Astronomers are exploring a new way to measure the universe’s expansion rate using the gravitational-wave background detected by pulsar timing arrays.
An illustration of the evolution of the universe from the Big Bang (left) to today (right).
Photo Credit: NASA
The astronomers measure the expansion by using the local supernovae and the cosmic microwave measurements, which give varying results on the expansion. The Hubble tension discrepancy poses a major challenge to the researchers whose answer is yet to be arrived at. According to some researchers, the third method that can be employed to calculate the Hubble constant is the application of the gravitational waves emitted by the mergers of black holes.
According to the new research, the North American Nanohertz Observatory (NANOGrav) has turned our galaxy into a giant gravitational-wave antenna by monitoring precisely timed pulsars. This “cosmic hum” likely comes from countless merging supermassive black holes. Researchers propose treating that background statistically as a “stochastic siren” that encodes the expansion rate. In this picture, a lower H₀ (slower expansion) means more mergers in a given volume, strengthening the hum. Applied to current LIGO/Virgo data, the method already prefers a faster expansion. Though still preliminary, as detectors improve, it should directly “hear” the background and measure H₀ precisely, potentially settling the tension.
The Hubble tension might signal new physics: possibilities include a brief “early dark energy” burst, exotic dark-sector interactions, or even subtle modifications to gravity. Astronomers are using every clue they can: refined “standard candles” and independent gravitational-wave “standard sirens.” For example, LIGO's neutron-star merger gave an H₀ consistent with supernovae, while the new pulsar-timing hum provides a third check. So far, these gravitational-wave H₀ estimates fall within the current uncertainty range. By combining standard candles, standard sirens, and the cosmic hum, scientists hope to learn whether the tension demands exotic new cosmology or just better measurements.
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