Wired: cosmic voids probe dark energy

Wired reported on May 22 that some cosmologists see cosmic voids — the vast, underdense regions between galaxy filaments and clusters — as unusually useful places to test dark energy, gravity and the long-running Hubble tension. The case for voids is not that they are empty in an absolute sense, but that they contain less matter than most of the cosmic web, which can make some signals easier to model. Researchers have been making that argument in review papers and new modeling work for several years, while large surveys including DESI and Euclid are building the maps needed to test it at scale. DESI says it will obtain spectra for tens of millions of galaxies and quasars, and ESA says Euclid will observe billions of galaxies across more than a third of the sky. ### Why would emptier regions tell astronomers more? Cosmic voids are defined by what they lack: galaxies, gas and dark matter compared with the denser structures around them. A 2019 review paper described voids as “promising laboratories” for cosmology because their low-density environments are especially sensitive to diffuse components such as dark energy and neutrinos, and because some modified-gravity effects are expected to stand out more strongly there. (arxiv.org) Modified-gravity models often include “screening” effects that hide departures from general relativity in dense regions such as galaxies or clusters. A review of voids in modified-gravity scenarios said those screening mechanisms fade in lower-density environments, making voids suitable places to look for differences between standard gravity and alternatives. A 2026 arXiv paper by Tommaso Moretti, Noemi Frusciante, Giovanni Verza and Francesco Pace extended that line of work with a hydrodynamical description of spherical void evolution in modified gravity. (arxiv.org) ### What does this have to do with dark energy? DESI says its primary cosmology mission is to study the nature of dark energy by measuring how cosmic expansion and structure growth change over time. Euclid says it is dedicated to investigating the accelerating expansion of the universe and the nature of dark energy, dark matter and gravity. Voids matter to that effort because dark energy becomes relatively more important where matter is sparse. (arxiv.org) A March 2026 ScienceDaily summary of new research put the point directly: in cosmic voids, dark energy “becomes dominant” relative to matter, and the voids expand as cosmic acceleration pushes space outward. That does not mean voids alone can settle the dark-energy question, but it helps explain why researchers treat them as complementary probes alongside baryon acoustic oscillations, weak lensing and supernova measurements. (desi.lbl.gov) ### Where does the Hubble tension enter? The Hubble tension is the mismatch between expansion-rate estimates inferred from the early universe and measurements based on the nearby universe. Nature’s report on the “Tensions in Cosmology 2025” conference said the Hubble constant remained one of the field’s central anomalies in the era of precision data from DESI, JWST and other facilities. (sciencedaily.com) Some researchers have proposed that large-scale inhomogeneities, including voids, could affect local expansion measurements. One recent arXiv paper argued that accounting for void-wall structure could help reconcile higher local Hubble constant estimates with lower values inferred from the cosmic microwave background. That is one proposed explanation, not a consensus result, but it shows why voids are now part of the Hubble-tension discussion. (nature.com) ### What observations are making this practical now? DESI says it is conducting a five-year redshift survey of roughly 40 million extragalactic sources over 14,000 square degrees, using spectra to build a 3D map of the universe out to about 11 billion light-years. The collaboration also says those maps support dark-energy measurements through baryon acoustic oscillations and redshift-space distortions, while creating opportunities for other cosmology studies. (arxiv.org) Euclid says it will create a large map of cosmic structure by observing billions of galaxies out to 10 billion light-years across more than a third of the sky. ESA says the mission is designed to reveal how the universe has expanded and how structure formed over cosmic history, with direct relevance to gravity and dark energy. Those wide, deep maps are the raw material needed to identify large void catalogs and compare their shapes, abundance and lensing signals with theory. (baas.aas.org) ### Have void measurements already shown anything concrete? A 2024 preprint using DESI Legacy Survey luminous red galaxies reported a 14-sigma detection of the lensing signal from stacked voids, rising to 17 sigma for some subsamples. That paper addressed a different cosmological question, but it showed that void signals can now be measured with high statistical significance in large survey data. (esa.int) DESI and Euclid will provide the next test. DESI’s public data pages list DR1 as the latest release, and Euclid continues to roll out survey products and science results as its sky map grows. Those releases will determine whether cosmic voids remain an intriguing side channel or become a standard part of the dark-energy and gravity toolkit. (data.desi.lbl.gov) (arxiv.org)