Imagine waking up one day to find out you live inside a vast, invisible bubble — a region of space so vast it stretches a billion light-years across, and yet so elusive it leaves no visible boundary. According to a provocative new study, that might just be our cosmic address.
This idea is meant to solve one of the biggest puzzles in cosmology: the Hubble tension. This long-standing discrepancy concerns two different ways of measuring how fast the universe is expanding. Measurements from the early universe — encoded in the cosmic microwave background, or CMB — predict a slower expansion rate than what astronomers observe in the local universe today with their telescopes.
The difference isn’t trivial. It’s large enough that some physicists have called it a crisis. And now, researchers from the University of Portsmouth, Bonn, and St. Andrews think they might have found a way to bridge this divide — by putting Earth inside a giant, underdense bubble of space.
A hole in the universe?
Dr. Indranil Banik and colleagues argue that our Milky Way might sit near the center of a “supervoid”, an immense region of space that contains fewer galaxies and less matter than average. Their model suggests that this void causes local galaxies to flow outward more rapidly, making the cosmic expansion rate appear faster here than in the rest of the universe.
“A potential solution to this inconsistency is that our galaxy is close to the centre of a large, local void,” said Dr. Banik, who presented the findings at the Royal Astronomical Society’s National Astronomy Meeting. “As the void is emptying out, the velocity of objects away from us would be larger than if the void were not there.”
This could be the missing piece to the Hubble tension puzzle. A local underdensity would subtly distort observations of galaxies and cosmic structures, inflating our measurement of the Hubble constant, whose value tells us how fast the universe is expanding per unit of distance.
But is there evidence for such a void?
Yes, and it may already have a name. Known as the KBC void (after researchers Keenan, Barger, and Cowie), this region was identified over a decade ago in galaxy number counts. It spans up to 300 megaparsecs (about a billion light-years) and appears to be about 20% less dense than the universe’s average. What’s more, this underdensity shows up in multiple wavelengths, including optical, infrared, X-ray, and radio observations.
But to support such a dramatic claim, the team had to do more than argue from the structure of local space. They turned to one of the most powerful observational tools in cosmology: baryon acoustic oscillations.
The Sound of the Big Bang
Often described as the “sound of the Big Bang,” BAOs are relics of pressure waves that once rippled through the hot plasma of the early universe. These sound waves stopped traveling when the universe cooled enough for neutral atoms to form, freezing their imprint into the distribution of matter. The resulting patterns serve as a cosmic yardstick — a “standard ruler” that astronomers use to measure distances and chart the universe’s expansion over time.
“These sound waves travelled for only a short while before becoming frozen in place once the universe cooled enough for neutral atoms to form,” said Banik. “They act as a standard ruler, whose angular size we can use to chart the cosmic expansion history.”
That’s exactly what Banik’s team did. They analyzed 20 years of BAO data, examining how this cosmic ruler behaves at different redshifts (or cosmic distances) in a universe that contains a giant local void. In such a scenario, gravity subtly distorts the distances we measure, making nearby expansion appear faster.
“A local void slightly distorts the relation between the BAO angular scale and the redshift,” 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.”
When all available BAO measurements were combined, the void model turned out to be about 100 million times more likely than a model that assumes no void and uses parameters designed to fit the Planck satellite’s CMB measurements.
Testing the Void
In their study published in 2023 in Monthly Notices of the Royal Astronomical Society, Banik and his co-authors took the idea further. They developed a semi-analytic model of a supervoid using alternative gravity theories, specifically MOND (Modified Newtonian Dynamics), rather than the standard ΛCDM cosmology.
MOND suggests that Newton’s law of gravity breaks down in extremely weak gravitational fields, such as those found on the largest cosmic scales. This theory can explain galactic rotation curves without invoking dark matter and predicts faster structure growth in the universe, which would make it easier for giant voids to form.
Using this framework, the team modeled how galaxies inside and around such a void would move. Crucially, they compared their predictions with a separate observational anomaly: the high “bulk flow” of galaxies on large scales.
Bulk flow is a measure of the average motion of galaxies in a given region. Standard cosmology predicts this flow should decrease with distance. But the scientists found the opposite: bulk flows increase out to 250 megaparsecs. That’s not supposed to happen unless, perhaps, we live inside a giant void.
“The rising bulk flow curve is unexpected in standard cosmology, causing 4.8σ tension at 200 Mpc,” the authors write. Their model, on the other hand, reproduces this rising curve naturally, without fine-tuning.
What does this mean for cosmology?
If the authors are right, this discovery could reshape our understanding of cosmic structure and the history of the universe’s expansion.
For one, the model doesn’t just solve the Hubble tension. It also addresses several other known problems with the standard ΛCDM cosmology. These include the surprisingly early formation of massive galaxy clusters like El Gordo, observed fast galaxy collisions, and even hints of anisotropies in the large-scale distribution of quasars.
“The existence of a deep and extended local void in the galaxy number counts and the fast observed bulk flows strongly suggest that structure grows faster than expected in ΛCDM on scales of tens to hundreds of millions of light-years,” wrote Banik.
That’s not all. Their findings are consistent with other recent studies showing that local measurements of the universe’s expansion start to diverge from the standard model at redshifts as high as 0.5 or even 1.0. That’s billions of years ago. In other words, the influence of this void might not be as local as previously thought.
Trouble in the Void
Still, the idea is not without controversy.
The standard model of cosmology assumes that matter is evenly distributed on very large scales. A void of this size and depth shouldn’t exist according to ΛCDM. A lot of stars need to align for this model to be adopted. That makes the proposal “a hard pill to swallow” for many cosmologists.
Even so, the idea is gaining traction as other explanations for the Hubble tension struggle. Tuning early-universe physics to allow for a higher Hubble constant risks contradicting other well-established observations, such as the ages of the oldest stars.
Yet, as Banik puts it, “The Hubble tension is largely a local phenomenon… so a local solution like a local void is a promising way to go about solving the problem.”
Perhaps the most compelling support comes from the BAOs themselves. These ancient ripples in the fabric of space, remnants of the universe’s earliest soundscape, now echo a surprising message: that something strange is happening close to home.
What’s next?
The researchers are already planning follow-up studies using full cosmological simulations within the MOND-based νHDM (neutrino Hot Dark Matter) framework. These simulations aim to test whether such voids can form naturally, and whether they lead to bulk flows and structure formations consistent with what we observe today.
More precise measurements of galaxy motion, better mapping of matter distribution, and refined cosmic chronometers could all help settle the debate. The next generation of sky surveys, like Euclid and the Vera C. Rubin Observatory’s LSST, may provide the data needed to confirm or falsify the model.
For now, the universe’s apparent acceleration may not be a uniform feature of space, but a product of our peculiar position within it.
