Cosmic Voids & Intergalactic Filaments
Cosmic voids are vast, roughly spherical underdense regions 20 to 100 megaparsecs across whose matter density is 10% to 20% of the cosmic mean. Together they occupy nearly 80% of the volume of the observable universe while containing less than 20% of its mass, because gravity continuously evacuates matter out of underdense regions and onto surrounding sheets and warm-hot intergalactic medium (WHIM) filaments.
Physical Mechanism & Dynamics
An underdense region in the early universe acts like an effective negative mass perturbation relative to the background density: it has a slightly lower local gravitational deceleration than the cosmic average, so its interior expands faster than the background Hubble flow. Matter inside the void streams outward toward the void boundary at 50–300 km/s, piling up into dense ridges where adjacent expanding voids collide to form cosmic sheets and filaments.
As a result, voids gradually become more spherical as they expand—the exact inverse of triaxial cluster collapse (Icke's theorem). Meanwhile, the filaments bordering voids collect diffuse baryonic gas shock-heated to 10⁵–10⁷ K, known as the warm-hot intergalactic medium (WHIM), which accounts for the long-sought 'missing baryons' in the low-redshift universe.
Quantitatively, this mechanism is governed by Void Outflow Peculiar Velocity (Linear Regime) (v_pec(r) = −(1 / 3) H(z) f(Ω_m) Δ(r) r, where f(Ω_m) ≈ Ω_m(z)^{0.55}). For an interior underdensity Δ(r) ≈ −0.8, v_pec points radially outward toward the surrounding filament wall.
Void Outflow Peculiar Velocity (Linear Regime)
v_pec(r) = −(1 / 3) H(z) f(Ω_m) Δ(r) r, where f(Ω_m) ≈ Ω_m(z)^{0.55}For an interior underdensity Δ(r) ≈ −0.8, v_pec points radially outward toward the surrounding filament wall.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Typical cosmic void diameter | 20 to 100 Mpc (65 to 325 Mly) | SDSS / BOSS watershed void catalogs (ZOBOV/VIDE) |
| Interior matter density (ρ_void / ρ̄) | 0.10 to 0.25 of cosmic mean | Weak lensing + galaxy counts in voids |
| Volume fraction of voids | ≈ 75%–80% of cosmic volume | N-body ΛCDM simulations + redshift surveys |
| Filament WHIM temperature | 10^5 to 10^7 K | O VI / O VII absorption & fast radio burst dispersion |
Observational Evidence & How Scientists Know
Redshift surveys identify thousands of voids using 3D Voronoi tessellation and watershed algorithms (such as ZOBOV). Because voids are nearly isotropic in real comoving space, measuring their apparent elongation along the line of sight versus across the sky provides a clean Alcock-Paczyński geometric test of dark energy and spatial curvature.
Photons from the cosmic microwave background passing through evolving voids experience a slight net gravitational redshift—the Integrated Sachs-Wolfe (ISW) effect—because dark energy causes the void's shallow gravitational potential to decay while the photon traverses it. Meanwhile, Fast Radio Bursts (FRBs) and X-ray absorption spectra confirm the ionized baryon column density along intergalactic filaments.
Across reference datasets (Planck 2018 Cosmological Parameters (PR3), SIMBAD Astronomical Database (CDS)), quantitative benchmarks for cosmic voids & intergalactic filaments are anchored by: Typical cosmic void diameter (20 to 100 Mpc (65 to 325 Mly), via SDSS / BOSS watershed void catalogs (ZOBOV/VIDE)); Interior matter density (ρ_void / ρ̄) (0.10 to 0.25 of cosmic mean, via Weak lensing + galaxy counts in voids); Volume fraction of voids (≈ 75%–80% of cosmic volume, via N-body ΛCDM simulations + redshift surveys); Filament WHIM temperature (10^5 to 10^7 K, via O VI / O VII absorption & fast radio burst dispersion).
Uncertainties, Systematics & Open Questions
Void boundaries depend on the tracer galaxy population and minimum luminosity threshold used to define the watershed, and the exact amplitude of the Integrated Sachs-Wolfe cold-spot signal in stacked supervoids remains an active observational test of ΛCDM.
Real Astronomical Benchmarks & Cross-Domain Links
Local Void
Adjacent ~45 Mpc underdense region bordering the Local Group that pushes our galaxy sheet at ~260 km/s.
Baryonic & Dark Inventory
Accounts for the WHIM filament gas that resolves the low-redshift missing baryon budget.
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- Planck 2018 Cosmological Parameters (PR3) (PR3 (2020 A&A 641, A6)) — Planck Collaboration, Aghanim, N., et al. 2020, A&A, 641, A6. Archive / Reference ↗
- SIMBAD Astronomical Database (CDS) (2026-10 release) — Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database). Archive / Reference ↗
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