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Local Void

The Local Void is a vast, low-density region in the local universe characterized by a paucity of galaxies and matter, bordering the Local Group of which the Milky Way is a part, and discovered in 1987 by astronomers R. Brent Tully and Richard Fisher through analysis of nearby galaxy distributions.^[1] It begins just 1 megaparsec (about 3.26 million light-years) away at the fringe of the Local Group, with its irregular extent spanning up to roughly 125 megaparsecs (about 407 million light-years) across in mapped dimensions.^[2]^[3] Lying predominantly behind the Milky Way's galactic disk toward the constellations of Virgo and Centaurus, the Local Void has long been obscured by interstellar dust and stars, complicating direct observations of its structure.^[4] Advancements, including the 2019 Cosmicflows-3 survey led by Tully and the updated 2023 Cosmicflows-4 survey, have utilized peculiar velocity measurements of over 55,000 galaxies to construct three-dimensional cosmographic maps, revealing the void as an expansive basin bounded by prominent filamentary superclusters such as the Perseus-Pisces and Norma-Pavo-Indus structures.^[2]^[5] This underdense volume, potentially expanding faster than the surrounding cosmic web due to reduced gravitational binding, exerts a significant dynamical influence on the Local Group's motion, contributing 200–250 km/s to its peculiar velocity away from the void and toward the Great Attractor.^[2] In broader cosmological contexts, the Local Void highlights the large-scale inhomogeneities in the universe's structure, aiding studies of cosmic expansion and relating to discrepancies in Hubble constant measurements through recent local void models (as of 2025).^[6]^[7]

Discovery and Characterization

Initial Discovery

The concept of cosmic voids as low-density regions within the large-scale structure of the universe emerged in the late 1970s, with pioneering studies identifying significant underdensities in galaxy distributions. In 1978, Stephen A. Gregory and Laird A. Thompson analyzed redshift data from the Kitt Peak National Observatory, revealing large empty regions devoid of bright galaxies, which they interpreted as voids spanning tens of megaparsecs. Concurrently, Mikhel Jõeveer, Jaan Einasto, and Erik Tago examined southern galactic hemisphere data, confirming the existence of extensive voids alongside superclusters, thus establishing voids as a fundamental component of the cosmic web. The Local Void, a particularly prominent underdensity adjacent to the Local Group, was specifically identified in 1987 by astronomers R. Brent Tully and J. Richard Fisher. Utilizing redshift surveys, including data from the Center for Astrophysics (CfA) Redshift Survey, they mapped nearby galaxy distributions and detected a vast region of low galaxy density extending from the Local Group. This discovery was detailed in their Nearby Galaxies Atlas, which compiled optical and radio observations to visualize the structure. Initial assessments placed the Local Void approximately 40-50 Mpc across, with its extent primarily toward the constellations of Virgo and Centaurus, bordering the Milky Way and encompassing a significant portion of the southern celestial sky. To confirm its relative emptiness, Tully and Fisher incorporated radio astronomy observations, particularly neutral hydrogen (HI) mapping, which revealed sparse detections of gas-rich galaxies in the region, supporting the underdense nature inferred from optical redshift data.

Modern Observations and Mapping

In the decades following its initial identification, surveys conducted in the 2000s significantly enhanced the mapping of the Local Void's boundaries through large-scale galaxy redshift observations. The 6dF Galaxy Survey (6dFGS), completed in 2006, provided spectroscopic data for over 125,000 galaxies across the southern sky, enabling detailed density field reconstructions that highlighted the void's underdense extent in the local volume up to approximately 15,000 km/s.^[8] Similarly, the Sloan Digital Sky Survey (SDSS), with its early data releases from 2000 onward, contributed photometric and spectroscopic measurements of millions of galaxies, refining the void's edges by identifying sharp transitions to surrounding filaments and walls through galaxy count contrasts.^[8] A pivotal advancement came in 2019 with the Cosmicflows-3 catalog, where Tully et al. utilized peculiar velocity measurements from over 17,000 galaxies to generate a comprehensive 3D map of the Local Void. This study revealed the void's irregular, elongated shape, with extensions reaching up to roughly 125 Mpc in certain directions, by modeling gravitational influences on galaxy motions to delineate underdense regions from nearby overdensities like the Virgo Cluster.^[2] Observations in 2022 further uncovered substructures within the Local Void through targeted studies of dwarf galaxies, suggesting the presence of faint filaments and walls that challenge the notion of complete emptiness. Analysis of isolated dwarf galaxies, such as those exhibiting unexpected kinematic behaviors, indicated embedded matter distributions forming tenuous connections across the void, potentially linking to broader cosmic web elements.^[9] Recent 2025 analyses incorporating baryon acoustic oscillation (BAO) measurements from ground-based telescopes have provided the most precise refinements to the Local Void's edges to date. By detecting faint echoes of the Big Bang in galaxy clustering patterns from surveys like SDSS and 6dFGS, these studies confirmed underdensity levels of 10-20% below the cosmic average, enabling sharper boundary definitions and validation of the void's impact on local expansion rates.^[10]

