7.1 Cosmic web
7 min read•august 20, 2024
The , a vast network of filaments, walls, and voids, forms the of the universe. This intricate pattern reveals the distribution of matter and provides crucial insights into the nature of and dark energy.
Understanding the cosmic web is essential for grasping the universe's evolution. By studying galaxy clusters at intersections, superclusters, and cosmic voids, scientists can unravel the complex interplay of gravity and expansion shaping our cosmos.
Large-scale structure of universe
- The large-scale structure of the universe refers to the patterns and arrangements of galaxies and galaxy clusters on the largest observable scales
- Consists of a complex network of filaments, walls, and voids known as the cosmic web
- Provides crucial insights into the distribution of matter, the nature of dark matter and dark energy, and the evolution of the universe over cosmic time
Filaments, walls, and voids
- Filaments are long, thin, thread-like structures of galaxies and dark matter that span vast distances between galaxy clusters
- Walls, also known as sheets, are two-dimensional structures composed of galaxies and dark matter that connect filaments
- Voids are vast, underdense regions that contain few galaxies and are surrounded by filaments and walls
Dark matter distribution
- Dark matter, which accounts for approximately 85% of the matter in the universe, plays a crucial role in shaping the cosmic web
- Dark matter halos form the backbone of the cosmic web, with galaxies forming within these halos
- The distribution of dark matter traces the large-scale structure, with higher concentrations in filaments and walls and lower concentrations in voids
Gravitational effects on cosmic web
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- Dark matter's gravitational influence is the primary driver of the formation and evolution of the cosmic web
- Dark matter halos attract baryonic matter, leading to the formation of galaxies and galaxy clusters along filaments and at nodes
- The gravitational pull of dark matter shapes the geometry and density of the cosmic web, creating the observed filamentary and wall-like structures
Mapping dark matter filaments
- is used to map the distribution of dark matter in the cosmic web
- Weak lensing surveys measure the subtle distortions in the shapes of background galaxies caused by the gravitational influence of foreground dark matter
- By analyzing these distortions, astronomers can reconstruct the three-dimensional distribution of dark matter and trace the filamentary structure of the cosmic web
Galaxy clusters in nodes
- Galaxy clusters, the largest gravitationally bound structures in the universe, form at the nodes or intersections of filaments in the cosmic web
- These nodes are regions of high matter density where multiple filaments converge, creating ideal conditions for the formation of massive galaxy clusters
- Galaxy clusters contain hundreds to thousands of galaxies, hot intracluster gas, and large amounts of dark matter
Properties of galaxy clusters
- Galaxy clusters have typical masses ranging from to solar masses and sizes of several megaparsecs
- The hot intracluster medium, heated to temperatures of to Kelvin, emits X-rays and can be used to study the properties of clusters
- Galaxy clusters are powerful gravitational lenses, distorting the light from background galaxies and providing a means to study their mass distribution
Cluster vs field galaxies
- Galaxies within clusters (cluster galaxies) experience different environmental influences compared to galaxies in less dense regions (field galaxies)
- Cluster galaxies are more likely to be elliptical or S0 (lenticular) galaxies, while field galaxies are more often spiral galaxies
- Galaxy interactions and mergers are more common in clusters, leading to the transformation of spiral galaxies into elliptical or S0 galaxies over time
Superclusters and great walls
- Superclusters are the largest known structures in the universe, consisting of multiple galaxy clusters and groups connected by filaments and walls
- Great walls are extensive, sheet-like concentrations of galaxies and dark matter that can span hundreds of millions of light-years
- Superclusters and great walls are not gravitationally bound structures but represent the largest-scale inhomogeneities in the cosmic web
Shapley and Sloan Great Wall
- The Shapley is one of the largest known structures in the local universe, containing over 20 galaxy clusters and spanning ~400 million light-years
- The Sloan Great Wall, discovered using data from the Sloan Digital Sky Survey (SDSS), is a vast, wall-like structure of galaxies spanning ~1.37 billion light-years
- These immense structures demonstrate the scale and complexity of the cosmic web and provide insights into the large-scale distribution of matter in the universe
Supercluster properties and evolution
- Superclusters are composed of multiple galaxy clusters, groups, and filaments, forming a complex network of structures
- The evolution of superclusters is driven by the gravitational collapse of matter on large scales, with smaller structures merging to form larger ones over time
- Superclusters are not gravitationally bound and will eventually disperse due to the accelerating expansion of the universe driven by dark energy
Cosmic voids
- Cosmic voids are vast, underdense regions in the universe that contain few galaxies and are surrounded by filaments and walls
- Voids are the largest structures in the cosmic web, with typical diameters ranging from ~10 to ~100 megaparsecs
- The low density of voids results from the gravitational influence of dark matter, which causes matter to flow away from regions and accumulate in filaments and walls
Void shapes and sizes
- Voids exhibit a variety of shapes, including spherical, ellipsoidal, and irregular geometries
- The size distribution of voids follows a power law, with smaller voids being more common than larger ones
