11.1 Cosmic web: filaments, sheets, and voids
4 min read•july 22, 2024
The cosmic web, a vast network of galaxies and , shapes the universe's large-scale structure. , , and form intricate patterns, with dark matter playing a crucial role. This invisible substance acts as a gravitational scaffold, guiding the formation of galaxies and clusters.
Evidence for the cosmic web comes from galaxy surveys, , and the Lyman-alpha forest. The structure evolves over time, starting with and growing through . Dark matter's influence is key, driving the formation and evolution of this cosmic tapestry.
The Cosmic Web
Components of cosmic web
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- Cosmic web is large-scale structure of universe consisting of galaxies, , and dark matter
- Filaments
- Elongated, thread-like structures of galaxies and dark matter connect galaxy clusters forming backbone of cosmic web
- Serve as pathways for gas and galaxies to flow towards nodes (intersections of filaments)
- Sheets
- Planar concentrations of galaxies and dark matter less dense than filaments often found between them
- Form at intersections of voids and can feed material into filaments
- Voids
- Vast, underdense regions with few galaxies occupy majority of volume in universe
- Surrounded by filaments and sheets
- Can span hundreds of millions of light-years in diameter (Boötes void, Eridanus Supervoid)
- Filaments
Dark matter's structural influence
- Dark matter accounts for ~85% of matter in universe interacting gravitationally but not electromagnetically playing crucial role in formation and evolution of cosmic web
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- Concentrations of dark matter surround galaxies and galaxy clusters providing gravitational scaffolding for cosmic web
- Serve as seeds for and influence gas accretion and star formation in galaxies
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- Act as highways along which galaxies and gas flow towards nodes contributing to formation of galaxy clusters at intersections
- Contain majority of dark matter in universe and can extend for hundreds of millions of light-years (Sloan Great Wall, Hercules-Corona Borealis Great Wall)
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Evidence for cosmic web
- Galaxy
- Map 3D distribution of galaxies using redshifts as proxy for distance revealing filamentary and sheet-like structures
- Examples include 2dF Galaxy Redshift Survey, Sloan Digital Sky Survey (SDSS), VIMOS Public Extragalactic Redshift Survey (VIPERS)
- Weak gravitational lensing
- Distortion of background galaxy images by gravitational influence of foreground matter allows mapping of dark matter distribution
- Surveys like Dark Energy Survey (DES) and Kilo-Degree Survey (KiDS) provide detailed maps of cosmic web
- Lyman-alpha forest
- Absorption features in spectra of distant quasars caused by neutral hydrogen gas along line of sight trace distribution of gas following dark matter structure
- Used to map intergalactic medium and study properties of cosmic web at high redshifts
Evolution of cosmic web
- Primordial density fluctuations
- Quantum fluctuations in early universe amplified by cosmic inflation seed formation of cosmic web
- Determine initial conditions for structure formation and set scale for future growth
- Gravitational instability
- Regions with slightly higher density than average expand more slowly due to self-gravity leading to collapse of matter into sheets, filaments, and halos
- Overdense regions continue to accrete matter from surroundings enhancing density contrast
- Hierarchical structure formation
- Smaller structures like galaxies form first and merge to create larger structures such as galaxy clusters
- Cosmic web becomes more pronounced and defined over time as structures grow and merge
- Future evolution
- Cosmic web will continue to evolve with galaxies and gas flowing along filaments towards nodes
- Voids will expand and become more underdense while filaments and nodes will become denser
- Eventually, dark energy may cause cosmic web to freeze in place as expansion accelerates and structures become isolated
Dark Matter and the Cosmic Web
Dark matter's structural influence
- Gravitational influence
- Dark matter's gravitational effects are primary driver of cosmic web formation with baryonic matter (gas and stars) following gravitational potential wells created by dark matter
- Responsible for depth and shape of potential wells that determine properties of galaxies and clusters
- Dark matter collapse
- Dark matter undergoes gravitational collapse earlier than baryonic matter due to lack of pressure support forming filaments and halos that define cosmic web
- Collapse occurs in stages with smaller halos forming first and merging to create larger structures over time
- Density distribution
- Distribution of dark matter determines location and size of voids, sheets, and filaments
- Higher density regions correspond to filaments and nodes while lower density regions form voids
- Density contrast between voids and filaments can reach factors of 10-100 or more
Key Terms to Review (23)
Alan Guth: Alan Guth is a prominent theoretical physicist and cosmologist best known for proposing the inflationary universe model in the 1980s, which provides a solution to several problems in cosmology, including the uniformity of the cosmic microwave background radiation. His work has significant implications for our understanding of quantum fluctuations, structure formation in the universe, the cosmic web, and the standard ΛCDM model.
