From the Local Void to the KBC Supervoid — Mapping the Universe’s Hidden Hollows
© Dhinakar Rajaram — Amateur Astronomer
Preface — My Journey into the Cosmic Voids
As an amateur astronomer, I have always been captivated not just by what the universe contains, but by the vast expanses it does not. Cosmic voids — enormous, silent, almost poetic absences — fascinate me precisely because they challenge our perception of the cosmos. My mission in writing this blog is to take these hidden structures to the masses, especially students and young astronomers, showing that emptiness is not trivial but a fundamental ingredient in the cosmic recipe.
Through this essay, I wish to present a structured, comprehensive view — a ready reckoner for learners and enthusiasts alike. Voids are more than curiosities; they help us test the limits of cosmology, probe dark energy, and refine the values of the universe’s expansion. To study nothingness is, paradoxically, to study everything.
I. Prologue — The Shape of Emptiness
“Then even nothingness was not, nor existence… There was neither death nor immortality then.” — Rig Veda 10.129
At the heart of creation lies a paradox: emptiness is not the absence of being, but the architecture upon which being itself unfolds. Between the luminous filaments of galaxies and the incandescent tapestries of clusters lies an abyss — vast, silent, and staggeringly immense — the cosmic voids.
Contrary to instinct, the universe is not primarily made of shining stars or radiant nebulae. Most of its expanse is an intergalactic wilderness — so immense that even imagination recoils. Yet these voids sculpt the geometry of the cosmos, defining where matter gathers and how the universe expands.
II. The Cosmic Web — A Universe of Filaments and Hollows
When astronomers began mapping galaxies through surveys like CfA and Sloan Digital Sky Survey (SDSS), they discovered that galaxies trace a delicate network — the cosmic web. This structure comprises filaments, walls, clusters, and between them, the yawning voids.
The universe resembles a colossal sponge: luminous threads marking where galaxies cluster, and dark cavities revealing where they do not. This pattern confirmed that our cosmos, though homogeneous in principle, is profoundly clumpy in practice — a grand orchestration of density and absence.
III. What Are Cosmic Voids?
A void is a region where the matter density (ρ) falls below the cosmic mean, often by 80–90%. Yet they are not true vacuums — they host faint dwarf galaxies, diffuse gas, and dark matter. Their distinguishing trait is faster expansion due to reduced gravitational pull. Typical voids measure 10–50 million light-years, but supervoids exceed 500 million.
IV. How We Detect Voids — Tracing the Shadows of Nothingness
Finding a nothing is paradoxical. Yet astronomers map voids through:
- Galaxy redshift surveys (CfA, 2dF, SDSS, 2MASS) — revealing filamentary walls and empty gaps.
- Peculiar velocities — galaxies drifting away from underdense regions, mapped via CosmicFlows.
- Baryon Acoustic Oscillations (BAO) — distortions in the cosmic “standard ruler” signalling underdensity.
- CMB Signatures — the Integrated Sachs–Wolfe effect leaves cold spots as photons pass through voids.
- Weak gravitational lensing — light diverges slightly in underdense zones.
- Void-finder algorithms — ZOBOV, VIDE, and WVF identify 3D voids statistically.
1. The Boötes Void
Discovered in 1981 by Robert Kirshner’s team, this vast region spans ~330 million light-years. Containing only a few dozen galaxies, it became the archetype of cosmic emptiness — proof that the universe is not uniform.
2. The Local Void
Adjacent to our Virgo Supercluster, ~150 million light-years wide. Our Local Group drifts away from it at ~260 km/s, subtly shaping local cosmic flows.
3. The Northern and Southern Local Supervoids
Each about 300 million light-years across, flanking our Local Sheet in opposite hemispheres. Together, they likely merge into a larger underdensity encompassing our region.
4. The Giant Void
Identified by Granett et al. (2008) near Canes Venatici, ~1.3 billion light-years wide, associated with the Sloan Great Wall and CMB cold imprints.
5. The KBC Void / Eridanus Supervoid
Proposed by Keenan, Barger & Cowie (2013). A possible 2-billion-light-year underdensity centred on Eridanus, enveloping the Milky Way itself. It may explain part of the “Hubble tension,” appearing as a faster local expansion region.
VI. The KBC Void in Detail — Our Possible Cosmic Basin
The KBC Void is a profound reimagining of our cosmic neighbourhood. Near-infrared surveys (2MASS, UKIDSS) show our region’s galaxy density ~30% below average within 1 Gly. Later simulations (Haslbauer, Banik & Kroupa) confirmed that such a basin could influence the measured Hubble constant.
Dimensions: ~1.8–2 Gly diameter, underdensity δρ/ρ ≈ −0.2 to −0.3, centred near Eridanus.
