2023-07-14 10:01:11
An international research program, including a team from Orléans and the Nançay observatory, was able to detect gravitational waves over the long term, according to results published on June 29. Opening the possibility of observing physical phenomena without light, but by measuring the deformation of space-time.
How to observe astrophysical phenomena when you can’t see anything? This is a problem that has been posed to astrophysicists for decades: the observation of the universe absolutely depends on the reception on Earth of electromagnetic waves, particularly light, infrared or radio waves.
Is it, therefore, possible to observe black holes, these celestial objects so heavy and so dense that even light cannot escape its gravitational attraction? An international research program appears to have provided a positive answer to this question, according to research published in multiple scientific journals on June 29.
Scientists – Europeans, Americans, Chinese, Japanese, Indians, South Africans, Australians – have thus detected what strongly resembles gravitational waves “low frequency“. Among these researchers, about ten work in France. In particular within the LPC2E, the laboratory of physics and chemistry of the environment and space (partnership between CNRSCNES and the University of Orléans) and at the Nançay radio telescope, in the Berry Sologne region.
Gravitational waves were first detected as early as 2016, proving then a prediction of Einstein’s theory of general relativity. The latter explains that celestial objects deform space-time according to their mass. If these masses (black holes for example) undergo significant accelerations, the deformation of space-time will propagate, like a ripple on the surface of water, but at the speed of light (about 300,000 km per second).
The waves detected in 2016 were only an extremely brief signal, the result of the merger of two black holes. This time, the signal is much more stretched in time. This is where the revolution lies. Because the study in the length of this signal can allow “to characterize its source“, explains Gilles Theureau, astronomer at the observatory of Paris/Nançay. To know, for example, “if it is a group of black holes, their size and position in space“. Even of “distinguish events from the primordial universe“.
Because such waves may have been emitted at huge distances, and have traveled billions of years before reaching the Earth, even while moving at the speed of light. The universe being 13.8 billion years old, observing phenomena as they took place even 5 billion years ago represents a significant leap in time. Towards a time when galaxies were more numerous and smaller.
Scientists thus estimate that the gravitational signal detected this year “is compatible with a signal from two supermassive black holes, i.e. each having more than a billion times the mass of the sun“, adds Gilles Theureau. It is by orbiting one around the other that these two black holes would emit this signal. Such black holes are generally observed in the center of galaxies, like our Milky Way . SO “observing binary black holes within a galaxy means that, at some point, it merged with another“, sums up Ismaël Cognard, research director at the CNRS in Orléans.
In other words, this discovery means that scientists now have a new means of observing the universe, which could go back further in time than any optical or radio telescope. “We talk about the study of the formation of galaxies over billions of years“, adds Ismaël Cognard. Even observations of “very beginnings of the universe, it is almost cosmology“.
The role of the Nançay radio telescope was essential in this discovery. To detect space-time deformations, scientists have relied on the observation of pulsars. The latter are residues, what remains of a very massive star which, when it has no more hydrogen to continue to shine, explodes in a supernova and ejects part of its mass into space. Its core condenses into an ultra-dense mush of neutrons. We are talking “one to twice the mass of the Sun in about thirty kilometers in diameter“, describes Gilles Theureau.
Sometimes these neutron stars spin so rapidly that they emit very powerful beams of radio waves at its poles. “As the star turns, it’s like a lighthouse by the sea.“This signal is normally extremely reliable. It was by studying interference in this signal that the Nançay scientists were able to determine that the space-time had been distorted along the way.
The conclusions of the scientific program still require confirmation and additional observations. As Ismaël Cognard, from the CNRS in Orléans, explains, “it takes time to confront everything, because the signals are very weak“. Anyway, now astrophysicists can see in complete darkness.
Interview by Xavier Naizet and Charly Krief.
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