When radio astronomer Jocelyn Bell discovered pulsars while doing her PhD with Antony Hewish in 1967 (which would get her the 1974 Nobel Prize in Physics, along with Martin Ryle, though Jocelyn Bell wasn’t there – sparking the origins of a controversy that reverberates to this day) you undoubtedly could not imagine that decades later she would indirectly open a window to a new astronomy, that of gravitational waves.
But she was able to witness on this June 29, 2023 with the Nobel Prize in Physics Kip Thorne, a pioneer in the study and discovery of gravitational waves, the resounding announcement of the discovery of some of these waves thanks to pulsars, an announcement by the members of the International Pulsar Timing Array ( IPTA), which brings together the collaborations of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA) in Australia, and the “Indian Pulsar Timing Array Project ( InPTA).
Pulsar, pulsating star corpses
Recall that pulsars are the corpses of massive stars that have exploded in SNSN II type supernovae. All that remains then is an analogue of an atomic nucleus, which can contain as much mass as the Sun in a sphere no more than a few tens of kilometers in diameter. When rotating rapidly, these objects also have a magnetic field that slows their rotation, but also causes them to emit a beam of radio waves that sweeps through space like a lighthouse. Some of these rays periodically cross the Earth and therefore appear in radio telescopes as very regular beeps, making some pulsars very stable cosmic clocks.
We know that since 2015, thanks to the work of researchers in the United States such as Kip Thorne, but also in France such as Alain Brillet and Thibault Damour, we have been able to discover and interpret on Earth the gravitational waves of the collisions of stars, black holes, neutron stars and neutron stars . These waves, which cause the room to vibrate like sound, air or waves on the water surface, have short wavelengths in the LigoLigo and VirgoVirgo detectors.
The European Space Agency (ESAESA) wants to observe longer wavelength waves on the horizon of the 2030s and with a detector in space as part of the LisaLisa project, which are very specific and which could have originated from the Big Bang and/or the movements of supermassive black holesSupermassive Black holes form binariesBinary systems at the heart of galaxiesGalaxies complete their merger. These giant black holes would talk about the evolution of galaxies.
Some explanations about pulsars. © Above ground
From Russian and American theorists to the gravitational wave hunters of Nançay
But we knew that from work in the late 1970s and early 1980s with researchers like the Russian Mikhail V. Sazhin of the mythical Sternberg Astronomical Institute (co-author with Dolgov and Zeldovich of a remarkable little introductory work on modern cosmology), and particularly the American Ron Hellings and George Downs that part of the spectrum of long-wavelength gravitational waves could undoubtedly be detected and measured using pulsars (This “pulsar network”-based detector is frequency-complementary to what will be accessible with LISA within a decade, with the first sensitive from nano-Hz to micro-Hz, the second from fractions from milli-Hz to tenths of a HzHz Thus, LISA will experience the merger events of massive black holes with a size of a few million or tens of millions of solar masses, like those at the center of the Milky Way, Milky Way (Sagitarius A*) when EPTA observes the close orbit of pairs of supermassive black holes (1-10 billion solar masses).
This is the so-called stochastic gravitational-wave background, a fluctuating and chaotic superposition of all waves emitted by supermassive black holes orbiting each other on a cosmological scale, but also potentially even more exotic objects and phenomena, such as cosmic strings or the gravitational waves of the Big Bang, as Futura explained in a previous article.
The basic idea for this detection is to use among the pulsars those that are the most stable and therefore behave like a kind of clock, the beeps of which are separated by regular time intervals. The passage of a gravitational wave, whose wavelength is measured in light-years, between a pulsar and an observer on Earth, vibrating the distances between the two celestial bodies, causes the arrival times of the pulses to vary. electromagnetic pulsars. We thus show that when observing nearby pulsars in the sky, the periodic delays and advances between the arrival of the radio pulses, measured over years, are correlated and allow the precise shape of gravitational waves at very low frequencies to be traced. Shape that encodes the nature of the sources of these waves.
However, the NANOGrav collaboration, which was the first to take to social media to announce that a major announcement is imminent, is not at the forefront of the International Pulsar Timing Array (IPTA). France itself plays an important role within the European Pulsar Timing Array (EPTA) with the legendary Nançay radio telescope, one of the largest in the world (5th and 2nd for Europe), and this is why Futura approached Gilles Theureau, Astronomer and astronomer at the Observatoire de Paris-PSL and one of the researchers involved in announcing the discovery of the gravitational-wave background that we can see together with his colleagues from Nançay at the beginning of the video below. He was kind enough to answer our questions.
As part of a global network observing pulsars, a European consortium published June 29, 2023 in the journal Astronomy and Astrophysics a set of results from data collected over a quarter-century from six of the world’s most sensitive pulsars. The data from the European consortium, as well as those from its American, Australian and Chinese colleagues, contain very solid evidence for the existence of gravitational waves, detected at very low frequencies, which would emanate from pairs of supermassive black holes at the center of merging galaxies. The French participation in this work is significant and includes the contribution of researchers from the Paris Observatory (PSL), the CNRS, the CEA, the University of Orléans and the University of Paris Cité. © The Paris Observatory
Futura: The great Nançay radio telescope began its career almost 60 years ago by studying the famous line 21 cm from hydrogen, which notably allowed the mapping of the Milky Way but was also used for the Seti program. Since when does he also research pulsars?
