A new detector-based approach explains how gravitational waves can be measured in a universe that is constantly changing.
Imagine trying to measure ripples on a pond while the pond itself is slowly changing shape. That is the difficulty scientists face when studying gravitational waves not as isolated signals from colliding black holes, but as part of a universe that is itself evolving.
Gravitational waves are extremely small distortions in spacetime. Their first direct detection in 2015 opened a new era in astronomy, allowing scientists to observe cosmic events that do not emit light. Since then, researchers have become adept at analyzing waves moving through relatively calm, empty regions of space, such as those produced by merging black holes.
In these well-understood cases, the situation is relatively straightforward. The wave is treated as a small disturbance on a stable background, and detectors measure the resulting stretching and squeezing of spacetime. In this framework, the “wave” and the “background” can be clearly distinguished.
Why the Expanding Universe Complicates the Problem
Cosmology changes the situation entirely. Instead of analyzing a wave moving through a quiet region of space, scientists must account for the universe as a whole—including spacetime itself and everything within it, such as stars, galaxies, black holes, and the large-scale cosmic structure.
In this context, the background is no longer static. The universe is expanding, matter is unevenly spread out, and continual fluctuations in density and motion affect spacetime. These factors make it difficult to clearly separate the background from a gravitational wave.
This raises a deceptively simple question: what exactly does a gravitational-wave detector measure when the entire universe is in motion?
A More Physical Definition of the Signal
Dr. Guillem Domènech and colleagues at the Institute of Theoretical Physics at Leibniz University Hannover (LUH) have proposed a detector-based framework to address this issue.
Instead of beginning with abstract mathematical descriptions of gravitational fields, the team concentrated on what an actual experiment would measure. Their setup involves two freely falling test masses—such as atomic clocks—linked by a beam of light. As a gravitational wave passes, it can subtly alter the time taken for light to travel between them, producing a detectable change in timing or frequency.
The researchers expressed this observable effect in a coordinate-independent form, accounting for terms up to second order in cosmic fluctuations. In doing so, they clarified how to describe the detector’s signal without mistaking mathematical artifacts for genuine physical phenomena.
“Gravitational-wave detectors measure variations in the frequency and arrival time of light beams,” explains lead author Guillem Domènech. “We compute these quantities precisely in an expanding spacetime and clearly separate what is physically observable from effects that depend on the chosen mathematical description. This makes theoretical predictions for future experiments both rigorous and dependable.”
Connecting Theory and Observation
The new framework provides a shared language for theorists and experimentalists to describe gravitational-wave measurements. In the simple case of nearly flat, quiet spacetime, it reproduces the standard signals detected by ground-based interferometers. In the more complicated cosmological setting, it ensures predictions remain directly connected to what a real detector would observe.
This makes the framework particularly valuable for detecting primordial gravitational waves and other faint signals distributed throughout the universe. It is also important for ongoing and future projects, including pulsar timing arrays and the space-based LISA observatory.
Read the original article on: Tech Xplore
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