Far from being little empty backwaters to study, these voids could solve some of the most persistent cosmic mysteries, such as the behavior of gravity, the nature of dark energy and the so-called Hubble tension, an observed mismatch in the expansion rate of the universe that has caused astronomers headaches for years.
“With emptiness, we have the power to tackle most of the interesting cosmological puzzles,” says Alice Pisani, a research professor of cosmology who works at the Center for Particle Physics in Marseille (CPPM) of the French National Center for Scientific Research. She adds that because there is less interference from matter, there is a “higher signal-to-noise” ratio in terms of what researchers can see.
The advent of new telescopes and advanced simulations has supercharged the field, inspiring a growing community of scientists around the world to specialize in voids as unique cosmological laboratories. Some experts argue that we may even live inside a vast void, a situation that could change our view of the universe in consequential ways.
For places defined by sparseness, voids are becoming cosmological heavyweights, where the laws of physics can be observed with unusual clarity.
“From a cosmological point of view, this is a very exciting time,” says Pisani.
What are cosmic voids?
After the Big Bang, the universe was a homogeneous soup of subatomic particles. But over millions of years, as matter cooled and solidified into atoms, the faint outlines of the cosmic web began to emerge.
Over billions of years, the Web gravitationally attracted gas clouds, galaxy clusters, and other cosmic objects. As more material is drawn into the web, the gaps between its fibers widen, creating voids.
Smaller “subvoids” may open up between galaxy clusters, where they may span only 10 or 20 million light years. But empty spaces can be large. huge. The Boötes Void, also known as the “Great Nothing”, extends over 300 million light years.
Calling them cosmic voids can be “misleading,” says Pisani, “because we tend to think that void means empty. But in reality, the voids we see are never empty. Inside those less dense regions are very small low-mass galaxies.” For example, the Bootes Void contains a few dozen galaxies – although this is still far fewer than the thousands that would be expected in a similarly sized region.
Because they are relatively devoid of material, cosmic voids remained out of observation until the late 1970s. Up to that point, the positions of galaxies had been mapped as 2D points on the sky, but the development of 3D maps of galaxy distribution revealed the outlines of the cosmic web for the first time, highlighting the presence of voids.
In recent years, several new telescope surveys have triggered an explosion of new void discoveries, such as the Dark Energy Survey Instrument (DESI) in Arizona, and the European Euclid Space Telescope. These instruments are expected to map more than 100,000 voids in space, offering an unprecedented glimpse of these structures. Yet these surveys will still capture only a fraction of the many millions of voids that exist in the observable universe.
“In the last 10 years, the field has really evolved significantly with new technologies,” says Nico Schuster, CPPM’s cosmologist and cosmic void expert. “All of this really enables us to observe much more galaxies than before, and it really allows us to probe deeper into the cosmic web, find more voids, and resolve them better.”
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