Physical Characteristics

Location and Extent

The Local Void is positioned adjacent to the Local Group, immediately neighboring our home supercluster structure and beginning approximately 1 Mpc from the Milky Way galaxy. The Milky Way resides on the edge of this underdense region as part of the Local Sheet, a flattened array of galaxies spanning roughly 10 Mpc in thickness that serves as one of the void's bounding walls. This proximity places the void in direct contact with the local cosmic environment, influencing the distribution of nearby matter.^[2]^[11] In terms of angular position, the void's center is approximately at right ascension 19^h and declination +3°, extending across a significant portion of the sky largely obscured by the Milky Way's disk. The nearer portions correspond to redshift distances from z ≈ 0.001 to z ≈ 0.003, reflecting recessional velocities of a few hundred km/s. These coordinates highlight the void's location in the general direction of the constellations Aquila and Hercules, with parts reaching toward Eridanus.^[12]^[2] The extent of the Local Void is irregular, with a characteristic diameter of approximately 60 Mpc (about 200 million light-years), though estimates range from 45 to 60 Mpc based on isodensity contours. Its dimensions in supergalactic coordinates measure roughly 69 × 51 × 60 Mpc, with one near edge located about 23 Mpc from the Local Group and the far side extending up to around 70 Mpc. This scale underscores its role as a major underdensity in the local universe.^[2] The Local Void borders the Virgo Supercluster to one side, separated by filamentary structures such as the Perseus-Pisces chain, and integrates into the broader cosmic web as a key node in the filament-void network. This positioning embeds the void within the hierarchical structure of large-scale cosmic features, where it contrasts with denser supercluster concentrations.^[2]

Density Profile and Structure

The Local Void exhibits a density profile characterized by significant underdensities, with the core regions showing density contrasts δ ≈ -1.1 to -1.89 relative to the cosmic mean, corresponding to local densities of approximately 10-20% of the average universe.^[2] This near-total absence of bright galaxies in the interior gives way to a gradual increase in density toward the edges, where contrasts shallow to δ ≈ -0.2, forming bounding "walls" of higher-density structures such as the Virgo, Perseus-Pisces, and Local superclusters.^[2] These gradients are traced through peculiar velocity fields and galaxy distributions, revealing multiple local minima that indicate a non-uniform underdensity.^[2] Structurally, the Local Void possesses an elongated and irregular, blobby shape, spanning approximately 69 × 51 × 60 Mpc at the δ = -0.7 isodensity contour, with a total volume of about 2 × 10^5 Mpc³.^[2] Its form is complex, featuring possible sub-voids marked by distinct underdensity minima (e.g., in the Lacerta and Andromeda regions) and potential thin gas filaments connecting sparse internal structures, though the interior remains predominantly devoid of luminous matter.^[2] The void's orientation tilts toward the positive supergalactic X-direction and merges with adjacent underdense regions like the Hercules and Sculptor voids at shallower density levels, contributing to a networked cosmic web.^[2] Models suggest this structure is expanding, influenced by dark energy, which accelerates the divergence of its boundaries. Compared to larger supervoids such as the Boötes Void, the Local Void is shallower in its average density contrast and smaller in scale, with the latter featuring an extreme emptiness (fewer than 100 galaxies in a volume expecting thousands) over a diameter of roughly 100 Mpc, whereas the Local Void's proximity allows for more detailed mapping revealing subtle internal variations. Despite this, the Local Void remains significant locally due to its direct influence on the motion of the Local Group.^[2] In the Lambda-CDM cosmological framework, the Local Void's formation aligns with predictions from initial density fluctuations in the early universe, where small underdensities grow and expand faster than surrounding overdensities due to gravitational instability and the subsequent dominance of dark energy. Simulations demonstrate that such voids emerge naturally from primordial Gaussian fluctuations amplified over cosmic time, with the Local Void's properties— including its irregular shape and density profile—consistent with hierarchical structure formation without requiring deviations from the standard model.

