The expansion of the universe is a fundamental aspect of cosmology, yet a significant discrepancy in its measurement persists. Researchers at the University of Tokyo have developed an independent method for determining the universe’s expansion rate, providing compelling evidence that the existing discrepancy is more than a result of measurement errors. This new approach highlights a divide between two established techniques that yield different values for the Hubble constant, a critical figure in understanding cosmic expansion.
For decades, astronomers have used distance markers, such as supernovae, to gauge the Hubble constant, which is currently estimated at approximately 73 kilometres per second per megaparsec. This means that for every 3.3 million light years from Earth, objects recede at an additional 73 kilometres per second.
The challenge arises when scientists employ alternate methods to measure the same phenomenon. By examining the cosmic microwave background, which is ancient radiation from the Big Bang, researchers calculate a considerably lower expansion rate of 67 kilometres per second per megaparsec. The difference between these two values has been termed the Hubble tension, and its implications could point to new physics that remains to be fully understood.
New Method Employs Gravitational Lensing
Project Assistant Professor Kenneth Wong and his team at the University of Tokyo’s Research Centre for the Early Universe have utilized a technique called time delay cosmography to measure the Hubble constant. This method circumvents traditional distance ladders, providing an innovative perspective on cosmic expansion.
Time delay cosmography leverages the phenomenon of gravitational lensing, where massive galaxies bend light from objects situated behind them. When conditions are ideal, a single distant quasar can appear as multiple distorted images around the lensing galaxy. Each of these images travels different paths to reach Earth, resulting in varying time delays. By observing these subtle differences, astronomers can quantify the time taken for light to travel along each path.
By combining these observations with models of mass distribution in the lensing galaxy, the researchers can derive the universe’s expansion rate. The team analyzed eight gravitational lens systems, each featuring a massive galaxy distorting light from a distant quasar, using data from advanced telescopes, including the James Webb Space Telescope. Their findings align with the higher estimate of 73 kilometres per second per megaparsec from nearby observations, diverging from the 67 kilometres per second per megaparsec derived from early universe data.
The strength of this new method lies in its independence from traditional distance measurements. Should systematic errors affect either the distance ladder or cosmic microwave background analysis, this technique remains robust. The fact that it corroborates contemporary observations rather than early universe estimates bolsters the argument that the Hubble tension reflects genuine physical phenomena.
Future Research Directions
The current level of precision for this new measurement stands at approximately 4.5 percent. To confirm the Hubble tension definitively, researchers aim to improve this accuracy to between 1-2 percent. Achieving this goal involves analyzing more gravitational lens systems and refining models of mass distribution within lensing galaxies. A significant source of uncertainty arises from the complex arrangement of mass within these galaxies, although researchers typically use profiles that are consistent with existing observations.
This work represents a culmination of decades of international collaboration among various observatories and research teams. If the Hubble tension is validated, it could signify a transformative moment in cosmology, potentially reshaping our understanding of the universe’s evolution. As researchers continue to explore these intriguing discrepancies, the quest for answers promises to deepen our comprehension of the cosmos and the fundamental laws governing it.
