11.1 Observational Evidence for Dark Matter
3 min read•august 9, 2024
Dark matter, the invisible cosmic glue, shapes our universe in ways we're just beginning to understand. Galaxies spin faster than they should, and clusters pack more punch than their visible parts suggest. It's like there's a hidden dance partner, leading the cosmic waltz.
From to , the evidence for dark matter is stacking up. The collision and observations add more weight to this mysterious matter's existence. It's a cosmic puzzle we're still piecing together.
Galactic Evidence
Rotation Curves and Velocity Dispersion
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- Galactic rotation curves demonstrate unexpected velocity distributions in galaxies
- Outer regions of galaxies rotate faster than predicted by visible matter alone
- measures random motions of stars within galaxies
- Higher velocity dispersion in galaxy outskirts indicates presence of additional unseen mass
- Observations reveal flat rotation curves extending far beyond visible galactic disks
- Flat rotation curves suggest presence of surrounding galaxies
- Dark matter halos extend well beyond visible galactic boundaries
- Halo mass estimated to be 5-10 times greater than visible galactic mass
Mass-to-Light Ratio and Dark Matter Distribution
- compares total mass of a galaxy to its luminous output
- Higher mass-to-light ratios indicate presence of non-luminous matter
- Typical spiral galaxies have mass-to-light ratios of 10-20 solar masses per solar luminosity
- Elliptical galaxies often exhibit even higher mass-to-light ratios
- Dark matter halo forms extended, roughly spherical region around galaxies
- Halo density profile typically modeled as Navarro-Frenk-White (NFW) profile
- NFW profile characterized by central cusp and gradual density decrease with radius
- Alternative halo models include and
Gravitational Lensing
Principles and Applications of Gravitational Lensing
- Gravitational lensing occurs when massive objects bend light from distant sources
- Based on Einstein's theory of general relativity
- produces multiple images or arcs of background objects
- causes subtle distortions in shapes of background galaxies
- temporarily amplifies light from background stars
- Gravitational lensing allows mapping of dark matter distribution in clusters and large-scale structures
- Provides independent method to estimate mass of lensing objects
- Lensing effects more pronounced for more massive and compact objects
The Bullet Cluster: A Compelling Case for Dark Matter
- Bullet Cluster formed by collision of two galaxy clusters
- Observed in visible light, X-rays, and through gravitational lensing
- X-ray observations show hot gas separated from visible galaxies due to collision
- Gravitational lensing reveals mass concentration doesn't match visible matter or hot gas
- Lensing indicates most mass located in regions containing galaxies, not hot gas
- Provides strong evidence for existence of dark matter
- Challenges alternative theories of gravity (MOND) that attempt to explain galaxy dynamics without dark matter
- Similar observations made in other colliding clusters (Abell 520, MACS J0025.4-1222)
Large-Scale Structure
Galaxy Cluster Dynamics and Mass Estimates
- Galaxy clusters contain hundreds to thousands of galaxies bound by gravity
- Cluster masses estimated through various methods
- relates kinetic energy of galaxies to cluster's gravitational potential energy
- X-ray observations of hot intracluster gas provide another mass estimate
- Gravitational lensing offers independent measure of cluster mass
- All methods consistently indicate more mass than accounted for by visible matter
- Typical clusters contain 80-85% dark matter by mass
- Velocity dispersion of galaxies within clusters higher than expected from visible mass alone
Cosmic Microwave Background and Structure Formation
- Cosmic microwave background (CMB) radiation provides snapshot of early universe
- CMB temperature fluctuations reflect density variations in early universe
- Amplitude and angular scale of fluctuations sensitive to cosmic matter composition
- Observations of CMB anisotropies consistent with presence of dark matter
- Dark matter played crucial role in
- Enabled matter to clump together before recombination, overcoming radiation pressure
- Hierarchical structure formation model explains observed large-scale structure
- Simulations incorporating dark matter successfully reproduce observed of galaxies, clusters, and filaments
Key Terms to Review (20)
Bullet Cluster: The Bullet Cluster is a pair of colliding galaxy clusters that provides compelling evidence for the existence of dark matter. This cosmic collision offers insights into how dark matter behaves differently from ordinary matter, helping to reinforce the theory that a significant amount of the universe's mass is composed of non-luminous, unseen matter.
Cosmic microwave background: The cosmic microwave background (CMB) is the afterglow radiation from the Big Bang, permeating the universe and providing a snapshot of the infant cosmos about 380,000 years after the event. This faint glow of microwave radiation is crucial for understanding the early universe's conditions, the formation of cosmic structures, and the overall evolution of the cosmos.
Cosmic web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of filaments composed of dark matter and galaxies that interconnect and surround enormous voids. This structure illustrates how matter is distributed in the universe, revealing the underlying gravitational forces and cosmic evolution over time.
Dark matter halos: Dark matter halos are vast, roughly spherical regions surrounding galaxies, composed primarily of dark matter that does not emit or absorb light. These halos play a crucial role in the structure and formation of galaxies, as they provide the necessary gravitational influence to hold galaxies together and facilitate their growth over time.
Einasto Profile: The Einasto profile is a mathematical function used to describe the density distribution of dark matter in galaxies and galaxy clusters, characterized by its smooth, exponential decline. This profile provides a more accurate representation of the density of dark matter compared to the traditional Navarro-Frenk-White (NFW) profile, particularly at large radii. It plays a critical role in understanding the distribution and structure of dark matter halos, which is essential for interpreting observational evidence related to dark matter.