Contents of the Local Void

Known Galaxies

The Local Void harbors a sparse population of galaxies, with over 80 confirmed inner members (nearest neighbor distance >2 Mpc) identified as of 2019, all of which are dwarf galaxies exhibiting low surface brightness and no evidence of large spirals or clusters.^[2]^[13] These isolated systems have evolved in relative solitude, resulting in distinct star formation histories shaped by limited interactions.^[14] Prominent examples include the dwarf irregular galaxy KK 246 (also known as ESO 461-36), located approximately 7 Mpc from the Milky Way. This HI-rich, metal-poor galaxy features an absolute blue magnitude of M_B \approx -14, an extended neutral hydrogen disk spanning about 6 kpc, and a gas-to-light ratio M_{\rm HI}/L_B \approx 1.3.^[15] Its recession velocity of 427 km s^{-1} reflects the outward expansion characteristic of void environments.^[15] Another key member is the blue compact dwarf ALFAZOAJ1952+1428 (PGC 5060431), situated at roughly 8.4 Mpc with M_B = -13.7 and a compact HI diameter of about 2 kpc. This metal-poor system ([\rm Fe/H] = -1.16) displays moderate ongoing star formation and a gas-to-light ratio of 0.3, underscoring its isolation-driven properties.^[14] Its heliocentric recession velocity of 278 km s^{-1} further indicates participation in the void's dynamical expansion.^[14] These galaxies were primarily discovered through blind HI mapping surveys, such as the Arecibo Legacy Fast ALFA (ALFALFA) survey conducted in the 2000s and 2010s, which targeted low-density regions to uncover faint, gas-rich dwarfs otherwise missed by optical searches.^[14] The overall low density of the Local Void explains this relative scarcity, with galaxy counts per unit volume roughly 10 times below the cosmic mean.^[2]

Other Objects and Features

Observations using radio telescopes, such as the Arecibo Legacy Fast ALFA (ALFALFA) survey, have detected sparse traces of neutral hydrogen (HI) within the Local Void, primarily in the form of extended disks around known dwarf galaxies and occasional isolated clouds with anomalous velocities. These detections suggest the presence of filamentary structures, potentially remnants of the cosmic web's lower-density extensions, though the overall HI content remains exceptionally low compared to denser regions of the universe.^[16] Models of the Local Void's structure indicate a significant underdensity in dark matter compared to the cosmic average, which contributes to the region's accelerated expansion relative to surrounding structures. This inferred dark matter distribution arises from simulations and observational constraints on large-scale structure, where voids like the Local Void exhibit reduced gravitational binding, allowing matter to flow outward more readily.^[17] Potential non-galactic features within the Local Void include ultra-diffuse galaxies or an intravoid medium, though detections remain elusive due to the region's low surface brightness. Recent studies of nearby voids have identified red ultra-diffuse galaxies using multi-wavelength imaging, hinting at similar faint, extended objects that could populate the Local Void's interior.^[18] Additionally, gravitational lensing analyses in void environments suggest the possibility of hidden baryonic matter, though no specific observations confirm such structures directly in the Local Void.^[19] The rarity of stellar or active objects in the Local Void emphasizes its profound emptiness, with luminous tracers limited to a handful of well-studied dwarf galaxies and negligible contributions from stars, quasars, or other energetic phenomena beyond these.^[20] This scarcity aligns with the void's underdense nature, where gravitational collapse is insufficient to form dense stellar populations or active galactic nuclei on large scales.^[21]

