Each time a new spectral band has opened up in the domain of electromagnetic waves for’astrophysics, resulting in new discoveries. We can cite in this respect the beginnings of radio astronomy andastronomy X which made it possible to discover the quasarsthe neutron stars and the first candidates for the title of black hole. We can therefore expect similar revelations with gravitational astronomy.
The detectors Ligo and Virgo allow you to explore the spectrum from gravitational waves with some frequencies roughly between 1 and 1,000 Hz. By combining these observations with instruments operating with electromagnetic waves to carry out multi-messenger astronomy, it has also been possible to verify certain models that can account for certain gamma-ray bursts, in the occurrence those involving collisions of neutron stars also manifesting themselves in the form of kilonovae.
Another spectral band, with lower frequencies ranging from 10−5 Hertz at 1 Hertz will be accessible with the eLisa mission in space and it should be talkative about emissions gravitational waves involving supermassive black holes. But that would have to wait until the 2030s…
…Or maybe not for part of this band because we think we can use the pulsar of the Milky Way in the 1 to 100 nHz band, in particular within the framework of the collaboration North American Nanohertz Observatory for Gravitational Waves (NANOGrav) or that of theEuropean Pulsar Timing Array (Eta).
Another similar perspective, in a domain around the microhertz () is also looming on the horizon as we can be convinced with two publications to be consulted in free access on arXiv. They are somewhat reminiscent of an idea put forward many years ago and which Futura had already talked about in the previous article below.
Fifty years after Neil Armstrong’s first step, the instruments deployed on the Moon by the Apollo 11 mission are still used by French scientists. Thanks to reflective panels placed on the lunar ground, they measure the distance that separates our planet from its satellite. The key is valuable lessons on the rotation of the Moon or the composition of its core. © CNRS
From supermassive black holes to primordial cosmology
To understand what it is all about, we must remember that gravitational waves are periodic deformations of thespace-time which behaves like an elastic medium. The passage of a gravitational wave will therefore stretch and compress space in an oscillating way and therefore the distances that a ray of light and quite simply also stretching and compressing a material body.
However, it so happens that for about 50 years and following the Apollo program and the arrival of soviet moon roversthere are retroreflectors on the surface of our satellite capable of reflecting directly in their direction ofimpact laser pulses. We can therefore calculate the Earth-Moon distance very precisely by measuring the round trip time of the laser pulses on Earth.
This is what teams from the Institute of Celestial Mechanics and Ephemeris Calculation – IMCCE (Paris Observatory – PSL / CNRS / Sorbonne University) and the Géoazur laboratory (OCA) have been doing for many years with the telemetry laser Moon of the Côte d’Azur Observatory, located on the Calern plateau. This allows them to calculate the movement orbital and rotational of the Moon with an uncertainty of the order of a centimeter over 10 years.
But it turns out that the astrophysicists relativists have demonstrated by calculation that the random combination of gravitational waves in the region around the microhertz, coming from many sources in the observable cosmos and which constitutes a kind of background noise stochastic as the experts call it, can cause the orbital parameters of the Earth-Moon system to evolve.
The passage of these waves would therefore transform these two celestial bodies into a kind of material body oscillating in a partly random way but with an exploitable signature allowing us to affirm that we can clearly see the influence of gravitational waves and nothing else disturbing the orbital parameters of these celestial bodies. This requires combining the equations celestial mechanics in relativity with gravitational waves and the famous Fokker-Planck equation initially used to describe Brownian motion but which has other long-known applications in astrophysics as demonstrated by a famous article by the great Indian astrophysicist Chandrasekhar (Stochastic Problems in Physics and Astronomy).
Not only the gravitational waves of supermassive black holes binaries would thus be detectable but also other sources more exotic like a phase transition of the first order in the content of theUniverse very essential.
Earth could be used to detect gravitational waves
Article of Laurent Sacco published on 03/18/2014
A fraction of Earth-scale seismic noise could in theory come from the background of gravitational waves produced by sources scattered throughout the cosmos. It would therefore be possible to observe and measure this background with the global network of seismometers. The idea was put forward decades ago by the physicist Freeman Dyson. It has been put into practice again recently.
Freeman Dyson recently celebrated its 90th birthday. He is one of the most original minds of the XXand century. Student of the famous mathematician Godfrey Hardy in Cambridge and admirer of Tractatus logico–philosophicus of Ludwig Wittgenstein, he first made a name for himself in quantum field theory. He was indeed the first to understand the importance and the merits of the work of Richard Feynman on thequantum electrodynamics relativist, of which he gave a more rigorous form. This allowed him to land a lifetime position at Princeton University without even having a doctorate.
His scientific contributions then focused on a wide variety of fields. For example, he was an important member of the project Oriona spacecraft that would have been propelled by nuclear explosions, and we owe the concept of dyson sphere. Two researchers have just revived a brilliant idea that Dyson had in 1969. It concerns the theory of general relativity.
Weber bars
At the time, relativistic astrophysics gained its letters of nobility with the discovery of quasarspulsars and cosmic radiation. We are in the midst of a revival of studies on the general relativity, and we are beginning to take the concept of a black hole very seriously. One of the most important predictions of general relativity is that of the existence of gravitational waves. The physicist Joseph Weber got down to the task of detecting them in the 1960s. For this, he used metal bars in aluminum weighing approximately one ton, placed under vacuum and isolated as much as possible from sources of vibrations terrestrial. They are now called Weber bars.
In principle, if a strong gravitational wave resulting from a violent astrophysical phenomenon (such as the collision of two black holes) crossed the Solar system, it should make material objects vibrate by distorting the structure of space-time. The effect is very weak, and you have to make sure that the metal bars you use are really well insulated. Weber repeatedly thought he detected gravitational waves, but these were errors. Nowadays, we hunt them with giant detectors like Virgo and Ligowhich are based on a different detection principle: the measurement of fringes ofinterference with lasers.
The results are currently negative, and they set limits on the intensities and the frequency bands where we could detect gravitational waves. It will most likely be necessary to go through the eLisa project for gravitational astronomy to really take off. However, in 1968, Dyson pointed out that there is a giant, natural detector of gravitational waves: the Earth.
Cosmic Gravitational Wave Background
The Earth can indeed be compared to an elastic body in rotation capable of vibrating in response to the passage of a gravitational wave. Dyson had wondered if these vibrations could give a clear signal in the form ofseismic waves recordable by seismometers. Given the uncertainties of the time, his calculations showed that it might not be impossible with frequencies in the hertz range.
Michael Coughlin of Harvard University (Cambridge, Massachusetts) and Jan Harms ofIstituto Nazionale di Fisica Nucleare (INFN) in Florence, Italy, used the modern global network of seismometers to re-examine this question. It was for them to estimate this time the background noise ofgravitational waves from all over the cosmos in a frequency band between 0.05 and 1 Hz.
Unfortunately, they found nothing. All they did was put a new limit on the background noise in this frequency band. It is not very constraining if we compare it to those placed on other strips. But as the researchers explain in an article on arxivit represents an improvement by a factor of the order of a billion compared to the previous limit for the same frequency band.
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