The KBC void (also known as the Keenan–Barger–Cowie void) is a giant cosmic underdensity — a vast region of space that contains significantly fewer galaxies, stars, and matter than the cosmic average. It was proposed in 2013 by Ryan Keenan, Amy Barger, and Len Cowie, based on observations of galaxy distributions.
Here’s what makes the KBC void important and intriguing:
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What is the KBC Void?
- A giant underdense region in the local universe, spanning roughly 1 to 2 billion light-years in diameter.
- Centered relatively near us — we are believed to be located within or near the center of this void.
- Estimated to be 20% to 50% less dense than the average matter density of the universe.
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Where is it?
- It encompasses a significant portion of the local universe, including our own Milky Way galaxy.
- It is not completely empty — it still contains galaxies and dark matter, but fewer than expected for a region of its size.
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Why does it matter?
- Hubble Tension:
- The KBC void has been proposed as a possible explanation for the Hubble tension — the discrepancy between the locally measured Hubble constant (H₀) and the one inferred from cosmic microwave background (CMB) measurements.
- If we are in an underdense region, light from distant galaxies would have passed through less gravitational potential, leading to apparent acceleration and higher local H₀ measurements.
- Challenge to Cosmology:
- Standard ΛCDM cosmology does not easily predict such a large and deep void in the local universe.
- Raises questions about whether local structure affects cosmological measurements more than previously assumed.
- Implications for Cosmic Isotropy:
- If the void is real and we are near its center, it might challenge the Copernican Principle, which says that we do not occupy a special place in the universe.
- However, models have been proposed to show that such a void could exist without violating this principle.
🔭 Observational Evidence:
- Based on galaxy counts and redshift surveys.
- Supported by some infrared surveys and measurements of Type Ia supernovae.
- However, it’s still debated — not all cosmologists agree that the void exists, or that it is large enough to account for the Hubble tension.
📚 Further Reading:
- Original paper: Keenan, Barger, & Cowie (2013), “Evidence for a ~300 Mpc Scale Under-density in the Local Galaxy Distribution”.
- Related work includes comparisons with supervoids like the CMB Cold Spot and large-scale structure simulations.
Would you like a comparative note between the KBC void and other cosmic voids or possible consequences for theories like inflation or alternative gravity models?