Giles Theureau: Since the late 1980s and early 1990s, particularly following the dissertation work of Ismaël Cognard, already dealing with the timing of millisecond pulsars, approaching the topic of gravitational-wave background detection following the ideas of Hellings and Downs in 1983.
Work on pulsars, not necessarily intended to capture this background, was developed later at Nançay until they now account for about 70% of the available observing time.
Nançay’s contribution to the European Pulsar Timing Array (EPTA) program is also about 70% and it should be noted that EPTA was established in 2006 before the collaboration of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in the United States.
We use more radio telescopes than NANOGrav and for Nançay we are about ten people, mainly from the University of Orléans, but also researchers from the CEA and the Laboratory of Astroparticles and Cosmology-Cosmology (APC) in Paris, dedicated to the analysis of data and data focus their interpretation. Some of these people had already worked on gravitational waves and their detection with Ligo, Virgo and ESA’s future Laser Interferometer Space Antenna (Lisa).
How to hunt pulsars in Nançay. © Above ground
Futura: Why did you use millisecond pulsars?
Giles Theureau: Because they are very stable cosmic clocks. When discovered, they were initially disconcerting because they were spinning very quickly, in just over a millisecond or so, as if they had just formed when there were no more supernova remnants around them. It is also known that young pulsars possess an intense magnetic field that decreases over time while slowing the rotation of neutron stars.
We finally understood that the millisecond pulsars were old pulsars whose rotation was accelerated again by the accumulation of matter from a companion star, accompanied by a transfer of angular momentum. Although its magnetic field weakens with age, its rotation decreases at a slower rate than that of an ordinary pulsar.
The millisecond pulsars we have selected no longer accumulate significant matter, as they are coupled with other neutron stars or white dwarf stars. And since their interiors are stabilized and they have a particularly crystallized crust, they also produce few “glitches” (in French “glitches”, “snags”), an English term that describes the violent variation of a pulsar’s rotation period to a sudden one Coupling between the inner superfluid and the crust, causing a rotational hiccup.
There are “burst-burst” type events, mergers, or very close flybys of two black holes that can produce bursts similar to glitches, but we know how to distinguish them. One or more consecutive glitches could be mistaken for gravitational “noise” but would only be observed in one pulsar at a time and would be uncorrelated.
Futura: In the jargon of physicists, whether in particle physics or in astrophysics, to announce a discovery it is generally necessary to achieve 5 Sigma, but all global collaborations brought together within IPTA have results whose statistics be approximately between 3 and 4 sigma. So why announce that the gravitational wave background has already been observed?
Giles Theureau: Ideally, the probability of observing the randomly received signals should be about 0.00006%, or about the chance of getting the same number eight times in a row when you roll the dice. But all teams observe similar waveforms and since we use different instruments, this cannot be due to an inherent error. We are therefore confident that within a year or two, when we have aggregated all our observations on a global scale, we will get these 5 Sigmas.
Giles Theureau: More so with the increase in computing power to analyze the observed signals and extract the effects of gravitational waves than with advances in electronics. The electromagnetic pulses emitted by the pulsars are in fact disturbed by their journey through the plasma of the interstellar medium and it is therefore necessary to be able to filter these disturbances effectively (see the video from the Paris Observatory above). In Nançay in 2008, a cluster of 77 CPUs was replaced by a set of four graphics cards for this purpose.
In the case of Nançay, we will expand the reception band of radio reception, which will allow us to be instantaneously more sensitive and therefore have more precise timing, directly affecting our sensitivity to gravitational waves. In general, of course, we will refine the shape of their spectrum by collecting other observations with other radio telescopes.
EPTA has the advantage over NANOGrav that it can observe the same pulsar with five different radio telescopes, identifying systematic effects that would otherwise go unnoticed. The combination of the data (thanks in particular to Nançay) allows for an unprecedented rate of 1-2 days on average. This gives access to frequencies around 10-6 Hz while other programs are limited to around 10-7 Hz.
By specifying the shape of the signals, we can also specify their possible origins and try to decide between several hypotheses.
So if the wave background is rather the product of phenomena during the Big Bang, for example during an inflation phase or during the phase transition phase transition of a plasma composed of quarks, quarks and gluons, which on cooling transforms into protons, protons and neutrons, we see a background more isotropic-isotropic Waves. On the other hand, if they are supermassive black holes, some are closer to the Milky Way and even appear as point sources on the celestial canopy.
Some dark matter theories, such as ultralight particles, also predict that the passage of concentrations of this matter in the Milky Way between the solar system and pulsars will affect pulsar signals due to the gravitational field gravity of these concentrations. Precisely what is being said in the ultralight dark matter models is that the gravitational potential of our own galaxy should oscillate at a specific frequency, affecting the optical path of the pulsars’ signal, mimicking, for example, the emission of a single pair of black holes. The current results show that this type of dark matter does not exceed a few tens of percent of the total dark matter content. Research on this is also in progress.
Presentation in 2017 by its director Stéphane Corbel of the radio astronomy station of the Paris Observatory in the Sologne in Nançay: a fleet of instruments dedicated to the observation of the universe in different radio wave bands, unique in France, from the pioneer radio telescopes to the most innovative devices. © Com Nancay