The universe’s expansion rate is under scrutiny as researchers at the University of Tokyo introduce a novel method for measuring it. This approach provides compelling evidence that the discrepancy between two established measurement techniques—resulting in differing values for the Hubble constant—may reflect a genuine phenomenon rather than mere measurement errors.
For decades, astronomers have relied on traditional distance markers, such as supernovae, to determine how quickly the universe is expanding. These “distance ladders” estimate the Hubble constant at approximately 73 kilometres per second per megaparsec. This means that for every 3.3 million light years from Earth, celestial objects appear to recede at a rate of 73 kilometres per second.
The Hubble Tension Explained
The conflict arises when scientists adopt an alternative method to calculate the expansion rate. By studying the cosmic microwave background, which is ancient radiation from the Big Bang, researchers estimate the expansion rate to be around 67 kilometres per second per megaparsec. The difference between these two values is referred to as the Hubble tension, and understanding this gap is crucial as it could indicate new physics that remains unexplained.
Project Assistant Professor Kenneth Wong and his colleagues at the University of Tokyo’s Research Centre for the Early Universe have now utilized a technique called time delay cosmography. This method circumvents traditional distance ladders entirely and instead exploits the phenomenon of gravitational lensing, where massive galaxies bend light from distant objects behind them.
In optimal conditions, a single distant quasar can appear as multiple distorted images around the lensing galaxy. Each image follows a different pathway to reach Earth, resulting in varying travel times. By observing identical changes in these images occurring slightly out of sync, astronomers can measure the time difference between the paths. This data, combined with estimates of mass distribution in the lensing galaxy, enables researchers to calculate the universe’s expansion rate.
New Findings and Future Implications
The research team meticulously analyzed eight gravitational lens systems, each featuring a massive galaxy distorting light from a distant quasar. They utilized data from advanced telescopes, including the James Webb Telescope, to conduct their measurements. The results were aligned with the 73 kilometres per second per megaparsec figure derived from nearby observations, notably differing from the 67 kilometres per second per megaparsec derived from early universe studies.
The robustness of this new method suggests it remains unaffected by potential systematic errors that might complicate either traditional distance ladder measurements or cosmic microwave background analyses. The alignment with present-day observations strengthens the hypothesis that the Hubble tension represents a genuine aspect of the universe’s expansion.
Currently, the precision of this measurement stands at roughly 4.5 percent. To definitively confirm the existence of the Hubble tension, researchers aim to enhance this accuracy to between 1 and 2 percent. Achieving this will involve examining additional gravitational lens systems and refining models of mass distribution in the lensing galaxies. The greatest uncertainty arises from the unknown distribution of mass within these lensing galaxies, although researchers typically operate under established observational profiles.
This work is the culmination of decades of international collaboration among various observatories and research teams. Should the Hubble tension be validated as a real phenomenon, it could usher in a new era of understanding in cosmology, fundamentally altering our grasp of how the universe evolves.
