14.1 Evidence for dark matter in galaxies and clusters
2 min read•july 25, 2024
Dark matter, the invisible cosmic enigma, plays a crucial role in shaping our universe. From galaxy to , evidence mounts for this elusive substance that outweighs visible matter by a factor of five.
The and showcase dark matter's influence on large-scale structure. Meanwhile, the provides a snapshot of the early universe, revealing dark matter's fingerprints in temperature fluctuations and acoustic oscillations.
Galactic Evidence for Dark Matter
Galaxy rotation curve anomalies
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- Rotation curves plot orbital velocities of stars and gas vs. distance from galactic center reveal unexpected behavior
- Keplerian decline predicts velocity decrease with distance beyond visible mass contradicted by observations
- Flat rotation curves show constant orbital velocities at large radii implying more mass than visible matter accounts for (NGC 3198)
- Newtonian dynamics predictions fail to match observed curves based on visible mass distribution necessitating additional unseen mass
- Mass discrepancy in galaxies increases with radius as ratio of total mass to visible mass grows suggesting extends beyond visible disk
Gravitational lensing in clusters
- Massive objects' gravitational fields bend light creating strong lensing effects like multiple images or Einstein rings (Abell 2218)
- Weak lensing subtly distorts background galaxy shapes allowing mass distribution mapping
- Galaxy cluster observations reveal stronger lensing effects than expected from visible mass alone indicating presence of dark matter
- collision provides direct empirical proof of dark matter's existence by showing separation of dark matter from visible matter
Cosmic Structure and Dark Matter
Dark matter in cosmic structure
- Dark matter's gravitational influence amplifies primordial density fluctuations in early universe seeding
- Cold dark matter model proposes non-relativistic particles form small structures first leading to bottom-up hierarchical growth
- Cosmic web emerges as dark matter forms backbone of large-scale structure creating filaments sheets and voids in matter distribution ()
- Dark matter halos act as seeds for galaxy formation driving evolution through mergers and accretion
- Galaxy clusters represent largest gravitationally bound structures with dark matter dominating cluster mass at about 85% ()
Dark matter evidence from CMB
- Cosmic Microwave Background relic radiation captures matter distribution at recombination through temperature fluctuations
- Acoustic oscillations result from interplay between radiation pressure and gravitational attraction with oscillation amplitude affected by dark matter content
- Angular power spectrum statistically measures CMB temperature fluctuations constraining cosmological parameters through peak positions and heights
- Dark matter signatures in CMB manifest as enhanced odd peaks due to gravitational effects
- Planck satellite results provide precise CMB anisotropy measurements confirming CDM model with significant dark matter component (~26% of universe's energy density)
Key Terms to Review (18)
Bullet Cluster: The Bullet Cluster refers to a pair of colliding galaxy clusters, known as 1E 0657-56, which provides compelling evidence for the existence of dark matter. The interaction of these clusters demonstrates the presence of unseen mass that affects their dynamics, allowing scientists to infer that a significant amount of the mass is not accounted for by visible matter.
Cold dark matter (cdm): Cold dark matter (cdm) is a hypothetical form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects on visible matter. It is considered 'cold' because it moves slowly compared to the speed of light, which allows for the formation of structures like galaxies and galaxy clusters in the universe.
Coma Cluster: The Coma Cluster is a large cluster of galaxies located approximately 320 million light-years away in the constellation Coma Berenices. It is one of the richest and most studied galaxy clusters, serving as crucial evidence for the existence of dark matter, as its visible mass alone cannot account for the high velocities of its galaxies.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe with a nearly uniform glow of microwave radiation. It serves as a snapshot of the universe when it was just 380,000 years old, providing vital clues about its early conditions, structure, and expansion. The CMB plays a crucial role in understanding the universe's constituents, its expansion over time, and influences our comprehension of dark matter and dark energy.
Cosmic web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of interconnected filaments of dark matter and galaxies. This structure forms a complex pattern of voids, filaments, and clusters, shaping how matter is distributed throughout the universe. Understanding the cosmic web is crucial for studying galaxy formation, the behavior of dark matter, and the overall evolution of cosmic structures.