- The shape and size of voids are determined by the complex interplay of gravitational forces and the initial density fluctuations in the early universe
Galaxy properties in voids
- Galaxies found within voids (void galaxies) are typically smaller, less massive, and more isolated than galaxies in denser regions
- Void galaxies have higher specific star formation rates and are more likely to be blue, gas-rich, and disk-dominated compared to galaxies in filaments and clusters
- The unique properties of void galaxies provide insights into galaxy formation and evolution in low-density environments
Formation and evolution
- The formation and evolution of the cosmic web are governed by the gravitational instability of matter in an expanding universe
- Primordial density fluctuations in the early universe, seeded by quantum fluctuations during inflation, serve as the initial conditions for structure formation
- As the universe expands, these density fluctuations grow through gravitational instability, with overdense regions collapsing to form galaxies, clusters, and the cosmic web
Gravitational instability and collapse
- Gravitational instability is the process by which small density perturbations in the early universe grow over time due to the self-gravity of matter
- Overdense regions, where the density is slightly higher than the average, will attract more matter and collapse under their own gravity
- As overdense regions collapse, they form gravitationally bound structures such as dark matter halos, galaxies, and galaxy clusters
Hierarchical structure formation
- The formation of the cosmic web follows a hierarchical process, with smaller structures forming first and subsequently merging to create larger structures
- In the early universe, small dark matter halos form and merge to create larger halos, which then merge to form even larger structures such as galaxy clusters and superclusters
- This bottom-up process, known as hierarchical structure formation, is a key prediction of the cold dark matter (CDM) model and is supported by observations of the cosmic web
Cosmological implications
- The study of the cosmic web provides crucial insights into the nature of the universe, including the properties of dark matter and dark energy
- The large-scale structure of the universe is a sensitive probe of cosmological parameters, such as the matter density, the dark energy equation of state, and the initial conditions of the universe
- By comparing observations of the cosmic web with theoretical models and simulations, astronomers can test and refine our understanding of the fundamental physics governing the universe
Cosmic web as probe of cosmology
- The statistical properties of the cosmic web, such as the power spectrum and correlation functions, encode information about the underlying cosmology
- Measurements of the cosmic web, through galaxy surveys and weak lensing studies, can constrain key cosmological parameters and test theories of gravity and dark energy
- The evolution of the cosmic web over time, as traced by galaxies at different redshifts, provides a powerful tool for studying the expansion history of the universe and the growth of structure
Constraints on dark energy and matter
- The geometry and growth of the cosmic web are sensitive to the nature of dark energy, which drives the accelerating expansion of the universe
- By measuring the clustering of galaxies and the evolution of the cosmic web, astronomers can constrain the dark energy equation of state and test alternative models of gravity
- The properties of the cosmic web also depend on the nature of dark matter, such as its interaction cross-section and particle mass
- Comparing observations of the cosmic web with simulations based on different dark matter models can help distinguish between various dark matter candidates and provide insights into the nature of this mysterious component of the universe
Key Terms to Review (18)
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 early universe when it was just about 380,000 years old. This faint glow, detected in the microwave part of the electromagnetic spectrum, is crucial for understanding the formation and evolution of structures in the universe, linking various aspects of cosmology and astrophysics.
Cosmic Web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of galaxies, clusters, and superclusters interconnected by filaments of dark matter and gas, along with vast voids in between. This intricate structure highlights the distribution of matter and energy in the universe and plays a crucial role in understanding its formation and evolution.
Dark Matter: Dark matter is a mysterious and invisible substance that makes up about 27% of the universe's mass-energy content, playing a critical role in the formation and structure of galaxies. While it does not emit, absorb, or reflect light, its presence is inferred from its gravitational effects on visible matter and cosmic structures. Understanding dark matter is essential for explaining phenomena like the movement of stars in galaxies and the overall arrangement of the universe.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in establishing the field of extragalactic astronomy and is best known for Hubble's law, which describes the expansion of the universe. His work not only led to the classification of galaxies but also revolutionized our understanding of the cosmos, connecting various concepts like the cosmic web and the cosmological principle.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light profile and an ellipsoidal shape, often containing older stars with little to no ongoing star formation. These galaxies range from nearly spherical to elongated structures and are usually found in larger galaxy clusters. Their structure and composition connect them to various cosmic phenomena and the broader structure of the universe.
