Through some groundbreaking advances to the field of cosmology, we may have found a way to solve the longstanding H0 tension. This tension points to the disparity between the measured accelerating rate of the universe’s expansion and what is forecast by the standard cosmological model. Dr. Indranil Banik of the University of Portsmouth has a colorful one. He proposes that Earth and the Milky Way may actually be located inside of a giant void, and this could explain why we observe a higher expansion rate locally than in more distant regions. We argued this theory at NAM 2025. In doing so, it might uncover key information about our universe’s current age—about 13.8 billion years.
The Hubble tension comes from a discrepancy in measurements of the universe’s rate of expansion. Indeed, the standard cosmological model would predict a slower expansion rate than what has been observed in the nearby galaxies. Dr. Banik notes that while this is a local curiosity, it begs the question of how matter is distributed throughout the universe. Consequently, it is now up to researchers to clarify the consequences of a local void and its effects on measurements of cosmic expansion.
Understanding Hubble Tension
Hubble tension is the term used to describe the discrepancy between the two measurements of how fast our universe is expanding. High precision measurements from the cosmic microwave background (CMB) indicate that the universe is expanding more quickly. By contrast, measurements of the expansion rate in our local galactic neighborhood suggest a much higher rate of expansion. This inconsistency has puzzled astronomers for years.
Dr. Banik elaborated on the nature of this tension, stating, “The Hubble tension is largely a local phenomenon, with little evidence that the expansion rate disagrees with expectations in the standard cosmology further back in time.” This delicate combination of observation and inference demonstrates that the early universe in fact conforms to expectations from cosmological models. Subsequent measurements find big surprises beyond those forecasts.
This is in line with the standard model of cosmology, which assumes that matter must be distributed uniformly on extremely large scales. If Earth and the solar system are near the center of a local void of at least a billion light-years across, we have an amazing opportunity. This situation might explain why on-the-ground measurements find a much faster expansion rate.
The Local Void Hypothesis
The explanatory model posits that Earth is located within a huge cavity in space. As for this hole, it has a density roughly 20% below the universe’s average. Such an emptiness could produce scorching spots that make cosmic growth appear to be accelerating when referenced towards different areas.
Dr. Banik noted that “a potential solution to this inconsistency is that our galaxy is close to the center of a large, local void.” Cosmologists do not take the existence of this void lightly. It’s upending what we thought we knew about the evenness of matter distribution across the vast expanse of the universe.
To back up this theory, Dr. Banik presented new data on baryon acoustic oscillations (BAOs). BAOs are acoustic waves generated in the early universe, and they serve as a “standard ruler” to measure cosmic distances. He explained, “These [sound waves] traveled for only a short while before becoming frozen in place once the universe cooled enough for neutral atoms to form.”
Implications for Cosmology
A local void does much more than just account for Hubble tension. It aids our understanding of how to figure out what the real age of the universe is. Dr. Banik and his coworkers examined BAOs from the past 20 years. Their results indicate that a void model is much more favored than a void-free model by CMB data from the Planck satellite, a flagship CMB satellite telescope.
“A local void slightly distorts the relation between the BAO angular scale and the redshift,” Dr. Banik explained. “The velocities induced by a local void and its gravitational effect slightly increase the redshift on top of that due to cosmic expansion.”
This research illustrates the need to understand local voids. It might be instrumental in improving measurements of the expansion of the universe, helping to solve problems like Hubble tension. Dr. Banik emphasized, “By considering all available BAO measurements over the last 20 years, we showed that a void model is about one hundred million times more likely than a void-free model.”