Type Ia supernovae appear slightly brighter here, BAO scales mildly stretched — signatures of faster expansion within the void.
VII. Nested Emptiness — How Voids Interconnect
Voids form hierarchies: the Local Void nests inside the Northern and Southern Supervoids, all embedded within the KBC complex. Galaxies on these boundaries flow outward, producing coherent velocity shears that subtly affect distance measures and our motion relative to the CMB.
VIII. Voids and the Hubble Tension — When Expansion Varies by Address
The local Hubble constant (~73 km/s/Mpc) exceeds the cosmic one (~67 km/s/Mpc). A giant underdensity naturally yields a faster local expansion: gravity is weaker, the scale factor grows slightly quicker. Lemaître–Tolman–Bondi models show a 30% void can raise H₀ by ~5 km/s/Mpc — matching part of the observed offset.
However, such a vast void is rare under ΛCDM, explaining only part of the tension. Other hypotheses (early dark energy, neutrino physics) complement this environmental view.
IX. The Anomalies — Voids and the Universe’s Uneasy Symmetries
The CMB Cold Spot in Eridanus, about 10° wide and 70 μK colder, aligns strikingly with the KBC/Eridanus Supervoid — possibly an ISW imprint. Local underdensities may also contribute to our peculiar velocity (~630 km/s) and the CMB multipole alignments known as the “Axis of Evil.”
X. Voids in ΛCDM Cosmology — Order within Emptiness
ΛCDM simulations (Millennium, Illustris, TNG300) reproduce voids naturally. Median diameters 20–50 Mpc, contrasts −0.8. Supervoids of gigaparsec scale are statistically rare (~1% probability), making them valuable tests of cosmic homogeneity.
Voids are clean laboratories for studying dark energy, modified gravity, and isolated galaxy evolution — their galaxies are bluer, smaller, more star-forming.
XI. Alternative Cosmologies and Theoretical Interpretations
- LTB Models: Inhomogeneous universes where a central void mimics cosmic acceleration.
- Modified Gravity / MOND: Predicts naturally larger voids without exotic dark matter.
- Early Dark Energy / Neutrinos: Alternative fixes for the Hubble tension, compatible with mild local underdensity.
- Cosmic Variance: Perhaps we simply inhabit a statistically rare but plausible low-density patch.
XII. Philosophical and Poetic Reflections — The Metaphysics of Nothing
If galaxies are the syllables of the universe’s speech, voids are its pauses. They remind us that most of reality — atomic to astronomical — is structured emptiness. To study voids is to understand the grammar of existence, how absence defines form. From the Nasadiya Sukta to modern cosmology, humanity’s gaze into nothingness remains a quest for meaning.
XIII. Epilogue — The Quiet Geometry of the Universe
We may inhabit not a bustling cosmic hub but a tranquil depression — a rarefied alcove from which to observe the drama of galaxies. Yet it is precisely this serenity that allows thought to flourish. To gaze upon the night sky from within a void is to listen to the universe’s symphony from its quietest hall.
Suggested Reading and References
- Keenan, R. C., Barger, A. J., & Cowie, L. L. (2013). ApJ, 775:62 — Evidence for a ~300 Mpc Scale Local Underdensity.
- Haslbauer, M., Banik, I., & Kroupa, P. (2020–2022). MNRAS — The KBC Void and the Hubble Tension.
- Tully, R. B. et al. (2019). AJ, 158, 50 — Cosmicflows-3: Velocity Fields and Local Structure.
- Granett, B. R., Neyrinck, M. C., & Szapudi, I. (2008). ApJL, 683:L99 — The Giant Void and ISW Correlations.
- Böhringer, H. et al. (2021). A&A, 651:A74 — Local Density Variations in X-ray Galaxy Clusters.
- Nadathur, S. (2020). Physics Reports, 841, 1–76 — Voids in the Large-Scale Structure of the Universe.
- Planck Collaboration (2020). A&A, 641, A6 — Planck 2018 Results: Cosmological Parameters.
© Dhinakar Rajaram | Amateur Astronomer | All rights reserved.
© 2025 Dhinakar Rajaram — All Rights Reserved.
All original text, illustrations, and poster designs in this article are © Dhinakar Rajaram.
Unauthorised copying, reproduction, redistribution, or use in any form, digital or print, is strictly prohibited without prior written permission.
Image credits:
• “Galaxy Superclusters and Galaxy Voids” map — Courtesy of Wikimedia Commons (used under fair use / public domain for educational reference).
• All other images, diagrams, and the poster “The Great Cosmic Voids — Inside the Universe’s Vast and Silent Chambers” — © Dhinakar Rajaram.
This blog post and its contents are intended solely for educational, research, and non-commercial purposes under fair use principles.
No comments:
Post a Comment