The Milky Way may be in a huge bubble of vacuum

Earth and the entire Milky Way may be inside a huge cosmic void , a sort of bubble whose expansion rate is greater than that of the outside . This hypothesis is supported by a study of the ' sound' of the Big Bang, conducted by astronomers Indranil Banik of the University of Portsmouth and Vasileios Kalaitzidis of the University of St Andrews in the United Kingdom. The results, published in the Monthly Notices of the Royal Astronomical Society, were presented at the Royal Astronomical Society meeting in Durham.

The theory represents a potential solution to the " Hubble tension ," the discrepancy between measurements of the universe's expansion rate , obtained through observations of Type Ia supernovae in the local universe, and those derived from data on the echo of the Big Bang, the cosmic microwave background radiation. "One possible solution is that our galaxy is located near the center of a large local void," Banik explains. "This would cause a gravitational pull of matter outward from the void , at a higher density, such that the void empties over time. As the void empties, the speed of objects moving away from us would be greater than if there were no vacuum. This then gives the appearance of a faster local expansion rate."

For this idea to hold up, Earth and the Solar System would have to be located near the center of a bubble with a radius of a billion light-years and a density 20% lower than the average for the universe . Indeed, direct galaxy counting in the near-infrared supports this hypothesis, as the density of the local universe appears to be lower than that of neighboring regions. However, the existence of such a vast and deep void is controversial, as it does not easily fit with the standard cosmological model, which predicts that matter should be more uniformly distributed on such large scales.

Nonetheless, the new data presented in the study show that the possibility of a local vacuum fits well with the "sound" of the Big Bang, namely baryon acoustic oscillations (BAOs), fluctuations in the distribution of ordinary matter in the universe created by pressure waves propagating through the primordial plasma. These waves, similar to sound waves, left an imprint on the distribution of matter and serve as a ruler for tracing the history of cosmic expansion. "Considering all the BAO measurements available over the past 20 years," Banik emphasizes, "we have shown that a model with a vacuum is about a hundred million times more likely than a model without vacuums."