Baryon Acoustic Oscillations: Baryon acoustic oscillations refer to the regular, periodic fluctuations in the density of baryonic matter (normal matter) in the early universe, which arose from the interplay between gravity and pressure waves in the primordial plasma. These oscillations left an imprint on the large-scale structure of the universe, influencing galaxy formation and distribution.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe and providing a snapshot of the early cosmos when it was just 380,000 years old. This faint glow, almost uniform across the sky, carries crucial information about the universe's formation, composition, and expansion, connecting various areas of cosmological research and theories.
Cosmic Voids: Cosmic voids are large, empty regions of space that contain very few galaxies and are a significant part of the universe's large-scale structure. They are surrounded by denser regions known as filaments and sheets, forming a vast cosmic web that defines the distribution of matter in the universe. These voids can stretch tens of millions of light-years across, contributing to our understanding of cosmic evolution and the dynamics of dark energy and matter.
Dark Matter: Dark matter is an unseen form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. It plays a crucial role in the structure and evolution of the universe, influencing galaxy formation, cosmic expansion, and the distribution of galaxies within the cosmic web.
Dark matter filaments: Dark matter filaments are vast, thread-like structures in the cosmic web that contain a significant amount of dark matter, connecting clusters and superclusters of galaxies. These filaments are part of the larger cosmic architecture, serving as highways for galaxies and influencing their formation and distribution throughout the universe.
Dark matter halos: Dark matter halos are large, invisible structures composed primarily of dark matter that surround galaxies and galaxy clusters, providing the necessary gravitational pull to hold visible matter together. These halos play a vital role in shaping the cosmic web, influencing the formation of filaments and sheets while also creating vast voids between them. Understanding dark matter halos helps explain how galaxies form and evolve within the larger structure of the universe.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in the development of modern cosmology, particularly known for discovering that the universe is expanding. His work provided crucial evidence for the Big Bang theory and established the relationship between redshift and distance, transforming our understanding of the cosmos.
Filaments: Filaments are massive, thread-like structures in the universe that form part of the cosmic web, connecting galaxy clusters and superclusters. These structures are critical in understanding the large-scale distribution of matter and energy in the universe, as they define the locations of galaxies and dark matter. Filaments are surrounded by vast voids and are interspersed with sheets of galaxies, creating a complex network that influences the formation and evolution of cosmic structures.
Galaxy clusters: Galaxy clusters are large groups of galaxies bound together by gravity, typically containing dozens to thousands of individual galaxies, along with gas, dust, and dark matter. These clusters are essential for understanding the large-scale structure of the universe, as they serve as key points in the cosmic web and provide insights into the distribution of mass and the effects of dark matter and dark energy.
Galaxy formation: Galaxy formation is the process by which galaxies develop and evolve from initial density fluctuations in the early universe. This complex journey involves the accumulation of gas and dark matter, leading to the birth of stars and the establishment of galactic structures like spiral arms and bulges. Understanding this process sheds light on the cosmic timeline and the large-scale structure of the universe.
Gravitational Instability: Gravitational instability refers to the process by which small density fluctuations in the universe, driven by gravitational forces, grow over time to form larger structures like galaxies and galaxy clusters. This phenomenon is crucial for understanding how matter clumps together under gravity's influence, leading to the formation of the cosmic web of structures we observe today.
