After 25 years of data collection by six radio telescopes, the announcement has fallen: dramatically “stretched” gravitational waves have been detected, indicative of the merger of supermassive black holes at the cores of galaxies. the first!
This result, obtained by a consortium of European astronomers, is published in the journal Astronomy and astrophysics. The announcement was coordinated with similar releases from other collaborations around the world, namely the Australian (PPTA), Chinese (CPTA) and North American (NANOGrav) collaborations.
Gravitational waves, these “ripples” in space-time that propagate like ripples on the surface of water, are generated by catastrophic phenomena, such as the merger of black holes. As they orbit each other faster and faster, these massive stars warp the fabric of space-time. These distortions propagate to us and can be detected by ultra-sensitive instruments, such as LIGO and Virgo interferometers. But these detectors can only perceive relatively short waves, coming from “mini” black holes or colliding neutron stars. However, as impressive as they are, these disasters are nothing compared to a frontal collision between two galaxies, for example.
To detect low-frequency waves, which would precisely emit collisions on gigantic scales, it was necessary to be cunning. the key? Pulsars, these super-dense stars that emit regular radio pulses, are a bit like metronomes. By carefully examining its pulsations, thanks to 25 years of data, scientists have been able to detect the trail of the passage of these giant gravitational waves. They carry about a million times more energy than the waves detected by LIGO and Virgo, and their oscillation frequencies span decades.
Quebec Sciences Victoria Caspi, an astrophysicist from McGill University and an active member of NANOGrav, was asked to decipher the discovery. It turns out that his doctoral dissertation in the 1990s was specifically about the possible use of pulsars for detecting this type of wave… “It gives me great pleasure and satisfaction to see that this work has contributed to the foundations for today’s success.”
***
Quebec Science: How are these gravitational waves different from those first detected in 2015 by LIGO?
Victoria Caspi: The gravitational waves detected by LIGO have a length of about 50 to 10,000 km. The waves detected by NANOGrav have a wavelength from 10 to 200 light years! It’s very different.
QS If it is “stretched”, it is due to the merger of supermassive black holes, not stellar black holes. We are talking about things billions of times larger than the mass of the sun! Did we know that these black holes can merge?
vk These results confirm this hypothesis. On the one hand, we know that the universe is full of galaxies, and that these galaxies sometimes interact and merge. On the other hand, we also believe – or at least our hypothesis – that every galaxy has a supermassive black hole at its center.
If these two concepts are correct, it is expected that sometimes two supermassive black holes can merge and produce this type of gravitational wave. The challenge, of course, was finding a way to detect it. And that’s what this business did!
So this is very strong evidence for the existence of these waves, and thus confirms the validity of our assumptions about the universe.
QS How are pulsars good tools for detecting low-frequency gravitational waves?
vk Pulsars are great at this because they produce very regular signals. So much so that if they were used as watches, they would be far more accurate than the best Rolex watch! In fact, many of the pulsars studied by the consortium are comparable, in terms of regularity, to the best atomic clocks.
When a gravitational wave passes between a radio telescope on Earth and a pulsar, the apparent frequency of the pulsar is slightly affected.
QS How can the Canadian CHIME radio telescope, with which you work, be useful for this type of observation?
vk CHIME is heavily involved in the NANOGrav group and provides monitoring of the respective sources on an almost daily basis. It is exceptional because it allows the entire planetarium in the northern hemisphere to be observed every day, and the cosmos to be studied in various ways, including by searching for fast radio bursts, another mysterious astrophysical phenomenon. Although the CHIME data do not appear in the data used for the work published today, they will certainly appear in future NANOGrav publications.