Influence on the Local Universe

Dynamical Effects

The Local Void's underdensity drives peculiar motions in the local universe, notably contributing to the Local Group's overall velocity field. Observations indicate that the Local Sheet, of which the Local Group is a part, moves away from the void at a peculiar velocity of approximately 200–250 km/s, as decomposed from the cosmic microwave background dipole and galaxy distance surveys. This outflow results from the combined effects of gravitational attraction toward denser filamentary structures beyond the void and the expansive push from the void's own growth, which evacuates surrounding material.^[11]^[2] The void also perturbs the local Hubble flow, accelerating expansion rates in its vicinity and thereby complicating redshift-based distance estimates. Within and near the Local Void, peculiar velocities induced by the underdensity enhance the apparent recession speeds of galaxies, yielding a locally elevated Hubble constant compared to the global average. This distortion can lead to systematic biases in measuring cosmic expansion, with models showing outflow velocities up to several hundred km/s altering the interpreted Hubble parameter by several percent.^[2] The Local Void influences the Milky Way's environment by shaping the Local Sheet's flattened morphology and alignment, acting as the adjacent underdense region that funnels material into a coherent wall-like structure. Dynamical models suggest this void-driven compression has aligned the Local Sheet's plane, with the Milky Way embedded within it and on a trajectory continuing away from the void over cosmic timescales. This process affects the coherence of nearby galaxy orbits, potentially stabilizing the sheet against further disruptions.^[22] N-body simulations of Local Group evolution highlight void-induced outflows that redistribute dwarf satellite galaxies, channeling their progenitors into planar configurations aligned with the Local Sheet. These models demonstrate how the void's expansion generates bulk flows exceeding 200 km/s, stripping low-mass halos and influencing the accretion of dwarfs onto hosts like the Milky Way, thereby explaining observed satellite plane alignments without invoking exotic physics.^[22]

Cosmological Significance

The Local Void, often considered part of or extended into the larger KBC void or Local Hole, has been proposed in 2025 studies as a key factor in the Hubble tension, the discrepancy between local measurements of the Hubble constant (H₀ ≈ 73 km/s/Mpc) and those inferred from the cosmic microwave background (CMB, H₀ ≈ 67 km/s/Mpc).^[23] This underdensity, estimated at around 20% less matter within a region spanning up to 300 Mpc, induces peculiar velocities that accelerate local expansion by approximately 8-10%, effectively reconciling the differing H₀ values without invoking modifications to the standard cosmological model.^[24] Models fitting direct distance tracers, such as those from the CosmicFlows-4 catalog, yield local H₀ estimates of 70-72 km/s/Mpc in void scenarios, reducing the tension to within 1-3σ of observations.^[23] In the ΛCDM framework, the Local Void serves as a testbed for inhomogeneities, with baryon acoustic oscillation (BAO) scales revealing distortions in redshift-distance relations caused by void-induced bulk flows.^[10] These flows inflate redshifts in underdense regions, leading to apparent deviations from the homogeneous Planck cosmology predictions for the isotropic distance measure D_V, particularly at low redshifts where χ² values drop from 75.7 (void-free) to 47-51 in void models, corresponding to a tension reduction from 3.3σ to 1.1-1.4σ.^[10] Such tests highlight how local voids can mimic global expansion effects, challenging the assumption of cosmic homogeneity on scales up to several hundred Mpc. Debates persist over whether the Local Void alone accounts for the Hubble tension or signals deeper anomalies in ΛCDM, with some analyses arguing that void models fail to fully alleviate discrepancies without additional parameters, as they do not outperform homogeneous cosmologies in all datasets.^[25] Conversely, 2025 research positions the void as a "pristine laboratory" for probing dark energy, where void-wall inhomogeneities produce backreaction effects that simulate evolving dark energy (w₀ > -1, w_a < 0), aligning with DESI observations of weakening acceleration at z < 1 and offering insights into spatial variations without new physics. Future surveys like the James Webb Space Telescope (JWST) and Euclid are poised to test violations of void homogeneity, mapping underdense regions to refine expansion rate anisotropies and distinguish local effects from global cosmology.^[26] Euclid's wide-field imaging and spectroscopy will quantify void size functions and galaxy-void correlations, potentially confirming H_loc ≈ 1.05 H_global for deep voids (δ_void ≈ -0.9) and clarifying the Local Void's role in broader tensions.^[26]