Cosmological simulations: Cosmological simulations are sophisticated computer models used to study the formation and evolution of cosmic structures in the universe, including galaxies, galaxy clusters, and large-scale cosmic web structures. These simulations utilize physical laws and observational data to replicate how matter interacts over vast time scales, helping scientists understand phenomena like dark matter, dark energy, and the overall dynamics of the universe.
Dark Energy Survey: The Dark Energy Survey (DES) is an astronomical survey designed to investigate the nature of dark energy and its effects on the expansion of the universe. By mapping galaxies, measuring their distribution, and analyzing cosmic structures, the survey aims to provide critical insights into how dark energy influences the universe's behavior, particularly in relation to dark matter in galaxies and clusters.
Dark matter halo: A dark matter halo is a region surrounding a galaxy that is composed primarily of dark matter, which does not emit light or energy and is thus invisible to telescopes. This halo plays a crucial role in the gravitational dynamics of galaxies, affecting their rotation curves and overall structure, while also serving as a major component in the formation and clustering of galaxies throughout the universe.
Density Profile: A density profile describes how the density of matter varies with distance from the center of an astronomical object, such as a galaxy or galaxy cluster. It provides insight into the distribution of both visible and dark matter, revealing critical information about the gravitational effects that shape these celestial structures and offering evidence for the existence of dark matter in the universe.
Galaxy clusters: Galaxy clusters are large groups of galaxies held together by gravity, containing anywhere from a few dozen to thousands of galaxies, along with gas and dark matter. They are the largest known gravitationally-bound structures in the universe, playing a critical role in understanding the large-scale structure of the cosmos and the distribution of matter, including dark matter, across different scales.
Gravitational Lensing: Gravitational lensing is the bending of light from a distant object, such as a galaxy or quasar, by the gravitational field of a massive object, like a galaxy cluster or black hole, situated between the observer and the distant source. This phenomenon provides crucial insights into the distribution of mass in the universe, including dark matter, and influences our understanding of cosmic structures and the evolution of galaxies.
Mass-to-light ratio: The mass-to-light ratio is a measure that compares the mass of an astronomical object to its luminosity, providing insights into its composition and structure. This ratio helps astronomers understand how much mass is present in stars or galaxies compared to the light they emit, which can indicate the presence of dark matter or the efficiency of star formation.
Millennium simulation: The millennium simulation is a large-scale computer simulation of the formation and evolution of cosmic structures in the universe, designed to understand how galaxies and clusters develop over time. It incorporates the influence of dark matter and dark energy, providing insights into the distribution and dynamics of matter in the universe, as well as helping to explain observations related to galaxy formation and cluster dynamics.
Rotation curves: Rotation curves are plots that show how the rotational velocity of objects in a galaxy varies with distance from the galaxy's center. These curves are essential for understanding the dynamics of galaxies, revealing how the mass is distributed within them and providing key insights into the presence of dark matter, particularly when the observed velocities do not match the expected values based on visible matter alone.
Spherical symmetry: Spherical symmetry refers to a situation where an object or system exhibits uniform properties in all directions from a central point. This means that if you were to rotate the object around any axis, its appearance and characteristics would remain unchanged. In astrophysics, spherical symmetry is often an idealization used when studying gravitational systems, as many celestial objects, like stars and galaxies, approximate this symmetry due to their mass distribution.
Structure formation: Structure formation refers to the process by which matter in the universe evolves from small density fluctuations in the early universe to the large-scale structures we observe today, such as galaxies, galaxy clusters, and superclusters. This process is heavily influenced by gravitational forces and the distribution of dark matter, shaping the cosmic web. Understanding structure formation is crucial to explaining how cosmic structures evolved and how they relate to phenomena like cosmic microwave background radiation, dark matter evidence, and potential dark matter candidates.
Velocity dispersion: Velocity dispersion refers to the range of velocities that stars or other objects within a system, such as a galaxy or cluster, exhibit around the average velocity of that system. This concept is essential in understanding the dynamics and structure of galaxies and galaxy clusters, as it provides insights into the mass distribution and presence of dark matter in these cosmic structures.
Vera Rubin: Vera Rubin was an American astronomer known for her pioneering work on the rotation curves of galaxies, which provided critical evidence for the existence of dark matter. Her research transformed our understanding of the Milky Way's structure and dynamics and influenced how galaxies are classified and studied, as well as how we perceive galaxy clusters and large-scale cosmic structures.