Filaments, “walls,” and huge void regions extending on massive scales – these reflect the seeds of early density fluctuations
Observing the night sky, the billions of stars we see mostly belong to our own Milky Way. But beyond our galaxy lies an even broader spectacle – the cosmic web – a vast 'fabric' of galaxy clusters, filaments, and empty spaces stretching across hundreds of millions of light-years. This large-scale structure arises from tiny density fluctuations in the early Universe, amplified over cosmic time by gravity.
In this article, we discuss how galaxy clusters form, how they fit into the cosmic web of filaments and “walls,” and the nature of the vast voids between them. By understanding how matter is distributed on the largest scales, we reveal key aspects of the Universe's evolution and structure.
1. Formation of large-scale structures
1.1 From primordial fluctuations to the cosmic web
Shortly after the Big Bang, the Universe was extremely hot and dense. Tiny quantum fluctuations, possibly arising during inflation, created slight over- and under-dense regions in the nearly uniform matter and radiation. Later, dark matter began to cluster around these excess regions; as the Universe expanded and cooled, baryonic matter (ordinary matter) sank into the dark matter's “gravitational wells,” accentuating density differences.
This is how the cosmic web we know today was formed:
- Filaments: Long, narrow strands of galaxies and galaxy groups stretched along the 'spines' of dark matter.
- Sheets (“Walls”): Two-dimensional structures located between filaments.
- Voids: Huge, low-density regions with few galaxies; they occupy most of the Universe's volume.
1.2 The ΛCDM framework
The most accepted cosmological model ΛCDM (Lambda Cold Dark Matter) states that dark energy (Λ) drives the accelerated expansion of the Universe, while non-relativistic (cold) dark matter dominates structure formation. In this scenario, structures form hierarchically — smaller halos merge into larger ones, creating the large-scale structures we observe. The distribution of galaxies on these scales closely matches modern cosmic simulations, confirming ΛCDM predictions.
2. Galaxy clusters: giants of the cosmic web
2.1 Definition and properties
Galaxy clusters – the most massive gravitationally bound structures in the Universe, typically hosting hundreds or even thousands of galaxies across several megaparsecs. Key features:
- Lots of dark matter: ~80–90% of the cluster's mass consists of dark matter.
- Hot intracluster medium (ICM): X-ray observations reveal enormous amounts of hot gas (107–108 K) filling the space between galaxies.
- Gravitational binding: The total mass is sufficient for members to remain bound despite the expansion of the Universe, so a cluster is a kind of "closed system" over cosmic timescales.
2.2 Formation through hierarchical growth
Clusters grow by accreting smaller groups and merging with other clusters. This continues in the present epoch. Since clusters form at the nodes of the cosmic web (where filamentary structures intersect), they become the "cities" of the Universe, and the surrounding branches (filaments) supply them with matter and galaxies.
2.3 Observation methods
There are several ways astronomers detect and study galaxy clusters:
- Optical surveys: Large redshift surveys, such as SDSS, DES, or DESI, search for large galaxy concentrations.
- X-ray observations: Hot intracluster gas emits intense X-rays, so the Chandra and XMM-Newton missions are especially important for detecting clusters.
- Gravitational lensing: The enormous mass of a cluster bends the light of background objects, providing an independent way to determine the total mass of the cluster.
Clusters act as important cosmic laboratories – by measuring their number and distribution at different epochs, fundamental cosmological parameters can be obtained (e.g., the amplitude of density fluctuations σ8, matter density Ωm, and properties of dark energy).
3. Cosmic web: filaments, "sheets," and voids
3.1 Filaments: matter highways
Filaments – elongated, string-like structures of dark matter and baryons that direct the movement of galaxies and gas toward cluster centers. They can range from a few to tens or hundreds of megaparsecs. Along these filaments, smaller galaxy groups and clusters "hang" like "beads on a string," where mass is further concentrated at intersections.
- Density contrast: In filaments, the density exceeds the cosmic average by several or tens of times, although they are not as dense as clusters.
- Flow of gas and galaxies: Gravity causes gas and galaxies to move along filaments toward massive nodes (clusters).
3.2 "Sheets" or "Walls"
Sheets (or "Walls"), located between filaments, are large-scale two-dimensional structures. Some observed cases, such as the Great Wall, extend over hundreds of megaparsecs. Although not as narrow or dense as filaments, they connect areas between sparser strands and voids.
3.3 Voids: cosmic "cavitation" regions
Voids – huge, almost empty spaces where the number of galaxies is much lower compared to filaments or clusters. Their size can reach tens of megaparsecs, occupying most of the volume of the Universe but containing only a small fraction of its mass.
- Structure in voids: Voids are not completely empty. They also contain dwarf galaxies or small filaments, but densities can be ~5–10 times lower than average.
- Cosmological significance: Voids are sensitive to the nature of dark energy, alternative gravity models, and small-scale density fluctuations. Recently, voids have become a new frontier for testing deviations from the standard ΛCDM.
4. Evidence supporting the cosmic web
4.1 Galaxy redshift surveys
Large-scale redshift surveys conducted in the late 1970s and early 1980s (e.g. CfA Redshift Survey) revealed “Great Walls” of galaxy clusters and voids, now called voids. Current larger programs like 2dFGRS, SDSS, DESI have surveyed millions of galaxies, leaving no doubt that their distribution matches the web pattern created by cosmic simulations.
4.2 Cosmic microwave background (CMB)
CMB anisotropy studies (Planck, WMAP, and earlier missions) confirm initial fluctuation properties. When these fluctuations are evolved forward in time in simulations, they grow into the cosmic web pattern. The high precision of CMB measurements allows determining the nature of density seeds shaping large-scale structure.
