piece of evidence for the Big Bang theory. This radiation, which is remarkably uniform in all directions, provides insights into the early universe's conditions and supports the existence of dark matter and dark energy as it reveals the universe's large-scale structure and evolution.
Cosmic web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of galaxies, galaxy clusters, and dark matter filaments that interconnect in a complex, web-like pattern. This structure plays a crucial role in understanding the distribution of matter and the dynamics of cosmic evolution, as it is influenced by dark matter and dark energy throughout the universe's history.
Cosmological constant: The cosmological constant is a term introduced by Albert Einstein in his equations of general relativity, representing a constant energy density filling space homogeneously. This concept plays a significant role in modern cosmology, particularly in understanding the accelerated expansion of the universe and the nature of dark energy, which is believed to drive this phenomenon.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is believed to be responsible for the accelerated expansion of the universe. It plays a crucial role in understanding how the cosmos behaves, particularly when considering observations related to the movement of galaxies and the cosmic microwave background radiation. By influencing the dynamics of cosmic expansion, dark energy ties into the framework of both the Big Bang Theory and our understanding of gravity on cosmic scales.
Dark matter: Dark matter is a type of matter that does not emit, absorb, or reflect any electromagnetic radiation, making it invisible and detectable only through its gravitational effects on visible matter. This mysterious substance constitutes about 27% of the universe's total mass-energy content and plays a crucial role in the formation and structure of galaxies and cosmic structures.
Fuzzy dark matter: Fuzzy dark matter is a theoretical form of dark matter that consists of ultra-light bosons, which are particles with integer spin that are less massive than typical dark matter candidates. This concept suggests that instead of being composed of point-like particles, dark matter may have wave-like properties, resulting in a smooth, fuzzy distribution. Fuzzy dark matter models aim to address some of the shortcomings of traditional dark matter theories, particularly in explaining the behavior of galaxies and cosmic structures.
Galactic rotation curves: Galactic rotation curves are graphical representations that show how the rotational speed of stars and gas in a galaxy varies with distance from its center. These curves are significant in understanding the distribution of mass within galaxies and highlight discrepancies between observed speeds and the predictions made by Newtonian dynamics, indicating the presence of dark matter.
Gravitational lensing: Gravitational lensing is the phenomenon where light from a distant object, like a galaxy or quasar, is bent around a massive object, such as a galaxy cluster or black hole, due to the object's gravitational field. This bending effect can create multiple images, magnify, or distort the appearance of the distant object, allowing astronomers to study the mass and structure of the intervening object and the universe itself. It provides valuable insights into the distribution of dark matter and the nature of cosmic structures.
Integrated Sachs-Wolfe Effect: The Integrated Sachs-Wolfe Effect refers to the phenomenon where the gravitational potential wells of large-scale structures in the universe affect the temperature of cosmic microwave background (CMB) radiation as it travels through them. This effect is crucial for understanding how dark energy influences the evolution of the universe and contributes to the overall energy density, impacting both dark matter and dark energy dynamics.
Lambda-cdm model: The lambda-cdm model is a cosmological model that describes the universe's large-scale structure and evolution, incorporating dark energy (represented by the Greek letter lambda, \(\Lambda\)) and cold dark matter (cdm). This model suggests that the universe is flat, expanding at an accelerating rate due to dark energy, while dark matter plays a critical role in structure formation by influencing gravitational interactions.
Large-scale structure: Large-scale structure refers to the vast formations of galaxies, galaxy clusters, and superclusters that make up the universe. These structures are not randomly distributed; instead, they exhibit a web-like pattern where galaxies are interconnected by filaments of dark matter and gas, with vast voids in between. Understanding these structures helps scientists learn about the distribution of dark matter and the overall evolution of the universe.
Modified Newtonian Dynamics (MOND): Modified Newtonian Dynamics (MOND) is a theoretical framework that proposes alterations to Newton's laws of motion and gravitation, particularly at low accelerations, to address discrepancies observed in galaxy rotation curves without invoking dark matter. It suggests that gravity behaves differently than expected under traditional physics in certain conditions, particularly at the edges of galaxies. MOND challenges the standard model of cosmology and offers an alternative perspective on galactic dynamics and the unseen mass problem.
Observational Astronomy: Observational astronomy is the branch of astronomy that involves the systematic observation and analysis of celestial objects and phenomena. This field relies on telescopes and other instruments to gather data about stars, planets, galaxies, and the cosmos as a whole, which helps scientists understand the universe's structure, evolution, and the nature of dark matter and dark energy.
Phantom Energy: Phantom energy refers to a theoretical form of dark energy that has an equation of state with a parameter less than -1, leading to an accelerated expansion of the universe at an increasing rate. This concept is linked to the idea that such energy could contribute to the observed phenomena of cosmic acceleration, challenging our understanding of gravity and the fundamental nature of energy in the universe.
Primordial black holes: Primordial black holes are hypothetical black holes formed in the early universe due to density fluctuations shortly after the Big Bang. These black holes could vary in mass from very small to several solar masses and are considered as potential candidates for dark matter, connecting them to the mysteries of dark energy and the universe's structure.
Quintessence models: Quintessence models are theoretical frameworks in cosmology that propose a dynamic form of dark energy, which changes over time and is responsible for the accelerated expansion of the universe. Unlike the cosmological constant, which is static, quintessence suggests that dark energy could vary in density and influence as the universe evolves. These models help to explain the mysterious behavior of cosmic acceleration and its implications for the fate of the universe.
Self-interacting dark matter: Self-interacting dark matter is a theoretical form of dark matter that allows for interactions between its particles, which could lead to observable effects in cosmic structures. This concept proposes that dark matter is not only gravitationally interacting but can also scatter off itself, potentially altering the distribution and dynamics of galaxies and clusters. These interactions may help resolve certain discrepancies between observed galaxy behaviors and predictions made by standard dark matter models.
Sterile neutrinos: Sterile neutrinos are hypothetical particles that do not interact via the standard weak interactions like other known neutrinos, making them 'sterile' in a sense. They are proposed as a component of dark matter and could help explain certain anomalies in neutrino physics, including the behavior of oscillations observed in experiments.
Type Ia supernovae: Type Ia supernovae are explosive events that occur in binary star systems, typically involving a white dwarf that accumulates mass from its companion until it reaches a critical limit, resulting in a catastrophic explosion. These supernovae are important for understanding the universe because they have consistent peak brightness, making them valuable as 'standard candles' for measuring cosmic distances and studying the expansion of the universe.
Vera Rubin: Vera Rubin was an American astronomer renowned for her pioneering work on galaxy rotation rates, which provided some of the first strong evidence for the existence of dark matter. Her observations showed that galaxies rotate at such speeds that the visible matter alone could not account for the gravitational forces required to hold them together, suggesting a significant amount of unseen mass. This groundbreaking research fundamentally changed our understanding of the universe and the role of dark matter in cosmology.
Weak lensing surveys: Weak lensing surveys refer to observational techniques used in astronomy to detect and study the effects of gravitational lensing on distant light sources caused by massive objects, like galaxies or clusters of galaxies. These surveys help map the distribution of dark matter by analyzing how light from background galaxies is distorted and stretched as it passes near massive foreground structures, providing insights into both dark matter and dark energy in the universe.
Weakly Interacting Massive Particles (WIMPs): Weakly interacting massive particles (WIMPs) are hypothetical particles that are considered one of the leading candidates for dark matter. They are characterized by their large mass and their ability to interact only through the weak nuclear force and gravity, making them incredibly difficult to detect. WIMPs play a crucial role in explaining the missing mass in the universe and how galaxies form and evolve.