Gravitational Lensing: Gravitational lensing is the phenomenon where the light from a distant object, such as a galaxy or quasar, is bent around a massive object, like a galaxy cluster, due to the effects of gravity. This bending of light can create multiple images, magnify the brightness of the source, and provide valuable insights into the distribution of mass in the universe, especially dark matter and its role in cosmic structure.
Hierarchical clustering: Hierarchical clustering is a method of organizing data into a tree-like structure based on similarity, where data points are grouped in a way that reflects their relationships. This approach is vital for understanding the large-scale structure of the universe, as it helps visualize how small fluctuations evolve into larger structures and how galaxies and galaxy clusters are interconnected, forming a cosmic web.
Inflation Theory: Inflation Theory is a cosmological model that proposes a rapid exponential expansion of the universe during the first moments after the Big Bang, occurring roughly between 10^{-36} and 10^{-32} seconds. This theory explains the large-scale structure of the universe, including the distribution of galaxies and the existence of cosmic web features like filaments, sheets, and voids. By addressing key problems such as the flatness and horizon issues, inflation helps to describe how the universe evolved into its current state, leading to the observed patterns of matter and energy.
Lambda cold dark matter model: The lambda cold dark matter (ΛCDM) model is the leading cosmological model that describes the large-scale structure and evolution of the universe. It incorporates the effects of dark energy (represented by the cosmological constant lambda, Λ) and cold dark matter, which together account for the observed phenomena in the universe such as galaxy formation, cosmic expansion, and the cosmic web.
Merger history: Merger history refers to the sequence of events and interactions that lead to the merging of cosmic structures, such as galaxies, over time. This process is crucial for understanding the evolution of the universe, as it helps explain the large-scale structures observed today, including filaments, sheets, and voids that make up the cosmic web.
Primordial density fluctuations: Primordial density fluctuations refer to small variations in the density of matter in the early universe, which are thought to be the seeds for the large-scale structure we observe today. These fluctuations are crucial for understanding how matter clumped together to form galaxies, stars, and other cosmic structures, and they play a significant role in models explaining cosmic evolution, the cosmic web, and baryon acoustic oscillations.
Redshift surveys: Redshift surveys are observational studies that map the distribution of galaxies in the universe by measuring their redshifts, which indicate how fast they are moving away from us due to the expansion of the universe. By analyzing these redshifts, astronomers can determine the distance and velocity of galaxies, helping to reveal the large-scale structure of the universe, including cosmic filaments, sheets, and voids, as well as the role of dark matter in structure formation.
Sheets: In the context of cosmology, sheets refer to large, flat structures in the cosmic web that are formed by the gravitational attraction of dark matter and baryonic matter. These sheets are part of the intricate arrangement of galaxies, filaments, and voids that shape the universe on a grand scale, acting as surfaces where galaxies and galaxy clusters are densely concentrated.
Structure Formation: Structure formation refers to the process by which matter in the universe organizes into structures such as galaxies, clusters of galaxies, and the large-scale cosmic web. This concept is crucial for understanding how the universe evolved from a homogeneous state after the Big Bang into the rich, complex structures we observe today, influenced by dark matter, dark energy, gravity, and various physical laws.
Superclusters: Superclusters are massive groupings of galaxies, often consisting of dozens to hundreds of galaxy clusters that are gravitationally bound together. These structures are part of the larger cosmic web, which includes filaments, sheets, and voids, highlighting the complex arrangement of matter in the universe. Superclusters represent some of the largest known structures in the cosmos and help astronomers understand the distribution of galaxies and the evolution of the universe.
Voids: Voids are large, nearly empty regions in the universe that exist between clusters of galaxies and other large-scale structures. These expansive spaces are critical to understanding the distribution of matter and energy in the cosmos, revealing a significant aspect of the universe's overall structure and evolution.