One of the greatest scientific discoveries is the expansion of the universe. This expansion drives the universe’s evolution, transforming it from the hot, and uniform state to the cold and clustered phase we observe today. It is a cornerstone of modern cosmology and makes the Big Bang theory a household name. The current expansion rate, known as the Hubble constant, named after Edwin Hubble, who discovered it in 1929 by observing the apparent motions of nearby galaxies, is of fundamental importance in modern cosmology.

Figure 1. Local measurements of 𝐻!. Credit: NASA, ESA and A. Feild (STScI).

Recently, a significant discrepancy has emerged in the measured values of the Hubble constant. These measurements can be divided into two categories: local measurements (Figure 1) based on light from relatively nearby objects, such as supernovae, and global measurements derived from observation of the cosmic microwave background assuming the standard cosmological model. While local measurements report values around 73 ± 1 km/s/Mpc, meaning that two galaxies separated by 1 Mpc (about 3.3 million light-years) appear to recede from each other at a rate of 73 km per second, global measurements prefer 67 ± 0.5 km/s/Mpc.

The discrepancy referred to as the Hubble Tension has generated considerable excitement and attention because it may indicate the existence of new physics beyond the standard cosmological model and the Standard Model of Particle Physics.

A recent study conducted by a team of researchers from the Department of Physics at the Chinese University of Hong Kong, led by Professor Ming-Chung Chu and PhD student Wangzheng Zhang, introduces a new method for measuring the Hubble constant. The team also includes Dr. Shek Yeung, a CUHK PhD graduate from 2022, along with collaborators Dr. Shihong Liao, who received his PhD from CUHK in 2015 and is affiliated with the National Astronomical Observatories of China, and Dr. Hui-Jie Hu from the University of the Chinese Academy of Sciences. Their findings, published in The Astrophysical Journal Letters, detail a novel approach that analyzes the mutual motions of galaxy pairs to measure the Hubble constant.

Uncovering the hidden dynamics of galaxy pairs

Instead of measuring the Hubble constant by observing the apparent motion of individual galaxies, as Edwin Hubble did, the researchers focused on the relative motions of pairs of galaxies (Figure 2). These motions are affected by both their mutual gravitational interactions
and the overall expansion of the universe. The team utilized a statistical measure known as pairwise velocity, which describes the relative
motion between galaxy pairs and provides valuable insights into the structure and evolution of the universe. By examining this
motion, the researchers were able to infer key details about the large-scale dynamics of the universe, including the Hubble constant.

The team used advanced computer simulations to model pairwise velocities across different expansion scenarios. They compared the results of these simulations with observational data collected from galaxy surveys. Their findings indicated that pairwise velocity could be used as an independent and accurate measurement of the Hubble constant, offering a new perspective on the Hubble Tension.

Figure 2. An illustration showing galaxy pairwise velocity in the SDSS galaxy map.

A New, More Reliable Approach to Measuring the Hubble Constant

This new method focuses on the relative velocity between galaxies within pairs, effectively minimizing common biases, including those introduced by our motion relative to the Hubble expansion. As a result, this approach is more reliable and complements existing methods for measuring the Hubble constant.

“We are excited not only because our method offers a new approach to studying the universe’s expansion, but also because it creates new opportunities for testing cosmological models and potentially measuring the mass of cosmological neutrinos,” said Wangzheng Zhang. “The
dynamics of galaxy pairs provide fresh insights into how the universe evolves. This understanding will be invaluable in tackling some of the most profound puzzles in cosmology,” added Professor Chu.

The new measurement places the Hubble constant at 75.5 km/s/Mpc, with an uncertainty of less than 1.4 km/s/Mpc. This result is consistent with other local measurements. This has reaffirmed the Hubble Tension with greater significance, further challenging the standard cosmological model.

This research result is available at https://iopscience.iop.org/article/10.3847/2041-8213/ad9aa7.