The expansion of the universe is a fundamental aspect of cosmology, yet recent measurements have produced conflicting results. Researchers at the University of Tokyo have developed an innovative method that provides compelling evidence suggesting the discrepancy in expansion rates is a genuine phenomenon rather than an error in measurement.
For decades, astronomers have utilized distance markers, notably supernovae, to determine the universe’s expansion rate, known as the Hubble constant. This method indicates an expansion rate of approximately 73 kilometres per second per megaparsec, meaning that for every 3.3 million light years away from Earth, objects recede at a speed of 73 kilometres per second faster.
The conflict arises when the expansion rate is calculated using an alternative approach: examining the cosmic microwave background (CMB), the ancient radiation from the Big Bang. This technique yields a significantly lower rate of 67 kilometres per second per megaparsec. The difference between these two values is referred to as the Hubble tension, a crucial issue as it may indicate unknown aspects of physics.
Innovative Method Utilizes Gravitational Lensing
Project Assistant Professor Kenneth Wong and his team at the University of Tokyo’s Research Centre for the Early Universe have introduced a new measurement technique called time delay cosmography. This method circumvents traditional distance ladders entirely by leveraging the phenomenon of gravitational lensing.
In gravitational lensing, massive galaxies bend the light from distant objects, creating multiple distorted images of a single quasar. When conditions align perfectly, these images appear around the lensing galaxy, each image following a different path and taking varying amounts of time to reach Earth. By observing identical changes in these images that occur out of sync, astronomers can calculate the time difference between the paths. This data, combined with estimates of mass distribution within the lensing galaxy, allows for the determination of the universe’s expansion rate.
The research team analyzed eight gravitational lens systems that featured massive galaxies distorting the light from distant quasars. Using data from advanced telescopes, including the James Webb Space Telescope, the measurement produced a value consistent with the higher rate of 73 kilometres per second per megaparsec, aligning with present-day observations.
Implications for Cosmology and Future Research
The alignment of this new method with contemporary measurements, rather than those from the early universe, strengthens the argument that the Hubble tension reflects real physical phenomena. Currently, the precision of this measurement stands at approximately 4.5 percent, but confirming the tension with greater certainty requires reducing this figure to 1-2 percent. Achieving this will involve analyzing additional gravitational lens systems and refining mass distribution models of the lensing galaxies.
The greatest uncertainty lies in the arrangement of mass within these lensing galaxies, although researchers base their assumptions on consistent observational profiles. This work signifies decades of international collaboration among various observatories and research teams, highlighting the collective effort to understand the cosmos.
If the Hubble tension is validated, it could signal a new era in cosmology, reshaping our understanding of how the universe evolves. As researchers continue to explore these discrepancies, they may uncover new physics that enhances our comprehension of the universe’s expansion.
