For decades, one cosmic assumption has remained mostly untouched: that the part of the universe we inhabit is representative of the whole. Telescopes peer outward and find broad uniformity on vast scales. The cosmic microwave background appears smooth. Galaxies, clusters, and voids all fall within expected bounds of distribution. But a quiet discrepancy, known as the Hubble tension, has forced a closer look at what might be happening right beneath our feet. And according to a new analysis published in Monthly Notices of the Royal Astronomical Society, the problem may lie in the ground rules of that assumption.
The study, led by Indranil Banik and Vasileios Kalaitzidis, takes a direct swing at one of cosmology’s most persistent puzzles. Measurements of the universe’s expansion rate, known as the Hubble constant, don’t agree depending on how they’re taken. When astronomers use the cosmic microwave background, they get one number. But when they measure local galaxies and supernovae, they get a higher value. The mismatch is not small. It has resisted resolution for years.
What Banik and Kalaitzidis propose is striking in its simplicity. Instead of new physics, modified dark energy, or hidden early-universe mechanisms, they explore whether the problem arises because we live inside a vast underdensity: a local void. In this region, the number of galaxies and the amount of matter is significantly lower than the cosmic average. It’s not just theoretical. This structure has already been observed. Known as the KBC void, it was identified more than a decade ago using near-infrared galaxy counts.
In a void like this, space expands faster locally because there’s less gravity to counter the outward motion. This would make objects in our region appear to recede more quickly than they actually do in the wider universe. It would also inflate redshifts of nearby galaxies, an effect that could explain why local measurements of the Hubble constant skew higher than predictions based on early-universe data.
To test this idea, the researchers didn’t just compare a few data points. They compiled and analyzed baryon acoustic oscillation (BAO) measurements from the past 20 years. BAOs are subtle but powerful features in the large-scale structure of the universe, left behind by sound waves in the early cosmos. They serve as a kind of cosmic yardstick, offering a consistent way to track distances across time and space.
The team modeled how a local void would distort these measurements. In particular, they looked at how it would alter the apparent distance to objects at various redshifts. A key metric in this analysis is the parameter D_V, which combines angular and radial distance estimates into a single isotropically averaged figure. If a local void exists, it should cause a systematic shift in D_V at low redshifts.
That’s exactly what they found. The BAO data doesn’t line up cleanly with the standard cosmological model. Instead, it begins to deviate below redshift 0.3, right where a local void would exert the most influence. Using three different models for the void’s density profile (Exponential, Gaussian, and Maxwell-Boltzmann), the researchers ran predictions and found all three provided a better match to the observational data than the homogeneous model. The best fit reduced the tension between models and observations from 3.3 sigma to around 1.1.
One of the more revealing aspects of this study is how it interacts with multiple BAO observables. Not just the isotropic average, but also transverse and radial BAO features were analyzed. The most pronounced and consistent discrepancy appeared in the transverse direction, precisely where angular measurements are less sensitive to redshift contamination. This pattern would be difficult to produce if the universe were smooth on all scales.
Other datasets seem to support this interpretation. Redshift gradients, for instance, appear anomalously high when measured locally. Galaxy counts are lower than expected in large sky surveys near Earth. Observations of bulk flow, how groups of galaxies move as a whole, also suggest a region of enhanced outward motion. The KBC void alone could account for an apparent increase of 11 percent in the measured local expansion rate.
To further cross-check the implications, the authors used models developed in earlier work by Haslbauer, Banik, and Kroupa, who simulated voids under modified gravity conditions. These simulations showed that structure formation on scales above 100 Mpc could produce the kind of voids needed to explain the Hubble tension without violating constraints from the cosmic microwave background. Importantly, the Planck cosmology, based on early-universe measurements, remains valid outside the void, preserving the success of large-scale predictions.
There’s no need to throw out the Lambda-CDM model. Instead, the local void hypothesis suggests that its parameters apply globally but not necessarily locally. The observed discrepancy would simply arise because we live in an unrepresentative patch of the universe.
BAO data are uniquely suited to test this. Unlike redshift-independent distance measures or supernova luminosities, BAO signals are tied to a fixed scale set in the early universe. This makes them less vulnerable to systematic drift over time. But they can still be distorted if space itself is distorted. In the presence of a local underdensity, even this standard ruler appears warped.
Interestingly, the study also looked at the Alcock-Paczynski parameter, which compares transverse and radial BAO signals to detect any shape distortion in observed structures. While current data here are less precise, future surveys may sharpen this tool enough to rule definitively for or against the void scenario.
Not all data fit perfectly. A measurement at redshift 0.122 deviated from the void model, though it also stood out as a potential outlier even in the homogeneous model. Another, based on the Ho’oleilana structure, supports the void hypothesis but has large uncertainties. Still, the overall statistical picture favors a model where Earth lies near the center of a 300 Mpc-wide underdensity.
If correct, the implications are significant. A local void changes the way we interpret cosmic distances. It calls for caution when comparing local and global measurements. It may also alter estimates of other parameters, including the age of the universe, the density of dark energy, and the curvature of space.
For now, the local void hypothesis doesn’t replace the standard model. It refines it. Banik and Kalaitzidis don’t suggest new particles or forces. They suggest that what we see depends on where we are. A simple idea, backed by decades of data, that reshapes one of modern cosmology’s most stubborn puzzles.
Source:
Banik, I., & Kalaitzidis, V. (2025). Testing the local void hypothesis using baryon acoustic oscillation measurements over the last 20 years. Monthly Notices of the Royal Astronomical Society, 540(1), 545–561. https://doi.org/10.1093/mnras/staf781