References

[1]
UH astronomer and team map vast void in cosmic neighborhood
Jul 22, 2019 · In 1987 Tully and Richard Fisher noted the existence of the Local Void, which has become widely accepted but remains poorly studied because it ...
[2]
Cosmicflows-3: Cosmography of the Local Void - IOPscience
Jul 22, 2019 · The Local Void has a substantial dynamical effect, causing a deviant motion of the Local Group of 200–250 km s−1. The combined perturbations due ...
[3]
Astronomers map our local cosmic void | Space - EarthSky
Aug 7, 2019 · That is 125 megaparsecs with each megaparsec equal to a distance of ... size and shape of an extensive empty region they call the Local ...
[4]
Mysteries of the Local Void: Scientists Map a Vast Emptiness Around ...
Jul 23, 2019 · However, it was difficult to observe the Local Void since it is located behind the center of the Milky Way from our perspective here on Earth.Missing: definition | Show results with:definition
[5]
Next Stop: Voids | NASA Blueshift - Astrophysics Science Division
Jul 30, 2013 · The closest void to us on Earth is the Local Void (clever, right?). This guy is 150 million light years across and sits at the edge of our ...
[6]
The local hole revealed by galaxy counts and redshifts
The 6dFGS-SGC region contains the Perseus-Pisces supercluster, whilst the SDSS-NGC region contains the Coma cluster. Figure 1.
[7]
The voids closest to us may not be entirely empty - Phys.org
Dec 14, 2022 · A team of researchers used simulations that represent the local volume of our universe to study the seven nearest voids to the Milky Way.
[8]
Testing the local void hypothesis using baryon acoustic oscillation measurements over the last twenty years
### Summary of 2025 BAO Measurements for Local Void
[9]
Our Peculiar Motion Away from the Local Void - IOPscience
The peculiar velocity of the Local Group of galaxies manifested in the cosmic microwave background dipole is found to decompose into three dominant components.
[10]
Kinematics of the Local cosmic void - Astrophysics
### Summary of Local Void Information
[11]
DRAINING THE LOCAL VOID - IOPscience
DRAINING THE LOCAL VOID. Luca Rizzi, R. Brent Tully, Edward J. Shaya, Ehsan Kourkchi, and Igor D. Karachentsev. Published 2017 January 19 • © 2017.
[12]
KK 246: A DWARF GALAXY WITH AN EXTENDED H i DISK IN THE ...
We have found that KK 246, the only confirmed galaxy located within the nearby Tully Void, is a dwarf galaxy with an extremely extended H i disk and signs of an ...
[13]
(PDF) Neutral hydrogen in cosmic voids - ResearchGate
Aug 6, 2025 · It is concluded that the voids do not contain a cosmologically significant population of H I protogalaxies, and evidence consistent with the ...
[14]
The dark matter of galaxy voids - Oxford Academic
We find that voids in galaxy surveys always correspond to underdensities in the dark matter, though the centres may be offset. When this offset is taken into ...
[15]
Discovery of a red ultra-diffuse galaxy in a nearby void based ... - HAL
Jun 26, 2023 · Both galaxies have compatible distances and redshifts, share a high peculiar velocity (∼1000 km s−1) and lie within a void of radius 7 Mpc. The ...
[16]
Void lensing as a test of gravity | Phys. Rev. D
Jul 13, 2018 · Recent bounds on the gravitational wave propagation speed using a binary neutron star merger [25–28] have strongly constrained modified gravity ...
[17]
Local Void: for or against ΛCDM? - Oxford Academic
The emptiness of the Local Void has been put forward as a serious challenge to the current standard paradigm of structure formation in Λ cold dark matter (CDM).
[18]
[1401.6459] The Local Void: for or against LCDM? - arXiv
Jan 24, 2014 · The emptiness of the Local Void has been put forward as a serious challenge to the current standard paradigm of structure formation in LCDM.Missing: minimal objects
[19]
The formation of Local Group planes of galaxies - Oxford Academic
Key to the formation of the planar structures has been the evacuation of the Local Void and consequent build-up of the Local Sheet, a wall of this void. Most of ...<|control11|><|separator|>
[20]
[2506.10518] Testing the local supervoid solution to the Hubble ...
Jun 12, 2025 · We evaluate models for the KBC void capable of resolving the Hubble tension with a background Planck cosmology. We fit these models to direct distances.
[21]
The Local Void Hypothesis: A Potential Resolution to the Hubble ...
Jul 21, 2025 · The Local Void Hypothesis suggests that the Milky Way is situated within the KBC void, a 2-billion-light-year underdense region with 20 ...Missing: H0 | Show results with:H0
[22]
Alleviating the Hubble tension with a local void and transitions of the ...
Aug 6, 2025 · In this underdense region, one will feel a locally faster expansion rate compared to the cosmic average.Missing: enhanced | Show results with:enhanced
[23]
(PDF) What Impact Do Voids Have on the Expansion of the Universe?
Mar 31, 2025 · 3.1 Large-Scale Surveys and Void Identification. Recent surveys such as SDSS, DESI, and Euclid have mapped void. distributions, showing ...<|control11|><|separator|>