Portsmouth University researchers suggest Earth may reside within a massive cosmic void, potentially explaining the Hubble tension – a discrepancy in universal expansion rate measurements that has puzzled astronomers for years.
Researchers propose that Earth and the entire Milky Way galaxy could be situated within an enormous cosmic void, causing the universe to expand more rapidly in our region compared to surrounding areas.
This theory offers a potential resolution to the Hubble tension – a significant discrepancy between observed and predicted rates of universal expansion – and may help scientists confirm the universe’s age of approximately 13.8 billion years.
The research, presented at the Royal Astronomical Society’s National Astronomy Meeting (NAM) in Durham, was spearheaded by Dr Indranil Banik from Portsmouth University’s Institute of Cosmology and Gravitation. The study demonstrates that sound waves from the universe’s earliest moments – described as “essentially the sound of the Big Bang” – lend credence to this hypothesis.
Edwin Hubble first introduced the Hubble constant in 1929 to quantify the universe’s expansion rate, measured by observing celestial objects’ distances and their recession velocities.
The challenge arises when comparing measurements: extrapolating data from the distant, early universe to present day using standard cosmological models suggests a slower expansion rate than observations of nearby, recent universe regions indicate.
“A potential solution to this inconsistency is that our galaxy is close to the centre of a large, local void,” Dr Banik explained.
“This would cause matter to be pulled by gravity towards the higher density exterior of the void, leading to the void becoming emptier with time. As the void empties out, the velocity of objects away from us would be larger than if the void were not there, giving the appearance of a faster local expansion rate.”
For this theory to be viable, Earth and our solar system would need to be positioned near the centre of a void approximately one billion light-years in radius, with a density roughly 20% below the universal average.
Galaxy counting supports this theory, as the number density in our local universe appears lower than in neighbouring regions. However, such a large and deep void contradicts the standard cosmological model, which predicts more uniform matter distribution on such vast scales.
Dr Banik noted that the Hubble tension appears to be “largely a local phenomenon, with little evidence that the expansion rate disagrees with expectations in the standard cosmology further back in time”.
Despite the controversy, new data presented by Dr Banik at NAM 2025 shows that baryon acoustic oscillations (BAOs) – the “sound of the Big Bang” – support the local void concept.
“These sound waves travelled for only a short while before becoming frozen in place once the universe cooled enough for neutral atoms to form,” he explained.
“They act as a standard ruler, whose angular size we can use to chart the cosmic expansion history. A local void slightly distorts the relation between the BAO angular scale and the redshift, because the velocities induced by a local void and its gravitational effect slightly increase the redshift on top of that due to cosmic expansion alone.”
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The research team’s analysis of BAO measurements from the past 20 years revealed that a void model is approximately one hundred million times more likely than a void-free model designed to fit cosmic microwave background observations from the Planck satellite.
Researchers will now compare their local void model with alternative methods for estimating the universe’s expansion history, including cosmic chronometers.
This approach involves studying galaxies that have ceased star formation. By examining their light spectra, scientists can determine stellar compositions and proportions. Since more massive stars have shorter lifespans, their absence in older galaxies provides a method for establishing galactic age.
Scientists can then combine this age data with the galaxy’s redshift – the stretching of light wavelengths – to determine how much the universe expanded whilst light travelled towards us, illuminating the universe’s expansion history.
The University of Portsmouth‘s Institute of Cosmology and Gravitation (ICG) stands as a recognised international centre of research excellence, bringing together over 70 researchers – faculty, postdoctoral fellows, and PhD students – investigating some of the universe’s most profound mysteries.
These range from the earliest moments following the Big Bang to large-scale galactic structures, dark energy, and gravitational waves. The REF 2021 assessment confirmed the institute’s world-class impact, rating 100% of ICG research as world-leading or internationally excellent.
The institute contributes to major international projects including Euclid, LISA (Laser Interferometer Space Antenna), the LIGO gravitational wave detectors, and the Dark Energy Spectroscopic Instrument (DESI).
