Biases from Astrophysical Environmental Effects in Standard-Siren Cosmology
This study demonstrates that unmodelled line-of-sight acceleration of compact binaries in dense astrophysical environments systematically biases standard-siren measurements of the Hubble constant toward lower values, creating a significant systematic error that persists even when averaging over large event catalogues.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The universe is expanding, and for decades, astronomers have been trying to measure exactly how fast. This rate, known as the Hubble constant, is a fundamental number that tells us the age and size of the cosmos. To find it, scientists traditionally use two different methods that act like cosmic rulers. One method looks at the very early universe, using the leftover glow from the Big Bang to calculate how fast space should be stretching. The other looks at the nearby universe, observing exploding stars and other bright objects to measure the expansion directly. Strangely, these two methods give different answers, a puzzle known as the Hubble tension. It is possible that the difference comes from a flaw in how we measure, or it could mean our understanding of physics is incomplete.
A third, independent way to measure this expansion has recently emerged using gravitational waves. These are ripples in space-time caused by violent cosmic events, such as the collision of two neutron stars. When these stars crash, they send out a signal that tells us exactly how far away they are, without needing to climb a ladder of other measurements to get there. If we can also find the galaxy where the crash happened and determine how fast that galaxy is moving away from us, we can calculate the expansion rate of the universe with high precision. This method is called standard siren cosmology, and with the next generation of detectors, it promises to be one of the most accurate tools we have. However, for this tool to work perfectly, we must be sure that nothing is distorting the signal as it travels to us.
A team of researchers has now investigated a subtle but potentially dangerous source of distortion. They focused on what happens when two neutron stars are not floating alone in empty space, but are instead orbiting each other inside a dense, crowded environment, such as the swirling disk of gas around a supermassive black hole at the center of a galaxy. In these busy neighborhoods, the stars are not just moving in a simple circle; they are being pushed and pulled by the gravity of nearby objects and the drag of the surrounding gas. This causes the stars to accelerate along the line of sight, meaning they speed up or slow down relative to us in a way that changes over time. The researchers wanted to know if this acceleration would trick our detectors into giving the wrong answer about the distance to the stars, and consequently, the wrong answer about the expansion of the universe.
To find out, the scientists created a computer simulation of fifty binary neutron star collisions. They imagined these stars were located in the centers of galaxies, where they experienced different levels of acceleration caused by their environment. They then ran these simulated signals through the data analysis software that future detectors, like the Einstein Telescope and Cosmic Explorer, will use to study real events. Crucially, the software they used was designed for the ideal case: it assumed the stars were moving in a perfect vacuum with no external forces acting on them. The researchers wanted to see what would happen if the real world was messier than the model assumed.
The results showed that ignoring the environmental push and pull creates a systematic error. When the software analyzed signals that actually contained acceleration, it consistently misjudged the distance to the stars. It calculated that the stars were farther away than they really were. Because the distance is a key part of the calculation for the expansion rate, this error caused the final estimate of the universe's expansion to be too low. The researchers found that this bias does not disappear when you look at more events. In fact, the opposite happens. When you combine the data from many stars, the statistical uncertainty gets smaller, making the result look more precise. But if every single measurement is shifted in the same wrong direction, the final answer becomes a very precise, very confident, but completely wrong number.
The study quantified just how much of a problem this could be. They found that even if only a small fraction of the observed stars are affected by this acceleration, the error can become significant. For example, if ten percent of the stars in a sample of fifty are moving with a specific type of acceleration, the final result could be off by more than three times the expected margin of error. If the acceleration is stronger, even a smaller fraction of affected stars can cause a large shift. The researchers noted that this effect is particularly tricky because the direction of the acceleration—whether the stars are speeding up toward us or slowing down—does not cancel the error out. The analysis tends to overestimate the distance regardless of the direction, leading to a consistent underestimation of the expansion rate.
This finding suggests that the environment where these stars merge is not just a background detail, but a critical factor in cosmology. If future detectors observe hundreds of these events, the small biases from individual stars could add up to a major systematic error that skews our entire understanding of the universe's expansion. The researchers conclude that to achieve the high precision promised by next-generation detectors, we must update our models to include these environmental effects. By doing so, we can not only correct the measurement of the Hubble constant but also learn more about the dense, chaotic regions where these cosmic collisions take place. The path to a precise measurement of the universe's expansion now requires us to look more closely at the crowded neighborhoods where the stars live.
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