Fritz Zwicky: Fritz Zwicky was a Swiss astronomer who made significant contributions to astrophysics in the early 20th century, particularly known for his pioneering work on the existence of dark matter. He introduced the concept of 'missing mass' when observing galaxy clusters, leading to important discussions on the unseen mass that affects gravitational interactions in the universe.
Galactic rotation curves: Galactic rotation curves are graphs that depict the rotational speeds of stars and gas in a galaxy as a function of their distance from the galactic center. These curves are crucial in understanding how mass is distributed within galaxies and provide key evidence for the existence of dark matter, as they often reveal unexpected flatness in rotation speeds at large distances from the center.
Gravitational Lensing: Gravitational lensing is the bending of light from distant objects due to the gravitational field of a massive object, such as a galaxy or cluster, located between the observer and the light source. This phenomenon allows astronomers to study the distribution of mass in the universe, providing insights into various cosmic structures and the nature of dark matter.
Isothermal Sphere: An isothermal sphere is a theoretical model describing a region of space where the temperature remains constant throughout, particularly in the context of gravitational systems. This concept is crucial in understanding how mass is distributed in galaxies and galaxy clusters, and it plays a key role in the study of dark matter's influence on visible matter in the universe.
Lambda Cold Dark Matter Model: The Lambda Cold Dark Matter (ΛCDM) model is a widely accepted cosmological framework that describes the evolution of the universe, combining the effects of dark energy, represented by the cosmological constant (Λ), and cold dark matter. This model explains how the universe expanded from an initial hot, dense state and evolved into the large-scale structure we observe today, including galaxies, clusters, and voids. It provides insights into cosmic phenomena and supports the existence of dark matter through its ability to explain the observed gravitational effects in the cosmos.
Mass-to-light ratio: The mass-to-light ratio is a measure that compares the total mass of an astronomical object, such as a galaxy, to its total luminosity. This ratio helps astronomers understand the distribution of matter within galaxies and provides crucial insights into the presence of dark matter, as well as the dynamics of galactic structures and their evolution over time.
Microlensing: Microlensing is a phenomenon that occurs when a massive object, such as a star or black hole, passes in front of a more distant light source, temporarily magnifying and distorting the light from that source due to gravitational effects. This effect is significant in studying both dark matter and exoplanets, as it provides a unique way to detect objects that might otherwise remain hidden, offering crucial evidence for their existence and properties.
Navarro-Frenk-White Profile: The Navarro-Frenk-White profile is a mathematical model used to describe the density distribution of dark matter in galaxies, particularly in the context of halo formation. This profile suggests that dark matter halos have a specific structure where the density falls off with radius in a characteristic way, allowing for predictions about the gravitational effects of dark matter on visible matter in galaxies.
Strong lensing: Strong lensing is a phenomenon that occurs when a massive object, like a galaxy or cluster of galaxies, significantly warps the space around it, causing the light from more distant objects to be bent and magnified. This effect allows astronomers to observe distant galaxies and other cosmic structures that would otherwise be too faint to detect, providing crucial evidence for the existence of dark matter and helping to map its distribution in the universe.
Structure Formation: Structure formation refers to the process by which the universe evolves from a nearly uniform state after the Big Bang to the complex structures we see today, such as galaxies, clusters of galaxies, and large-scale cosmic filaments. This process is influenced by gravitational forces, dark matter, and baryonic matter, and it plays a critical role in understanding the distribution and dynamics of matter in the universe.
Velocity dispersion: Velocity dispersion is a measure of the range of velocities of objects within a system, typically used in astrophysics to understand the dynamics of galaxies and clusters. It provides insight into how quickly stars or galaxies are moving in relation to one another, reflecting the gravitational interactions and mass distribution within these systems. Understanding velocity dispersion is crucial for analyzing galaxy cluster properties and for supporting evidence related to dark matter's influence on galactic dynamics.
Vera Rubin: Vera Rubin was an influential American astronomer known for her pioneering work in studying galaxy rotation curves and providing key evidence for the existence of dark matter. Her groundbreaking observations revealed that the rotational speeds of galaxies did not decrease with distance from their centers as expected, suggesting that a significant amount of unseen mass was present. This discovery has had profound implications for our understanding of the universe, highlighting the importance of dark matter in shaping galactic dynamics and structure.
Virial theorem: The virial theorem is a fundamental principle in astrophysics that relates the average kinetic energy of a system to its average potential energy. It provides insight into the stability of various astrophysical systems, including stars, galaxies, and clusters, by showing how the forces at play within these systems balance over time.
Weak lensing: Weak lensing refers to the subtle distortion of background light from distant galaxies due to the gravitational influence of foreground mass, such as galaxy clusters. This effect is not strong enough to produce noticeable images but can be measured statistically to reveal the distribution of dark matter and the large-scale structure of the universe. By studying weak lensing, scientists can gain insights into galaxy cluster properties, provide evidence for dark matter, and understand cosmic shear effects.
WIMP Theory: WIMP theory proposes that Weakly Interacting Massive Particles (WIMPs) are a primary candidate for dark matter, which makes up a significant portion of the universe's mass. These particles are predicted to have mass and interact through the weak nuclear force and gravity, but not through electromagnetic forces, making them elusive and hard to detect directly. WIMP theory connects to the search for dark matter by providing a framework for understanding the missing mass needed to explain various astrophysical observations.