Probe the cosmos using slow gravitational waves

Physics 16, 117

The path of a gravitational wave passing close to a galaxy can be deflected, producing multiple signals that could help next-generation detectors measure the expansion of the Universe.


Twin signals. A single event, such as a black hole merger, can look like two when gravitational waves follow paths that bend around a foreground galaxy on their way to Earth.

The cosmic expansion rate, defined by the Hubble constant, has become a matter of contention, as different measurement methods give different values. New method proposes using gravitational-wave events, especially those that are ‘slow’ from an intermediate galaxy, to determine the Hubble constant and other cosmological parameters [1]. This statistical approach would require many thousands of gravitational wave events, so it will only become viable when future gravitational wave detectors come online. The technique would probe the Universe at an early epoch that is between time periods targeted by other methods.

Gravitational lensing is commonly observed for light waves. The most dramatic examples are some quasars that appear in multiple locations in the sky: their light propagates along different paths around a foreground galaxy. The same bending should occur with gravitational waves, but the effect has not yet been observed. “This will happen in the coming years,” says Parameswaran Ajith of the International Center for Theoretical Sciences at the Tata Institute of Fundamental Research in India. His confidence comes from the fact that current detectors will soon be cataloging hundreds of gravitational-wave events a year, and at least some of these events will be located behind a foreground galaxy that will act as a lens.

A lensed event will be detected as two gravitational wave signals coming from the same part of the sky but with the second one arriving a few minutes to a few weeks after the first. This expected time delay reflects the slight difference in signal path lengths around the foreground galaxy. Researchers have previously considered using time delays in slow gravitational wave signals as a cosmological probe [2]but these proposals have assumed that the underlying event is observed in both gravitational waves and light, a rare event (see Point of View: Neutron Star Merger Seen and Heard).


Hidden wave detector. The underground Einstein Telescope, a third generation gravitational wave detector, is scheduled to be built in Europe in 2030.

Ajith and his colleagues have devised a new method that doesn’t require light-based observation; instead, he looks for a cosmological fingerprint in the statistics of slow gravitational wave events. The idea is that the rate of expansion of the Universe affects where the sources of gravitational events (particularly black hole mergers) are located relative to the galaxies that act as lenses. The rate of expansion also affects the distances between Earth and slow events, which in turn determine time delays. Assuming a model for the spatial distribution of galaxies, the researchers show that the fraction of gravitational-wave events that are slowed down depends on the Hubble constant and other parameters such as the average density of matter in the Universe. They also discover that the probability distribution of time lags depends on these cosmological parameters. For example, it appears that a larger Hubble constant would result in a higher fraction of slow events and a shift in the time delay distribution to smaller values ​​than would be the case for a smaller Hubble constant.

However, performing such statistical analysis will require data on many thousands of gravitational wave events, which will not be available anytime soon. Researchers are eyeing future detectors, such as the Einstein Telescope, an underground project proposed in the mid-1930s. This “third generation” of facilities are expected to be 10 times more sensitive than current observatories and could collect up to 100,000 events a year.

The researchers believe it will be worth the wait, as gravitational-wave lensing offers several advantages over current techniques for studying cosmology. For one thing, gravitational waves aren’t affected by the dust and gas that dampen light waves, so they have the potential to see far into the Universe’s past. In particular, gravitational-wave lensing could offer a measure of cosmic expansion at an early epoch, termed the ‘high redshift’, when galaxies were just starting to form. This time period lies between the earliest and latest epochs that correspond to conflicting measurements of the Hubble constant: the earliest measurements are from the CMB, while the latest measurements are based on supernova observations.

“The detection of strongly slow gravitational waves would be an important scientific breakthrough, giving observable access to the high-redshift Universe,” says gravitational wave analyst Jonathan Gair of the Max Planck Institute for Gravitational Physics in Germany. However, he wonders whether the long lead times of this statistical approach will make it less competitive with other ever-improving techniques. However, gravitational-wave physicist Chris Van Den Broeck of Utrecht University in the Netherlands believes this method has the potential to have a big impact on cosmological studies because it “adds a completely new method of measurement” with several assumptions and uncertainties.

Michael Schirber

Michael Schirber is Corresponding Editor forPhysics magazine based in Lyon, France.


  1. St. Jana et al.Cosmography using strongly slow gravitational waves from binary black holes, Phys. Rev. Lett. 130261401 (2023).
  2. K.Liao et al.Editor’s Correction: Precision Cosmology from Future Lensing Gravitational Waves and Electromagnetic Signals, Nat. Common. 82136 (2017).

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