Main points
- An international team of researchers has developed a new method for measuring the Hubble constant using gravitational waves, allowing us to more accurately calculate the expansion rate of the Universe.
- The new approach, called stochastic siren, uses the gravitational wave background and already allows us to rule out slow expansion scenarios by shifting the value of the Hubble constant into the Hubble voltage region.
Scientists Close to Resolving Hubble Tension / Unsplash / Jeremy Thomas
An international team of researchers has developed a new method for measuring the expansion rate of the Universe using gravitational waves. The approach improves the accuracy of calculations of the Hubble constant and may help explain one of the main problems in modern cosmology – the so-called Hubble voltage.
For several decades, scientists have known that the universe is expanding . The rate of this expansion is described by the Hubble constant . However, the values obtained from data from the early universe do not match measurements based on the later cosmic stage. This contradiction is known as the Hubble tension and is considered one of the most important open problems in cosmology. Phys.org writes about this.
Will the new method help resolve the Hubble tension?
A team of astrophysicists and physicists from the University of Illinois Urbana-Champaign and the University of Chicago has proposed a new way to calculate the Hubble constant using gravitational waves – weak fluctuations in space-time that occur when massive objects, including black holes, collide.
Physics professor Nicolás Yunes called the result very significant and emphasized that an independent measurement of the Hubble constant is critical to resolving the current tension. According to him, the proposed approach improves the accuracy of estimates based on gravitational waves.
Co-author of the study, Professor Daniel Holz , said that scientists have effectively created a new tool for cosmology. Using the background hum of gravitational waves from merging black holes in distant galaxies, it is possible to obtain information about the age and composition of the Universe.
The study has been accepted for publication in the journal Physical Review Letters and is also available on the arXiv server.
As measured previously
Historically, methods for determining the expansion rate have been divided into two main approaches – electromagnetic observations and gravitational wave analysis.
The so-called standard candle method uses supernovae – bright explosions of stars at the end of their lives. The distance to the supernova and its speed of retreat allow us to calculate the expansion rate.
As Scienceblog writes, with the development of gravitational wave detectors, another approach has emerged – the standard siren method. The distance to the collision of black holes can be determined by the gravitational wave signal. However, the speed of the object's retreat is difficult to measure directly – for this you need to record the light emission of the event or locate the galaxy where it occurred.
Gravitational waves are detected by the international network LIGO-Virgo-KAGRA Collaboration , which brings together over 2,000 scientists.
If the Hubble tension cannot be removed, it could mean that current models of the early universe need to be revised. Possible explanations include early dark energy, dark matter interacting with neutrinos, or changing dark energy dynamics over time.
What is the essence of the new approach?
The researchers proposed using not individual collisions, but the so-called gravitational wave background – the total signal from a large number of events that current detectors cannot yet record individually.
According to lead author Bryce Cousins , by analyzing the frequency of recorded black hole mergers, we can estimate the number of those that remain beyond the sensitivity of instruments. This total invisible signal forms the background.
The team showed that at lower values of the Hubble constant, the volume of space in which collisions occur is smaller and the density of events is higher. This means a stronger gravitational wave background . If no background is detected, this allows us to reject too low values of the expansion rate.
The method was called stochastic siren , because the events that form the background occur randomly.
Based on the available LVK data, the team showed that the absence of a fixed background already allows for the exclusion of slow expansion scenarios. Combining the new approach with measurements of individual black hole collisions gave a more precise estimate of the Hubble constant and shifted its value into the Hubble voltage region.
It is expected that over the next six years, the sensitivity of detectors will increase to the point where the gravitational wave background can be directly detected. Even until then, improving the upper limits of the signal will gradually allow us to refine estimates of the Hubble constant and better investigate the nature of cosmic expansion.