New study suggests Milky Way may lie within giant cosmic void

For nearly a decade, astronomers have been grappling with a persistent mystery known as the “Hubble tension”—a mismatch in measurements of the universe’s expansion rate. Observations of nearby galaxies suggest the cosmos is expanding faster than calculations based on the early universe predict.

A new analysis presented this week at the Royal Astronomical Society’s National Astronomy Meeting (NAM 2025) offers a bold explanation: Earth, the Milky Way, and the surrounding region may lie inside an enormous under-dense area—a cosmic void roughly a billion light-years wide, Caliber.Az reports, citing Earth.com.

This giant void, if real, could cause galaxies near us to appear to move away faster than expected, providing a natural explanation for the faster expansion measured locally without requiring changes to fundamental physics.

The “Hubble constant” defines the rate at which the universe expands today. Scientists use two main methods to measure it: one looks outward from the present day by measuring distances to nearby objects like supernovae; the other extrapolates forward from the early universe using the cosmic microwave background (CMB) radiation and the standard Lambda-CDM cosmological model.

The problem is that these methods yield conflicting values—around 73 km/s/Mpc from local measurements versus about 67 km/s/Mpc from early-universe data—far beyond what random errors can explain.

Dr. Indranil Banik of the University of Portsmouth, lead author of the new study, explains that if the Milky Way lies in a region about 20% less dense than the cosmic average, gravity would pull matter outward toward denser regions, making local galaxies appear to recede faster.

To explain the Hubble tension, the void would need to be vast—on the order of one billion light-years across. While such a large under-density challenges the conventional cosmological model, which predicts matter should be more evenly spread on large scales, local galaxy surveys have noted a relative deficit of galaxies that supports the idea.

The research team examined “baryon acoustic oscillations” (BAOs)—fossilised sound waves imprinted on galaxy distributions shortly after the Big Bang. By analysing two decades of BAO data, they found that the giant-void model fits observations about 100 million times better than the standard homogeneous model based on Planck satellite data.

Though the BAO evidence is compelling, it is not definitive. The team plans to test the void hypothesis further using “cosmic chronometers”—galaxies with well-understood aging stellar populations—to map the universe’s expansion history more precisely.

Scepticism remains among cosmologists, as simulations suggest large voids of this scale are unlikely, and ongoing surveys may fill the apparent gaps in local galaxy counts.

Still, the void hypothesis is attractive because it is testable: future observations will confirm or rule out its existence. If confirmed, astronomers may need to reconsider what “average” means in the cosmos, potentially resolving the Hubble tension and refining estimates of the universe’s age.

Next-generation telescopes and surveys promise to shed more light on this cosmic mystery, bringing us closer to understanding the true nature of our universe.

By Vugar Khalilov