Hubble Tension Prompts New Measurement Methods and Theories

Scientists continue to investigate the persistent difference in measurements of the Hubble constant, a key value describing how fast the universe is expanding. This research is important because resolving the discrepancy could improve understanding of cosmic history and the universe’s structure.

Traditional approaches to measuring the Hubble constant use nearby objects such as supernovae, which yield a value near 73 km/s/Mpc, while calculations based on the cosmic microwave background suggest a lower rate of about 67 km/s/Mpc. This ongoing difference, known as the "Hubble tension," has prompted the development of new measurement strategies and theoretical models.

One recent method uses time delays observed in gravitationally lensed quasars to determine the expansion rate. This approach produces a value close to 73 km/s/Mpc, aligning with local measurements but differing from early-universe estimates. The result adds further evidence to the ongoing debate about the true rate of expansion.

Another study measured the Hubble constant using Mira variable stars, achieving a precision of 3.7%. This finding supports the results from Cepheid-based local measurements and suggests that the observed tension is unlikely to be caused by measurement errors alone.

What the numbers show

• Local measurements of the Hubble constant are around 73 km/s/Mpc
• Early-universe estimates from the cosmic microwave background are about 67 km/s/Mpc
• Mira variable star studies measured the constant to within 3.7% accuracy
• Lensed quasar time delay method also yields approximately 73 km/s/Mpc

Researchers have also introduced a "stochastic siren" method, which uses the gravitational-wave background from binary black hole mergers to provide an independent measurement of the Hubble constant. This method combines constraints from non-detection of the gravitational-wave background with data from individual merger events to refine the expansion rate estimate.

When applied to data from the LVK Collaboration, the stochastic siren approach has helped rule out lower values of the Hubble constant and increased the precision of the measurement. This technique offers a new way to probe cosmic expansion without relying on traditional electromagnetic observations.

To explain the persistent difference between measurement methods, some researchers have proposed theoretical models. One such model suggests that the universe may be rotating extremely slowly, completing a single revolution every 500 billion years. This rotation could reconcile the conflicting Hubble constant values without contradicting current observations.

Another hypothesis proposes that Earth is located within a cosmic void about 2 billion light-years wide and roughly 20% less dense than average. According to this idea, the local underdensity could accelerate the observed expansion rate, potentially contributing to the Hubble tension.

* This article is based on publicly available information at the time of writing.