Cosmic Voids Offer New Data on Gravity, Dark Energy, and Expansion Rate

Cosmic voids are under-dense regions between filaments of the cosmic web that formed after the Big Bang as matter clustered under gravity. These regions contain fewer galaxies than surrounding areas and allow clearer observations of certain physical processes.

Alice Pisani, a research professor at the Centre for Particle Physics in Marseille, stated that voids provide a high signal-to-noise ratio because there is less interference from matter. She noted that voids can be used to address questions about gravity, dark energy, and the Hubble tension.

After the Big Bang, matter cooled and formed the cosmic web over billions of years. Gaps between filaments widened as matter accumulated, creating voids that range from 10 million light years to more than 300 million light years across, such as the Boötes Void.

Pisani said the term “void” can be misleading because these regions still contain very low-mass galaxies. The Boötes Void contains a few dozen galaxies rather than the thousands expected in a comparable volume.

Voids remained difficult to observe until three-dimensional galaxy maps were developed in the late 1970s. Recent surveys by the Dark Energy Survey Instrument in Arizona and the European Euclid space telescope are expected to map more than 100,000 voids.

Nico Schuster, a cosmologist at the same Marseille center, said that improved telescopes and simulations now allow researchers to observe more galaxies and model hundreds of thousands of voids, an order of magnitude increase over earlier capabilities.

Because voids contain little matter, they provide simpler conditions for testing modified gravity theories and measuring the effects of dark energy. Researchers track the motion of galaxies and dark matter halos through voids to compare observations with model predictions.

Schuster has studied how neutrino motion in voids can be observed with reduced interference. Pisani said voids are the first regions of the universe dominated by dark energy, making its effects more measurable there.

Indranil Banik of the University of Portsmouth has argued that this location could explain the Hubble tension by affecting local measurements of cosmic expansion. Banik stated that more researchers are considering the void hypothesis. Pisani and Schuster said the idea merits further study, though they are not yet convinced it is correct.

Banik added that decisive tests of the hypothesis are expected within the next 10 years.