The universe is vast, mysterious, and filled with countless galaxies, stars, and other celestial objects. But what about the empty spaces between them? These enormous regions, known as cosmic voids, are among the largest structures in the universe—yet they remain some of the least understood. Scientists estimate that voids make up about 80% of the universe’s volume, yet they contain very little matter. While they aren’t completely empty, they have much lower densities of dark matter compared to the rest of the universe.
One of the largest known voids, the Boötes Void, spans a staggering 330 million light-years across, with only a handful of galaxies inside. So far, scientists have discovered over 6,000 cosmic voids, but their nature and role in cosmic evolution remain major areas of research.
Formation of Cosmic Voids
Right after the Big Bang, both normal and dark matter were distributed relatively evenly throughout the universe. However, tiny quantum fluctuations created slight variations in density.
- Denser regions had more gravitational pull, attracting more matter and forming galaxy clusters.
- Less dense regions had weaker gravity, causing them to lose matter over time.
As a result, the universe developed a filament-like structure called the cosmic web, where matter is concentrated along these filaments, forming galaxies and clusters. Meanwhile, the voids—large, nearly empty regions—expanded as the universe evolved.
Why Are Cosmic Voids Expanding?
Unlike galaxy clusters, which are held together by gravity, voids are expanding—and at an accelerating rate. But why?
1. Dark Energy
Cosmic voids appear to be filled with an excess of dark energy, the mysterious force driving the accelerated expansion of the universe. Since dark energy acts as a repulsive force, voids may be growing due to an overflow of this energy.
2. Low Gravity
With a lower concentration of matter, voids experience less gravitational pull. This allows them to expand more freely compared to denser regions of the universe.
3. The Motion of Surrounding Galaxies
The strong gravitational pull of nearby galaxies pushes them away from voids, further contributing to their expansion.
Some physicists propose an alternative idea: perhaps dark energy isn’t needed at all. Instead, they suggest that different regions of the universe expand at different rates, creating the illusion of acceleration. However, data from the Cosmic Microwave Background (CMB), supernovae, and large-scale cosmic structures align well with the dark energy model.
How Do Scientists Detect Cosmic Voids?
Though cosmic voids are mostly empty, astronomers have developed two major methods to detect and study them:
1. Baryon Oscillation Spectroscopic Survey (BOSS)
BOSS is a cosmological survey that maps the large-scale structure of the universe using galaxies and quasars. It relies on Baryon Acoustic Oscillations (BAO)—sound waves from the early universe that left a distinct imprint on matter distribution. These waves created a natural spacing of 150 megaparsecs (Mpc) between galaxy clusters, which also helps in identifying cosmic voids.
2. Integrated Sachs-Wolfe Effect
When CMB photons pass through voids, they lose energy due to the region’s low gravitational pull. This results in colder-than-average areas in the CMB (blue regions), helping scientists map out cosmic voids.
What Can We Learn from Cosmic Voids?
1. Mapping Dark Matter
Since voids contain very little normal matter, any gravitational lensing effects observed within them are almost purely due to dark matter. By studying how light bends around voids, scientists can map the dark matter distribution at different distances (times) and track the universe’s expansion history.
2. Understanding Dark Energy
Since dark energy plays a key role in cosmic void expansion, analyzing voids could offer new insights into one of the biggest mysteries in cosmology—the force that is driving the accelerated expansion of the universe.
Do Different Methods Give Different Expansion Rates?
One of the biggest unresolved mysteries in cosmology is the Hubble Tension—the disagreement in expansion rate measurements from different methods. Scientists use several key techniques to measure cosmic expansion:
1. Type Ia Supernovae
These exploding stars serve as standard candles—since they have a known brightness, astronomers can compare their actual brightness with their observed brightness to measure their distances. Their redshift helps determine how much the universe has expanded since their explosion.
2. Baryon Acoustic Oscillations (BAO), Cosmic Voids, and the CMB
Each method provides slightly different expansion rates, contributing to Hubble Tension. Since voids are dominated by dark matter and dark energy, their growth could reveal new physics we don’t fully understand yet.
New Discoveries: Intergalactic Gas Inside Voids
Recent studies have found that, although voids contain very little intergalactic gas, the gas that does exist is hotter than expected. One possible explanation is that jets from supermassive black holes are heating the gas inside voids. This discovery adds yet another layer of complexity to our understanding of these massive cosmic structures.
Are We Living in a Cosmic Void?
Earth is not located inside a true void, but some scientists suggest we may be in a relatively underdense region known as the KBC Void. If true, this could help explain some discrepancies in expansion rate measurements and contribute to the ongoing debate over Hubble Tension.
Conclusion: Why Studying Voids Matters
Cosmic voids are the largest yet least understood structures in the universe. Their study could unlock new insights into dark matter, dark energy, and the fundamental nature of the cosmos. Additionally, voids could help us explore subatomic particles like neutrinos, which may play a crucial role in cosmic evolution.
In many ways, voids can be thought of as “giant laboratories of the cosmos”—vast, empty regions where nature’s most profound mysteries could finally be solved.
sources:
https://www.iflscience.com/the-gas-in-cosmic-voids-might-be-hotter-than-expected-46677
https://www.scientificamerican.com/article/how-analyzing-cosmic-nothing-might-explain-everything
https://www.caltech.edu/about/news/cosmic-web-lights-up-in-the-darkness-of-space