A new study has found that a hidden bias in galaxy models can skew gravitational-wave measurements of how fast the universe is expanding.
That finding complicates one of astronomy’s most promising ways to settle the fight over the Hubble constant – the rate at which the universe is expanding today.
Where the bias starts
In the team’s tests, the trouble appeared whenever galaxy catalogs ran thin beyond nearby cosmic neighborhoods.
Using 46 dark-siren events, Cezary Turski, a doctoral researcher at Ghent University (UGent) showed how missing galaxies bend the odds.
Because catalogs lose track of faint and distant galaxies, the calculation starts filling gaps with a brightness model that can steer results.
Here is where the back-door bias enters, and it becomes hardest to ignore when a merger lands beyond the catalog’s comfort zone.
How dark sirens work
Gravitational waves can tell astronomers how far away a merger happened, giving them a fresh route to the universe’s expansion rate.
Bernard Schutz’s 1986 proposal showed that those signals could measure that rate without the usual ladder of overlapping distance estimates.
When no flash of light reveals the host galaxy, astronomers use dark standard sirens, mergers matched statistically to many possible galaxies.
As a result, black-hole mergers can join the census, but missing galaxies still seep into the final uncertainty.
Galaxy catalog limits
Existing galaxy catalogs are not blank maps, but they stop being trustworthy surprisingly early on the cosmic scale.
The widely used GLADE+ catalog stitches together six surveys, giving astronomers a broad map of nearby galaxies.
Once redshift, a measure of how much space has stretched, climbs toward 0.1, that map starts losing firm support for many hosts.
From that point on, the unseen part of the universe stops being a side issue and starts steering the calculation.
Galaxies evolve over time
Astronomers handle the missing light with a Schechter function, a formula that describes how galaxy brightness is distributed.
A fixed version assumes that galaxy populations look the same across cosmic time, even as the universe ages and thins.
That shortcut breaks down because young galaxies and older galaxies do not fill the sky in the same mix.
Instead, the hidden galaxies get weighted the wrong way, and the expansion estimate drifts with them.
Bias enters calculations
Inside the calculation, a prior, the starting set of odds before the data bite, tells the code where host galaxies are likely.
Missing galaxies change that prior because the model decides how many unseen hosts should exist at each distance.
An evolving brightness distribution pushes those background odds differently than a fixed one, especially where the catalog contributes little direct help.
So the bias appears before anyone changes the raw gravitational-wave signal or its measured distance.
Bias grows with distance
Distance made the problem stand out most clearly in the paper’s farthest and least catalog-supported mergers.
For nearby events, real galaxies still carry much of the weight, so the brightness model has less room to tug the answer.
Farther out, the host list thins fast, and the assumed shape of the hidden galaxy population starts acting like a substitute map.
For future detectors, that dependence is a warning, because they will hear many more distant mergers than today’s instruments do.
One fix, one trade
UGent’s team also tested a partial fix by letting the Hubble estimate and the merger population evolve together.
Using 42 black-hole events, they found that this joint fit removed the direct bias in the expansion rate.
Even then, the low-redshift rate parameter changed with the luminosity model, showing that one uncertainty had partly swapped places with another.
Instead, it revealed exactly where the bookkeeping still goes astray inside a more ambitious analysis.
Why this matters
The stakes reach beyond one narrow modeling problem because gravitational waves are supposed to arbitrate a stubborn dispute over cosmic expansion.
Planck’s measurement puts the Hubble constant, the current cosmic expansion rate, near 67.4 on astronomers’ usual scale.
Another result placed the local value near 73.04, a gap large enough to keep cosmologists arguing.
Under that condition, a gravitational-wave answer built on the wrong galaxy model would fail to settle the fight.
Sharper maps ahead
The latest catalog already uses 141 events in dark-siren expansion tests, showing how quickly the data volume is rising.
Deeper sky surveys should catch more host galaxies directly, reducing how often cosmology depends on guessed populations beyond telescope reach.
At the same time, more sensitive detectors will hear mergers from much farther away, expanding the very territory where missing galaxies matter most.
In that sense, catalog depth and galaxy modeling become part of the instrument, not just background paperwork.
What changes now
UGent’s result leaves gravitational-wave cosmology promising but more dependent on unseen galaxies than many readers might expect.
Future analyses will need deeper catalogs, evolving galaxy models, and careful population fits if gravitational waves are going to serve as a deciding measure.
The study is published in Monthly Notices of the Royal Astronomical Society.
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