Filament: A filament refers to the massive, thread-like structures found in the cosmic web, composed primarily of dark matter and galaxies. These filaments connect clusters of galaxies and form a vast network that shapes the universe's large-scale structure, influencing how matter is distributed across cosmic distances. They are fundamental in understanding the formation and evolution of galaxies as they provide the pathways for gas and other materials to flow into galaxy clusters.
Galaxy cluster: A galaxy cluster is a large grouping of galaxies that are bound together by gravity. These clusters can contain anywhere from a few dozen to thousands of galaxies, along with hot gas and dark matter, making them the largest known structures in the universe. They play a significant role in understanding galaxy morphology and the overall cosmic web due to their interactions and the influence they exert on their environment.
Gravitational Lensing: Gravitational lensing is a phenomenon that occurs when a massive object, such as a galaxy or a cluster of galaxies, bends the light from a more distant object due to its gravitational field. This effect not only magnifies and distorts the image of the background object but can also provide crucial information about the mass and distribution of dark matter in the lensing object, connecting it to various cosmic structures and dynamics.
Hierarchical formation: Hierarchical formation refers to the process in which structures, such as galaxies, develop through a series of merging events, creating larger systems from smaller components. This concept explains how smaller galaxies and star systems come together over time to form larger and more complex galaxies, revealing insights into the evolution of cosmic structures. It plays a crucial role in understanding the formation and evolution of different galaxy types, the interactions between galaxies, and the large-scale structure of the universe.
Inflationary theory: Inflationary theory proposes that the universe underwent a rapid exponential expansion just after the Big Bang, driven by a high-energy field known as 'inflaton.' This theory helps explain several observations about the universe, such as its large-scale structure, uniformity, and the distribution of galaxies, connecting it to concepts like the cosmic web, voids, cosmological probes, and large-scale structure surveys.
Lambda cold dark matter model: The lambda cold dark matter model, often abbreviated as \( \Lambda CDM \), is the prevailing cosmological model that describes the large-scale structure and evolution of the universe. It combines the effects of a cosmological constant, denoted by \( \Lambda \), which represents dark energy, with cold dark matter, a non-baryonic form of matter that interacts only through gravity, playing a crucial role in shaping cosmic structures like galaxies and clusters.
Large-scale structure: Large-scale structure refers to the organization and distribution of matter in the universe on scales larger than individual galaxies, encompassing clusters, superclusters, and the cosmic web. This framework helps us understand how galaxies and other cosmic structures form and evolve under the influence of gravitational forces and dark matter.
Merger: A merger is the process where two or more galaxies combine to form a single, larger galaxy. This phenomenon plays a crucial role in galaxy evolution, as mergers can trigger star formation, alter galaxy morphology, and influence the overall dynamics of galaxies within their environment.
Spectroscopy: Spectroscopy is the study of the interaction between light and matter, particularly focusing on how light is absorbed, emitted, or scattered by atoms and molecules. This technique allows astronomers to analyze the composition, temperature, density, and motion of celestial objects, providing crucial insights into their physical properties and behaviors.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its distinct spiral arms that wind outward from a central bulge, typically containing a mix of young and old stars, gas, and dust. These galaxies are often rich in star formation, particularly in the arms, and are one of the most common galaxy types observed in the universe. Their structure and formation provide insights into the evolutionary processes of galaxies and their environments.
Supercluster: A supercluster is a massive group of galaxies that are gravitationally bound together, typically containing dozens or even hundreds of galaxy clusters. Superclusters are among the largest known structures in the universe and play a crucial role in understanding the large-scale structure of the cosmos, particularly in relation to the cosmic web and the distribution of galaxies across vast distances.
Vera Rubin: Vera Rubin was an influential American astronomer known for her pioneering work on the rotation curves of galaxies, which provided critical evidence for the existence of dark matter. Her groundbreaking observations helped reshape our understanding of galaxy dynamics and the structure of the universe, connecting closely to various concepts including galaxy formation and the cosmic web.
Void: A void refers to vast, relatively empty regions of space that contain few or no galaxies, typically found in the large-scale structure of the universe. These expansive areas are crucial in understanding the distribution of galaxies and their environments, highlighting how matter is organized in the cosmic web, where galaxies are often found along filaments and in clusters while voids remain largely unpopulated.