4.3 Gravitational lensing and weak lensing
Weak lensing studies track subtle distortions of background galaxy shapes caused by intervening matter. CFHTLenS, KiDS, and other projects revealed that mass distributes according to the same web pattern traced by galaxy locations, further confirming that dark matter on large scales arranges similarly to baryons.
5. Theoretical and simulation approaches
5.1 N-body simulations
Dark matter N-body simulations naturally highlight the cosmic web “skeleton,” where billions of particles gravitationally collapse forming halos and filaments. Key highlights:
- Emergence of the “Web”: Filaments connect through dense regions (clusters, groups), reflecting gravitational flow dynamics from outer areas.
- Voids: Form in underdense regions where matter flows repel material, further emphasizing the voids.
5.2 Hydrodynamics and galaxy formation
Adding hydrodynamics (gas physics, star formation, feedback) to N-body codes better reveals how galaxies distribute in the cosmic web:
- Filamentary gas inflow: In many simulations, cool gas flows along filaments into forming galaxies, promoting star formation.
- Feedback effects: Supernovae and AGN outflows can disturb or heat the inflowing gas, modifying the local network structure.
5.3 Remaining issues
- Small-scale issues: Phenomena such as the core-cusp problem and the "too-big-to-fail" problem highlight discrepancies between ΛCDM predictions and observations of some local galaxies.
- Cosmic voids: Detailed modeling of void dynamics and the smaller structures within them remains an active area of research.
6. Evolution of the cosmic web over time
6.1 The early epoch: high redshifts
Just after reionization (z ∼ 6–10), the cosmic web was not yet so prominent, but still visible from the distribution of small halos and emerging galaxies. Filaments may have been narrower and sparser, but they still channeled gas flows toward protogalactic centers.
6.2 The maturing web: intermediate redshifts
At about z ∼ 1–3, filamentary structures are much more pronounced, feeding rapidly star-forming galaxies. Clusters form quickly, merging into increasingly massive systems.
6.3 The present epoch: nodes and void expansion
Today we see mature clusters as nodes in the web, while voids have significantly expanded under the influence of dark energy. Many galaxies reside in dense filaments or cluster environments, but some remain isolated deep in voids, evolving along very different paths.
7. Galaxy clusters as cosmological probes
Because galaxy clusters are the most massive bound structures, their abundance at different epochs of the Universe is very sensitive:
- Density of dark matter (Ωm): More matter means more intense cluster formation.
- Amplitude of density fluctuations (σ8): Stronger fluctuations lead to faster formation of massive halos.
- Dark energy: It affects the growth rate of structures. If there is more dark energy in the Universe, clusters form more slowly at later times.
Thus, observational data of galaxy clusters, i.e., their number, mass (measured by X-rays, lensing, or the Sunyaev–Zel’dovich effect), and evolution with redshift allow for determining robust cosmological parameters.
8. The cosmic web and galaxy evolution
8.1 Environmental conditions
The cosmic web environment strongly influences galaxy evolution:
- In cluster centers: Large velocity differences, ram pressure stripping, and mergers often quench star formation, so these regions are rich in large elliptical galaxies.
- "Feeding" from filaments: Spiral galaxies can continue to actively form stars if they constantly receive new gas from filaments.
- Void galaxies: Isolated, slower evolving, retaining gas longer and continuing star formation into the cosmic future.
8.2 Chemical enrichment
Galaxies forming in dense nodes experience many starbursts and feedbacks, ejecting metals into the intercluster medium or filaments. Even void galaxies are somewhat enriched through sporadic outflows or cosmic flows, though more slowly than in denser regions.
9. Future directions and observations
9.1 Next-generation large surveys
LSST, Euclid, and the Nancy Grace Roman Space Telescope will study billions of galaxies, providing an extremely precise 3D map of the cosmic fabric. Enhanced lensing data will allow even clearer determination of how dark matter is distributed.
9.2 Deep filament and void observations
Detection of the Warm–Hot Intergalactic Medium (WHIM) in filaments still poses challenges. Future X-ray missions (e.g., Athena) and improved UV or X-ray spectroscopy may reveal the mist of gas bridges between galaxies, ultimately showing the "missing baryons" in the cosmic web.
9.3 Precision void cosmology
The field of void cosmology is also developing, aiming to use void properties (size distribution, shapes, velocity flows) to test alternative gravity theories, dark energy models, and other non-ΛCDM variants.
10. Conclusion
Galaxy clusters, visible in the nodes of the cosmic web, along with filaments, "sheets," and voids arranged between them, form the Universe's "construction" on the largest scales. These structures originated from small density fluctuations in the early Universe, amplified by gravity acting on dark matter and the expansion caused by dark energy.
Today we see a dynamic cosmic web, full of huge clusters, tangled filaments containing many galaxies, and vast, almost empty voids. These enormous "construction" forms not only reflect the importance of gravitational laws on intergalactic scales but are also essential for testing cosmological models and our understanding of how galaxies evolve in the densest or sparsest regions of the Universe.
Links and further reading
- Bond, J. R., Kofman, L., & Pogosyan, D. (1996). "How filaments are woven into the cosmic web." Nature, 380, 603–606.
- de Lapparent, V., Geller, M. J., & Huchra, J. P. (1986). "A slice of the universe." The Astrophysical Journal Letters, 302, L1–L5.
- Springel, V., et al. (2005). "Simulations of the formation, evolution and clustering of galaxies and quasars." Nature, 435, 629–636.
- Cautun, M., et al. (2014). "The cold dark matter cosmic web." Monthly Notices of the Royal Astronomical Society, 441, 2923–2944.
- Van de Weygaert, R., & Platen, E. (2011). "Cosmic Voids: Structure, Dynamics and Galaxies." International Journal of Modern Physics: Conference Series, 1, 41–66.