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Pulsar timing measures background chorus of low-pitch gravitational waves

A milestone in astronomy: for the first time, astronomers have found traces of gravitational waves with an extremely low pitch. These arise when supermassive black holes at the heart of galaxies orbit each other. The measurement technique used is a valuable addition to Earth-based observatories such as the future Einstein Telescope.

More than twenty-five years of observations with the pulsar timing technique show that the universe is filled with a murmuring background chorus of extremely low-pitched gravitational waves. This was announced by the European collaboration EPTAGW and the Australian, Chinese and North American teams PPTA, CPTA and NANOGrav on 29 June.

The gravitational waves measured by EPTAGW and their colleagues are so low in pitch that they could only originate from slowly orbiting supermassive black holes, say the researchers. Those are specimens millions of times heavier than the Sun and can be found at the centre of galaxies like our Milky Way.

Gravitational waves can be measured by observing their effect on pulsars. Credit: Danielle Futselaar, MPIfR
Gravitational waves can be measured by observing their effect on pulsars. Credit: Danielle Futselaar, MPIfR

Pulsar timing

Gravitational waves are ripples in the fabric of the universe, like the waves in a pond caused by throwing in pebbles. They occur when huge masses such as black holes orbit each other or even collide. Gravitational waves therefore provide a stunning glimpse into the universe’s most extreme events.

The pulsar timing technique used in this research measures gravitational waves by accurately tracking pulsars over long periods of time. These are extinct stars that flash very regularly as they spin on their axis. Passing gravity waves cause us on Earth to see that blink rate change. Thanks to that effect, EPTAGW astronomers were able to use dozens of such cosmic clocks as a gravitational wave detector.


Earth-based observatories such as the US LIGO, the European Virgo and the future Einstein Telescope can only measure gravitational waves caused by much smaller black holes and neutron stars with a few times the mass of the sun. Such interferometers can, however, zoom in much better on the sources of gravitational waves.

In the Dutch newspaper de Volkskrant, research member Gemma Janssen of the Dutch institute ASTRON calls the discovery a completely new way of looking at the universe: “You could say that LIGO can only hear the high tones of a flute. We listen to the low tones of a double bass. To fathom the whole composition, of course, you need everything.”

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